No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Abstract
Rapid recruitment of small fish after biomanipulation in warm lakes may delay the reestablishment of submerged macrophytes, not least at high nutrient concentrations. Success has recently been obtained in controlling phosphorus (P) loading to many lakes, but nitrogen (N) inputs often remain high. To determine the interactive effects of N loading and the abundance of small-sized fish on the growth of the submerged macrophyte Vallisneria natans, we conducted an outdoor mesocosm experiment with a factorial design on the north shore of Lake Taihu, China. The experiment involved two densities of small crucian carp – low (10 g m− 2) and high (40 g m− 2) – crossed with two levels of N loading – present-day external nutrient loading (P: 5 μg L− 1 day− 1, N: 130 μg L− 1 day− 1) and P: 5 μg L− 1 day− 1 with a three times higher N loading (N: 390 μg L− 1 day− 1). The results showed that nitrogen-fish interactions significantly hindered the growth of V. natans, particularly at the high N loading. At low N loading, high densities of fish decreased the relative growth rate, mean leaf length, leaf mass and root mass of V. natans by 16%, 5%, 8%, and 23%, respectively, compared with these measures at low fish densities. The effect of fish was even stronger when N loading was high, with decreases of 232%, 32%, 57%, and 47% for the respective plant growth measures. The stronger effect at high N loading was attributed to higher turbidity due to enhanced phytoplankton biomass and to increased consumption or damage of plants by the fish in response to the more nutrient-enriched plant tissue. Our results indicate that high abundance of small crucian carp in warm lakes may reduce the resilience of submerged macrophytes to external N loading, thereby lowering the chances of successful restoration by biomanipulation.
... However, single restoration measures, such as reducing exogenous nutrient loads or planting submerged plants alone, do not lead to plant recovery (Hilt et al., 2006). High fish activity levels (herbivory, disturbance, etc.) may jeopardize restoration efforts Dorenbosch & Bakker, 2012;Gu et al., 2018). Recently, the fast recovery of fish biomass and dominance by benthivorous fish in subtropical and tropical lakes have been widely observed (Clavero et al., 2023;Roshni et al., 2022). ...
... Benthivorous fish have a high potential for causing sediment resuspension, which harms water characteristics (Badiou & Goldsborough, 2015;. However, some previous studies have provided conflicting evidence, such as PO 4 -P increase He et al., 2022), decrease (Badiou & Goldsborough, 2015), or no change (Gu et al., 2018). Overlying water Chl a increase (Badiou & Goldsborough, 2015;Ren et al., 2022) or no change . ...
... A single factorial design experiment was conducted with three densities of fish (FD), that is, 0, 1, and 4 M. anguillicaudatus (0, 7, and 28 g m −2 ), respectively ( Figure S1). The densities were chosen based on observations during field surveys in lakes in the middle and lower reaches of the Yangtze River (Gu et al., 2018). At the beginning of the experiment, 192 shoots of plants were planted in 12 tanks; each tank contained 16 shoots, which were chosen based on observations during field surveys in Liangzi Lake (Qian et al., 2014). ...
Submerged macrophytes play a key role in the restoration of shallow eutrophic lakes. However, in some subtropical lakes, benthivorous fishes dominate the fish assemblages and influence the growth of submerged plants. A comprehensive understanding of the direct and indirect effects of benthivorous fishes on submerged plants is important. We conducted mesocosm experiments to examine the effects of three densities of benthivorous fish, Misgurnus anguillicaudatus, on the water properties, the growth, asexual reproduction, and the germination of turions of Potamogeton crispus L. Our results showed that fish disturbance increased TN, TP, PO4-P, NH4-N, and NO3-N of the water, raising the extinction coefficient K, Chl a, and the periphyton biomass. Benthivorous fish disturbance reduced the total biomass, root length, relative growth rate (RGR), and branching number while increasing the plant height of P. crispus. The P stoichiometric homeostasis coefficient (H P) (except turions) and H N was lower in plant tissues due to fish disturbance. Benthivorous fish disturbances promoted turions formation (e.g., increased turions total numbers and biomass) of P. crispus. Moreover, P. crispus exhibited transgenerational plasticity for benthivorous fish affecting turion emergence. The maximum final germination rate occurred only when fish density in the mother plant grow experiment matched that in the turion germination experiment. Turions generated by P. crispus disturbed by low-density fish exhibited increased germination rates. Our findings suggest that controlling benthivorous fish reduces its indirect and direct effects on submerged vegetation, facilitating the successful restoration of these plants.
... The canopy-forming species can, therefore, capture sufficient light, and plenty of light may relieve plants from NH 4 -related physiological stress, as suggested in previous mesocosm experiments Yuan et al., 2016). Moreover, it has been found that fish disturbance did not affect the relative growth rate (RGR) of canopy-forming H. verticillate (Chen et al., 2020a) but hindered the growth of rosette-forming V. natans (Gu et al., 2018). Therefore, we hypothesised that 1) low light conditions caused by fish disturbance might aggravate NH 4related stress on the population growth of submersed macrophytes, in other words create a negative synergistic effect; 2) the strategies of different growth forms of submersed macrophyte might have different tolerance levels to the combined effect of high NH 4 -N stress and fish disturbance. ...
... These contrasts the findings in several other studies (e.g., Badiou and Goldsborough, 2015;Chen et al., 2020a) but it may reflect the high biomass of macrophytes, buffering the effect of fish disturbance and nutrient excretion (Kaldy, 2014). Previous studies have showed that fish disturbance has positive (He et al., 2017;Gu et al., 2018), neutral (Parkos et al., 2003;Chen et al., 2020b) and negative effects (Wahl et al., 2011;Badiou and Goldsborough, 2015) on the content of Chl a in the water column. In our study, the content of Chl a was not affected by fish disturbance, which may be explained by the following mechanisms: 1) the rather dense submersed macrophytes may control phytoplankton by producing allelochemicals that suppress phytoplankton growth rates (Vanderstukken, et al., 2014), and 2) submersed macrophytes may control phytoplankton by competing for nutrients (Jeppesen et al., 1998;Declerck et al., 2007). ...
... Our results are different from those of previous studies showing that fish disturbance inhibited the growth of V. natans (Gu et al., 2018). However, as a canopy-forming species, the leaves of M. spicatum are more abundant in the upper layer of the water column and sparse in the lower part. ...
Benthivorous fish disturbance and nitrogen loading are two important factors that influence the community structure of submersed macrophytes, but their interactive effect is not well elucidated. We conducted an outdoor mesocosm experiment to examine the individual and combined effects of these two factors on the growth of two submersed macrophytes of different growth forms, i.e., the rosette-forming Vallisneria natans and the canopy-forming Myriophyllum spicatum. The treatments involved two levels of fish (Carassius auratus) disturbance crossed with two levels (0 and 12 g NH4Cl per month) of ammonium (NH4 ⁺-N) loading. For M. spicatum, we found that maximum height (MH) was reduced by 30.7%, 26.4%, and 51.0% in fish addition alone (F) and nitrogen addition treatments (N, F + N), respectively, compared with the control (C) treatment. The density of M. spicatum declined by 20%, 62% and 68.8% in the F, N and F + N treatment. The above-ground biomass (AGB) of M. spicatum respectively reduced by 56.7%, 94%, and 96.5% in the F, N and F + N treatments, and the roots/shoots ratio (R/S) increased by 114%, and 176% in N and N + F treatments, respectively. Regarding V. natans, only the MH in the N treatment was reduced (71.9%), and the density exhibited a reduction of 59.1% and 64.5% in the N and F + N treatments, respectively. The AGB of V. natans was significantly lower in the N (90.3%) and N + F (78.4%) treatments compared with the C treatment, while increased by 60.3% in F treatment. The R/S of V. natans increased by 227%, and 74.4% in the N and F + N treatments compared with the C treatment. The interactive effect of fish disturbance and high N on MH and AGB of V. natans and density of M. spicatum were antagonistic. However, the interactive effect on density and BGB of V. natans and AGB of M. spicatum were negatively synergistic. Moreover, fish activity significantly increased the concentration of total suspended solids (TSS) in the water, while total nitrogen (TN), ammonium, total phosphorus (TP), light, pH and salinity were unaffected. When fish and nitrogen were combined, TN, TP, TSS and salinity increased significantly, while pH decreased. Our study reveals that the interactive effects of fish disturbance and high N are synergistic and/or antagonistic, suggesting that the same stressor interaction may vary from synergistic to antagonistic depending on the response variables and growth forms of the macrophytes examined. Our study contributes to the understanding of how different factors can interact with each other and affect submersed macrophytes in aquatic ecosystems. This is timely and relevant knowledge, considering the range of multiple stressors involved in the decline of aquatic ecosystems worldwide at present.
... Sediment resuspension increases the turbidity of the water and thus reduces underwater light availability impacting submerged macrophytes (Lorenz et al., 2013;Gu et al., 2016;Chen et al., 2020a). Besides, resuspension may also promote internal nutrient release from the sediment, leading to increased phytoplankton growth and potentially deteriorated water quality (Roozen et al., 2007;Gu et al., 2016Gu et al., , 2018He et al., 2017). ...
... Hence, the positive effects of submerged macrophytes on water quality can be offset by the negative effects of benthivorous fish that may hamper submerged macrophyte growth and expansion (Gu et al., 2018;Chen et al., 2020aChen et al., , 2020b. In a mesocosm study, treatments with the macrophyte Vallisneria natans had the best water quality, while it was worst when benthic fish were present (Ren et al., 2022). ...
... As a result of the relative high N and P fluxes, phytoplankton growth boosted in the controls, where cyanobacteria increased from <20 μg Chl a L − 1 to ~100 μg Chl a L − 1 and chlorophytes from <10 μg Chl a L − 1 to ~50 μg Chl a L − 1 . This is in line with other studies showing that nutrient release from the sediment to the overlying water caused by small crucian carp can significantly promote the growth of phytoplankton, especially cyanobacteria and chlorophytes (Gu et al., 2016(Gu et al., , 2018He et al., 2017;Han et al., 2021). V. natans significantly reduced cyanobacteria, and slightly lowered chlorophyte biomass. ...
Bioturbation by omni-benthivorous fish often causes sediment resuspension and internal nutrient loading, which boosts phytoplankton growth and may lead to a shift of clear water lakes to a turbid state. Removal of large-sized omni-benthivorous individuals is a lake restoration measure that may revert lakes from a turbid to a clear water state, yet the rapid reproduction of small omni-benthivorous fish in tropical and subtropical shallow lakes may impede such lake recovery. In lake restoration, also a combination of lanthanum-modified bentonite (LMB) and planting submerged macrophytes has been used that may synergistically improve lake water quality. How the combined effect works in the presence of small omni-benthivorous fish has not been studied, which is needed given the high abundances of small omni-benthivorous fish in (sub)tropical lakes. We conducted a two-by-two factorial mesocosm experiment with and without the submerged macrophytes Vallisneria natans and with and without LMB, all in the presence of small crucian carp. At the end of the experiment, turbidity in the V. natans, LMB and combined LMB + V. natans treatments had decreased by 0.8%, 30.3% and 30.9%, respectively, compared with the controls. In addition, the nitrogen (N) and phosphorus (P) release from the sediment in the combined LMB + V. natans treatments had decreased substantially, by 97.4% and 94.3%, respectively, compared with the control. These N and P fluxes were also significantly lower in the combined LMB + V. natans treatments than in the sole LMB treatment (88.1% and 82.3%) or the V. natans treatment (93.2% and 90.3%). Cyanobacteria in the overlying water in the combined LMB + V. natans treatments significantly decreased by 84.1%, 63.5% and 37.0%, respectively, compared with the control and the sole LMB and V. natans treatments. Our results show that LMB and submerged macrophytes complement each other in effectively improving the water quality, even in the presence of small omni-benthivorous fish.
... Lake eutrophication is accompanied by an increase in fish density and a shift from piscivorous-dominated communities towards planktivorous-and benthivorous-dominated communities (Jeppesen et al., 2000). Benthivorous fish can adversely affect submerged macrophytes through several pathways: direct physical disturbance by uprooting, sediment resuspension increasing turbidity during their feeding on benthic invertebrates, preying on zooplankton and periphyton grazers leading to increased shading of macrophytes by phytoplankton and periphyton (Phillips et al., 2016;Scheffer et al., 1993; and direct macrophyte consumption (Gu et al., 2018;. ...
... The presence of juvenile common carp substantially reduced the abundance of the submerged macrophyte P. crispus, which is in line with previous studies (Gu et al., 2018;. ...
... This reduction was most likely caused by several known effects of common carp (Vilizzi et al., 2015), such as increased water turbidity due to sediment disturbance and increased phytoplankton (Gu et al., 2018) and periphyton biomass, probably due to fish predation on zooplankton and snails ( Figure S2; Phillips et al., 2016). These effects reduced the light availability for submerged macrophytes (Figure 4). ...
Multiple stressors such as climate change and eutrophication are responsible for the global decline in macrophytes in lakes. The loss of this key component can result in turbid conditions and a loss of important ecosystem functions and services, particularly in shallow lakes. Benthivorous fish, which can increase in abundance during eutrophication, can adversely affect macrophytes through physical disturbance, cascading effects on turbidity, suspended and attached algae (phytoplankton and periphyton) and direct consumption. However, whether warming amplifies their effects on macrophytes and can trigger regime shifts remains unexplored.
Here, we tested the single and combined effects of warmer water (+4.5°C) and the widespread benthivorous juvenile common carp Cyprinus carpio on two different types of aquatic macrophytes in 24 mesocosms (2,500 L each). We monitored phytoplankton, periphyton, turbidity and the abundance of the submerged curly leafed pondweed Potamogeton crispus and the floating‐leaved water chestnut Trapa bispinosa during their growing season. These species dominated successively in spring and summer.
Warming alone advanced the growing season of P. crispus by 17 days. Juvenile carp decreased the abundance of the more palatable P. crispus, but promoted the abundance of T. bispinosa, supporting an ecosystem shift to a dominance of floating‐leaved macrophytes. Fish also substantially increased water turbidity and the biomass of phytoplankton and periphyton. Warming amplified juvenile carp effects on turbidity and submerged macrophytes, but also decreased the abundance of floating‐leaved macrophytes leading to an overall macrophyte decline and increase in water turbidity.
Synthesis and applications. Our study provides the first experimental evidence for a warming‐induced regime shift from clear‐water conditions dominated by submerged or floating/floating‐leaved macrophytes to a turbid state in shallow aquatic ecosystems. The regime shift was triggered by the impacts of warming on benthivorous fish (juvenile common carp) rather than on macrophytes. Lowering nutrient loading and other measures to reduce the abundance of benthivorous fish (e.g. fish removal and piscivorous fish restocking) thus may become increasingly important for the management of shallow lakes under global climate change.
... However, the ecological effects of benthivorous fish on the growth of submersed macrophytes and water quality are species-specific. For instance, Astronotus crassipinnis increased the number of plant fragments (Ribas et al., 2017), but Carassius carassius did not (Gu et al., 2018); C. carpio uprooted submersed macrophytes during spawning (Zambrano and Hinojosa, 1999), whereas Scardinius erythrophthalmus showed no negative effects on submersed macrophyte biomass (Dorenbosch and Bakker, 2012). The impacts of fish on macrophytes also depend on the fish biomass density. ...
... Fish impacts on macrophytes can be very negative at high fish biomass densities; however, their impact may be neutral or slightly positive at very low fish biomass densities (Wood et al., 2017). Moreover, some results are inconsistent, such as those regarding the concentrations of phosphate phosphorus (P-PO 4 ) (Badiou and Goldsborough, 2015;Gu et al., 2018) and chlorophyll a (chl a) (Kyeongsik et al., 2001;(Badiou and Goldsborough, 2015;Chen et al., 2019;Gu et al., 2018;He et al., 2017;Parkos et al., 2003;Wahl et al., 2011)). Therefore, we need to improve our understanding of the conditions under which fish would be expected to have corresponding impacts on submersed macrophyte growth and water quality. ...
... Fish impacts on macrophytes can be very negative at high fish biomass densities; however, their impact may be neutral or slightly positive at very low fish biomass densities (Wood et al., 2017). Moreover, some results are inconsistent, such as those regarding the concentrations of phosphate phosphorus (P-PO 4 ) (Badiou and Goldsborough, 2015;Gu et al., 2018) and chlorophyll a (chl a) (Kyeongsik et al., 2001;(Badiou and Goldsborough, 2015;Chen et al., 2019;Gu et al., 2018;He et al., 2017;Parkos et al., 2003;Wahl et al., 2011)). Therefore, we need to improve our understanding of the conditions under which fish would be expected to have corresponding impacts on submersed macrophyte growth and water quality. ...
... However, the ecological effects of benthivorous fish on the growth of submersed macrophytes and water quality are species-specific. For instance, Astronotus crassipinnis increased the number of plant fragments (Ribas et al., 2017), but Carassius carassius did not (Gu et al., 2018); C. carpio uprooted submersed macrophytes during spawning (Zambrano and Hinojosa, 1999), whereas Scardinius erythrophthalmus showed no negative effects on submersed macrophyte biomass (Dorenbosch and Bakker, 2012). The impacts of fish on macrophytes also depend on the fish biomass density. ...
... Fish impacts on macrophytes can be very negative at high fish biomass densities; however, their impact may be neutral or slightly positive at very low fish biomass densities (Wood et al., 2017). Moreover, some results are inconsistent, such as those regarding the concentrations of phosphate phosphorus (P-PO 4 ) (Badiou and Goldsborough, 2015;Gu et al., 2018) and chlorophyll a (chl a) (Kyeongsik et al., 2001;(Badiou and Goldsborough, 2015;Chen et al., 2019;Gu et al., 2018;He et al., 2017;Parkos et al., 2003;Wahl et al., 2011)). Therefore, we need to improve our understanding of the conditions under which fish would be expected to have corresponding impacts on submersed macrophyte growth and water quality. ...
... Fish impacts on macrophytes can be very negative at high fish biomass densities; however, their impact may be neutral or slightly positive at very low fish biomass densities (Wood et al., 2017). Moreover, some results are inconsistent, such as those regarding the concentrations of phosphate phosphorus (P-PO 4 ) (Badiou and Goldsborough, 2015;Gu et al., 2018) and chlorophyll a (chl a) (Kyeongsik et al., 2001;(Badiou and Goldsborough, 2015;Chen et al., 2019;Gu et al., 2018;He et al., 2017;Parkos et al., 2003;Wahl et al., 2011)). Therefore, we need to improve our understanding of the conditions under which fish would be expected to have corresponding impacts on submersed macrophyte growth and water quality. ...
... Previous studies have shown that most small-bodied fish species are either omnivorous or zooplanktivorous in shallow lakes within the MLYRB (Xie et al., 2005;Zhang, 2005), and that these fishes can affect water quality through various pathways (Yu et al., 2021a). For example, small-bodied omnibenthivorous fishes such as crucian carp and pond loach Misgurnus anguillicaudatus, which live near the bottom and feed mainly on the benthos, detritus and aquatic vegetation (Zhang, 2005), can suspend sediments leading to an increase in turbidity and a release of nutrients back into the water column, thus boosting the growth of algae (Gu et al., 2017;Ren et al., 2022). Small-bodied omniplanktivorous fishes (e.g., sharpbelly H. leucisculus and bitterling Acheilognathus macropterus) feed on a variety of food items including zooplankton, plant material, benthic invertebrates and periphyton (Yu et al., 2016(Yu et al., , 2020. ...
... Traditional biomanipulation, which aims to reduce the biomass of zooplanktivorous fishes to increase grazing pressure on phytoplankton by zooplankton, has been studied extensively in temperate lakes (Mehner et al., 2004;Søndergaard et al., 2008). In shallow subtropical lakes of the MLYRB, however, studies have mostly evaluated the food web effects of small-bodied omnibenthivorous fishes (Gu et al., 2017;Huang et al., 2020;Ren et al., 2022) or omniplanktivorous fishes (Zhou and Chen, 2015;Yu et al., 2020Yu et al., , 2021b, with minimal consideration of zooplanktivorous fishes. This is despite zooplanktivorous fishes outnumbering omnivorous fishes in many MLYRB lakes-Taihu Lake , Gehu Lake and Biandantang Lake are notable examples. ...
The proliferation of small-bodied fishes in lakes is often accompanied by deterioration of water quality and ecosystem function. However, the potential impacts of different types of small-bodied fish species (e.g., obligate zooplanktivores and
omnivores) on subtropical lake ecosystems in particular have been overlooked mainly due to their small size, shorter life spans and lower economic value. Therefore, we conducted a mesocosm experiment to elucidate how plankton communities and water
quality respond to different types of small-bodied fishes, including a common zooplanktivorous fish (thin sharpbelly Toxabramis swinhonis) and other small-bodied omnivorous fishes (bitterling Acheilognathus macropterus, crucian carp Carassius
auratus and sharpbelly Hemiculter leucisculus). During the experiment, the mean weekly total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (CODMn), turbidity, chlorophyll-a (Chl.α) and trophic level index (TLI) values were
generally higher in treatments where fish were present compared to treatments where fish were absent, but responses varied. At the end of the experiment, phytoplankton abundance and biomass and the relative abundance and biomass of cyanophyta were higher while the abundance and biomass of large-bodied zooplankton were lower in the fish-present treatments. Moreover, the mean weekly TP, CODMn, Chl.α and TLI values were generally higher in treatments with the obligate zooplanktivore, thin
sharpbelly, when compared to treatments with omnivorous fishes. Also, the ratio of zooplankton to phytoplankton biomass was the lowest, and the ratio of Chl.α to TP was the highest in treatments with thin sharpbelly. Collectively, these general findings indicate that an overabundance of small-bodied fishes can have adverse effects on water quality and plankton communities and that small-bodied zooplanktivorous fishes likely induce stronger top-down effects on plankton and water quality than omnivorous fishes. Our results emphasise that small-bodied fishes should be monitored and or controlled if overabundant when managing or restoring shallow subtropical lakes. From the perspective of environmental protection, the combined stocking of different piscivorous fish species that feed in different habitat types could be a way forward to control small-bodied fishes with different feeding habits, but more research is needed to assess the feasibility of this approach.
... In the present study, the significant increase in Chl a concentration in the presence of juvenile crucian carp may be explained by a combination of three factors. Firstly, juvenile crucian carp disturbed the sediment and the resuspension promoted the release of nutrients from pore water, e.g., TN, TP, and SRP, which promoted the growth of phytoplankton (Gu et al., 2016(Gu et al., , 2018He et al., 2017). Similarly, a stimulated growth of cyanobacteria is often recorded due to more nutrient resuspension after a typhoon passed in Lake Taihu, China (Zhu et al., 2014). ...
... Crucian carp may also exert direct effects on V. natans that may inhibit growth, such as potential physical damage, uprooting, or direct grazing of V. natans. Some studies have found submerged macrophyte residues in the stomachs of juvenile crucian carp (Gu et al., 2016(Gu et al., , 2018. ...
• The re-establishment of submerged macrophytes is crucial in shallow lake restoration transforming a turbid into a clear water system, but it might be inhibited by juvenile omni-benthivorous fish. These fishes disturb the sediment, and even uproot new sprouts, creating turbid water through sediment resuspension and release of nutrients stored in the sediment. Lanthanum modified bentonite (LMB), an in-situ capping material, can immobilise phosphate and consolidate sediments, potentially weakening the negative effects of juvenile omni-benthivorous fish.
• We hypothesised that applying LMB would improve water quality and promote the growth of submerged macrophytes by eliminating or weakening the increase of suspended solids, nutrient concentration, and algal reproduction due to juvenile omni-benthivorous fish disturbance. To test our hypothesis, a two-by-two factorial mesocosm experiment with two densities of juvenile crucian carp (absence and 10 g/m³) and two doses of LMB (absence and 800 g/m²) was conducted.
• Juvenile crucian carp significantly inhibited the growth and reproduction of the macrophyte Vallisneria natans by reducing the underwater light and direct effects such as uprooting. No significant inhibition or promotion by LMB on the growth of V. natans in the absence of juvenile crucian carp was observed. However, LMB significantly inhibited the fish-induced sediment resuspension and nutrients increases. Consequently, algal production was reduced and underwater light conditions were improved, which promoted the growth and reproduction of V. natans, e.g., more ramets, biomass, and leaves of V. natans.
• Our results indicated that submerged macrophytes could benefit from LMB application under high intensity disturbance by juvenile omni-benthivorous fish, thereby conducive to the maintenance of a clear water state in shallow lakes.
... daily as aqueous solutions of potassium nitrate (KNO 3 ), which is the main inorganic nitrogen in Lake Taihu (Xu et al., 2021b). The loading of external N (130 μg L − 1 d − 1 ) was based on Gu et al. (2018) and He et al. (2021), and simulated external influxes in Lake Taihu . To prevent unrealistic high temperatures occurring, each bucket was wrapped with insulation cotton. ...
... The changes in DMB ULB , AI ULB specified in (ii) can lead to RGR ULB , NAR ULB ≤ 0 (Lamont et al., 2023). Since, according to J. Gu et al. (2018), negative leaf growth is accompanied by a decrease in plant RGR, then winter wheat with RGR ULB , NAR ULB < 0 reached full ripeness the fastest. Simultaniously, LAD ULB , BMD ULB changed reciprocally, relative to NAR ULB , RGR ULB . ...
The relative and absolute importance of a number of traits, in particular, agrophysiological, morpho-functional, at the level of individual organs and parts of an integral plant, and/or sowing for the development of features of biological traits, and other agroecologically significant components of the crop production process, has been discussed in research papers for a long time. The purpose of the study was to search for agroecologically significant signs of growth of the upper leaf blades (ULB), which can empirically and potentially determine the development of the grain dry mass (GDM) of winter wheat under “model” conditions of biological agrotechnical influences designated as biological fertiliser systems. Methods used in the research: methodological approaches of field experiments, gravimetric, convective drying, and stochastic methods. The development of GDM was largely driven by potentially scalable integral growth traits of ULB – leaf area duration, biomass duration (LADULB, BMDULB, respectfully) or their combinations with potentially non-scalable features of the average growth rate ULB – net assimilation rate, relative growth rate (NARULB, RGRULB, respectively). It is also highly probable that LADULB may play a central role in the development of RGRULB or BMDULB (but not NARULB). The coordination of RGRULB with NARULB was not excluded, although it was overly complicated. The construction of such and similar studies in the line of an exhaustive explanation of consistent systemic and mechanistic predeterminations of the production process with signs of ULB growth under various agrotechnical and biological influences will improve discursive and mathematical simulation constructs that can characterise and integrate the differential effects of plant components on photosynthesis of leaf cover, crown, and ultimately on the processes of development of components of the final biological and economic yield of winter wheat
... Fish grazing and disturbance have a notable impact on the growth of submerged plants (Chen et al., 2020;Gu et al., 2018;Li et al., 2023;Liu et al., 2018;Ren et al., 2022). Both experiments revealed a significant decrease in the biomass of submerged plants in the presence of fish ( Fig. 1a; Fig. 2a). ...
Biological manipulation, involving fish stockings, is commonly used to counteract the deterioration of submerged vegetation in eutrophic lakes. Nevertheless, the non-consumptive effects (NCEs) of stocked carnivorous fish are often overlooked. Using a controlled experimental system, we investigated the NCEs of a native carnivorous fish, snakehead (Channa argus), on two key biological factors, herbivore-dominated grass carp (Ctenopharyngodon idella) and disturbance-dominated loach (Misgurnus anguillicaudatus), influencing submerged plants growth. Additionally, we conducted a meta-analysis on predation risk and primary productivity. The results reveal that predation risk induces oxidative stress damage and affects grass carp growth. Non-significant changes in cortisol and glucose may be linked to predation risk prediction. Simultaneously, predation risk reduces fish feeding and disturbance behavior, relieving pressure on submerged plants to be grazed and disturbed, thereby supporting plant development. The presence of submerged plants, in turn, enhances loach activity and influences water body characteristics through negative feedback. Furthermore, the meta-analysis results indicate the facilitative effect of predation risk on primary producers. Our findings contribute to the understanding of biological manipulation theory. We demonstrate that the predation risk associated with introducing carnivorous fish can promote the growth of submerged plants through behaviorally mediated indirect effects. This highlights the potential utility of predation risk in lake restoration efforts.
... However, upper trophic levels are often limited to different species of arthropods, not fish, in microcosms and even mesocosms due to size limitations. Yet, the importance of including fish is evident from some mesocosm experiments studying effects of climate change and nutrient loading using stickleback which predicted severe effects on fish survival (and predation) (Moran et al., 2010;Moss, 2010), but then other studies using bighead carp or crucian carp did not (Gu et al., 2018;He et al., 2018). Thus, using generalities about consequences to fish based on limited studies can be dangerous. ...
An endless list of new chemicals are entering nature, which makes it an impossible task to assess all possible mixture combinations at all possible concentrations and conditions that are leading to the ubiquitous anthropogenic impacts on the aquatic environment resulting from deteriorating water quality. Therefore, ecotoxicology is moving more toward a mechanistic understanding of toxicological processes, using trait-based approaches and sublethal molecular and physiological endpoints to understand the mode of action of pollutants and the adverse outcomes at the organismal and population level. These molecular and physiological endpoints can be used as biomarkers, applicable in the field. This brings ecotoxicological research much closer to conservation physiology. Understanding the relationships between chemical reactivity in the water and in organisms, and assessing the consequences at higher levels, allows conservation physiologists and managers to take the right restoration measures for an optimal improvement of the aquatic habitats of concern. In this chapter we discuss the role which the promising approach of mechanistic-based Adverse Outcome Pathways (AOPs) can play in ecotoxicological research. It studies a pathway of events, from the direct interaction of a chemical with a molecular target, through subsequent intermediate events at cellular, tissue, organ and individual organism levels which then result in an Adverse Outcome (AO) relevant to ecotoxicological risk assessment and regulatory decision-making. In this context, we also discuss the importance of modeling, including bioavailability based and effect based models. Finally, we reflect on the possibilities that meta-analysis has to offer to detect unifying physiological processes, as well as interesting outliers.
... We had expected that crucian carps would impose strong negative effects on the growth of the submerged macrophytes as described in a previous study (Gu et al. 2018). However, we did not observe this in our study. ...
Submerged macrophytes are vital components in shallow aquatic ecosystems, but their abundances have declined globally. Shading by periphyton and phytoplankton/turbidity plays a major role in this decline, and the competing aquatic primary producers are subject to the complex influence of multiple stressors such as increasing temperatures, nutrient loading and herbicides. Their joint impact has rarely been tested and is difficult to predict due to potentially opposing effects on the different primary producers, their interactions and their grazers. Here, we used 48 mesocosms (2500 L) to simulate shallow lakes dominated by two typical submerged macrophytes, bottom-dwelling Vallisneria denseserrulata and canopy-forming Hydrilla verticillata, and associated food web components. We applied a combination of nutrient loading, continuous warming, heat waves and glyphosate-based herbicides to test how these stressors interactively impact the growth of submerged macrophytes, phytoplankton and periphyton as competing primary producers. Warming or heat waves alone did not affect phytoplankton and periphyton abundance, but negatively influenced the biomass of V. denseserrulata. Nutrient loading alone increased phytoplankton biomass and water turbidity and thus negatively affected submerged macrophyte biomass, particularly for V. denseserrulata, by shading. Glyphosate alone did not affect biomass of each primary producer under ambient temperatures. However, heat waves facilitated phytoplankton growth under combined nutrient loading and glyphosate treatments more than continuous warming. As a consequence, H. verticillata biomass was lowest under these conditions indicating the potential of multiple stressors for macrophyte decline. Our study demonstrated that multiple stressors interactively alter the biomass of primary producers and their interactions and can eventually lead to a loss of macrophyte communities and shift to phytoplankton dominance. These results show the risks in shallow lakes and ponds in agricultural landscapes and underline the need for multiple stressor studies as a base for their future management.
... During eutrophic conditions, phytoplankton and periphyton growth is promoted, which can substantially reduce the growth of submerged macrophytes via increased light competition (Zhang et al., 2020). Furthermore, changes in light availability and nutrient loading may modify plant nutrient content, the carbon (C):nutrient [nitrogen (N) and phosphorus (P)] composition, and growth rates (Gu et al., 2018;Velthuis et al., 2017;Zhang et al., 2020). However, during winter, when temperatures are low, algae growth may be temperature limited (Edwards et al., 2016), and eutrophication may thus have less impact during these early stages of submerged macrophyte establishment. ...
Submerged macrophytes play a key role in maintaining a clear‐water phase and promoting biodiversity in shallow aquatic ecosystems. Since their abundance has declined globally due to anthropogenic activities, it is important to include them in aquatic ecosystem restoration programs. Macrophytes establishment in early spring is crucial for the subsequent growth of other warm‐adapted macrophytes. However, factors affecting this early establishment of submerged macrophytes have not been fully explored yet. Here, we conducted an outdoor experiment from winter to early spring using the submerged macrophytes Potamogeton crispus and Vallisneria spinulosa to study the effects of shading, nutrient loading, snail herbivory (Radix swinhoei), and their interactions on the early growth and stoichiometric characteristics of macrophytes. The results show that the effects strongly depend on macrophyte species. Biomass and number of shoots of P. crispus decreased, and internode length increased during low light conditions, but were not affected by nutrient loading. P. crispus shoot biomass and number showed hump‐shaped responses to increased snail biomass under full light. In contrast, the biomass of the plant linearly decreased with snail biomass under low light. This indicates an interaction of light with snail herbivory. Since snails prefer grazing on periphyton over macrophytes, a low density of snails promoted growth of P. crispus by removing periphyton competition, while herbivory on the macrophyte increased during a high density of snails. The growth of V. spinulosa was not affected by any of the factors, probably because of growth limitation by low temperature. Our study demonstrates that the interaction of light with snail herbivory may affect establishment and growth of submerged macrophytes in early spring. Macrophyte restoration projects may thus benefit from lowering water levels to increase light availability and making smart use of cold‐adapted herbivores to reduce light competition with periphyton. Our study demonstrates that the interactions of light with snail herbivory, but not nutrient loading, may affect establishment and growth of submerged macrophytes in early spring. Our study thus has implications for recovery of submerged macrophytes in deteriorative water bodies.
... Crucian carp (Carassius carassius L.) is a widespread omni-benthivorous fish species in subtropical and tropical freshwater waterbodies and constitute a major threat to the water clarity in warm shallow lakes (Gao et al., 2014). Studies have shown that juvenile crucian carp can enhance eutrophication by unrooting submerged macrophytes, disturbing sediments, enhancing internal loading, and preying on herbivorous zooplankton and zoobenthos (Gu et al., 2016(Gu et al., , 2018He et al., 2017;Han et al., 2020). In hypereutrophic lakes, however, gut content analysis showed that crucian carp also ingest bloom-forming and/or filamentous phytoplankton (Kolmakov and Gladyshev, 2003;Liu, 2008). ...
Biomanipulation by piscivore stocking has been widely used to combat eutrophication in north temperate lakes, but its applicability in warm lakes has not yet been well elucidated. Here, we used experimental mesocosms to test the effects of a native benthi-piscivore (snakehead, Channa argus Cantor) on water clarity under subtropical conditions where small omni-benthivorous fish like crucian carp (Carassius carassius L.) prevail. Our results showed that, despite of a great reduction of crucian carp biomass, snakehead stocking did not create a strong trophic cascade as neither (herbiv-orous) zooplankton biomass nor their grazing pressure, indicated by biomass ratio of (herbivorous) zooplankton to phytoplankton, changed significantly. Moreover, snakehead stocking significantly increased water non-algal turbidity as well as nutrient and chlorophyll-a concentrations, suggesting that these benthi-piscivores also disturbed sediments like crucian carp did. Our study showed that biomanipulation by stocking of snakehead does not facilitate clear-water state in warm shallow lakes, even on the short-term.
... where W f (g) and W i (g) are the average final and initial mass of the snails or macrophytes in each aquarium, respectively, in grams (Gu et al.,2018). The effects of macrophyte species, grazing treatment, and their interaction on the biomass and RGR of macrophytes were determined using two-way ANOVA with post hoc LSD tests for multiple comparisons. ...
The relationships between producers (e.g., macrophytes, phytoplankton and epiphytic algae) and snails play an important role in maintaining the function and stability of shallow ecosystems. Complex relationships exist among macrophytes, epiphytic algae, phytoplankton, and snails. We studied the effects of snail communities (consisting of Radix swinhoei, Hippeutis cantori, Bellamya aeruginosa, and Parafossarulus striatulus) on the biomass of phytoplankton and epiphytic algae as well as on the growth of three species of submerged macrophytes (Hydrilla verticillata, Vallisneria natans, and one exotic submerged plant, Elodea nuttallii) in a 90-day outdoor mesocosm experiment conducted on the shore of subtropical Lake Liangzihu, China. A structural equation model showed that the snail communities affected the submerged macrophytes by grazing phytoplankton and epiphytic algae (reduction in phytoplankton Chl-a and epiphytic algal abundance), enhancing the biomass of submerged macrophytes. Highly branched macrophytes with high surfaces and morphologies and many microhabitats supported the most snails and epiphytic algae (the biomass of the snail communities and epiphytic algae on H. verticillata was greater than that on V. natans), and snails preferred to feed on native plants. Competition drove the snails to change their grazing preferences to achieve coexistence.
... For example, Razlutskij et al. [20], through 72 days of outdoor experiments, found that introducing Carassius auratus increased the biomass of Phyt. Recent ecological studies have revealed significant negative effects of crucian carp (Cruc) on the water quality and ecological states of shallow lakes, e.g., increasing nutrient levels, leading to reduced water clarity [21,22]. The current international guidance on the application of biological manipulation technology to the process of lake ecological restoration remains far from sufficient [23]. ...
Maintaining the health of lake ecosystems is an urgent issue. However, eutrophication seriously affects lakes’ ecological functions. Eutrophication is also the main target of lake ecological restoration. It is vital to carry out research on lake eutrophication control and energy flow evaluation in ecosystems scientifically. Based on in situ survey results for the aquatic life data for Lianshi Lake from 2018 to 2019, the Ecopath model was used to establish an evaluation index system for the typical biological chain to screen out the key species in the water ecosystem, and the fuzzy comprehensive evaluation (FCE) method was used to screen all the biological chains controlling algae. A combination of the FCE coupled with the Ecopath screening method for typical biological chains for algal control was applied to the Lianshi Lake area; the results show that the typical biological chain for algal control is phytoplankton (Phyt)–zooplankton (Zoop)–macrocrustaceans (Macc)–other piscivorous (OthP). Upon adjusting the biomass of Zoop and Macc in the typical biological chain for algal control to three times that of the current status, the ecological nutrition efficiency of Phyt was increased from 0.308 to 0.906. The material flow into the second trophic level from primary producers increased from 3043 to 8283 t/km2/year. The amount of detritus flowing into primary producers for sedimentation decreased from 7618 to 2378 t/km2/year. Finally, the total primary production/total respiratory volume (TPP/TR) decreased from 9.224 to 3.403, the Finn’s cycle index (FCI) increased from 13.6% to 17.5%, and the Finn’s average energy flow path length (FCL) increased from 2.854 to 3.410. The results suggest that the problem of eutrophication can be solved by introducing Zoop (an algal predator) and Macc to a large extent, resulting in improved ecosystem maturity. The research results can facilitate decision making for the restoration of urban lake water ecosystems.
... One way to restore eutrophicated lakes following external nutrient loading reduction is therefore to re-establish the submerged macrophytes by direct transplantation (Liu et al., 2018;Li et al., 2021), but several studies report difficulties in reinstating clear water status by the transplantation of macrophytes alone (Lauridsen et al., 2003;Paice et al., 2016;Knopik and Newman, 2018), due to the initial low survival of the macrophyte seedlings transplanted or the instability of the macrophyte community in the following years (Larned et al., 2006). One possible reason is that the stirring of benthic fish promoted re-suspension of fine detritus from the sediment and thus reduce water transparency, inhibiting the growth of all primary producers especially for the macrophyte seedlings (Gu et al., 2018). Although both macrophytes and periphyton serve as food for snails, periphyton constitute the majority of their diets (Lodge, 1986;Cao et al., 2014;Yang et al., 2020). ...
In shallow eutrophic lakes, submersed macrophytes are essential for maintaining a clear water state, and they are affected markedly by fishes directly through herbivory and indirectly by fish-invertebrate-periphyton complexity, a pathway that presently is not well understood in subtropical lakes but probably vital to lake managements. We conducted a mesocosm study involving benthic fish (Misgurnus anguillicaudatus), snails (Radix swinhoei) and submersed macrophyte (Vallisneria natans), aiming to examine whether benthic fish is detrimental to reestablishment of clear-water macrophyte-dominated state in eutrophic degraded lakes. In addition, we aimed to investigate the cascading effect that benthic fish might have on periphyton and phytoplankton and to what extent snails can alleviate this effect. Our results showed that benthic fish promoted nutrient release from the sediment and thereby facilitated the growth of phytoplankton and periphyton, leading to reduced growth of submerged macrophytes due to shading. Snails consumed the periphyton attached on the leaves of macrophytes, thereby being beneficial to the plant growth, albeit it could not fully counteract the adverse effects from benthic fish. The water quality indicators in terms of nutrients concentrations, phytoplankton biomass and light extinction coefficient along the water column was affected primarily by benthic fish, followed by macrophytes and snails. To target a clear-water condition, the water quality was best at the presence of macrophytes alone or in combination with snails, and worst at the presence of benthic fish. Our results implied that the removal of benthic fish should be a useful ecological restoration method for rehabilitation of submersed macrophytes and water quality improvement in subtropic, eutrophic, shallow lakes following external nutrient loading reduction.
... verticillata: 0.718 ± 0.027 g, M. spicatum: 0.746 ± 0.020 g, and C. caroliniana 0.641 ± 0.017 g; means ± SD). After 1 week, all of the shoots had survived and adapted to the new environment (Gu et al. 2018;Huang et al. 2018;Li et al. 2016). The water depth in each bucket was then increased to 75 cm (measured from the bottom of the bucket). ...
Aims
The submerged plant species Carolina fanwort (Cabomba caroliniana) has become a dominant invasive aquatic plant in the Lake Taihu Basin (LTB) in China. Introduced species may escape their original specialist enemies and encounter fewer enemies in their new environment. They were assumed to have suffered less herbivory than native species as they are relatively unpalatable (the enemy release hypothesis (ERH)). The objective of this study was to compare the responses of C. caroliniana with those of co-occurring native species to herbivory from native herbivores.
Methods
We conducted a mesocosm experiment to record the responses of C. caroliniana and two commonly co-occurring native submerged plant counterparts, water thyme (Hydrilla verticillata) and Eurasian watermilfoil (Myriophyllum spicatum), to herbivory by two native generalist gastropod snails, Radix swinhoei and Sinotaia quadrata. Plant morphological traits (total biomass, shoot/root (S/R) biomass ratio and relative growth rate (RGR)) and physiological traits (leaf total nonstructural carbohydrate (TNC), lignin, and cellulose) were recorded.
Important Findings
The snail S. quadrata rarely influences the plant traits of the three submerged plants. With the increasing numbers of R. swinhoei treatments, most of the plant traits of H. verticillata and M. spicatum changed, while those of C. caroliniana showed a relatively stable fluctuation. This result indicated that C. caroliniana was more resistant to herbivory by the snail R. swinhoei, which is consistent with the ERH hypothesis. This finding indicates that herbivorous snail species contributes the invasion of C. caroliniana, which potentially alters the species composition of submerged plants in the plant community.
... Pomacea canaliculata are generalist herbivores that can exert top-down control on a wide range of aquatic plants, including various submerged macrophytes species (Fang et al., 2010;He & Liu, 2016;Wong et al., 2010). Their effects on N concentrations may trigger other negative interactions, e.g. between macrophytes and benthivorous fish (Gu et al., 2018). ...
• In shallow lakes, submerged macrophytes can stabilise clear‐water conditions and prevent phytoplankton blooms. Nutrient enrichment can promote their abundance. Above critical thresholds in water nutrient concentrations, however, submerged macrophytes have collapsed in many lakes worldwide with negative consequences for important ecosystem functions. The mechanisms behind this decline are complex and subject to debate.
• We conducted mesocosm and laboratory experiments to investigate the top‐down effect of invasive herbivorous snails (Pomacea canaliculata) on native submerged macrophytes (Vallisneria natans) under low and moderate nutrient treatment. We tested whether effects of nutrient addition to the water column on snail herbivory were influenced by shading by phytoplankton and epiphyton, macrophyte elemental tissue content and snail nutrient release.
• We found that herbivorous snails had strong negative top‐down effects on macrophytes, which were amplified by nutrient enrichment in the water column leading to higher macrophyte consumption rates. In moderate nutrient treatment, macrophyte consumption was faster and snails grew better than in low nutrient treatment. Additionally, snail treatments had higher nitrogen concentrations in the water leading to a negative feedback loop.
• Our results suggest that snail herbivory may increase the chance for macrophyte collapse and shifts of shallow lakes to turbid states, and this effect occurs at lower snail densities when nutrient concentrations increase. Shallow lakes are thus severely at risk from projected increases in nutrient loading and spread of invasive herbivorous molluscs.
... Strong water movement can affect shoot elongation and plant development directly, and thereby indirectly influence the growth of submerged macrophytes via sediment resuspension (Kankanamge et al., 2011;Bertrin et al., 2017). Similarly, fish disturbance inhibits the growth of these plants by increasing nutrient loading in the water column via sediment resuspension (Cu et al., 2018;Chen et al., 2020). These phenomena may result in changes to both light and nutrient availability given the strong sediment resuspension that occurs in shallow lakes. ...
The allocation of limiting elements among plant organs is an important aspect of the adaptation of plants to their ambient environment. Although eutrophication can extremely alter light and nutrient availability, little is known about nutrient partitioning among organs of submerged macrophytes in response to eutrophication. Here, we analyzed the stoichiometric scaling of carbon (C), nitrogen (N), and phosphorus (P) concentrations among organs (leaf, stem, and root) of 327 individuals of seven common submerged macrophytes (three growth forms), sampled from 26 Yangtze plain lakes whose nutrient levels differed. Scaling exponents of stem nutrients to leaf (or root) nutrients varied among the growth forms. With increasing water total N (WTN) concentration, the scaling exponents of stem C to leaf (or root) C increased from <1 to >1, however, those of stem P to root P showed the opposite trend. These results indicated that, as plant nutrient content increased, plants growing in low WTN concentration accumulated leaf C (or stem P) at a faster rate, whereas those in high WTN concentration showed a faster increase in their stem C (or root P). Additionally, the scaling exponents of stem N to leaf (or root) N and stem P to leaf P were consistently large than 1, but decreased with a greater WTN concentration. This suggested that plants invested more N and P into stem than leaf tissues, with a higher investment of N in stem than root tissues, but eutrophication would decrease the allocation of N and P to stem. Such shifts in plant nutrient allocation strategies from low to high WTN concentration may be attributed to changed light and nutrient availability. In summary, eutrophication would alter nutrient allocation strategies of submerged macrophytes, which may influence their community structures by enhancing the competitive ability of some species in the process of eutrophication.
... This may lead to higher phytoplankton biomass and higher light attenuation (Menezes et al., 2010), potentially hampering the growth of submerged macrophytes (Gu et al., 2016). Studies on the effects of direct grazing by omnivores on macrophytes have so far focused on large-sized species, such as common carp (Cyprinus carpio) (Lougheed et al., 1998;Miller and Crowl, 2006) and crucian carp (Carassius carassius) (Gao et al., 2017b;Gu et al., 2018). The impacts of small omnivores, commonly found among the macrophytes (Ye et al., 2006;Cai et al., 2019), on submerged macrophytes have rarely been investigated. ...
Recovery of submerged macrophytes has been considered a key factor in the restoration of shallow eutrophic lakes. However, in some subtropical restored lakes, small omnivorous fish dominate the fish assemblages and feed in part on submerged macrophytes. Knowledge of the effects of small omnivores on the growth of submerged macrophytes is scarce and their responses are potentially species-specific， i.e. the growth of some species may be hampered by fish grazing while growth of others may be promoted by the nutrients becoming available by fish excretion. We conducted mesocosm experiments to examine the effects of the small omnivorous bitterling Acheilognathus macropterus, a common species in restored subtropical lakes in China, on nutrient concentrations and the growth of four species of submerged macrophytes (Hydrilla verticillata, Vallisneria denseserrulata, Ceratophyllum demersum and Myriophyllum spicatum). We found that the bitterling significantly increased nutrient concentrations via excretion and thereby enhanced the net growth of the less grazed nuisance macrophyte M. spicatum. In contrast, the net growth of C. demersum was reduced by the bitterling, most likely due to grazing as indicated by gut content analyses. Dominance by bitterling may, therefore, pose a threat to the long-term success of lake restoration by provoking a shift in the submerged macrophyte community towards nuisance species through selective grazing. Nutrient excretion may potentially also stimulate the growth of phytoplankton and periphyton, hampering the growth of submerged macrophyte.
... In general, submersed macrophytes in Chinese lakes are subjected to low-light stress due to lake eutrophication and further shading from phytoplankton blooms, and the distribution area of submersed macrophytes has broadly declined in recent years (Ni, 2001;Zou et al., 2013). The recovery of submersed macrophytes faces many difficulties, and the successful recovery of submersed macrophytes in an entire lake is rare (Qin, 2007;Søndergaard et al., 2007;Gu et al., 2017). Overall, more data regarding low-light stress adaptation mechanisms are needed to increase the probability of submersed macrophyte recovery in lakes. ...
... In sediment with a high organic matter content, higher temperature might increase sediment organic matter mineralization (Gudasz et al. 2010, Sobek et al. 2017), leading to higher nutrient availability for plants (Fisher et al. 2005, Alsterberg et al. 2012. Similarly, with increased nutrient loading to the water layer, aquatic plants can also accumulate nutrients (Gu et al. 2016, Gu et al. 2018). Both forms of increased nutrie nt availability may lead to a decrease of the C:nutrient ratio with temperature rise as long as increasing temperature, as a result, the plant C:nutrient ratio increases with temperature. ...
... In general, submersed macrophytes in Chinese lakes are subjected to low-light stress due to lake eutrophication and further shading from phytoplankton blooms, and the distribution area of submersed macrophytes has broadly declined in recent years (Ni, 2001;Zou et al., 2013). The recovery of submersed macrophytes faces many difficulties, and the successful recovery of submersed macrophytes in an entire lake is rare (Qin, 2007;Søndergaard et al., 2007;Gu et al., 2017). Overall, more data regarding low-light stress adaptation mechanisms are needed to increase the probability of submersed macrophyte recovery in lakes. ...
The decline in submersed macrophytes induced by low-light stress is ubiquitous in mid-lower Yangtze lakes in China. However, the trade-offs among the adaptation mechanisms used by submersed macrophytes to low-light stress remain unclear. Moreover, the experimental period used in most previous studies was relatively short, and plant traits were not monitored multiple throughout the experimental period. In the present study, we examined the growth of a representative submersed macrophyte, Potamogeton maackianus, under four light regimes (2.8%, 7.1%, 17.1% and 39.5% of ambient light) over a course of 12 months and assessed various plant traits at monthly and quarterly intervals. The results showed that P. maackianus exhibited a lower leaf moisture content and a higher stem moisture content under decreased light conditions. Under the lowest light regime, P. maackianus had low soluble carbohydrate (SC) contents in the leaves and low starch contents in the stems at all the seasons. P. maackianus showed different allometric relationships among different treatments reflected different adaption strategies resulting from different light environment. P. maackianus exhibited a relatively stable biomass allocation pattern characterized by a continual increase in the stem mass fraction of total biomass under relatively high light transmittance (17.1% and 39.5%), which is a response of P. maackianus that can ensure a high initial biomass in the next spring season. However, under low light transmittance, the biomass allocation pattern fluctuated early in the experiment, and the amplitude increased with decreases in the light transmittance (2.8% and 7.1%). P. maackianus exhibited trade-offs between the stem (plant height) and leaf mass (leaf area) fractions of the total biomass, which help improve the adaptation of the macrophyte to a low-light environment. Our results can be useful for estimating light-use conditions of P. maackianus according to regression relationships between the leaf/stem mass fractions of the total biomass and time.
... Both warming (temperature rise) and eutrophication (nutrient enrichment) affect aquatic plant nutrient content and subsequent stoichiometry. Nutrient enrichment in the environment significantly increases the plant nutrient content (Dorenbosch and Bakker, 2011;Dülger et al., 2017), and decreases the C:nutrient ratios (Gu et al., 2016;Velthuis et al., 2017;Gu et al., 2018). However, studies on the impact of warming on aquatic plant C:nutrient ratios are scarce and yield contradictory results (Cross et al., 2015;Velthuis et al., 2017). ...
The abundance and stoichiometry of aquatic plants are crucial for nutrient cycling and energy transfer in aquatic ecosystems. However, the interactive effects of multiple global environmental changes, including temperature rise and eutrophication, on aquatic plant stoichiometry and palatability remain largely unknown. Here, we hypothesized that (1) plant growth rates increase faster with rising temperature in nutrient-rich than nutrient-poor sediments; (2) plant carbon (C): nutrient ratios [nitrogen (N) and phosphorus (P)] respond differently to rising temperatures at contrasting nutrient conditions of the sediment; (3) external nutrient loading to the water column limits the growth of plants and decreases plant C:nutrient ratios; and that (4) changes in plant stoichiometry affect plant palatability. We used the common rooted submerged plant Vallisneria spiralis as a model species to test the effects of temperature and nutrient availability in both the sediment and the water column on plant growth and stoichiometry in a full-factorial experiment. The results confirmed that plants grew faster in nutrient-rich than nutrient-poor sediments with rising temperature, whereas external nutrient loading decreased the growth of plants due to competition by algae. The plant C: N and C: P ratios responded differently at different nutrient conditions to rising temperature. Rising temperature increased the metabolic rates of organisms, increased the nutrient availability in the sediment and enhanced plant growth. Plant growth was limited by a shortage of N in the nutrient-poor sediment and in the treatment with external nutrient loading to the water column, as a consequence, the limited plant growth caused an accumulation of P in the plants. Therefore, the effects of temperature on aquatic plant C:nutrient ratios did not only depend on the availability of the specific nutrients in the environment, but also on plant growth, which could result in either increased, unaltered or decreased plant C:nutrient ratios in response to temperature rise. Plant feeding trial assays with the generalist consumer Lymnaea stagnalis (Gastropoda) did not show effects of temperature or nutrient treatments on plant consumption rates. Overall, our results implicate that warming and eutrophication might interactively affect plant abundance and plant stoichiometry, and therefore influence nutrient cycling in aquatic ecosystems.
Rebuilding a clear-water state dominated by submerged macrophytes is essential for addressing eutrophication, yet the impact of benthic fish on water quality is complex. We conducted two experiments to explore the interaction of submerged plants and benthic fish on the water quality. Experiment I investigated the water clearing effects of submerged macrophytes with varying coverage (from 0% to 40%) before and after the removal of benthic fish. Experiment II explored the impacts of benthic fish at different densities on aquatic ecosystems with and without submerged macrophytes. The results showed that an increase in submerged macrophytes coverage significantly enhanced the reduction of some major water quality parameters. We assert that the coverage of submerged macrophytes should not be lower than 40% to establish and sustain a clear-water state in shallow lakes. However, benthic fish significantly weaken the ability of submerged macrophytes to improve water quality. Surprisingly, the presence or absence of macrophytes may reverse the role of benthic fish in freshwater ecosystems. When macrophytes are present, benthic fish can cause water quality to deteriorate. Conversely, when macrophytes are absent, benthic fish with a density of ≤ 10 g/m3 can restrict the growth of phytoplankton by directly consuming algae or by disturbing sediments to increase turbidity, thereby potentially improving water quality. But the detrimental effects of benthic fish with higher densities may gradually outweigh their benefits to water clarity. Therefore, the percentage of submerged macrophyte cover in combination with the density of benthic fish play crucial roles in shaping the ecological effects of benthic fish and overall ecosystem dynamics. These findings underscore the importance of understanding ecosystem interactions and have practical implications for the management of shallow lakes.
This research examines the Jiefang Park Water Environment Remediation Project, a pioneering initiative in urban lake rehabilitation. The project is anchored in a fish-plant symbiosis system, which has significantly improved water quality, upgrading it from Category V to Category III, and in certain zones, achieving the commendable Category II. This enhancement is quantitatively marked by a notable reduction in nitrogen and phosphorus levels, and a marked increase in water transparency, with submerged plants covering over 70% of the lakebed.A key innovation is the utilization of a machine learning model to predict chlorophyll-a concentration, a vital water quality metric. The model’s accuracy is underscored by its R² values, ranging from 0.23 to 0.99, and RMSE values between 6.921 and 0.237, with the best performance at point 1. Additionally, comprehensive measurement data highlight the project’s effectiveness. Water transparency significantly improved from baseline levels, as evidenced by increased dissolved oxygen levels at various sampling sites, indicative of restored ecological equilibrium in the lake. This study represents an exemplary fusion of ecological restoration and advanced data analytics, signifying a substantial advancement in urban lake restoration. It underscores the potential of combining synergistic ecological approaches with technological innovation to further sustainable urban environmental management.
When restoring shallow lakes, the recovery of submerged macrophytes is of vital importance to obtain clear water conditions. However, post-restoration recovery of small omnivorous fish can increase water nutrient concentrations through excretion. The impact of these recycled nutrients on submerged macrophytes is not well-studied, and it may depend on fish food of varying nutrient contents. We studied the effects of Acheilognathus macropterus on the growth and nutrient stoichiometry of Vallisneria denseserrulata and Hydrilla verticillata. The experiment had six treatments: two controls without recycled nutrients and two nutrient treatments for each of the plant species, respectively. We found that fish released more phosphorus after consuming low N:P food and more nitrogen after high N:P food. The recycled nutrients significantly promoted the growth of both macrophyte species. Interestingly, macrophytes showed higher growth rates in treatments with nutrients from the low N:P food source. Our study, therefore, suggests that nutrient subsidy from fish excretion can significantly stimulate the growth of submerged macrophytes, the magnitude of the effect being determined by the N:P ratio of the fish food. The realization of this growth potential, however, also depends on the phytoplankton response to these nutrients, that expectedly will be weaker at high macrophyte coverage.
The invasive red swamp crayfish Pro-cambarus clarkii (Girard, 1852) can affect freshwater ecosystems, their macrophyte communities in various ways. To examine the effects of red swamp crayfish on clearwater macrophyte-dominated lakes, we performed a 28-day mesocosm experiment, without and with presence of red swamp crayfish, with or without access to sediments using netting. All in the presence of one of the following three different submerged macrophytes: Myriophyllum spicatum L., Vallisneria denseserrulata (Makino) Makino, or Hydrilla verti-cillate (L.f.) Royle. This experimental design allowed a partial separation of the effects of disturbing the sediment using netting from other factors such as excretion and direct harm of the plants. Nutrients and total suspended solids concentrations were lower in the crayfish-with-net than in the crayfish-without-net treatments but had similar concentrations in the crayfish with net and crayfishless-without-net treatments. Compared to the crayfish-with-net treatment, there were more broken leaves in the crayfish-without-net treatments for V. denseserrulata and H. verticillata, while no difference was found for M. spicatum, likely reflecting their different growth forms. Our results suggest that the sediment disturbance by the crayfish was more important than their plant handling ,consumption and nutrient excretion for an observed water quality change from clear to turbid
External nutrient loading can cause large changes in freshwater ecosystems. Many local field and laboratory experiments have investigated ecological responses to nutrient addition. However, these findings are difficult to generalize, as the responses observed may depend on the local context and the resulting nutrient concentrations in the receiving water bodies. In this research, we combined and analysed data from 131 experimental studies containing 3054 treatment‐control abundance ratios to assess the responses of freshwater taxa along a gradient of elevated nutrient concentrations. We carried out a systematic literature search in order to identify studies that report the abundance of invertebrate, macrophyte, and fish taxa in relation to the addition of nitrogen, phosphorus, or both. Next, we established mixed‐effect meta‐regression models to relate the biotic responses to the concentration gradients of both nutrients. We quantified the responses based on various abundance‐based metrics. We found no responses to the mere addition of nutrients, apart from an overall increase of total invertebrate abundance. However, when we considered the gradients of N and P enrichment, we found responses to both nutrients for all abundance metrics. Abundance tended to increase at low levels of N enrichment, yet decreased at the high end of the concentration gradient (1–10 mg/L, depending on the P concentration). Responses to increasing P concentrations were mostly positive. For fish, we found too few data to perform a meaningful analysis. The results of our research highlight the need to consider the level of nutrient enrichment rather than the mere addition of nutrients in order to better understand broad‐scale responses of freshwater biota to eutrophication, as a key step to identify effective conservation strategies for freshwater ecosystems.
Subtropical lakes are often dominated by benthi-omnivorous fish (e.g., crucian carp) that may substantially disturb sediments and thereby increase water turbidity and nutrient concentrations. Piscivores stocking is sometimes used to control these benthi-omnivores in the expectation that they will reduce their adverse effects on the water quality. However, in shallow subtropical lakes the benthic foraging activities of piscivorous fish may also disturb sediments, a topic that has not yet been well elucidated. Here, we conducted a 6-day mesocosm experiment to explore the bioturbation effects of two native subtropical piscivorous fish (i.e., snakehead, Channa argus and mandarin fish, Siniperca chuatsi) in prey fish (crucian carp, Carassius carassius) present and absent scenarios. Our results showed that in mesocosms without prey, both snakehead and mandarin fish greatly increased the concentrations of suspended solids. However, the concentrations of total, organic and inorganic suspended solids were significantly higher in the mesocosms with snakehead than in those with mandarin fish, suggesting that snakehead has higher bioturbation effects than mandarin fish. When crucian carp were present, we found that both snakehead and mandarin fish significantly reduced prey abundance, whereas piscivore stocking did not significantly decrease the suspended solids and nutrient concentrations. Our study revealed differential effects of two subtropical piscivorous fish species on sediment disturbance and water quality, and we suggest that for lake restoration with piscivores stocking, mandarin fish are a better option than snakehead fish as their predation effect was equal but the strength of sediment disturbance by mandarin fish was lower than that of snakehead.
In shal
low eutrophic lakes, submersed macrophytes are essential for maintaining a clear water state and they are significantly affected by benthic fish disturbance, light availability, and sediment types. We conducted a mesocosm experiment with benthic fish (Misgurnus anguillicaudatus), two light regimes, and submerged macrophyte (Vallisneria natans) growing in two sediment types to investigate the ecological effects of benthic fish and light regimes on water quality and the growth of submersed macrophyte. Our findings indicated that the benthic fish increased the concentrations of total nitrogen, total phosphorus, and total dissolved phosphorus in the overlying water. The effects of benthic fish on ammonia–nitrogen (NH4+-N) and chlorophyll a (Chl-a) contents were related to light regimes. Fish disturbance indirectly promoted the growth of macrophytes growing in sand by increasing NH4+-N content in overlying water. However, the increasing Chl-a content stimulated by fish disturbance and high light regime reduced the growth of submersed macrophytes growing in clay due to shading. Macrophytes with different sediments had different strategies coping with light. Plants growing in sand responded to low light mainly by adjusting the leaf and root biomass allocation, whereas plants growing in clay responded to low light by physiologically adjusting the soluble carbohydrate content. The findings of this study might help restore lake vegetation to some degree, and using nutrient-poor sediment might be an appropriate method to avoid the detrimental effects of fish-mediated disturbances on the growth of submerged macrophytes.
To maintain submerged macrophyte beds (SMBs), a sound understanding of the factors that influence their succession is required. The aim of this study was to determine what influenced the succession of SMBs in a tidal river system, namely the Pearl River Delta, China (PRD). Surveys were carried out from 2015 to 2020 and a range of environmental (16) and ecological parameters were measured that could support an examination of the distribution and variation in SMBs. The coverage area, species richness, and biomass were measured at the community level, while nine morphological traits of the dominant species, Vallisneria denseserrulata, were measured at the population level. The results showed that, in 2015, the SMB community was distributed in sporadic patches and the community was dominated by multiple species. In 2018, V. denseserrulata grew continuously from February to August in all sections but its growth through the overwintering period (December) followed two patterns: recession (Pattern 1) or continuous growth (Pattern 2). Principal component analysis highlighted that the clonal growth ability, sexual propagation allocation, and perennial growth were related to different patterns of V. denseserrulata growth in the overwintering period. Pattern 1 was characterized by a low clonal growth ability and no sexual propagation, while Pattern 2 was characterized by a strong clonal growth ability and sexual propagation over winter. In 2019, the community was dominated by a single species of V. denseserrulata and macrophytes had disappeared from most river sections. The results from redundancy analysis implied that the decline in the SMBs from 2015 to 2020 was probably related to increases in the (1) TP concentrations in the sediment, (2) occupation of SMB habitats, and (3) annual river runoff in pre-flooding periods.
Lake eutrophication and cyanobacterial blooms are water ecological and environmental problems for a long time to come, due to the synergistic effects of human activities and global climate change. The outbreak of lake cyanobacterial blooms would entail water ecological risk and drinking water crisis, thus curative methods to suppress the proliferation of cyanobacteria bloom are imperative. Nutrient control, shallow lake regime shifts and biological manipulation are theoretical basis for the long-term strategies for cyanobacteria blooms prevention and control. Accordingly, external and internal nutrient loading reduction, ecological restoration and biomanipulation are sustainable control and management approaches. In addition, methods used for emergency situations can mitigate the intensity of cyanobacterial bloom, mainly including physical and chemical approaches. Based on the review of the domestic and international cases on cyanobacterial bloom control, we sort out the applicable conditions, cyanobacterial mitigation efficiency and performance cost of each approach. In fact, lake eutrophication management and cyanobacterial bloom mitigation are a long-term and systematic process. These in-lake strategies should invariably be accompanied by nutrient input reductions. The continuous monitoring of lake water quality and water ecology as well as the building of prediction model support the further adjustment of strategies and measures to ensure long-term success and sustainability.
Re-establishment of submerged macrophytes and especially charophyte vegetation is a common aim in lake management. If revegetation does not happen spontaneously, transplantations may be a suitable option. Only rarely have transplantations been used as a tool to support threatened submerged macrophytes and, to a much lesser extent, charophytes. Such actions have to consider species-specific life strategies. K-strategists mainly inhabit permanent habitats, are perennial, have low fertility and poor dispersal ability, but are strong competitors and often form dense vegetation. R-strategists are annual species, inhabit shallow water and/or temporary habitats, and are richly fertile. They disperse easily but are weak competitors. While K-strategists easily can be planted as green biomass taken from another site, rare R-strategists often must be reproduced in cultures before they can be planted on-site. In Sweden, several charophyte species are extremely rare and fail to (re)establish, though apparently suitable habitats are available. Limited dispersal and/or lack of diaspore reservoirs are probable explanations. Transplantations are planned to secure the occurrences of these species in the country. This contribution reviews the knowledge on life forms, dispersal, establishment, and transplantations of submerged macrophytes with focus on charophytes and gives recommendations for the Swedish project.
Small fish are highly associated with submerged macrophytes but may potentially hamper their growth due to nutrient excretion that stimulate growth of phytoplankton and periphyton growth. We conducted a mesocosm experiment to elucidate the effects of the small omnivore Chinese bitterling Acheilognathus macropterus on the growth of phyto-plankton, periphyton and the submerged macrophyte Vallisneria denseserrulata. The treatments were fish-less as well as low (LF) and high (HF) fish density. We found that the concentrations of nutrients and the phytoplankton biomass increased substantially in both fish treatments, leading to a significantly higher light attenuation compared with the control. Moreover, bitterling substantially enhanced the biomass of peri-phyton on plant leaves. Consequently, the relative growth rate (RGR) of V. denseserrulata was significantly suppressed in HF, while RGR in the LF treatment did not differ significantly from the controls. However, the bitterling also stimulated the ramet production of V. denseserrulata, significantly. Our results indicate that Chinese bitterling reduce the RGR of V. denseserrulata under high fish density condition. Therefore, the density of Chinese bitterling should be kept low in order to reduce the negative effects of the Handling editor: André Padial Supplementary Information The online version of this article contains supplementary material available (https://doi. 123 Hydrobiologia https://doi.org/10.1007/s10750-021-04643-5(0123456789().,-volV) (01234567 89().,-volV) fish on the RGR of submerged macrophytes (e.g. V. denseserrulata), when restoring lakes by plant transplantation.
Lanthanum-modified bentonite (LMB) is widely used for eutrophication control and has demonstrated good efficiency in some eutrophic lakes. However, the efficiency of LMB on eutrophication control in some eutrophic lakes, where the structure of food webs is mainly dominated by omni-benthivorous fish, remains ambiguous. Omni-benthivorous fish usually disturbs sediment and promotes the release of internal nutrients, the effect of which on the efficacy of LMB remains to be studied. Thus, a 30-day mesocosm experiment was conducted to determine whether omni-benthivorous fish disturbance and LMB would cause antagonistic responses. LMB significantly reduced dissolved P concentration in overlying water, converting mobile P to bound P in the surface layer of sediment in the absence of crucian carp (Carassius carassius). However, there were significantly negative interaction effects between LMB and crucian carp. Although LMB still effectively reduced the total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP) concentrations of overlying water in the presence of crucian carp, it had limited efficacy on inhibiting the increased concentrations of suspended solids, particulate nutrients, and chlorophyll a (Chl a) due to crucian carp disturbance. Furthermore, the crucian carp disturbance also increased the risk of mobile P releasing from surface sediment, whether with or without LMB application. The results indicated that the efficacy of LMB was insufficient to offset the negative effect of omni-benthivorous fish disturbance on eutrophication control. Hence, the omni-benthivorous fish also need to be considered for eutrophication control in shallow eutrophic lakes. Some measures need to be taken to control the biomass of omni-benthivorous fish.
Constructed wetlands (CWLs) are widely used for water environment protection. In some cases, CWL animals can help improve CWL treatment efficiency and contribute to CWL maintenance and management. However, while plants, microorganisms, and substrates in CWLs have received much attention, animals have been largely ignored. Therefore, the aims of this review are to determine the roles wetland animals play in the water environmental protection of CWLs. This study introduced species of wetland animals and the main factors that can affect their survival. The way in which CWL animals affect pollutants was discussed in detail from four perspectives: adsorption and bioaccumulation, bioturbation, and the influence of CWL animals on plants and microorganisms. The characteristics of CWL animals that can be used for biological monitoring are summarized, and the use of CWLs for the protection of wetland biodiversity is also discussed. Finally, some prospects are proposed for future research. This study will help researchers better understand the role of CWL animals in CWLs and encourage researchers to focus on studies of wetland animals.
Shallow lakes are dominated by small omnivorous fish, but the roles of these small fish in aquatic ecosystems are not well-known. A small omnivorous bitterling (Acheilognathus macropterus) has been found to be dominant after lake restoration in shallow lakes. We conducted a mesocosm experiment to examine the effects of bitterling on water quality and plankton communities. Bitterling significantly increased the concentrations of nutrients, chlorophyll a (Chla) and suspended solids (TSS), and decreased the light intensity. The abundance and biomass of phytoplankton in the fish-present treatment were significantly higher than the controls, with Microcystis spp. and Dolichospermum spp. dominating the phytoplankton community. However, bitterling did not significantly affect the abundance and biomass of zooplankton, although the presence of bitterling shifted the community structure to dominance by small-sized species. Density-dependent effects of bitterling were observed on the concentrations of nutrient, TSS, organic suspended solids and Chla and on light intensity, but were not found for plankton and ratio of zooplankton to phytoplankton biomass. Our study indicates that A. macropterus can negatively affect water quality and facilitate the dominance of cyanobacteria. Therefore, monitoring and control of these small omnivorous species should be emphasized when restoring or managing shallow lakes.
Submerged macrophytes and phytoplankton assemblage play significant roles in the functioning of aquatic ecosystems. An experiment was carried out in Beijing in order to further evaluate the environmental factors that affect the growth of submerged macrophytes and phytoplankton assemblage. Submerged macrophytes (i.e., Vallisneria natans, Hydrilla verticillata, and Ceratophyllum demersum) constructed the growth system with some controllable influencing factors (i.e., the flow rate and water depth gradient). The flow rates were set separately as 4 L/h (1#), 6 L/h (2#), and 12 L/h (3#), while the water depth gradient was of 0.5–1.7 m in eutrophic water. Generally, all macrophytes could grow normally in the experiment, and the system could maintain and improve the effluent quality. The average removal rates of NH3-N, COD, NO3-N, TN, and TP were about 90%, 33%, 65%, 45%, and 40%, respectively. Seen from the results of the water depth gradient experiments, it is indicated that Vallisneria natans grows better in shallow water (0.5 m) and moderate shallow water (0.7 m) area, with an average relative growth rate (ARGA) of 57%. Hydrilla verticillata and Ceratophyllum demersum grow better in moderate deep water (1.2 m) and deep water (1.7 m) area (ARGA of 66% and 64%, respectively). Results of the flow rate experiments showed that the moderate flow rate (6 L/h) was the best for those three macrophytes’ growth. As the fitting results of the rapid light curves (RLCs) showed that the utilization of light and the tolerance to strong light were different for these macrophytes, if they are ranked in the order of the utilization and the tolerance from strong to weak, they are Hydrilla verticillata, Ceratophyllum demersum, and Vallisneria natans. Microbial analyses indicated that the overall system diversity of the experimental groups have been improved after cultivation of macrophytes. However, the accumulated Cyanobacteria caused by the low flow rate (1#) would lead to the suppression of microbial organics decomposition and nutrient metabolism in the macrophytes. To sum up, the results of this study provided theoretical guidance and technical support for the restoration of submerged macrophytes in eutrophic water.
Small omnivorous fish often dominate in subtropical shallow lakes, and they may affect the community structure of aquatic organisms on at least two trophic levels. However, in the study of aquatic food webs in subtropical lakes, most ecologists have focused on the effects of large-sized omnivorous species (e.g. common carp), studies of small-sized species being scarce. We conducted a mesocosm experiment with two treatments (fish presence and absence) to examine the effects of a small-sized omnivore, bitterling (Acheilognathus macropterus), on phytoplankton, zooplankton and benthic macroinvertebrates. Our results showed that bitterling presence significantly increased the chlorophyll a concentration and biomass of phytoplankton, which became dominated by cyanobacteria (mainly Aphanizomenon spp.) that accounted for >99% of both total phytoplankton abundance and biomass. Both the abundance and biomass of zooplankton were also higher in the fish-present treatment, but small rotifers became dominant, and the zooplankton:phytoplankton biomass ratio decreased, indicating less grazing on phyto-plankton. Moreover, both the abundance and biomass of benthic macroinvertebrates (tubificids) were higher in the bitterling-present treatment than in the controls, which is opposite to the situation found when omni-benthivorous fish (e.g. crucian carp) dominate. Higher biomass of tubificids may, in turn, result in higher sediment nutrient release. Our study suggests that A. macropterus, and maybe also other bitterling species, can alter both pelagic and benthic assemblages via both top-down and bottom-up control effects and lead to more turbid water in eutrophic lakes. Thus, more attention should be paid to these small omnivorous species in the restoration and management of shallow subtropical lakes.
Sustainable development, while simultaneously sustaining the ability of natural systems to provide ecosystem services on which the economy and society depend, is one of the most important development goals. In this review, the theories of five sustainable development pathways are presented, followed by a discussion on the influences of involuntary and voluntary practices. Specifically, this paper reviews the literature on limits to growth and on planetary boundaries, examines the critique of green growth, and outlines the debate between degrowth and a-growth. Then, the importance of economic recession as an involuntary instrument and working time reduction policies as a voluntary instrument in mitigating environmental pressure is examined. Policy implications are highlighted in the final section.
High nitrogen (N) loading may contribute to recession of submerged macrophytes in shallow lakes; yet, its influences vary depending on environmental conditions. In August 2013, we conducted a 28-day factorial-designed field mesocosm experiment in Lake Taihu at the Taihu Laboratory for Lake Ecosystem Research (TLLER) to examine the effects of high N loading on the growth of Vallisneria natans in systems with contrasting sediment types. We ran the experiments with two levels of nutrient loading—present-day external nutrient loading (average P: 5 µg·L −1 ·day −1 , N: 130 µg·L −1 ·day −1) and P: 5 µg·L −1 ·day −1 , and with three times higher N loading (N: 390 µg·L −1 ·day −1) and used sediment with two contrasting nutrient levels. V. natans growth decreased significantly with increasing N loading, the effect being dependent, however, on the nutrient status of the sediment. In low nutrient sediment, relative growth rates, leaf biomass and root biomass decreased by 11.9%, 18.2% and 23.3%, respectively, at high rather than low N loading, while the decline was larger (44.0%, 32.7% and 41.8%, respectively) when using high nutrient sediment. The larger effect in the nutrient-rich sediment may reflect an observed higher shading of phytoplankton and excess nutrient accumulation in plant tissue, though potential toxic effects of the high-nutrient sediment may also have contributed. Our study confirms the occurrence of a negative effect of increasing N loading on submerged plant growth in shallow nutrient-enriched lakes and further shows that this effect is augmented when the plants grow in nutrient-rich sediment. External N control may, therefore, help to protect or restore submerged macrophytes, especially when the sediment is enriched with nutrients and organic matter.
Fish removal has been used to restore temperate lakes, and positive effects on ecological state and water clarity have frequently been recorded in many lakes. Recently, a supplementary measure, transplantation of submerged macrophytes after fish removal, has been applied to restore warm Chinese shallow lakes in order to compensate for the expected lack of increasing grazing control of phytoplankton after the biomanipulation. These measures have successfully shifted turbid warm lakes to a clear water state, but little is known about the responses to restoration of key physico-chemical variables. We analyzed the seasonal variation in nutrient concentrations in two subtropical and one tropical biomanipulated shallow Chinese lakes subjected to restoration. In all three lakes, a marked decline occurred in the concentrations of lake total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), and chlorophyll a (Chl a), while the transparency (SD:WD ratio, Secchi depth to water depth ratio) increased. A clear water state was established, lasting so far for 7 to 23 months, and TN, TP, Chl a, and TSS levels in the three restored lakes decreased to, on average, 49%, 58%, 41%, and 18% of the level prior to restoration and/or the level in a reference lake, respectively, while the annual mean SD:WD ratio exhibited a 1.5–4 fold increase. In conclusion, lake restoration by transplantation of submerged macrophytes after fish removal had major positive effects on the physico-chemical variables in our study lakes. However, continuous control of omnivorous and herbivorous fish biomass is recommended as the fish typically present in warm, shallow lakes to some extent feed on submerged macrophytes, when available.
Biomanipulation based on removal of coarse fish, piscivorous fish stocking and sometimes also planting of submerged macrophytes has been used to restore temperate eutrophic shallow lakes. However, in warmer lakes, omnivorous fish are more abundant and apparently less well controlled by the piscivores. We investigated the food web structure and energy pathways of fish in the restored part of subtropical Lake Wuli, China, using gut contents analysis (GCA) and the IsoSource model based on stable isotope analysis (SIA) data. We found that omnivores dominated the fish community in terms of numbers. GCA showed that cyclopoid copepods constituted the main food item for the planktivores, while all adult omnivorous fish fed mainly on macrophytes. The IsoSource SIA model supported these results. Furthermore, piscivores consumed shrimps rather than juvenile omnivores, and the SIA analysis revealed no trophic links between piscivores and adult omnivores or zooplanktivores. We conclude that macrophytes constituted an important food item for omnivores, potentially promoting population growth of omnivores as control by piscivores was weak. This may yield a high predation pressure on both zooplankton and on macrophytes, possibly preventing the establishment of a stable macrophyte state following restoration of eutrophic lakes unless the fish density is regularly controlled.
Fluid macro-instability reflects fluid axial energy and mass transfer behavior. To reveal the unsteady flow rule in a mixer-settler with rigid-flexible impeller, oil and water were used as working fluid, and the wavelet analysis and Matlab numerical calculation were adopted to analyze the power spectrum of pressure fluctuation signal in the mixing tank. The influences of rigid-flexible impeller on the frequency of macro-instability were studied, and CFD simulation was adopted to explore the flow field structure in the mixer-settler. Experiments showed that, while the agitation speed (N) of rigid-flexible impeller was lower than 250 r·min-1, the relationship between macro-instability frequency (fMI) and N was linear. When N was higher than 250 r·min-1, much air was sucked into the liquid and fMI disappeared, so the flow field became multi scale structure and resulted in serious emulsification, which was unfavorable for the operation of clarification. Compared with rigid impeller, the rigid-flexible one can enhance the macro-instability and improve the efficiency of fluid mixing and energy transfer. Computational simulation shows that, the rigid-flexible impeller could obviously increase the blade suction ability and axial movement of fluid, realizing efficient mixing and avoiding excessive stirring of the fluid, which is helpful to mixing and clarification.
1. Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However , the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short-term studies (days to a few months).
2. We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Ca-bomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN) without nitrogen addition (mean TN = 1.9 mg L À1), low nitrogen (LN) addition (mean TN = 3.5 mg L À1) and high nitrogen (HN) addition (mean TN = 5.5 mg L À1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L À1) during the experiment in all treatments.
3. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load.
4. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phy-toplankton and periphyton in the summer, while there was no indication of direct N toxicity.
Lake Griffin received discharges for
decades from muck farms developed on former
floodplain wetlands, leading to hypereutrophic conditions.
Management actions included wetland restoration
of farmland to reduce nutrient discharges, and
harvesting of gizzard shad to remove nutrients in fish
biomass and reduce nutrient recycling from sediments.
Despite a reported susceptibility to wind-driven
sediment resuspension, there have been substantial
improvements in water quality in Lake Griffin,
including decreases in nutrient concentrations, chlorophyll-
a, and cyanobacterial biovolume, and increases
in water transparency. Water quality improvements in
Lake Griffin were substantially greater than occurred
in ten comparison lakes. External nutrient load
reduction was the primary factor contributing to water
quality improvement, although there was evidence of
an effect of shad harvesting, including correlations
between shad catch per unit effort and nutrient
concentrations, and an estimated effect of biomass
removal and recycling reduction accounting for about 40%
of the external load during the harvest period. Net
production of total nitrogen in the lake was strongly
related to external total phosphorus loading, indicating
phosphorus limitation of nitrogen fixation. The response
of Lake Griffin indicates that the combination
of external nutrient load reduction and biomanipulation
can result in sustained improvements in water
quality in shallow subtropical lakes.
We examined the interactions of the common carp (Cyprinus carpio L.) and nutrient additions on water quality, sedimentation rates, and submerged macrophyte biomass in mesocosms in Delta Marsh, Manitoba, Canada. We wanted to determine if carp and nutrients interacted synergistically to increase phytoplankton biomass. A two-by three duplicated, factorial design had the following treatments: (1) control mesocosms with no carp or
nutrient additions; (2) low carp density and no nutrient additions; (3) high carp density and no nutrient additions; (4) no carp and nutrient additions; (5) low carp density and nutrient additions; and (6) high carp density and nutrient additions. The presence of carp increased ammonia concentrations, turbidity, and phytoplankton biomass as expected but did not increase total reactive phosphorus concentrations. The presence of carp did not appear to interact synergistically with nutrient additions to increase phytoplankton as has been suggested by others. In mesocosms with high carp density and receiving nutrient enrichment, phytoplankton appeared to be suppressed relative to mesocosms receiving nutrient enrichment only, and nutrient enrichment and low carp density. Overall, the presence of carp appears to mimic the effects of eutrophication. Our results demonstrate that carp can cause a shift from a clear, macrophyte dominated state to a turbid phytoplankton-dominated state at a biomass of less than 600 kg ha-1.
The abundance of primary producers is controlled by bottom-up and top-down forces. Despite the fact that there is consensus that the abundance of freshwater macrophytes is strongly influenced by the availability of resources for plant growth, the importance of top-down control by vertebrate consumers is debated, because field studies yield contrasting results. We hypothesized that these bottom-up and top-down forces may interact, and that consumer impact on macrophyte abundance depends on the nutrient status of the water body. To test this hypothesis, experimental ponds with submerged vegetation containing a mixture of species were subjected to a fertilization treatment and we introduced consumers (mallard ducks, for 8 days) on half of the ponds in a full factorial design. Over the whole 66-day experiment fertilized ponds became dominated by Elodea nuttallii and ponds without extra nutrients by Chara globularis. Nutrient addition significantly increased plant N and P concentrations. There was a strong interactive effect of duck presence and pond nutrient status: macrophyte biomass was reduced (by 50 %) after the presence of the ducks on fertilized ponds, but not in the unfertilized ponds. We conclude that nutrient availability interacts with top-down control of submerged vegetation. This may be explained by higher plant palatability at higher nutrient levels, either by a higher plant nutrient concentration or by a shift towards dominance of more palatable plant species, resulting in higher consumer pressure. Including nutrient availability may offer a framework to explain part of the contrasting field observations of consumer control of macrophyte abundance.
Electronic supplementary material
The online version of this article (doi:10.1007/s00442-014-3047-y) contains supplementary material, which is available to authorized users.
While numerous reports exist on the results of lake restoration initiatives in temperate regions, only a few exist from subtropical lakes. We present results of the lake restoration of shallow, subtropical Lake Wuli, China, conducted between 1999 and 2010. After restoration, annual average concentrations of total nitrogen, total phosphorus (TP), and chlorophyll a and the chemical oxygen demand declined significantly, though summer TP remained high. Suspended solids increased significantly over the years, whereas transparency decreased, though not significantly so. The contribution of cryptophytes to total phytoplankton biomass decreased, while the proportion of cyanobacteria, especially potentially N 2 -fixing species, increased. Rotifers were superseded by crustaceans as the dominant taxon of the zooplank-ton community. Enhanced abundance of Daphnia spp., appearance of Leptodora kindti, and increased biomass ratios of zooplankton to phytoplankton, calanoids to cyclopoids, and nauplii to copepods in the post-restoration period indicate reduced fish pre-dation and stronger top-down control of phytoplank-ton. However, the increase in non-algal turbidity, probably caused by the higher biomass of benthivor-ous fish, apparently prevented the re-establishment of submerged macrophyte communities. We conclude that removal of fish, particularly benthivorous species, will further improve water quality in this and other subtropical shallow lakes.
Laboratory aquarium experiments were conducted to study the effects of water column nutrient enrichment on the growth of Potamogeton maackianus A. Been, a dominant submersed macrophyte in Lake Donghu before the 1970s. The plant was grown in fertile sediment, gradients of water nutrients and different shade intensities for 40 days. At the end of the experiment, redox potential of the sediment was negatively related to the water nutrient concentration, while nitrogen and phosphorus concentration, and peroxidase ac- tivity of the plant tissue was positively related to the nutrient gradient. Plant growth reduced at light levels corresponding approximately to the depth range of 0.9-1.5 m given current water transparency within Lake Donghu. Within this stress zone, plant chlorophyll content, total shoot length increased primarily with the increased stress and peaked at 1.2 to 1.3 m depths, showing a narrower adaptive range of the plant than the low light stress range. The present study indicates that growth of P. maackianus is strongly inhibited by consequences of nutrient enrichment in water. Thus, eutrophication in Lake Donghu during 1970s may have played an important role in the disappearance of this species from the lake.
We removed gizzard shad (GS; Dorosoma cepedianum) from Lake Denham, a 104 ha hypereutrophic lake in the Ocklawaha chain of lakes in central Florida, to evaluate biomanipulation for restoration of shallow hypereutrophic Florida lakes. A commercial haul seine removed GS in winters 1990, 1991, and 1992. The 3-year total removal was 51,738 kg wet weight or 498 kg/ha, which reduced the stock of harvestable adult GS to less than 3% of the initial level. Water quality in Lake Denham improved progressively during the biomanipulation, and some improvements persisted through 2007 despite removal of a fish barrier in 1993. From 1989 until 1993, mean annual Secchi disk visibility in Lake Denham increased from 23 to 57 cm, mean annual chlorophyll a decreased from 143 to 40 ug/L, and mean annual total phosphorus declined from 194 to 93 ug/L. Neither land use nor rainfall changed significantly during the experiment, and similar improvements were not evident in other basin lakes during the same period. These findings and work elsewhere support our conclusion that removal of GS can be an effective component in the restoration of shallow hypereutrophic lakes. We postulate that the improvements were associated with reduced nutrient cycling and bioturbation rather than increased zooplankton predation on phytoplankton. Long-term improvements in water quality in Lake Denham indicate that effects of biomanipulation can persist when the benthivorous fish populations remain depressed.
Eutrophication resulting from high nutrient loading has been the paramount environmental
problem for lakes world-wide for the past four decades. Efforts are being made
in many parts of the world to reduce external nutrient loading via improved wastewater
treatment or diversion of nutrient-rich inflows. However, even after a reduction of the
external phosphorus loading, the effects obtained may be unsatisfactory. This may
reflect an insufficient reduction in the external nutrient loading to effectively limit phytoplankton
growth. However, the lack of success may also be due to chemical or biological
within-lake inertia preventing or delaying improvements. To overcome the
resilience and thereby reinforce recovery, a number of physico-chemical and biological
restoration methods have been developed.
In this chapter, we describe recent developments of biological restoration methods
related to eutrophication, their short-term and long-term effects, and discuss the
possibility of using combined physico-chemical and biological methods to improve
the long-term stability of restoration and to reduce restoration costs. As comprehensive
reviews of the effect of fish manipulation in cold temperate lakes are numerous, for
these waterbodies, we highlight recent results, including effects on biodiversity and
metabolism, and present new approaches of biomanipulation. Our particular focus is,
however, directed at biomanipulation in warm lakes and on combined treatments
which are far less well described in the literature.
The common carp Cyprinus carpio, a worldwide introduced benthivorous fish, has been implicated in the degradation of native environments through initiation of a shift to a phytoplankton-dominated turbid state, which is associated with dramatic biodiversity loss. This study combined surveys of ponds containing either low total biomass of small-sized carp or high densities of large-sized carp with an enclosure/exclosure experiment, in order to quantify the direct (trophic) and indirect (via habitat deterioration) impacts of carp on pond communities. High-density ponds supported substantially lower biodiversity and were more turbid than low-density ponds. The subsequent field experiment examined the effects of carp presence/absence and of clear-water versus moderately turbid conditions mediated by carp on the survival to metamorphosis of larval anurans Pelobates fuscus and Hyla arborea, on Zygoptera and Anisoptera densities, and on the biomass of submerged macrophytes. The presence of enclosed one-year old carp resulted in the complete elimination of larval anurans and the absence of Odonata. The effects of the habitat conditions were not significant, apart from better survival of P. fuscus in the moderately turbid carp exclosures than in clear water. Submerged plants were more abundant in clear-water than in turbid treatments, with a negligible effect of enclosures/exclosures. These results suggest that carp predation and related effects may be primarily responsible for animal diversity loss in invaded communities, as they may act prior to, or independent of, the ecosystem switch to a turbid phase.
Shallow lakes, one of the most widespread water bodies in the world landscape, are very sensitive to climate change. Several theories predict changes in community traits, relevant for ecosystem functioning, with higher temperature. The space-for-time substitution approach (SFTS) provides one of the most plausible empirical evaluations for these theories, helping to elucidate the long-term consequences of changes in climate.Here, we reviewed the changes at the community level for the main freshwater taxa and assemblages (i.e. fishes, macroinvertebrates, zooplankton, macrophytes, phytoplankton, periphyton and bacterioplankton), under different climates. We analyzed data obtained from latitudinal and altitudinal gradients and cross-comparison (i.e. SFTS) studies, supplemented by an analysis of published geographically dispersed data for those communities or traits not covered in the SFTS literature.We found only partial empirical evidence supporting the theoretical predictions. The prediction of higher richness at warmer locations was supported for fishes, phytoplankton and periphyton, while the opposite was true for macroinvertebrates and zooplankton. With decreasing latitude, the biomass of cladoceran zooplankton and periphyton and the density of zooplankton and macroinvertebrates declined (opposite for fishes for both biomass and density variables). Fishes and cladoceran zooplankton showed the expected reduction in body size with higher temperature. Life history changes in fish and zooplankton and stronger trophic interactions at intermediate positions in the food web (fish predation on zooplankton and macroinvertebrates) were evident, but also a weaker grazing pressure of zooplankton on phytoplankton occurred with increasing temperatures. The potential impacts of lake productivity, fish predation and other factors, such as salinity, were often stronger than those of temperature itself. Additionally, shallow lakes may shift between alternative states, complicating theoretical predictions of warming effects. SFTS and meta-analyses approaches have their shortcomings, but in combination with experimental and model studies that help reveal mechanisms, the " field situation" is indispensable to understand the potential effects of warming.
We conducted a study to examine the relationship between common carp ( Cyprinus carpio) exclusion, water quality, zooplankton, and submergent macrophytes. Twelve 50-m 2 in situ experimental enclosures were installed in degraded Cootes Paradise Marsh during the carp spawning period in 1995. Enclosures were stocked with two or three carp of similar size, ranging from 13 to 59 cm and in total biomass from 23 to 2100 kg/ha. Turbidity, total phosphorus, and total ammonia concentrations increased predictably with total carp biomass in the enclosures. Although carp had no direct effect on zooplankton community structure, increased turbidity and nutrient load associated with carp activity resulted in reduced total zooplankton biomass. We developed a relationship between species richness and water turbidity for 19 wetlands in the Great Lakes basin which indicated that above an apparent threshold of 20 NTU, there were less than five species of submergent plants, while a more diverse community existed in less turbid systems. We predict that water turbidity in Cootes Paradise Marsh may not be reduced below this threshold value of 20 NTU following carp exclusion. We emphasize the need to consider other factors that may contribute to increases in water turbidity and nutrient concentrations, including wind resuspension and substrate characteristics. Résumé : Nous avons étudié la relation entre l'exclusion des carpes ( Cyprinus carpio), la qualité de l'eau, le zooplancton et les macrophytes submergés. Douze enclos expérimentaux in situ de 50 m 2 ont été aménagés dans le marais Cootes Paradise, un
Shallow lakes, the most abundant lake type in the world, are very sensitive to climatic changes. The structure and functioning of shallow lakes are greatly impacted by submerged plants, and these may be affected by climate warming in various, contrasting, ways.
Following a space-for-time substitution approach, we aimed to analyse the role of aquatic (submerged and free-floating) plants in shallow lakes under warm climates. We introduced artificial submerged and free-floating plant beds in five comparable lakes located in the temperate zone (Denmark, 55–57 °N) and in the subtropical zone (Uruguay, 30–35 °S), with the aim to study the structure and dynamics of the main associated communities.
Regardless of differences in environmental variables, such as area, water transparency and nutrient status, we found consistent patterns in littoral community dynamics and structure (i.e. densities and composition of fish, zooplankton, macroinvertebrates, and periphyton) within, but substantial differences between, the two regions. Subtropical fish communities within the macrophyte beds exhibited higher diversity, higher density, smaller size, lower relative abundance of potentially piscivores, and a preference for submerged plants, compared with otherwise similar temperate lakes. By contrast, macroinvertebrates and cladocerans had higher taxon richness and densities, and periphyton higher biomass, in the temperate lakes. Several indicators suggest that the fish predation pressure was much stronger among the plants in the subtropical lakes. The antipredator behaviour of cladocerans also differed significantly between climate zones. Submerged and free-floating plants exerted different effects on the spatial distribution of the main communities, the effects differing between the climate zones. In the temperate lakes, submerged plants promoted trophic interactions with potentially positive cascading effects on water transparency, in contrast to the free-floating plants, and in strong contrast to the findings in the subtropical lakes.
The higher impact of fish may result in higher sensitivity of warm lakes to external changes (e.g. increase in nutrient loading or water level changes). The current process of warming, particularly in temperate lakes, may entail an increased sensitivity to eutrophication, and a threat to the high diversity, clear water state.
Phytoplankton abundance and biomass can be explained as a result of spatial and temporal changes in physical and biological
variables, and also by the externally imposed or self-generated spatial segregation. In the present study, we analyzed contrasting-season
changes in the phytoplankton communities of five subtropical shallow lakes, covering a nutrient gradient from oligo-mesotrophy
to hypereutrophy, using a morphologically based functional approach to cluster the species. Six environmental variables accounted
for 46% of the total phytoplankton morphological groups variance, i.e., turbidity (Secchi disk), conductivity, total phosphorus,
total nitrogen, total zooplankton abundance, and herbivorous meso:microzooplankton density ratio. The differences in resource
availability and zooplankton abundance among the systems were related with important changes in phytoplankton composition
and structure. Within phytoplankton assemblages, adaptations to improve both light and phosphorus/nitrogen uptake were important
in nutrient-poor systems; while grazing-avoidance mechanisms, such as colonial forms or bigger individuals, seemed relatively
important only in eutrophic Lake Blanca, where light was not a limiting factor. However, this was not observed in the nutrient-rich
Lake Cisne, where low light availability (due to clay resuspension and dark water color) was identified as the main structuring
factor. Our results suggest that the composition of phytoplankton morphologically based functional groups appear to reliably
describe the trophic sate of the lakes. However, other factors, such as nonbiological turbid condition, or zooplankton composition,
may interact rendering interpretations difficult, and therefore, deserve further studies and evaluation.
KeywordsPhytoplankton-Morphology-based groups-Subtropical lakes
Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes
in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects
of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution.
We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community
structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter
life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e.
stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes
imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in
the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified
in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent
in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and
good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research
is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction
of the nutrient load.
KeywordsAquatic food webs-Sub-tropical lakes-Piscivory-Planktivory-Benthivory-Eutrophication-Salinisation-Biomanipulation
We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added
nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass
being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5m
plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot
density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions
between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also
increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated
with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy
grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer
despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high
nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other
than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions
in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls.
KeywordsTropical seagrass-Herbivory-Shelter-Fish grazing
Developments in Hydrobiologia: Structure and Function of World Shallow Lakes
Reestablishment of submerged macrophytes is considered important when restoring shallow eutrophic lakes. To improve water clarity and consequently the growth conditions of macrophytes, removal of plankti-benthivorous fish has been used. In subtropical shallow lakes, however, rapid recruitment of small fish, especially benthivores during restoration, may hamper early reestablishment of submerged macrophytes. Crucian carp (Carassius carassius) and Vallisneria natans are common species dominated in subtropical shallow lakes. To investigate the effect of small benthivorous fish on the growth of Vallisneria natans, a 28-day outdoor controlled experiment was undertaken in 12 mesocosms with three densities of Carassius carassius − low (10 g m −2), high (40 g m −2) and no fish (all in four replicates). The results showed that the fish significantly increased chlorophyll-a concentrations and periphyton biomass in both fish treatments, most significantly at high density for Chl a. This concurs with an increase in nutrient concentrations , likely mediated by fish sediment disturbance and excretion, and a reduction of zooplankton biomass (less algal grazing). Increased concentrations of inorganic suspended solids with increasing fish density further enhanced turbidity, causing shading of the macrophytes. Accordingly, the relative growth rate, ramet number and root/leaf ratio of V. natans decreased significantly at low and high fish density compared