A comparison of shallow Danish and Canadian lakes and implications of climate change

1. Winter temperatures differ markedly on the Canadian prairies compared with Denmark. Between 1 January 1998 and 31 December 2002, average weekly and monthly temperatures did not drop below 0 °C in the vicinity of Silkeborg, Denmark. Over this same time, weekly average temperatures near Calgary, Alberta, Canada, often dropped below −10 °C for 3–5 weeks and the average monthly temperature was below 0 °C for 2–4 months. Accordingly, winter ice conditions in shallow lakes in Canada and Denmark differed considerably. 2. To assess the implications of winter climate for lake biotic structure and function we compared a number of variables that describe the chemistry and biology of shallow Canadian and Danish lakes that had been chosen to have similar morphometries. 3. The Danish lakes had a fourfold higher ratio of chlorophyll‐a: total phosphorus (TP). Zooplankton : phytoplankton carbon was related to TP and fish abundance in Danish lakes but not in Canadian lakes. There was no significant difference in the ratio log total zooplankton biomass : log TP and the Canadian lakes had a significantly higher proportion of cladocerans that were Daphnia. These differences correspond well with the fact that the Danish lakes have more abundant and diverse fish communities than the Canadian lakes. 4. Our results suggest that severe Canadian winters lead to anoxia under ice and more depauperate fish communities, and stronger zooplankton control on phytoplankton in shallow prairie lakes compared with shallow Danish lakes. If climate change leads to warmer winters and a shorter duration of ice cover, we predict that shallow Canadian prairie lakes will experience increased survivorship of planktivores and stronger control of zooplankton. This, in turn, might decrease zooplankton control on phytoplankton, leading to ‘greener’ lakes on the Canadian prairies.     

Indirect effects of fish community structure on submerged vegetation in shallow,eutrophic lakes: an alternative mechanism

The loss of submerged macrophytes during eutrophication of shallow lakes is a commonly observed phenomenon. The proximate reason for this decline is a reduction of available light due to increasing phytoplankton and/or epiphyton biomass. Here we argue that the ultimate cause for the transition from a macrophyte-dominated state to a phytoplankton-dominated state is a change in fish community structure. A catastrophic disturbance event (e.g. winterkill) acting selectively on piscivores, cascades down food chains, eventually reducing macrophyte growth through shading by epiphyton, an effect that is reinforced by increasing phytoplankton biomass. The transition back from the phytoplankton to the macrophyte state depends on an increase in piscivore standing stock and a reduction of planktivores. A conceptual model of these mechanisms is presented and supported by literature data and preliminary observations from a field experiment.     

Relationship of fish and macroinvertebrate assemblages to environmental factors: implications for community concordance

Community concordance describes similarity in distributions and abundances of organisms from different taxonomic groups across a region of interest, with highly concordant communities assumed to respond similarly to major environmental gradients, including anthropogenic stressors. While few studies have explicitly tested for concordance among stream-dwelling organisms, it frequently is assumed that both macroinvertebrates and fish respond in concert to environmental factors, an assumption that has implications for their management. We investigated concordance among fish and macroinvertebrates from tributaries of two catchments in southeastern Michigan having varied landscape characteristics. Classifications of fish and macroinvertebrate assemblages resulted in groups distinguished by differences in taxonomic characteristics, functional traits, and stressor tolerance of their respective dominant taxa. Biological groups were associated with principal landscape gradients of the study region, which ranged from forests and wetlands on coarse surficial geology to agricultural lands on finer, more impervious surficial geology. Measures of stream habitat indicated more stable stream flows and greater heterogeneity of conditions at site groups with catchments comprising forests and wetlands on the coarsest geology, but did not distinguish well among remaining site groups, suggesting that habitat degradation may not be the driving mechanism leading to differences in groups. Despite broadly similar interpretations of relationships of site groups with landscape characteristics for both fish and macroinvertebrates, examination of site representation within groups indicated weak community concordance. Our results suggest that explicit responses of fish and macroinvertebrates to landscape factors vary, due to potential differences in their susceptibility to controls or to differences in the scale at which landscape factors influence these organisms. Handling editor: Robert Bailey     

Differences in food webs and trophic states of Brazilian tropical humid and semi-arid shallow lakes: implications of climate change

Global warming may intensify eutrophication of shallow lakes by affecting nutrient loading, evaporation rates, and water level and thus produce major changes in food webs. We investigated to what degree food webs in tropical humid lakes differed from those in more eutrophic semi-arid lakes of the same latitude. Our results indicate that the catchment area-to-lake area ratio, nutrients, chlorophyll a, suspended solids, abundances of phytoplankton, zooplankton, and omnivorous fish as well as total fish catch per unit effort were all higher in the semi-arid lakes, whereas inlet water-to-evaporation ratio (proxy for water balance), water transparency, percentage macrophytes cover, and the piscivores:omnivores ratio were higher in the humid lakes. Our results suggest that reduced inlet water-to-evaporation ratio will increase lake eutrophication, which, in turn, as in temperate regions, will alter trophic structure of the freshwater community.

     

Unravelling fish community assembly in shallow lakes: Insights from functional and phylogenetic diversity

Reviews in Fish Biology and Fisheries - Global biodiversity loss has increased interest in identifying the patterns and mechanisms that shape community assembly. Growing empirical evidence...     

Substantial differences in littoral fish community structure and dynamics in subtropical and temperate shallow lakes

1. Fish play a key role in the functioning of temperate shallow lakes by affecting nutrient exchange among habitats as well as lake trophic structure and dynamics. These processes are, in turn, strongly influenced by the abundance of submerged macrophytes, because piscivorous fish are often abundant at high macrophyte density. Whether this applies to warmer climates as well is virtually unknown. 2. To compare fish community structure and dynamics in plant beds between subtropical and temperate shallow lakes we conducted experiments with artificial submerged and free‐floating plant beds in a set of 10 shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N), paired along a gradient of limnological characteristics. 3. The differences between regions were more pronounced than differences attributable to trophic state. The subtropical littoral fish communities were characterised by higher species richness, higher densities, higher biomass, higher trophic diversity (with predominance of omnivores and lack of true piscivores) and smaller body size than in the comparable temperate lakes. On average, fish densities were 93 ind. m⁻² (±10 SE) in the subtropical and 10 ind. m⁻² (±2 SE) in the temperate lakes. We found a twofold higher total fish biomass per unit of total phosphorus in the subtropical than in the temperate lakes, and as fish size is smaller in the former, the implication is that more energy reaches the littoral zone fish community of the warmer lakes. 4. Plant architecture affected the spatial distribution of fish within each climate zone. Thus, in the temperate zone fish exhibited higher densities among the artificial free‐floating plants while subtropical fish were denser in the artificial submerged plant beds. These patterns appeared in most lakes, regardless of water turbidity or trophic state. 5. The subtropical littoral fish communities resembled the fish communities typically occurring in temperate eutrophic and hypertrophic lakes. Our results add to the growing evidence that climate warming may lead to more complex and omnivory‐dominated food webs and higher density and dominance of smaller‐sized fish. This type of community structure may lead to a weakening of the trophic cascading effects commonly observed in temperate shallow lakes and a higher risk of eutrophication.     

Cladocera in shallow lakes from the Ecuadorian Andes show little response to recent climate change

Floating-leaved rhizophytes and pleustophytes are the first barrier to Sun’s rays and significantly affect the light regime of the water column. To evaluate these effects on light attenuation, the reflectance and transmittance spectra variability were examined according to the leaf traits within three plant groups: (1) seed plants with green abaxial surfaces; (2) seed plants with red abaxial surfaces; and (3) ferns with trichomes. Specific leaf area (SLA), chlorophyll a and b, and UV-B and UV-A-absorbing substances differed between these three groups. The ‘spectral signatures’ of floating-leaved seed plants are similar to those of terrestrial seed plants, with a peak in the green region and a pronounced ‘red edge’. Ferns transmitted more light along the whole spectrum compared to other species. Most reflectance and transmittance spectra variability of the first group was explained by SLA. In the second group, 36% of the reflectance spectra variability was explained by tissue density and carotenoids, and 48% of the transmittance spectra variability by carotenoids, anthocyanins and SLA. In ferns, the reflectance spectra variability was mainly explained by chlorophylls, and partly by trichome length and mesophyll thickness, with the transmittance spectra variability significantly affected by chlorophyll b.     

Predicting the effect of climate change on temperate shallow lakes with the ecosystem model PCLake

Global average surface temperatures are expected to rise by about 1.4–5.8°C from the present until the year 2100. This temperature increase will affect all ecosystems on earth. For shallow lakes—which can be either in a clear water or a turbid state—this climate change will expectedly negatively affect water transparency though the prediction is far from conclusive and experimental investigations elucidating the potential climatic effects on shallow lakes are still rare. The aim of this study was to further shape and sharpens hypotheses on the impact of climate change on shallow lakes by applying an existing and well-calibrated ecosystem model, PCLake. We focused on asymptotic model behaviour for a range of temperature and loading scenarios in a factorial design. We conclude that climate change will likely lead to decreased critical nutrient loadings. Combined with an expected increase in the external nutrient loading, this will increase the probability of a shift from a clear to a turbid state. As the model predicts a higher summer chlorophyll-a concentration, a stronger dominance of cyanobacteria during summer and a reduced zooplankton abundance due to climate change, the turbid state itself is likely to become even more severe.     

UV damage in shallow lakes: the implications of water mixing

The plankton of shallow lakes is expected to experience increasingexposure to ultraviolet radiation (UVR) within the next decades.In addition to stratospheric ozone depletion, decreases in dissolvedorganic carbon (DOC), caused by climate warming or acidification,will result in greater UVR penetration in many lakes. Circulationpatterns induced by moderate winds force the plankton of shallowlakes to be cycled through the whole water column. This translatesinto higher doses and fluctuating dosage rates. Here, we reportthe results of a series of ‘in situ’ UV exposureexperiments in which two zooplankton crustaceans (one capableof photorecovery and the other not) were incubated either atfixed depths or under simulated mixing. Our main goal was toassess the extent to which the results from static incubationscould be extrapolated to vertically moving zooplankton. Staticincubations accurately predicted the survival of moving individualsfor the species without photorecovery, but failed to do so forthe species capable of photorecovery. The fluctuations in dosagerate for high doses tended to increase the survival of the specieswith photorecovery. We contend that neglecting the effects ofvertical mixing may result in a biased perception of UVR effectsin shallow lakes.     

Effects of water-level regulation on the nearshore fish community in boreal lakes

The fish community in the littoral areas of eight regulated lakes and five reference lakes in Finland was sampled by electrofishing. No significant effect of winter drawdown on species richness was recorded across lakes. Total fish density for stony bottoms of the regulated and reference lakes averaged 19.3 and 32.7 individuals per 100 m², respectively, but this difference was not statistically significant. The combined proportion of littoral fish species, including minnow (Phoxinus phoxinus), bullhead (Cottus gobio), alpine bullhead (Cottus poecilopus), nine-spined stickleback (Pungitius pungitius), and stone loach (Barbatula barbatula), supplemented with zoobenthos feeders ruffe (Gymnocephalus cernuus) and young burbot (Lota lota), was much lower in the regulated lakes than in the reference lakes. Besides winter drawdown, other variables, such as nutrient level and lake size, affected the fish community. Guest editors: K. M. Wantzen, K.-O. Rothhaupt, M. M?rtl, M. Cantonati, L. G.-Tóth & P. Fischer Ecological Effects of Water-Level Fluctuations in Lakes     

Regime shifts in shallow lakes: the importance of seasonal fish migration

Shallow eutrophic lakes commonly exist in two alternative stable states: a clear-water state and a turbid water state. A number of mechanisms, including both abiotic and biotic processes, buffer the respective states against changes, whereas other mechanisms likely drive transitions between states. Our earlier research shows that a large proportion of zooplanktivorous fish populations in shallow lakes undertake seasonal migrations where they leave the lake during winter and migrate back to the lake in spring. Based on our past research, we propose a number of scenarios of how feedback processes between the individual and ecosystem levels may affect stability of alternative stable states in shallow lakes when mediated by fish migration. Migration effects on shallow lakes result from processes at different scales, from the individual to the ecosystem. Our earlier research has shown that ecosystem properties, including piscivore abundance and zooplankton productivity, affect the individual state of zooplanktivorous fish, such as growth rate or condition. Individual state, in turn, affects the relative proportion and timing of migrating zooplanktivorous fish. This change, in turn, may stabilize states or cause runaway processes that eventually lead to state shifts. Consequently, such knowledge of processes coupled to seasonal migration of planktivorous fish should increase our understanding of shallow lake dynamics.     

Role of food partitioning in structuring the zooplankton community in mountain lakes

Trophic-niche differentiation is often cited as a main factor in structuring zooplankton assemblages, although field evidence for this is rarely presented. The study was based on a survey of 29 Pyrenean lakes with altitudes ranging between 1,875 and 2,990 m carried out during July and August 2000. Because of the oligotrophic nature of these lakes, we aimed to confirm that food partitioning is a major factor in shaping zooplankton assemblages. We analysed the amino acid composition of six cladocera and seven copepod species. A discriminant analysis showed that each species could be distinguished according to its amino acid composition. A negative relationship between amino acid differentiation and co-occurrence among the cladocera and cyclopoid copepod was observed. In contrast, calanoids did not show any relationship and were characterised by a high amino acid differentiation between species. As the differences in the amino acid composition among zooplankton species indicate distinct food sources, the relationship found indicates that trophic-niche differentiation plays a key role in determining the assemblage of these zooplankton communities. Therefore exploitative competition, either at present or in the past by driving co-evolutionary histories, has been a significant factor in structuring the cladocera and cyclopoid communities in these oligotrophic lakes.     

Vulnerability of mires under climate change: implications for nature conservation and climate change adaptation

Wetlands in general and mires in particular belong to the most important terrestrial carbon stocks globally. Mires (i.e. bogs, transition bogs and fens) are assumed to be especially vulnerable to climate change because they depend on specific, namely cool and humid, climatic conditions. In this paper, we use distribution data of the nine mire types to be found in Austria and habitat distribution models for four IPCC scenarios to evaluate climate change induced risks for mire ecosystems within the 21st century. We found that climatic factors substantially contribute to explain the current distribution of all nine Austrian mire ecosystem types. Summer temperature proved to be the most important predictor for the majority of mire ecosystems. Precipitation—mostly spring and summer precipitation sums—was influential for some mire ecosystem types which depend partly or entirely on ground water supply (e.g. fens). We found severe climate change induced risks for all mire ecosystems, with rain-fed bog ecosystems being most threatened. Differences between scenarios are moderate for the mid-21st century, but become more pronounced towards the end of the 21st century, with near total loss of climate space projected for some ecosystem types (bogs, quagmires) under severe climate change. Our results imply that even under minimum expected, i.e. inevitable climate change, climatic risks for mires in Austria will be considerable. Nevertheless, the pronounced differences in projected habitat loss between moderate and severe climate change scenarios indicate that limiting future warming will likely contribute to enhance long-term survival of mire ecosystems, and to reduce future greenhouse gas emissions from decomposing peat. Effectively stopping and reversing the deterioration of mire ecosystems caused by conventional threats can be regarded as a contribution to climate change mitigation. Because hydrologically intact mires are more resilient to climatic changes, this would also maintain the nature conservation value of mires, and help to reduce the severe climatic risks to which most Austrian mire ecosystems may be exposed in the 2nd half of the 21st century according to IPCC scenarios.     

Resuspension of algal cells by benthivorous fish boosts phytoplankton biomass and alters community structure in shallow lakes

1. Positive effects of fish on algal biomass have variously been attributed to cascading top‐down effects and to nutrient enrichment by fish excretion. 2. Here, we used a combination of field and laboratory approaches to test an additional hypothesis, namely that the physical resuspension of settled algal cells by fish enhances algal biomass and alters community composition. 3. A multi‐lake survey showed that phytoplankton biomass and the fraction of motile algae increased with the concentration of inorganic suspended solids. This correlation could not be explained by wind‐induced resuspension because of the small size of the lakes. 4. In an enclosure experiment, chlorophyll‐a concentration, phytoplankton abundance and inorganic suspended solids increased significantly in the presence of Cyprinus carpio (common carp), but only if the fish had access to the sediment. No such effects were seen when a net prevented carp reaching the sediment. 5. The effects of enhanced nutrients and reduced zooplankton grazing as a result of fish feeding could not (fully) explain these observations, suggesting that the resuspension by carp of settled algae from a surface film on the sediment was the major factor in the outcome of the experiment. 6. An increase in diatoms and green algae (organisms with a relatively large sedimentation velocity) only in enclosures where carp could reach the sediment supported this view. 7. Several lines of evidence indicate that fish‐induced resuspension of algal cells from the sediment is an important mechanism that affects phytoplankton biomass and community composition in shallow lakes.     

Scale effects in species distribution models: implications for conservation planning under climate change

Predictions of future species' ranges under climate change are needed for conservation planning, for which species distribution models (SDMs) are widely used. However, global climate model-based (GCM) output grids can bias the area identified as suitable when these are used as SDM predictor variables, because GCM outputs, typically at least 50x50 km, are biologically coarse. We tested the assumption that species ranges can be equally well portrayed in SDMs operating on base data of different grid sizes by comparing SDM performance statistics and area selected by four SDMs run at seven grid sizes, for nine species of contrasting range size. Area selected was disproportionately larger for SDMs run on larger grid sizes, indicating a cut-off point above which model results were less reliable. Up to 2.89 times more species range area was selected by SDMs operating on grids above 50x50 km, compared to SDMs operating at 1 km2. Spatial congruence between areas selected as range also diverged as grid size increased, particularly for species with ranges between 20000 and 90000 km2. These results indicate the need for caution when using such data to plan future protected areas, because an overly large predicted range could lead to inappropriate reserve location selection.     

Disentangling climate change effects on species interactions: effects of temperature, phenological shifts, and body size

Climate-mediated shifts in species’ phenologies are expected to alter species interactions, but predicting the consequences of this is difficult because phenological shifts may be driven by different climate factors that may or may not be correlated. Temperature could be an important factor determining effects of phenological shifts by altering species’ growth rates and thereby the relative size ratios of interacting species. We tested this hypothesis by independently manipulating temperature and the relative hatching phenologies of two competing amphibian species. Relative shifts in hatching time generally altered the strength of competition, but the presence and magnitude of this effect was temperature dependent and joint effects of temperature and hatching phenology were non-additive. Species that hatched relatively early or late performed significantly better or worse, respectively, but only at higher temperatures and not at lower temperatures. As a consequence, climate-mediated shifts in hatching phenology or temperature resulted in stronger or weaker effects than expected when both factors acted in concert. Furthermore, consequences of phenological shifts were asymmetric; arriving relatively early had disproportional stronger (or weaker) effects than arriving relatively late, and this varied with species identity. However, consistent with recent theory, these seemingly idiosyncratic effects of phenological shifts could be explained by species-specific differences in growth rates across temperatures and concordant shifts in relative body size of interacting species. Our results emphasize the need to account for environmental conditions when predicting the effects of phenological shifts, and suggest that shifts in size-structured interactions can mediate the impact of climate change on natural communities.     

Effects of warming and nutrients on sediment community respiration in shallow lakes: an outdoor mesocosm experiment

1. Climate warming is expected to change respiration in shallow lakes but to an extent that depends on nutrient state. 2. We measured sediment respiration (SR) over the season in the dark on intact sediment cores taken from a series of flow‐through, heated and unheated, nutrient‐enriched and unenriched mesocosms. The natural seasonal temperature cycle ranged from 2 to 20 °C in the unheated mesocosms. In the heated mesocosms, the temperature was raised 4–6 °C above ambient temperatures, depending on season, following the A2 climate change scenario downscaled to the local position of the mesocosms, but enlarged by 50%. We further measured ecosystem respiration (ER) in the mesocosms based on semi‐continuous oxygen measurements. 3. SR changed over the season and was approximately ten times higher in summer than in winter. SR showed no clear response to warming in the nutrient‐enriched treatment, while it increased with warming in the unenriched mesocosms which also had lower fish densities. 4. ER was not affected by artificial warming or nutrient enrichment, but it was ten times higher in summer than in winter. 5. SR contributed 24–32% to ER. The SR:ER ratio was generally stimulated by warming and was higher in winter than in summer, especially in the nutrient‐enriched mesocosms. 6. Our results indicate that climate warming may lead to higher SR, especially in clear, macrophyte‐dominated systems. Moreover, the contribution of SR to ER will increase with higher temperatures, but decrease as the winters get shorter.     

Continental-scale patterns of nutrient and fish effects on shallow lakes: synthesis of a pan-European mesocosm experiment

1. Results are analysed from 11 experiments in which effects of fish addition and nutrient loading on shallow lakes were studied in mesocosms. The experiments, five in 1998, six in 1999, were carried out in six lakes, distributed from Finland to southern Spain, according to a standard protocol. 2. Effects of the treatments on 29 standard chemical, phytoplankton and zooplankton variables are examined to assess the relative importance of bottom‐up (nutrient enrichment) and top‐down (fish predation) effects. For each year, the experiments in different locations are treated as replicates in a meta‐analysis. Results of individual experiments are then compared in terms of the patterns of significant influences of nutrient addition and fish predation with these overall results (the baseline), and between years in the same location. 3. The overall meta‐analysis gave consistent results across the 2 years, with nutrient loading influencing all of the chemical variables, and on average 31% of primary producer and 39% of zooplankton variables. In contrast, fish influenced none of the chemical variables, 11% of the primary producer and 44% of the zooplankton variables. Nutrient effects on the system were thus about three times greater than fish effects, although fish effects were not inconsiderable. 4. The relative importance of nutrients and fish in individual experiments often differed between years at the same location and effects deviated to varying degrees from the baseline. These deviations were treated as measures of consistency (predictability) of conclusions in repeat experiments. Consistency increased southwards and this is interpreted as a consequence of more variable annual weather northwards. 5. The influence of nutrient loading was greater southwards and this was probably manifested through naturally greater annual macrophyte abundance in warmer locations in consequence of the longer plant growing‐season. There was no trend in the relative importance of fish effects with latitude but this may partly be an artefact of the simple fish community used. These findings suggest that nutrient control should be a greater priority than biomanipulation in the restoration of eutrophicated shallow lakes in warm temperate regions. 6. Starting conditions affected the outcome of experiments. High initial concentrations of total phosphorus and planktonic chlorophyll a concentration (created by local conditions prior to the experiment) led to de‐emphasis of the importance of nutrient loading in the experiment.     

Effects of temperature, salinity, and fish size on growth and consumption of juvenile bluefish

Consumption and growth rates of juvenile bluefish Pomatomus saltatrix increased with increasing temperature and decreased with increasing fish size in short-term (7 days) experiments. Salinity had no effect on growth or consumption rate in a short-term experiment. In a long-term (90 days) mesocosm experiment, consumption and growth rates declined with increasing body size. Predictive equations developed from short-term experiments did not adequately predict observed consumption rates in the mesocosm experiment. However, growth in the mesocosm experiment was similar to field growth. Also, mesocosm consumption rates and consumption rates calculated using field growth and mesocosm growth efficiencies were similar to published independent field estimates of consumption rate. Our results indicate that experiments to determine the effects of temperature and the allometry of body size on growth and consumption rates should be conducted over long time periods simulating field conditions. Juvenile bluefish have rapid growth and their individual cumulative consumption is large. This result suggests that bluefish may have a large effect on their prey populations. This effect has yet to be quantified.