ABSTRACT: Mitigating nutrient losses from anthropogenic nonpoint sources is today of particular importance for improving the water quality of numerous freshwater lakes worldwide. Several empirical relationships between land use and in-lake water quality variables have been developed, but they are often weak, which can in part be attributed to lack of detailed information about land use activities or point sources. We examined a comprehensive data set comprising land use data, point-source information, and in-lake water quality for 414 Danish lakes. By excluding point-source-influenced lakes (n = 210), the strength in relationship (R2) between in-lake total nitrogen (TN) and total phosphorus (TP) concentrations and the proportion of agricultural land use in the watershed increased markedly, from 10-12% to 39-42% for deep lakes and from 10-12% to 21-23% for shallow lakes, with the highest increase for TN. Relationships between TP and agricultural land use were even stronger for lakes with rivers in their watershed (55%) compared to lakes without (28%), indicating that rivers mediate a stronger linkage between landscape activity and lake water quality by providing a "delivery" mechanism for excess nutrients in the watershed. When examining the effect of different near-freshwater land zones in contrast to the entire watershed, relationships generally improved with size of zone (25, 50, 100, 200, and 400 m from the edge of lake and streams) but were by far strongest using the entire watershed. The proportion of agricultural land use in the entire watershed was best in explaining lake water quality, both relative to estimated nutrient surplus at agricultural field level and near-lake land use, which somewhat contrasts typical strategies of management policies that mainly target agricultural nutrient applications and implementation of near-water buffer zones. This study suggests that transport mechanisms within the whole catchment are important for the nutrient export to lakes. Hence, the whole watershed should be considered when managing nutrient loadings to lakes, and future policies should ideally target measures that reduce the proportion of cultivated land in the watershed to successfully improve lake water quality.
Ecological Applications 06/2012; 22(4):1187-200. · 5.10 Impact Factor
ABSTRACT: With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status
of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton
were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component
of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted
in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian,
and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which
cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right
metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are
important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore,
we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional
calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases
of fish, phytoplankton, macrophytes and benthic invertebrates.
KeywordsZooplankton–Eutrophication–Indicator–Water Framework Directive (WFD)–Lakes–Ecological state–Water quality
Hydrobiologia 05/2012; 676(1):279-297. · 1.78 Impact Factor
ABSTRACT: While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known
about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing
climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure
experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations.
In hypertrophic Lake Søbygård, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton
ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing
Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and
their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were
absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during
winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects
on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading
to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched
shallow lakes, not only during the summer season, but also during winter.
Hydrobiologia 04/2012; 664(1):147-162. · 1.78 Impact Factor
ABSTRACT: Climate change might have profound effects on the nitrogen (N) dynamics in the cultivated landscape as well as on N transport
in streams and the eutrophication of lakes. N loading from land to streams is expected to increase in North European temperate
lakes due to higher winter rainfall and changes in cropping patterns. Scenario (IPCC, A2) analyses using a number of models
of various complexity for Danish streams and lakes suggest an increase in runoff and N transport on an annual basis (higher
during winter and typically lower during summer) in streams, a slight increase in N concentrations in streams despite higher
losses in riparian wetlands, higher absolute retention of N in lakes (but not as percentage of loading), but only minor changes
in lake water concentrations. However, when taking into account also a predicted higher temperature there is a risk of higher
frequency and abundance of potentially toxic cyanobacteria in lakes and they may stay longer during the season. Somewhat higher
risk of loss of submerged macrophytes at increased N and phosphorus (P) loading and a shift to dominance of small-sized fish
preying upon the key grazers on phytoplankton may also enhance the risk of lake shifts from clear to turbid in a warmer North
European temperate climate. However, it must be emphasised that the prediction of N transport and thus effects is uncertain
as the prediction of regional precipitation and changes in land-use is uncertain. By contrast, N loading is expected to decline
in warm temperate and arid climates. However, in warm arid lakes much higher N concentrations are currently observed despite
reduced external loading. This is due to increased evapotranspiration leading to higher nutrient concentrations in the remaining
water, but may also reflect a low-oxygen induced reduction of nitrification. Therefore, the critical N as well as P loading
for good ecological state in lakes likely has to be lower in a future warmer climate in both north temperate and Mediterranean
lakes. To obtain this objective, adaptation measures are required. In both climate zones the obvious methods are to change
agricultural practices for reducing the loss of nutrients to surface waters, to improve sewage treatment and to reduce the
storm-water nutrient runoff. In north temperate zones adaptations may also include re-establishment of artificial and natural
wetlands, introduction of riparian buffer zones and re-meandering of channelised streams, which may all have a large impact
on, not least, the N loading of lakes. In the arid zone, also restrictions on human use of water are urgently needed, not
least on the quantity of water used for irrigation purposes.
KeywordsClimate change–Nitrogen loading–Lakes–Ecological state–Modelling–Streams–Subtropics–Temperate–Warm arid
Hydrobiologia 04/2012; 663(1):1-21. · 1.78 Impact Factor
ABSTRACT: We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and
production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting
area (from 13 to 80,800ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte
dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon
(DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and
morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances
content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton
variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in
the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation
is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot
be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than
in the pelagic zone of shallow lakes.
KeywordsHorizontal zonation-Physiological status-Bacterial morphotypes-Bacterial predation-Macrophytes-Phytoplankton
Hydrobiologia 04/2012; 646(1):311-326. · 1.78 Impact Factor
ABSTRACT: Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions
and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management
strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4–20years.
In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited
low mean summer levels (<2.2mgO2l−1). Oxygenation generally increased the hypolimnetic water temperature by 0.5–2°C, but in one lake it increased by 4–6°C. In
all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification.
The accumulation of phosphorus and ammonia typically decreased by 40–88%. In two lakes oxygenation was stopped for 1–2years
and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in
one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is
concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia
in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water
quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water
Hydrobiologia 04/2012; 625(1):157-172. · 1.78 Impact Factor
ABSTRACT: Trophic niche divergence is considered to be a major process by which species coexistence is facilitated. When studying niche
segregation in lake ecosystems, we tend to view the niche on a one-dimensional pelagic-littoral axis. In reality, however,
the niche use may be more complex and individual fidelity to a niche may be variable both between and within populations.
In order to study this complexity, relative simple systems with few species are needed. In this paper, we study how competitor
presence affects the resource use of brown trout (Salmo trutta) in 11 species-poor Faroese lakes by comparing relative abundance, stable isotope ratios and diet in multiple habitats. In
the presence of three-spined sticklebacks (Gasterosteus aculeatus), a higher proportion of the trout population was found in the pelagic habitat, and trout in general relied on a more pelagic
diet base as compared to trout living in allopatry or in sympatry with Arctic charr (Salvelinus alpinus). Diet analyses revealed, however, that niche-segregation may be more complex than described on a one-dimensional pelagic-littoral
axis. Trout from both littoral and offshore benthic habitats had in the presence of sticklebacks a less benthic diet as compared
to trout living in allopatry or in sympatry with charr. Furthermore, we found individual habitat specialization between littoral/benthic
and pelagic trout in deep lakes. Hence, our findings indicate that for trout populations interspecific competition can drive
shifts in both habitat and niche use, but at the same time they illustrate the complexity of the ecological niche in freshwater
KeywordsNiche complexity–Stable isotopes–Trout–Stickleback–Aquatic ecology–Faroe Islands
Environmental Biology of Fishes 04/2012; 93(3):305-318. · 0.91 Impact Factor
ABSTRACT: During the past 10–15years removal of plankti- and benthivorous fish (mainly roach, Rutilus rutilus, and bream, Abramis brama) has commonly been used as a method to improve the ecological quality of Danish lakes. Here, we examine the general and long-term
effects obtained after the removal of 41–1360kg fish ha−1 in 36 mainly shallow and eutrophic lakes. In lakes in which less than 200kg fish ha−1 were removed within a 3-year period only minor effects were observed, but at higher removal rates both chemical and biological
variables were markedly affected. The concentrations of chlorophyll a (Chla), total phosphorus (TP), total nitrogen (TN), and suspended solids (SS) decreased to 50–70% of the level prior to removal.
The most significant and long-lasting effects were found for SS and Secchi depth, whereas the most modest effects were seen
for Chla. This probably reflects an efficient and persistent reduction of the bream stock which reduced resuspension and SS,
while the biomass of roach returned to former levels, decreasing the zooplankton grazing with less control on Chla. Total
algal biomass also declined after fish removal, particularly that of cyanobacteria, whereas the biomass of cryptophytes increased,
indicating enhanced grazing pressure by zooplankton. The abundance and species number of submerged macrophytes increased in
the majority of the lakes. For most variables the effects of the fish removal were significant for 6–10years, after which
many lakes tended to return to pre-restoration conditions, probably mainly because of consistently high external and internal
phosphorus loading. We conclude that a sufficiently extensive removal of plankti- and benthivorous fish is an efficient tool
to create clear water; however, repeated fish removal is presumably required to obtain long-term effects in the most nutrient
Ecosystems 04/2012; 11(8):1291-1305. · 3.49 Impact Factor
ABSTRACT: The thermoelectric properties of a PtSb(2) single crystal containing a stoichiometric gradient were investigated. The gradient was produced by employing a Stockbarger synthesis technique. The gradient was observed through the use of spatial resolved Seebeck coefficient measurements and verified utilizing X-Ray Diffraction and Energy Dispersive X-Ray Spectroscopy. The correlation between Pt/Sb ratio and physical property parameters--Seebeck coefficient, mobility, resistivity and charge carrier concentration--was studied. Elemental analysis by Energy Dispersive X-Ray Spectroscopy, X-Ray Fluorescence and Inductively Coupled Plasma revealed Sb deficiency in the crystal, which explains the observed high charge carrier concentration and metallic properties. The transport properties were measured in the temperature range T = 20-300 K on a polycrystalline sample. Furthermore, ab initio theoretical calculations have been conducted to support the interpretation of the measurements.
Dalton Transactions 11/2011; 41(4):1278-83. · 3.84 Impact Factor
ABSTRACT: Summary1. Filamentous green algae (FGA) may represent an alternative state in high-nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain.2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature.3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m3) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 °C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short-term microcosms and laboratory experiments.4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a).5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non-limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals.6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.
Freshwater Biology 11/2010; 56(3):541 - 553. · 3.29 Impact Factor
ABSTRACT: A simple biomolecule-assisted hydrothermal approach has been developed for the fabrication of Bi(2)Te(3) thermoelectric nanomaterials. The product has a nanostring-cluster hierarchical structure which is composed of ordered and aligned platelet-like crystals. The platelets are approximately 100 nm in diameter and only approximately 10 nm thick even though a high reaction temperature of 220 degrees C and a long reaction time of 24 h were applied to prepare the sample. The growth of the Bi(2)Te(3) hierarchical structure appears to be a self-assembly process. Initially, Te nanorods are formed using alginic acid as both reductant and template. Subsequently, Bi(2)Te(3) grows in a certain direction on the surface of the Te rods, resulting in the nanostring structure. The nanostrings further recombine side-by-side with each other to achieve the ordered nanostring clusters. The particle size and morphology can be controlled by adjusting the concentration of NaOH, which plays a crucial role on the formation mechanism of Bi(2)Te(3). An even smaller polycrystalline Bi(2)Te(3) superstructure composed of polycrystalline nanorods with some nanoplatelets attached to the nanorods is achieved at lower NaOH concentration. The room temperature thermoelectric properties have been evaluated with an average Seebeck coefficient of -172 microV K(-1), an electrical resistivity of 1.97 x 10(-3) Omegam, and a thermal conductivity of 0.29 W m(-1) K(-1).
ACS Nano 05/2010; 4(5):2523-30. · 10.77 Impact Factor
ABSTRACT: Summary1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (Cmax) of plants as well as coverage and plant volume inhabited (PVI) as indicators of eutrophication.2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient-rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) <0.02 mg P L−1 and Chla <5 μg L−1 and therefore cannot be considered as reliable indicators of eutrophic conditions.3. Submerged macrophyte coverage, PVI and the Cmax were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L−1 plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L−1 only 29% of the lakes had coverage >10%. Cmax was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes.4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well-defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and Cmax, which might be used to track major changes in macrophyte communities and for lake classification.
Freshwater Biology 11/2009; 55(4):893 - 908. · 3.29 Impact Factor
ABSTRACT: Summary1. Fish community structure and habitat distribution of the abundant species roach, perch and ruffe were studied in Lake Nordborg (Denmark) before (August 2006) and after (August 2007) aluminium treatment to reduce internal phosphorus loading.2. Rapid changes in fish community structure, abundance and habitat distribution occurred following a decline in in-lake phosphorus concentrations from 280 to 37 μg P L−1 and an increase in Secchi depth transparency from 1.1 to 1.9 m (August). The proportion of perch in overnight gill net catches increased, whilst roach decreased, and the average weight of all key species increased.3. The habitat distribution of perch and roach changed from a high proportion in the upper pelagic and littoral zones in 2006, towards enhanced proportions in the deeper pelagic and profundal zone in 2007. The abundance of large-bodied zooplankton increased and the abundance of benthic invertebrates decreased in the same period, suggesting that the habitat shift was not induced by food limitation.4. Ruffe shifted from the littoral and upper profundal zones towards the deep profundal zone, likely reflecting an increased predation risk in the littoral zone and better oxygen conditions in the deep profundal.5. Our results indicate that enhanced risk of predation in the upper pelagic and the littoral zones and perhaps improved oxygen concentrations in the deeper profundal zone at decreasing turbidity are responsible for the observed habitat shift. The results indicate that fish respond rapidly to changes in nutrient state, both in terms of community structure and habitat use.
Freshwater Biology 08/2009; 55(5):1036 - 1049. · 3.29 Impact Factor
ABSTRACT: We sought to identify environmental factors influencing crustacean zooplankton species richness in brackish lagoons and to elucidate whether crustacean zooplankton species richness and trophic structure of brackish lagoons differ among two regions with contrasting temperatures. We sampled 35 and 42 brackish lagoons (salinity ranging from 0.3 to 55‰) in Mediterranean Catalonia (NE Spain) and northern-temperate Denmark, respectively. No significant differences were found in total crustacean zooplankton species richness or cladoceran richness between the climatic regions. Calanoid richness was higher in Denmark than in Catalonia, while cyclopoid richness was higher in Catalonia. Salinity was the most important variable associated with zooplankton species richness in both regions, richness of total zooplankton species, cladocerans and cyclopoids being negatively related with salinity. In both regions, a shift occurred from dominance of large filter feeding cladoceran species at low salinities to copepods and small cladoceran species at higher salinities. Cladoceran richness increased with increasing total phosphorus, but was not influenced by total nitrogen or chlorophyll-a. Trophic structure in Mediterranean brackish lagoons showed a more pronounced seasonal variation than in north temperate brackish lagoons. Our results imply that the indirect effects of climate warming, such as changes in salinity and hydrology, will have a larger impact on brackish lagoon ecosystems than the increase in temperature per se.
Ecography 07/2009; 32(4):692 - 702. · 4.19 Impact Factor
ABSTRACT: Lake restoration in Denmark has involved the use of several different restoration techniques, all aiming to improve lake water quality and establishing clear-water conditions. The most frequently used method, now used in more than 20 lakes, is the reduction of zooplanktivorous and benthivorous fish (especially roach (Rutilus rutilus) and bream (Abramis brama)) with the objective of improving the growth conditions for piscivores, large-sized zooplankton species, benthic algae and submerged macrophytes. Piscivore stocking (mainly Esox lucius (pike)), aiming especially at reducing the abundance of young-of-the-year fish, has been used in more than 10 lakes and frequently as a supplement to fish removal. Hypolimnetic oxidation, with oxygen and nitrate, has been undertaken in a few stratified lakes and sediment dredging, with the purpose of diminishing the internal phosphorus loading, has been experimented with in one large, shallow lake. Submerged macrophyte implantation has been conducted in some of the biomanipulated lakes to increase macrophyte abundance and distribution. Overall, the results from lake restoration projects, in the mainly shallow Danish lakes, show that external nutrient loading must be reduced to a level below 0.05–0.1 mg P L–1 under equilibrium conditions to gain permanent effects on lake water quality. By using fish removal, at least 80% of the fish stock should be removed over a period of not more than 1–2 years to obtain a substantial effect on lower trophic levels and to avoid regrowth of the remaining fish stock. Stocking of piscivores requires high densities (>0.1 individuals m–2) if an impact on the plankton level is to be obtained and stocking should be repeated yearly until a stable clear-water state is reached. The experiments with hypolimnetic oxygenation and sediment dredging confirm that internal phosphorus loading can be reduced. Experience from macrophyte implantation experiments indicates that protection against grazing by herbivorous waterfowl may be useful in the early phase of recolonization.
Lakes & Reservoirs Research & Management 10/2008; 5(3):151 - 159.
Journal of Applied Ecology 10/2007; 44(6):1089 - 1094. · 5.05 Impact Factor
ABSTRACT: In brackish lagoons, Daphnia is replaced by calanoid copepods (Eurytemora affinis, Acartia spp.) and rotifers when a certain threshold (depending on, for instance, fish density) is reached. We hypothesize that loss of Daphnia induces a regime shift from clear to turbid at high nutrient concentrations. We conducted a factorial designed enclosure experiment with contrasting salinities (0–16‰), low fish predation (one three-spined stickleback, Gasterosteus aculeatus, m−2) and three levels of nutrient loading in a shallow brackish lagoon. A change point analysis suggests a strong regime shift from a clear to a turbid state at 6–8‰ salinity at low and high loading, but not for the control. From the low to the high salt regime, chlorophyll a (Chla), Chla:total phosphorus (TP) and Chla:total nitrogen (TN) ratios shifted highly significantly for all nutrient treatments, and the bacterioplankton production followed the changes in Chla. These changes occurred parallel with a shift from cladoceran and cyclopoid copepod to rotifer dominance. Monitoring data from 60 Danish brackish lagoons show increasing Chla with increasing TP and TN as well as interactive effects of TN and salinity, peaking at intermediate salinity. A relatively weak effect of
salinity at low nutrient concentrations and the stronger effect at intermediate high salinity are in accordance with the experimental results. However, these data suggest a lower salinity threshold than in the experiment, which may be explained by a higher fish density. Our results have implications for the management of coastal lagoons both at present and in a future (predicted) warmer climate: (1) improved water quality can be obtained by reducing the nutrient loading or enhancing the freshwater input
to a level triggering a shift to Daphnia dominance (typically <2‰), (2) fish manipulation is probably not a useful tool for brackish lagoons, unless the salinity is below the threshold for a potential shift to a clear Daphnia dominated state, and (3) more abrupt changes will expectedly occur in low-saline coastal lagoons at increasing salinity during summer in a future warmer climate.
Ecosystems 10/2007; 10(1):48-58. · 3.49 Impact Factor
ABSTRACT: A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m−2 d−1 before the loading reduction and 125 mg N m−2 d−1 after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments.
Internationale Revue der gesamten Hydrobiologie und Hydrographie 01/2007; 77(1):29 - 42.
ABSTRACT: Shallow lakes respond to nutrient loading reductions. Major findings in a recent multi-lake comparison of data from lakes
with long time series revealed: that a new state of equilibrium was typically reached for phosphorus (P) after 10–15 years
and for nitrogen (N) after <5–10 years; that the in-lake Total N:Total P and inorganic N:P ratios increased; that the phytoplankton
and fish biomass often decreased; that the percentage of piscivores often increased as did the zooplankton:phytoplankton biomass
ratio, the contribution of Daphnia to Zooplankton biomass, and cladoceran size. This indicates that enhanced resource and predator control often interact during
recovery from eutrophication. So far, focus has been directed at reducing external loading of P. However, one experimental
study and cross-system analyses of data from many lakes in north temperate lakes indicate that nitrogen may play a more significant
role for abundance and species richness of submerged plants than usually anticipated when total phos-phorus is moderate high.
According to the alter-native states hypothesis we should expect ecological resistance to nutrient loading reduction and P
hysteresis. We present results suggesting that the two alternative states are less stable than originally anticipated. How
global warming af-fects the water clarity of shallow lakes is debat-able. We suggest that water clarity often will decrease
due to either enhanced growth of phytoplankton or, if submerged macrophytes are stimulated, by reduced capacity of these plants
to maintain clear-water conditions. The latter is supported by a cross-system comparison of lakes in Florida and Denmark.
The proportion of small fish might increase and we might see higher aggregation of fish within the vegetation (leading to
loss of Zooplankton refuges), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. Moreover,
lakes may have prolonged growth seasons with a higher risk of long-lasting algal blooms and at places dense floating plant
communities. The effects of global warming need to be taken into consideration by lake managers when setting future targets
for critical loading, as these may well have to be adjusted in the future. Finally, we highlight some of the future challenges
we see in lake restoration research.
12/2006: pages 239-252;
ABSTRACT: Summary1. The variable ecological response of lakes to reduced nutrient loading (oligotrophication) at sites in Europe and North America was discussed at a workshop held in Silkeborg (Denmark) in January 2003. Studies of lake oligotrophication were presented based on both long-term monitoring and data generated by palaeolimnological methods.2. This introduction to the special issue provides short summaries of a series of the papers presented and their limnological context. Results show that the majority of lakes had approached a new equilibrium of phosphorus (P) and nitrogen (N) concentrations 10–15 years (P) and 0–5 years (N) after a major reduction in loading, irrespective of hydraulic retention time. Phytoplankton biomass decreased and a shift towards meso-oligotrophic species dominance occurred. The fish responded surprisingly fast to the loading reduction in most lakes. As a result, the percentage of piscivores increased and total fish biomass declined markedly, which may explain an increase in the body size of cladocerans and an increase in the zooplankton to phytoplankton biomass ratio seen in many of the lakes.3. Monitoring has in general been initiated after the effects of eutrophication became apparent. In this context palaeolimnological techniques become very useful because they allow limnologists to extend time scales of coverage and to define restoration targets and baseline conditions. Moreover, lake sediments pre-dating anthropogenic disturbance can be used to examine ecological response to, for instance, climate variability, allowing problems associated with multiple stressors to be addressed.4. It is concluded that there is a great need for a synthetic, holistic approach to studying lake oligotrophication, combining multiple techniques of palaeolimnological sediment analysis with detailed but temporally limited long-term monitoring of chemical and biological variables. This is important, not least to assess future responses to nutrient loading reductions, as global warming will interact with a range of external stressors and ultimately affect lake management strategies to deal with the resultant feedbacks.
Freshwater Biology 09/2005; 50(10):1589 - 1593. · 3.29 Impact Factor