Article

Effects of small-sized crucian carp (Carassius carassius) on the growth of submerged macrophytes: Implications for shallow lake restoration

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Abstract

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 with the controls, but the effects did not depend on fish density. However, mean leaf length rose with increasing fish density, likely to allow the plants to obtain more light. Overall, our results show that not only large-bodied carp, as demonstrated frequently, but also small-sized crucian carp posed a constraint on submerged macrophyte reestablishment, and we conclude that crucian carp may hamper restoration efforts in subtropical shallow lakes. Restoration by biomanipulation should therefore target also small-sized crucian carp.

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... Thus, rapid recruitment of crucian carp has often been observed after fish removal in both subtropical and tropical shallow lakes (Gao et al., 2014). Deterioration of the light condition and grazing by crucian carp had significant negative effects on the growth of V. natans, resulting in reduced relative growth rate and ramet number and adjustment of a shift in the biomass allocation between shoots and roots (Gu et al., 2016). V. denseserrulata is an autochthonic perennial meadow-forming species in shallow lakes and often used in Chinese lake restoration projects (Liu et al., 2018;Zhou et al., 2016). ...
... As expected, TN and TP concentrations increased after fish stocking (Fig. 7), which is consistent with the findings in earlier studies showing an important role of fish for the nutrient levels in lakes (Cline et al., 1994;Dantas et al., 2018;Lougheed et al., 1998), and higher nutrient concentrations may stimulate phytoplankton growth as evidenced by the higher Chl.a in the fish mesocosms. A decreased grazing pressure on phytoplankton due to crucian carp predation on zooplankton may also have contributed to the increase in Chl.a (Gu et al., 2016;Schriver et al., 1995). Additionally, fish disturb the sediment in the process of foraging and locomotion, and the resuspension of algal cells from the sediment may be another possible reason for the higher Chl.a concentration in the two fish stocking treatments (Roozen et al., 2007). ...
... In the present study, longer leaf length, fewer ramets and leaves, and reduced biomass of V. denseserrulata were recorded in the presence of fish. These changes were consistent with the adaptation of other species in the Vallisneria genus (e.g., V. americana, V. spiralis, V. natans) to the low light conditions (Barko et al., 1982;Xiao et al., 2007;Gu et al., 2016). ...
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Both chemical phosphorus (P) immobilization and submerged macrophytes are widely used in lake restorations to reduce internal phosphorus loading. Their combined effects seem much stronger than used alone. However, benthivores fish populations recover fast and they may thus counteract the restoration effects. We conducted a mesocosm experiment with three densities of crucian carp (Carassius auratus gibelio) to study the effects of benthivorous fish on the combined effects of lanthanum (La) modified bentonite and submerged macrophyte Vallisneria denseserrulata in lake restoration: control (fish-free), low-density (45 g m⁻²) and high-density (83 g m⁻²). Crucian carp increased the concentrations of total phosphorus, total nitrogen, total suspended solids, and chlorophyll a and deteriorated the light climate in the water column compared with the controls. The porewater soluble reactive phosphorus concentrations in the deep sediment were two times higher in the high-density treatment than in the control group and the low-density treatment. Crucian carp increased the mobile phosphorus (mainly the NaOH-OP) in the 0-2 cm sediment, likely due to the dilution of La in the surface sediment as caused by accelerated downward transportation of La, and at the same time La-bound P (part of the HCl-P fraction) was mixed deeper into the sediment with increasing fish density. In the high-density mesocosms, plant biomass and density were reduced by 37% and 51%, respectively, which is another possible mechanism for the water quality deterioration. We conclude that it is important to control benthivorous fish when conducting lake restoration by combining phosphorus immobilization and submerged macrophytes.
... Because they feed on at least 2 trophic levels, omnivorous fishes may affect lake ecosystems through various pathways. The presence of omnivorous fishes can increase nutrient concentrations, turbidity (total suspended solids [TSS], especially inorganic suspended solids [ISS]), and the biomass of both phytoplankton (Drenner et al. 1998, Gu et al. 2016, Zhang et al. 2017, He et al. 2019 and periphyton (Gu et al. 2016), thereby suppressing the growth of submerged macrophytes (Miller andCrowl 2006, Qiu et al. 2019). In a study including both the omnibenthivorous Common Carp (Cyprinus carpio Linnaeus, 1758) and the omni-planktivorous filter-feeding Silver Carp (Hypophthalmichthys molitrix Valenciennes, 1844), Zhang et al. (2016) found that Common Carp substantially increased the biomass of pelagic algae but decreased the biomass of benthic algae (periphyton), whereas Silver Carp did not influence the pelagic or benthic algal biomass. ...
... Because they feed on at least 2 trophic levels, omnivorous fishes may affect lake ecosystems through various pathways. The presence of omnivorous fishes can increase nutrient concentrations, turbidity (total suspended solids [TSS], especially inorganic suspended solids [ISS]), and the biomass of both phytoplankton (Drenner et al. 1998, Gu et al. 2016, Zhang et al. 2017, He et al. 2019 and periphyton (Gu et al. 2016), thereby suppressing the growth of submerged macrophytes (Miller andCrowl 2006, Qiu et al. 2019). In a study including both the omnibenthivorous Common Carp (Cyprinus carpio Linnaeus, 1758) and the omni-planktivorous filter-feeding Silver Carp (Hypophthalmichthys molitrix Valenciennes, 1844), Zhang et al. (2016) found that Common Carp substantially increased the biomass of pelagic algae but decreased the biomass of benthic algae (periphyton), whereas Silver Carp did not influence the pelagic or benthic algal biomass. ...
... In the Mu-BF treatment, we added 2 mussels and 4 fish (similar biomass as in the mussel and bitterling treatments). The density of fish used was intermediate between the densities used in previous studies with omni-benthivorous fishes (He et al. 2017, Gu et al. 2016 and was close to the natural density (8.7 ind./m 2 ) of small fishes (e.g., A. macropterus, R. ocellatus, and Pseudorasbora parva Temminck and Schlegel, 1846) found in a natural lake (Ye et al. 2006). We collected bitterling from Lake Taihu and only used female adults of similar size in our experiment. ...
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Omnivorous fishes are prevalent in warm waters and may have strong impacts on water quality by excreting nutrients and reducing periphyton biomass. However, most studies have focused on large-sized species and overlooked the role of small omnivores. Filter-feeding mussels may modulate the negative effects of small omnivorous fishes on water quality, and stocking of mussels has been frequently used in shallow eutrophic freshwaters in China to improve the water clarity. However, the mechanisms behind such management practices are poorly studied. We conducted a mesocosm experiment to examine the ecosystem effects of the bitterling Acheilognathus macropterus as modulated by the mussel Sinanodonta woodiana, one of the mussels upon which it relies for breeding. We hypothesized that bitterling would exert negative effects on the lake environment, specifically higher phytoplankton biomass and lower water clarity, but that these effects might be alleviated by the filter-feeding activities of S. woodiana. In a 56-d mesocosm experiment with and without bitterling in the presence and absence of mussels, we found interactive effects of bitterling and mussels. In mesocosms with bitterling, nutrient concentrations, phytoplankton biomass, and total suspended solids (TSS) increased, but there were no changes in periphyton biomass in the mussel-free treatments. In contrast to the effects of large-sized omnivorous fishes reported from the literature, bitterling mainly affected TSS levels by increasing organic suspended solids rather than inorganic solids, indicating weak effects on sediment resuspension. However, the presence of mussels alleviated the negative effects of bitterling by decreasing nutrient levels , phytoplankton biomass, and TSS concentrations. Mussels alone had no effects on periphyton biomass, but the mussel-bitterling interactions boosted the growth of periphyton. Our study suggests that the negative effects of bitterling on water quality (e.g., increased nutrient concentrations and phytoplankton biomass) are alleviated by the presence of filter-feeding mussels, but the stimulatory interactive effects of mussels and bitterling on periphyton may impair the recovery of submerged macrophytes.
... Submerged macrophytes can, up to a certain nutrient load, effectively maintain a clear water state in temperate and (sub)tropical shallow lakes, e.g. by stabilizing the sediment, removing nutrients and inhibiting the growth of phytoplankton (Jeppesen et al., 1998;Van Donk and van de Bund, 2002;Gao et al., 2009;Levi et al., 2015;Liu et al., 2020a;Li et al., 2021). In contrast, benthivorous fish reduce the chances of a clear water state in shallow lakes, due to sediment disturbance and resuspension when foraging (Roozen et al., 2007;Lorenz et al., 2013;Gu et al., 2016;Han et al., 2020). 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). ...
... In contrast, benthivorous fish reduce the chances of a clear water state in shallow lakes, due to sediment disturbance and resuspension when foraging (Roozen et al., 2007;Lorenz et al., 2013;Gu et al., 2016;Han et al., 2020). 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). ...
... 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). ...
Article
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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.
... High-density juvenile omni-benthivorous fish would inhibit growth and colonisation of submerged macrophytes by reducing underwater light through sediment resuspension, promoting phytoplankton reproduction through releasing of sediment nutrients (bottom-up effect) and reducing grazing pressure of zooplankton on phytoplankton (topdown effect; Chen, Su, et al., 2020;Gu et al., 2016;He et al., 2017;Lorenz et al., 2013). Therefore, developing new measures to suppress the negative effects of juvenile omni-benthivorous fish disturbance on submerged macrophytes is critical. ...
... In the present study, water quality was significantly affected by juvenile crucian carp. Higher ISS concentration in the presence of juvenile crucian carp indicated that juvenile crucian carp caused sediment resuspension, thereby directly reducing the underwater light intensity (Chen, Su, et al., 2020;Gu et al., 2016;He et al., 2017;Lorenz et al., 2013). Previous research has reported decreased (Badiou & Goldsborough, 2015;Kyeongsik et al., 2001;Wahl et al., 2011), unchanged (Chen, Liu, et al., 2020;Chen, Su, et al., 2020), and increased Chl a concentrations in the presence of fish disturbance Roozen et al., 2007;Zhang et al., 2016). ...
... 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). ...
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• 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.
... The process often causes resuspension of sediments, resulting in an increase in turbidity, nutrient concentrations and algal biomass in the water column [8][9][10]. Furthermore, benthivorous fish also reduce zooplankton via direct grazing and/or indirect effects on suspended solids [11,12]. Therefore, benthivores have more complex influence on water physicochemical and biotic conditions than planktivorous fish [13,14]. ...
... The major decrease in zooplankton biomass indicates that besides predating on benthic organisms ( Figure 5), crucian carp could also reduce zooplankton, potentially affecting their control of pelagic phytoplankton. Crucian carp is an omnivore, and previous gut content analyses have revealed that it may prey directly upon zooplankton, creating a community shift in dominance from large-sized cladocerans and copepods to small-sized rotifers [12]. This was exactly the pattern observed in our study ( Figure 4). ...
... Despite the reduced top-down control of phytoplankton, water Chl-a concentrations did not increase correspondingly (Figure 3a) in these eutrophic mesocosms, in contrast to the results in similar mesocosm studies in which a clear increase in phytoplankton growth was found after introduction of crucian carp [12,15,22]. We attributed this discrepancy to the different trophic states of these mesocosm experiments. ...
Article
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Crucian carp (Carassius carassius) is a key fish species in most Chinese subtropical and tropical shallow lakes. Through sediment feeding, crucian carp could greatly change water turbidity and nutrient levels, as well as the abundance of herbivorous consumers, which may have important influences on seston element stoichiometry. However, so far, experimental studies on this topic are lacking. We conducted a 36-day mesocosm experiment to explore the effects of crucian carp on water physicochemical and biological properties, and C/N/P ratios in suspended particulate matter (SPM) under eutrophic conditions. Our results provided three major findings: (1) Crucian carp resuspended sediments and along with them, reduced light penetration and lower light/total phosphorus (TP) ratios. (2) Crucian carp reduced biomasses of both zooplankton and macrozoobenthos, whereas their effect on phytoplankton was weak, potentially because of resuspension-induced light limitation. (3) Both C/P and N/P ratios in SPM were significant lower in mesocosms with crucian carp than in fish-free controls, which may be attributed to the high contribution of P-rich sediments and low light to nutrient supply caused by fish-induced resuspension. Our results suggest that besides planktivorous fish, benthivore (e.g., crucian carp) in warm shallow waters could also affect pelagic C/N/P stoichiometry via sediment feeding, which may further influence energy transfer efficiency in lake food chain.
... Therefore, omnivorous fish feed on at least two trophic levels and may therefore impact lake ecosystems through various pathways. Benthic omnivores, such as common carp (Cyprinus carpio) and crucian carp (Carassius carassius), have been observed to increase nutrient concentrations and turbidity (Drenner et al. 1998;Gu et al. 2016;Zhang et al. 2017;He et al. 2019;Qiu et al. 2019) through disturbance of sediment when foraging. This induces resuspension of particulate matter, which indirectly increases the biomass of phytoplankton through the release of nutrients sequestered in the sediment into the water column. ...
... This induces resuspension of particulate matter, which indirectly increases the biomass of phytoplankton through the release of nutrients sequestered in the sediment into the water column. They also decrease the biomass of both zooplankton and benthic invertebrates via feeding (Gu et al. 2016;He et al. 2019), which also may lead to an increase in the phytoplankton biomass. Omni-planktivores, such as silver carp (Hypophthalmichthys molitrix) and bighead Responsible editor: Lotfi Aleya * Jinlei Yu jlyu@niglas.ac.cn carp (Hypophthalmichthys nobilis), are non-selective filter feeders, and they may significantly decrease the biomass of zooplankton and facilitate dominance of small-sized species in planktonic communities (Domaizon and Devaux 1999;Lu et al. 2002;Zhang et al. 2006). ...
... For instance, fishinduced increase in sediment resuspension, which is likely positively related to fish size, is one of the key processes affecting water quality in omnivorous fish-dominated lakes. Moreover, the effects of omnivorous fish on pelagic and benthic communities are also determined by fish size; for instance, the effects of crucian carp on the community structures of phytoplankton and zooplankton differed in studies involving different fish sizes (Gu et al. 2016;He et al. 2019). ...
Article
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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.
... More than that, due to breeding of fish throughout the years in the tropical and subtropical lakes, juvenile omni-benthivorous fish such as small crucian carp (Carassius carassius) tend to dominate lakes (FAO 2004(FAO -2015Gao et al. 2014;Liu et al. 2018;Fernando 1994). The presence of abundant juvenile omnibenthivorous fish would lead to intense sediment resuspension, which enhances the release of nutrients from the sediment and therefore reduces underwater transparency and light intensity (Lorenz et al. 2013;Gu et al. 2016;Chen et al. 2020). The effect of omni-benthivorous fish on water and sediment seems to be opposite to the efficiency of LMB. ...
... In addition, the recurring resuspension of settled phytoplankton through the benthivorous fish disturbance also increased Chl a concentration in overlying water (Roozen et al. 2007). Practically,cruciancarpcanalsoreducetheefficacyofLMBon waterthroughthe"top-downeffect."Zooplanktonconstitutes alargeproportioninthedietofcruciancarp(Kloskowski2011; Gu et al. 2016), and the presence of juvenile crucian carp can reduce the zooplankton biomass and the grazing pressure of zooplankton on phytoplankton, thereby promoting phytoplanktongrowth (He et al. 2017;Guetal.2018) and ultimately reducing the efficacy of LMB. ...
... We speculated that the differences of crucian carp growth between LMB + fish treatment and fish treatment were related to the food source of crucian carp. Some studies have shown that the application of LMB could reduce the abundance of zooplankton and benthic animals in the short term (Van Oosterhout and Lürling 2011; Waajen et al. 2017), and both of them are important food source for crucian carp (FAO 2004(FAO -2015Kloskowski 2011;Gu et al. 2016). Furthermore, the differences of crucian carp growth might be an indirect consequence of the differences in phytoplankton between LMB + fish treatment and fish treatment, as the small carp also forages on phytoplankton (Hirpo 2012). ...
Article
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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.
... In practice, multi-approaches are often employed simultaneously to alter the proportions of primary producers, food webs and water clarity (Chen et al., 2009;Gu et al., 2016;Knopik and Newman, 2018;Liu et al., 2018;Mormul et al., 2018), and it is therefore difficult to disentangle the specific effect of the different measures used. Transplantation of submersed macrophytes and rehabilitation of snails have been recommended (Mo et al., 2017;Mormul et al., 2018), but the success of such attempts may potentially be hampered by sediment disturbance by benthic fish. ...
... In the present study, the benthic fish (128 g m − 2 ) enhanced the phytoplankton and periphyton biomass and turbidity substantially. In subtropical eutrophicated lakes, the fish community is composed mainly of benthic omnivorous and planktivorous fish Gu et al., 2016) and their biomass can reach 390-810 g m − 2 (Suresh and Lin, 1992), and might be a key factor to impede the reinstatement of clear water when the macrophyte growth was suppressed by the increased phytoplankton. The removal of omnivorous fish (162 g m − 2 ) in combination with submersed macrophyte transplantation reduced the phytoplankton biomass greatly and rebuilt clear water state in a tropic eutrophic lake (Liu et al., 2018). ...
Article
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.
... such resuspension of sediment can potentially influence the structure and functioning of the lake ecosystem in several ways (scheffer, 1998) by reducing the light penetration through the water column (Roberts et al., 1995;Parkos et al., 2003). This may influence foraging strategies of visual orientated piscivores, i.e. reduce foraging rates, and thereby potentially uncouple trophic top down cascades (Miner and stein, 1993) and affect dispersal and growth of submerged macrophytes, due to less light available in the water column (Fisher et al., 2013;Gu et al., 2016). Another non-consumptive ecosystem effect of large bodied benthivorous fish is an increase in total phosphorus and nitrogen concentrations due to resuspension, releasing sedimentary nutrients to the water column, which ultimately can result in increased growth of algal biomass (Boström et al., 1982;Andersson et al., 1988;Parkos et al., 2003;Fisher et al., 2013;Gu et al., 2016). ...
... This may influence foraging strategies of visual orientated piscivores, i.e. reduce foraging rates, and thereby potentially uncouple trophic top down cascades (Miner and stein, 1993) and affect dispersal and growth of submerged macrophytes, due to less light available in the water column (Fisher et al., 2013;Gu et al., 2016). Another non-consumptive ecosystem effect of large bodied benthivorous fish is an increase in total phosphorus and nitrogen concentrations due to resuspension, releasing sedimentary nutrients to the water column, which ultimately can result in increased growth of algal biomass (Boström et al., 1982;Andersson et al., 1988;Parkos et al., 2003;Fisher et al., 2013;Gu et al., 2016). Hence, benthivorous fish can have profound effects on other trophic levels and play an important role for maintaining lakes in, or shifting lakes towards a turbid stable state, which may have severe negative impacts on biodiversity as well as ecosystem functioning (scheffer, 1990). ...
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The foraging activity of large-bodied benthivorous fish has been suggested to be of key importance for maintaining shallow lakes in a turbid state. Since especially the spring ecosystem dynamics determines the successive state of shallow lakes, such impact could depend on seasonality in benthivore activity patterns. However, we do not yet know to what extent the activity of large-bodied benthivorous fish affects lake turbidity across the year. In order to investigate seasonal dynamics in bream activity and its impact on water turbidity under natural conditions, bream daily activity was studied in a small (39 ha) shallow Danish lake using passive biotelemetry technology, i.e. a modified Passive Integrated Transponder (PIT)-tag antenna system. We tracked the activity of 448 benthivorous bream over a period of four years (2012 to 2016) and during the same period wind conditions, water turbidity and temperature was measured. Results showed a clear relationship between bream activity and water turbidity at water temperature below 15°C indicating that winter season activity of benthivorous bream may play an important role for maintaining lake ecosystems in a turbid state. Also wind speed and wind direction affected water turbidity, suggesting that wind induced resuspension can be important even in small shallow lakes. This is to our knowledge the first full-scale study under natural conditions to describe how bream activity influence lake turbidity on a day-to-day basis. Our findings also add a seasonal component to previous findings by showing that benthivorous feeding bream have the potential to increase water turbidity also in the winter season and thereby, ultimately, impact ecosystem functioning within shallow lakes.
... In aquatic ecosystems, submerged macrophytes also play a central role because of their numerous functions in nutrient and organic matter balance and food web regulation (Declerck et al. 2011;Lauridsen et al. 2015; Barbara et al. 2016). Macrophytes also reduce sediment suspension and water turbidity (Gu et al. 2016), thereby contributing to the maintenance of a clear water state. Therefore, submerged macrophytes are widely used to control water pollutions, including eutrophication (Gao et al. 2017). ...
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Simulation experiments were conducted using sediments collected from the Taihu Lake to determine the combined effects of submerged macrophytes Ceratophyllum demersum and phosphate-solubilizing bacteria (PSB) strain XMT-5 (Rhizobium sp.) on phosphorus (P) concentrations in overlying waters and sediments. After 30 days of experimental incubation, the total phosphorus (TP) and dissolved total phosphorus (DTP) concentrations of the overlying water subjected to AMB and AHMB treatments (both with the combined effects of PSB cells and submerged macrophytes) were generally lower than those of the AM (with individual effects of inoculated C. demersum) and AB (with individual effects of a smaller amount of inoculated PSB cells) control treatments but higher than that of the A (with no effects of inoculated PSB cells or C. demersum) and AHB (with individual effects of a larger amount of inoculated PSB) control treatments. The TP contents of the sediment in the AMB and AHMB treatments were significantly lower than those of the other control treatments. The TP contents of the C. demersum cocultured with the PSB strain XMT-5 cells in the AMB and AHMB treatments were all significantly higher than that of the AM treatment, indicating the enhancement of P uptake by submerged plants inoculated with PSB. The bacterial diversity structures of the rhizosphere sediment subjected to different treatments were also analyzed by the high-throughput sequencing method. According to the ACE and Chao 1 indices, the bacterial diversity in the AMB and AHMB treatments were the highest. Although many sources contributed to the decrease in the nutrient loads of the lake sediment, harvesting macrophytes inoculated with PSB cells prior to their senescence might constitute a significant in-lake measure for reducing internal P load.
... However, the abundance of juvenile fish is easy to increase due to the reduction of fish population during biomanipulation, and the water returns to turbid status frequently. The high abundance of juvenile fish, especially juvenile benthivorous fish, not only leads to sediment resuspension, reducing transparency and light intensity of water but also results in nutrients releasing from sediment and promotes phytoplankton reproduction, ultimately inhibiting growth of submerged macrophytes (Gulati et al. 2008;Lorenz et al. 2013;Gu et al. 2016). Additionally, juvenile fish also inhibits growth of submerged macrophytes through direct grazing, which would be enhanced by increasing nitrogen loading, potentially impeding the process of ecological restoration (Gu et al. 2018). ...
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Rapid expansion of juvenile fish after biomanipulation can delay the successful restoration of submerged macrophytes, leading to a turbid water status in subtropical shallow lakes. Aimed to test the effects of direct removal of juvenile crucian carp (Carassius carassius) on water quality and growth of two submerged macrophytes Vallisneria natans and Hydrilla verticillate, a short-term outdoor mesocosm experiment was conducted in the Taihu Laboratory for Lake Ecosystem Research (TLLER). The results indicated that the concentrations of TN, TP, suspended solids, and chlorophyll a decreased significantly with increasing removal density of juvenile crucian carp, thus resulting in a clear status of the water. Additionally, the mean relative growth rate of V. natans and H. verticillata in the low- and high-density removal treatments were higher than that in the controls. Moreover, the removal of juvenile crucian carp also significantly increased the stem length of V. natans, but no significant effect on that of H. verticillata. Meanwhile, the total number of V. natans and H. verticillata in the low- and high-density removal treatments was higher than that in the controls, but all of H. verticillata were lower than the initials. Our results indicated that removing juvenile crucian carp could improve the water quality, increasing relative growth rate, height, and reproduction of V. natans, and improving the survival rate of H. verticillata. The promotion of fish removal on the V. natans growth was greater than H. verticillata. The results also implied that it was necessary to continuously remove the juvenile benthivorous fish several times for restoring the submerged macrophytes in shallow lakes.
... We also found an increase in TSS and a reduction in the light intensity on the sediment surface linked with the presence of crucian carp. Crucian carp can resuspend sediment particles during benthic feeding and thus increase water turbidity and TSS levels in the water [27,44]. Likewise, He et al. [7] found that crucian carp enhanced sediment resuspension and increased TSS in the mesocosms with crucian carp relative to controls. ...
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The presence of omnivorous fish is known to affect aquatic ecosystems, including water quality. The effect, however, depends on the species in question, and our knowledge is limited on the effect of omnivorous crucian carp (Carassius auratus), a common and often the most numerous fish species in eutrophic shallow lakes in China. We conducted a 70-day outdoor experiment in mesocosms with and without crucian carp to examine whether this species adversely affects water quality by increasing the levels of total nitrogen (TN) and total phosphorus (TP), thereby stimulating the biomass of phytoplankton and increasing water turbidity. Compared with carp-free controls, the presence of crucian carp resulted in higher TN and TP in the water column, greater phytoplankton biomass and lower periphyton biomass, measured as chlorophyll a. Total suspended solids (TSS) also increased in the presence of fish. We conclude that crucian carp can increase TN and TP, enhance phytoplankton biomass, and increase water turbidity, thereby contributing significantly to the deterioration of the water quality. In addition to controlling external nutrient loading, the removal of crucian carp may help to improve water quality in warm shallow eutrophic lakes.
... Many studies have suggested omnivorous fish affect macrophytes indirectly by altering concentrations of nutrients and suspended solids (Drenner et al., 1998) which then modulate phytoplankton biomass and the light attenuation (Menezes et al., 2010), thereby affecting growth of submerged macrophytes (Gu et al., 2016). Studies investigating the effects of direct grazing by omnivorous fishes on macrophytes have focused on large body-size species such as common carp (Cyprinus carpio) (Lougheed and Chow-Fraser, 1998;Miller and Crowl, 2006) while neglecting the impacts of small omnivores like A. macropterus. ...
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Transplantation of submerged macrophytes has been widely used to improve water quality in restoring shallow lakes in China. However, in some lakes, small omnivorous fish predominated the fish assemblages and fed mainly on submerged macrophytes. Despite significant research examining grazing selectivity in herbivorous fishes, macrophyte feeding preferences of small omnivorous fishes are poorly understood. We conducted a mesocosm experiment to examine the effects of a prolific small omnivorous bitterling fish Acheilognathus macropterus on the relative growth rate (RGR) and biomass of submerged macrophytes ( Ceratophyllum demersum, Myriophyllum spicatum, Vallisneria denseserrulata, and Hydrilla verticillata ). Our results showed that the presence of A. macropterus significantly increased nutrient concentrations ( e.g. total nitrogen and total phosphorus). The RGR of C. demersum in the bitterling-present treatment was significantly lower than the controls, in the presence of other macrophyte species. Further, total biomass of the four species of macrophytes in the fish-present mesocosms was markedly lower than in the fish-absent treatment, suggesting considerable consumption of macrophytes by bitterling. Moreover, the percent biomass of V. denserrulata and H. verticillata were significantly enhanced by the presence of bitterling. Our findings suggest that A. macropterus may increase nutrient concentrations through excretion and reduce the biomass and RGR of certain submerged macrophytes which may shift macrophyte community structure via selective grazing.
... Omnivorous fish also affect macrophytes indirectly by altering the concentrations of nutrients and suspended solids (Drenner et al., 1998). 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). ...
Article
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 addition, as our principal aim was to experimentally elucidate the interaction terms of small fish and bivalves, we did not establish control treatments without fish and bivalves. However, our previous study provided basic data regarding the density-dependent effect of small crucian carp [79] and C. fluminea [80], J. Gu et al., unpublished data] on plant growth. In addition, some studies showed that the presence of mussels did not substantially affect the biomass of pelagic algae in a clear lake, but significantly affected the water quality by reducing the phytoplankton biomass (Chl-a) in a turbid lake, emphasizing that the mussel effect likely depends on the trophic state of the lake [61,81]. ...
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Increased recruitment of small-sized fish following biomanipulation by reducing the biomass of plankti-benthivorous fish, not least in (sub)tropical lakes, may deteriorate water quality and thereby potentially hamper the recovery of submerged macrophytes. Filter-feeding bivalves remove suspended particles from the water and may, thereby, somewhat or fully counteract this negative effect of the increasing abundance of small-sized fish. So far, only few studies have investigated the interactive effects of fish and bivalves on water clarity and macrophyte growth. We conducted a 2 × 2 factorial designed outdoor mesocosm experiment with two densities of small crucian carp Carassius carassius (low 10 g m −2 and high 40 g m −2) and two densities of bivalves Corbicula fluminea (low 204 g m −2 and high 816 g m −2). We found significant interactive effect of fish and bivalves on the growth of the macrophyte Vallisneria natans. In the low density bivalve regime, the relative growth rates, root mass, root:shoot ratio and number of tubers were 30.3%, 30.8%, 21.6% and 27.8% lower in the high than in the low density fish treatments, while the decrease was less pronounced in the high density bivalve regime: 1.2%, 8.7%, 2.1% and 13.3%, respectively. Thus, bivalves reduced the negative effects of fish, not least when bivalve density was high. The weaker effects of small fish on plants in the high-than in the low-density C. fluminea regime can be attributed to lower total suspended solids (TSS) and Chl a in the first week of the experiment. Better light conditions further stimulated the growth of benthic algae, potentially increasing the removal of nutrients from the water and reducing fish-driven resuspension of the sediment. In addition, high densities of C. fluminea also enriched the sediment total nitrogen (TN) and total phosphorus (TP) content, favouring plant growth as indicated by an increase in leaf tissue TN and TP contents. Our results demonstrate that filter-feeding bivalves can alleviate harmful effects of small fish by prolonging a clear-water state that facilitates submerged macrophyte growth. Addition of the bivalve C. fluminea can be a promising tool for the restoration of submerged macrophytes in shallow eutrophic lakes, in particular lakes containing small, rapidly reproducing fish that due to their small sizes are not capable of controlling the bivalves.
... The submerged macrophytes play a key role in the inland aquatic ecosystems (river and lakes) and are critical part of food webs Townsend et al. 2018;Vanderstukken et al. 2011). Submerged macrophytes are the main primary producers that provide food and habitat to microorganisms and aquatic animals present in rivers and lakes (Gu et al. 2016). Submerged macrophytes can adsorb and fix the suspended sediments and also improve the water transparency (Hilt and Gross 2008;Zhu et al. 2018) and decontamination (Saleh et al. 2020). ...
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This study investigated the effects of light intensity on growth and biomass production of three submerged macrophyte species including Hydrilla verticillata, Vallisneria natans, and Ceratophyllum demersum in different water strata. For this purpose, water was classified into six strata designed: 5 cm, 30 cm, 60 cm, 90 cm, 120 cm, and 150 cm in the field under natural conditions. Results revealed that very high and very low heat intensity affected the growth of submerged macrophyte species differently. The growth of macrophytes was inhibited because of the high light intensity at 5 cm water strata. However, at 150-cm water strata, the macrophytes were died due to low light penetration. Among macrophyte species, the biomass production of Hydrilla verticillata and Ceratophyllum demersum was highest in the water strata at 90 cm, while Vallisneria natans were died in water strata of 120 cm due insufficient illumination. Furthermore, Hydrilla verticillata and Ceratophyllum demersum showed resilience and grown normally in later stages. The highest height of Vallisneria natans (35.3 cm), including Hydrilla verticillata (45.8 cm) and Ceratophyllum demersum (28.2 cm), was observed in water strata at 30, 60, and 60 cm. This study revealed that high and low light intensity affects the chlorophyll contents of macrophytes that results an impact on their growth and biomass production.
... 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). ...
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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.
... However, the abundance of cladocerans showed a decreasing strength with increasing experimental duration. Indeed, fish can exert detectable effects on lower trophic levels and abiotic environmental factors in a short time (≤1 week), which has been widely found in many studies (Figueredo and Giani 2005, Wasserman et al. 2015, Gu et al. 2016. Cladocerans may show evolutionary adaptations that allow them to resist the top-down effects in the long term. ...
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Top‐down cascade effects are among the most important mechanisms underlying community structure and abundance dynamics in aquatic and terrestrial ecosystems worldwide. A current challenge is understanding the factors controlling trophic cascade strength under global environmental changes. Here, we synthesized 161 global sites to analyze how multiple factors influence consumer‐resource interactions with fish in freshwater ecosystems. Fish have a profound negative effect on zooplankton and water clarity but positive effects on primary producers and water nutrients. Furthermore, fish trophic levels can modify the strength of trophic cascades, but an even number of food chain length does not have a negative effect on primary producers in real ecosystems. Eutrophication, warming and predator abundance strengthen the trophic cascade effects on phytoplankton, suggesting that top‐down control will be increasingly important under future global environmental changes. We found no influence or even an increasing trophic cascade strength (e.g., phytoplankton) with increasing latitude, which does not support the widespread view that the trophic cascade strength increases closer to the equator. With increasing temporal and spatial scales, the experimental duration has an accumulative effect, while the experimental size is not associated with the trophic cascade strength. (https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/ecy.3370)
... In addition, as our principal aim was to experimentally elucidate the interaction terms of small fish and N loading, we did not establish control treatments in which plants grew with no stressors. However, our previous study provided basic data regarding the density-dependent effect of small crucian carp on plant growth (Gu et al., 2016a). The conditions prevailing at low fish density and low N loading reflect the present eutrophic conditions of Lake Taihu, China (Chen et al., 2009;Kang et al., 2013), as well as many other eutrophic aquatic systems containing small, rapidly reproducing fish (Havens and Beaver, 2011;Meerhoff et al., 2012). ...
Article
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.
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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.
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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.
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The disappearance of submerged aquatic vegetation caused by the use of purse seine in productive fishery has aroused the attention of the national government. In order to restore aquatic vegetation, the government has removed the seine nets on the Huayanghe Lake's surface. Here, fourteen cruises were conducted in the Huayanghe Lakes from 2016 to 2019 to study the response of water quality and phytoplankton communities to rapidly recovering aquatic vegetation. The results showed that the restoration of aquatic vegetation increased the Secchi depth (from 65.36 to 105.52 cm) and dissolved oxygen (from 8.98 to 12.17 mg/L), while the concentration of total nitrogen (from 0.98 to 0.53 mg/L), and total phosphorus (from 0.04 to 0.02 mg/L) decreased, resulting in a 35.3% and 11.6% reduction in the total phytoplankton density and chlorophyll ɑ, respectively. In addition, the restoration of aquatic vegetation significantly increased Shannon–Wiener diversity, Margalef richness indices by 51.6% and 40.1%. We also observed that phytoplankton density exhibited significant changes based on nonmetric multidimensional scaling analysis (NMDS). In Lake Huangda, the coverage of aquatic vegetation was as high as 80%. We also observed that after the restoration of aquatic vegetation, the proportion of cyanobacteria decreased significantly by 21%. Our study emphasizes that aquatic plants can alleviate eutrophication and increase the diversity of phytoplankton, thus providing guidance for the restoration and protection of water ecosystems in the lakes connected to the Yangtze River.
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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.
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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.
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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.
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Invasive common carp Cyprinus carpio has long been identified as a contributor to water quality deterioration, disrupted ecosystem processes, and shifts in biological assemblage structure. In contrast, little information is available regarding the effects of native benthivorous species on aquatic systems despite their functional similarity to common carp. Effects of common carp and the native black bullhead Ameiurus melas on sediment resuspension, nutrient concentrations, macrophytes, and assemblage structure of zooplankton and macroinvertebrates, were experimentally evaluated. We observed decreased water clarity, increased nutrient concentrations, decreased macrophyte biomass, and decreased benthic macroinvertebrate abundance and biomass associated with common carp, regardless of the presence of black bullhead. In contrast, black bullhead increased total phosphorus, and chlorophyll a concentrations and copepod abundance, but had little or no effect on other measured water quality and biological variables when compared to control conditions. Overall, experimental results suggest that although black bullhead tend to be tolerant of degraded ecosystems, they are not a source of physical changes to the environment known to be responsible for perpetuating degraded water quality (e.g., stable state shifts). Therefore, increased abundance of native species, such as black bullhead, following the invasion of common carp, may serve as an indicator of ecological conditions and should not to be assumed as causative.
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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.
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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.
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We assessed the importance of temperature, salinity, and predation for the size structure of zooplankton and provided insight into the future ecological structure and function of shallow lakes in a warmer climate. Artificial plants were introduced in eight comparable coastal shallow brackish lakes located at two contrasting temperatures: cold-temperate and Mediterranean climate region. Zooplankton, fish, and macroinvertebrates were sampled within the plants and at open-water habitats. The fish communities of these brackish lakes were characterized by small-sized individuals, highly associated with submerged plants. Overall, higher densities of small planktivorous fish were recorded in the Mediterranean compared to the cold-temperate region, likely reflecting temperature-related differences as have been observed in freshwater lakes. Our results suggest that fish predation is the major control of zooplankton size structure in brackish lakes, since fish density was related to a decrease in mean body size and density of zooplankton and this was reflected in a unimodal shaped biomass-size spectrum with dominance of small sizes and low size diversity. Salinity might play a more indirect role by shaping zooplankton communities toward more salt-tolerant species. In a global-warming perspective, these results suggest that changes in the trophic structure of shallow lakes in temperate regions might be expected as a result of the warmer temperatures and the potentially associated increases in salinity. The decrease in the density of large-bodied zooplankton might reduce the grazing on phytoplankton and thus the chances of maintaining the clear water state in these ecosystems.
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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.
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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.
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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
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Survival and growth of Vallisneria americana winterbuds was significantly related to both initial winterbud size and to the water column turbidity under which the plants were grown. Larger winterbuds showed better survival and better growth than did smaller ones. Turbidity likewise significantly impacted the survival and growth of the plants. Over the turbidity range of 0.2–45 NTU (53–7% total incident light), the plants were shown to have progressively poorer survival and to produce fewer rosettes and total number of leaves. Vallisneria americana seedlings were likewise influenced by turbidity. Under high turbidity conditions the seedlings had significantly higher mortality, while surviving plants produced fewer rosettes and accumulated less biomass than seedlings grown under low turbidity conditions. In addition, under turbid conditions the seedlings had to invest proportionally more energy into above-ground tissues.
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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.
Conference Paper
Fish manipulation has been used to restore lakes in the temperate zone. Often strong shortterm cascading effects have been obtained, but the long term-perspectives are less clear. Fish manipulation methods are far less advanced for warm lakes, and it is debatable whether it is, in fact, possible to create a trophic cascade in warm lakes due to the dominance and high densities of fast-reproducing omnivorous fish. However, other important aims of fish manipulation, for instance, the removal of benthic feeding fish, are to reduce disturbance of the sediment, which not only affects the nutrient level but also the concentration of suspended organic and inorganic matter with reduced clarity as a result, and hampers growth of submerged macrophytes. We conducted a biomanipulation experiment in two basins of Chinese Huizhou West Lake that has remained highly turbid after extensive nutrient loading reduction. A third basin was used as control (controltreatment pairing design). Removal of a substantial amount of plankti-benthivorous fish was followed by planting of submerged macrophytes and stocking of piscivorous fish. We found strong and relatively long-lasting effects of the restoration initiative in the form of substantial improvements in water clarity and major reductions in nutrient concentrations, particularly total phosphorus, phytoplankton and turbidity, while only minor effects were detected for crustacean zooplankton grazers occurring in low densities before, as well as after the restoration. Our results add importantly to the existing knowledge of restoration of warm lakes and are strongly relevant, not least in Asia where natural lakes frequently are used extensively for fish production, often involving massive stocking of benthivorous fish. With a growing economy and development of more efficient fish production systems, the interest in restoring lakes is increasing world-wide. We found convincing evidence that fish removal and piscivores stocking combined with transplantation of submerged macrophytes may have a substantial role in conservation and management of warm water lakes.
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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 Cabomba 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 �1), low nitrogen (LN) addition (mean TN = 3.5 mg L �1) and high nitrogen (HN) addition (mean TN = 5.5 mg L �1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L �1) 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 phytoplankton and periphyton in the summer, while there was no indication of direct N toxicity.
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Fishes play a functional role in structuring aquatic ecosystems through top-down and bottom-up processes. Adult common carp (Cyprinus carpio) are well recognized for their middle-out effects on aquatic ecosystems that can shift shallow lakes from the clear- to turbid-water stable state through benthic foraging activities. However, less is known about ecosystem effects of age-0 common carp. Age-0 common carp are planktivorous and can be highly abundant, suggesting that large year classes may also produce undesirable ecosystem effects. We evaluated the effects of four age-0 common carp densities (0, 175, 475, and 812 kg/ha) on water quality (ammonium, total phosphorus, total Kjeldahl nitrogen, and turbidity) and primary (macrophytes and phytoplankton) and secondary (zooplankton and benthic invertebrates) production. Common carp increased nutrient availability and phytoplankton production and decreased water transparency with effects increasing with carp biomass. Common carp also reduced macrophyte coverage and cladocera density (through effects on chydorus and ceriodaphnia) and body size, but effects were not density dependent. In contrast, common carp did not appear to affect copepod, rotifer, chironomid, or gastropod densities. These results suggest that even relatively low age-0 common carp densities (>175 kg/ha) may have many comparable ecosystem effects as adult carp, although effects may be accrued through different pathways.
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While the effects of lake restoration by fish manipulation are well-studied in the temperate zone, comparatively little information is available on this issue from tropical lakes. It may be expected that fish removal leads to faster recovery of the fish stock here than in temperate lakes due to more frequent and earlier reproduction, which may, in turn, delay positive effects of the restoration. We studied the community composition, feeding type and abundance of fish in three basins of a tropical shallow lake: one unrestored basin (UR) and two basins restored by fish manipulation and transplantation of submerged macrophytes. While omni-benthivorous fish dominated the biomass in the restored basins 3 and 5 years after restoration, planktivores were most abundant in the UR, although total fish biomass remained similar. One-way analyses of similarities based on fish species presence/absence, abundance and biomass data revealed significant differences in fish community composition among the restored basins and UR, and redundancy analyses further indicated that submerged macrophytes were a key driver behind this difference. Our results indicate that active implantation of submerged macrophytes to stabilise the fish community is a tool to consider when planning lake restoration by biomanipulation in the tropics.
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While considerable research has addressed the influence of sediment resuspension on shallow lake ecosystems, little attention has been paid to its potential impact on phytoplankton responses to nutrient loading. A factorial experiment was designed using field mesocosms to investigate the impacts of resuspension, nutrient loading and the interactions between them on standing crops of phytoplankton and community structure dynamics. Results showed that phytoplankton biomass increased with nutrient loading when no sediment resuspension occurred, but there was no significant biomass response to nutrient loading in treatments with strong sediment resuspension. The results also showed both sediment resuspension and nutrient loading to have significant effects on the structural dynamics of phytoplankton communities. Our results suggest that strong sediment resuspension can dampen the response of phytoplankton to nutrient loading in lakes such as Lake Taihu which experiences strong sediment resuspension due to wind action. These findings also suggest that in shallow eutrophic lakes with high nutrient concentrations or external loadings, measures aimed at reducing sediment resuspension are unlikely to be effective in aiding the re-establishment of macrophytes, because of the side effect of enhanced phytoplankton growth.
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Aquatic plants frequently encounter multiple stresses under natural conditions. Nuttall's water weed, Elodea nuttallii (Planch.) is a submerged aquatic macrophyte which has flexible ability to use different nutrient sources from various environments. However, recently the growth of E. nuttallii has been declining in waters of Japan and in the Chesapeake Bay, a large estuary in the United States. In the present experiment, we studied growth and survival capabilities of the plant under a gradient of redox conditions; from highly oxic (+400 to +440 mV) to extremely reduced (−180 to −120 mV) conditions. Reduced environment was prepared by adding glucose to growth medium and nitrogen gas bubbling, while the oxic environment was brought about by atmospheric air bubbling. In comparison to the oxic environment, growth rate and carbon–nitrogen content of the plants were significantly affected negatively at hypoxic and anoxic conditions. In hypoxic and anoxic environments, indole acetic acid (IAA), tissue nitrogen and chlorophyll levels were down-regulated, whereas hydrogen peroxide (H2O2), indole acetic acid oxidase (IAAO) and peroxidase (POD) levels were up-regulated. It was also found that high NH4–N concentrations (10–40 ppm) affect the growth rate and biochemical parameters of the plant; however, in hypoxic and anoxic treatments the effects were more severe. We conclude that E. nuttallii is poorly tolerant to hypoxia/anoxia. Moreover, oxygen stress combined with high ammonium concentration act as important factors influencing distribution and abundance of this species.
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Variations in the growth of dioecious Hydrilla verticillata (L.f.) Royle and Vallisneria americana Michx. were examined in a greenhouse under each of two levels (low and high) of light availability, sediment fertility, and inorganic carbon supply. Under low light (ca. 125 uE/m/s at midday), effects of sediment fertility and inorganic carbon availability on biomass production were relatively minor. Maximum biomass production in both species was achieved under high light (ca. 550 uE/m/s at midday) on fertile sediment with a ten-fold increase in ambient CO2, supply. However, with other treatment combinations, biomass and morphological responses differed considerably between species. Biomass production in Hydrilla was most responsive to CO2 supply, while in Vallisneria, production was most responsive to sediment fertility. Shoot elongation in response to low light was much more pronounced in Hydrilla than in Vallisneria. Under high light, Hydrilla maximized shoot density (number) with an accompanying increase in biomass production. In Vallisneria, however, shoot (plant) density diminished with increased biomass production under both light levels. In nature, the outcome of competition between these two important submersed macrophyte species may be influenced significantly by interactions among the studied variables.
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Describing the relative magnitude and controls of herbivory and decomposition is important in understanding the trophic transference, recycling, and storage of carbon and nutrients in diverse ecosystems. We examine the variability in herbivory and decomposition between and within a wide range of aquatic and terrestrial ecosystems. We also analyze how that variability is associated with differences in net primary production and producer nutritional quality. Net primary production and producer nutritional quality are uncorrelated between the two types of system or within either type. Producer nutritional quality is correlated to the percentage of primary production consumed by herbivores or percentage of detrital production decomposed annually, regardless of whether the comparison is made between the two types of systems or within either type of system. Thus, producer nutritional quality stands out as a consistent indicator of the importance of consumers as top-down controls of producer biomass and detritus accumulation and nutrient recycling. However, absolute consumption by herbivores and absolute decomposition (both in g C·m-2·yr-1) are often associated with absolute primary production and independent of producer nutritional quality, because the variability in net primary production across systems largely exceeds that in the percentage consumed or decomposed. Thus, primary production often stands out as an indicator of the absolute flux of producer carbon transferred to consumers and of the potential levels of secondary production maintained in the system. These patterns contribute to our understanding of the variability and control of herbivory and decomposition, and implications on carbon and nutrient cycling, in aquatic and terrestrial ecosystems. Furthermore, in view of their robustness, they may offer a template for global change models seeking to predict anthropogenic effects on carbon and nutrient fluxes.
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Survival and growth of Vallisneria americana winterbuds was significantly related to both initial winterbud size and to the water column turbidity under which the plants were grown. Larger winter buds showed better survival and better growth than did smaller ones. Turbidity likewise significantly impacted the survival and growth of the plants. Over the turbidity range of 0.2–45 NTU (53–7% total incident light), the plants were shown to have progressively poorer survival and to produce fewer rosettes and total number of leaves. Vallisneria americana seedlings were likewise influenced by turbidity. Under high turbidity conditions the seedlings had significantly higher mortality, while surviving plants produced fewer rosettes and accumulated less biomass than seedlings grown under low turbidity conditions. In addition, under turbid conditions the seedlings had to invest proportionally more energy into above-ground tissues.
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A two-year restoration period, large-scale ecological restoration demonstration engineering project was carried out in the near-shore zones of Wuli Bay, Taihu Lake. Various methods to restore the aquatic biodiversity and prevent ecological degradation were employed and their effects on water quality and aquatic plants were investigated. The results showed that water quality had been significantly improved in the demonstration zones. The concentrations of TN and TP were about half of those of the reference site in Wuli Bay. The water transparency was 30cm higher than that in the reference site. The species, cover and biomass of aquatic plants were also significantly increased in the demonstration zones.
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Recent findings suggest that herbivores select feeding sites of intermediate biomass in order to maximise their digestible nutrient intake as the result of the trade-off between forage quality and quantity ('forage maturation hypothesis'). We propose a reformulation of this hypothesis which recognises this trade-off, but also underlines that constraints due to body mass (i.e. metabolism and digestive constraints, size of the feeding apparatus) can lead to variations in grazing patterns. We tested this latter hypothesis experimentally in three species of herbivorous Anatidae of different body mass: the wigeon Anas penelope (in our study c. 620 g), the barnacle goose Branla leucopsis (c. 2000 g), and the greylag goose Anser anser (c. 3500 g). Each species was tested separately from 0600 to 0930 hours, in an enclosure with a mosaic of patches of grass of three different heights: short, medium and tall. The behaviour, and the location (i.e. patch) of each individual were recorded every 5 minutes. Our results show important interspecific differences in intake rates resulting in different feeding site selection: wigeon and barnacle goose fed fastest on the shortest swards, and selected short grass which was also of higher quality. Tall grass provided the highest dry matter intake rate and digestible protein intake for greylag geese, and they preferred these swards. These choices allowed the birds to maximise their digestible nitrogen intake rate rather than dry matter intake rate and our results thus underline the importance of nitrogen as a major currency for foraging decisions in herbivorous Anatidae. Since the birds selected the two extreme sward heights (instead of the medium one), the results give support to our hypothesis and underline the role of body size as an important cause of variations in patch selection in herbivorous Anatidae.
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This paper critically reviews the published works on lake restoration in north-western Europe, with the aim to highlight the causes of failures of lake biomanipulation, and to identify the main bottlenecks that have impeded progress. More importantly, we explore the prospects of applying new ecotechnological measures to lakes with a focus on shallow lakes. These complementary measures are: (1) reduction of sediment resuspension; (2) water-level management; and (3) the use in shallow lakes of bivalves as effective grazers on lake seston, especially when cyanobacteria are dominant. If the sustainability of the positive effects of biomanipulation is considered over a decade, there are probably more cases of failures than successes. The failures can be ascribed to several bottlenecks that include: (1) inadequate reduction of allochthonous phosphorus (P) and an increase in autochthonous P inputs, i.e. release of P from the lake sediments following reductions of external P inputs; (2) poor edibility of filamentous and colonial cyanobacteria to daphnids; (3) inadequate coverage of the lake area by macrophytes partly due to foraging on the macrophytes by both fish and birds; (4) ineffective reduction of planktivorous fish biomass and our inability to maintain the fish mass to a ‘low level’ for longer periods; and (5) failure of northern pike (Esox lucius) after its transplantation to the lakes to develop a population level that can control planktivorous fish to desired low levels. Three potentially complementary ecotechnological measures are discussed. The first such measure concerns prevention of sediment resuspension in lakes by creating islands in order to minimise the wind fetch to reduce the wave amplitude. The second measure involves allowing greater water-level fluctuations (WLF) in lakes as planned in lowland countries like the Netherlands; WLF are likely to allow more space for water, and may lead to improved water quality and higher biodiversity. The third ecotechnological measure relates to grazer populations that complement herbivorous zooplankton to regulate phytoplankton, particularly to control cyanobacterial blooms. For this, the bivalve Dreissena polymorpha appears to be a good potential candidate for grazing on phytoplankton, especially in shallow eutrophic lakes that are dominated by filamentous and toxic cyanobacteria (e.g. Planktothrix agardhii and Microcystis aeruginosa).