Daphnia Abundance Research Articles

Dynamic adaptation of phytoplankton vertical migration to changing grazing and nutrient conditions

Despite the ubiquitous occurrence of vertical migration of phytoplankton its quantitative significance remains poorly known. We eliminated a dense Daphnia population in a pond by introducing whitefish fingerlings, and assessed the effects on the vertical migration of dominating motile phytoplankton. At the highest abundance of Daphnia, cryptophytes reduced grazing losses by staying in the hypolimnion day and night, but Mallomonas species armoured by silica bristles remained in the epilimnion. After the fish introduction, phytoplankton was released from Daphnia grazing pressure, allowing cryptophytes to occur in the epilimnion also at noon. At the same time, increased phytoplankton biomass exacerbated the nutrient depletion. Cryptophytes compensated for that by migrating into the anoxic hypolimnion, whereupon their growth rates increased. The collapse of Daphnia was also associated with a temporary increase in nutrient regeneration by enzyme activities and decreases in total nutrient concentration and bacterial biomass in the whole water column. Our results show that cryptophytes can dynamically modify their vertical migration to balance between the exploitation of various nutrient resources and the risk of becoming eaten. Hypolimnetic nutrient resources can be quantitatively more important for phytoplankton than previously assumed.

Freshwater salinization reduces vertical movement rate and abundance of Daphnia: Interactions with predatory stress

Contaminants in human-dominated landscapes are changing ecological interactions. The global increase in freshwater salinity is likely to change predator-prey interactions due to the potential interactive effects between predatory stress and salt stress. We conducted two experiments to assess the interactions between the non-consumptive effects of predation and elevated salinity on the abundance and vertical movement rate of a common lake zooplankton species (Daphnia mendotae). Our results revealed an antagonism rather than a synergism between predatory stress and salinity on zooplankton abundance. Elevated salinity and predator cues triggered a >50% reduction in abundance at salt concentrations of 230 and 860 mg Cl−/L, two thresholds designed to protect freshwater organisms from chronic and acute effects due to salt pollution. We found a masking effect between salinity and predation on vertical movement rate of zooplankton. Elevated salinity reduced zooplankton vertical movement rate by 22–47%. A longer exposure history only magnified the reduction in vertical movement rate when compared to naïve individuals (no prior salinity exposure). Downward movement rate under the influence of predatory stress in elevated salinity was similar to the control, which may enhance the energetic costs of predator avoidance in salinized ecosystems. Our results suggest antagonistic and masking effects between elevated salinity and predatory stress will have consequences for fish-zooplankton interactions in salinized lakes. Elevated salinity could impose additional energetic constraints on zooplankton predator avoidance behaviors and vertical migration, which may reduce zooplankton population size and community interactions supporting the functioning of lake ecosystems.

The effects of Bythotrephes longimanus invasion on diets and growth of age‐0 yellow perch in Oneida Lake, New York

AbstractBythotrephes longimanus is a predatory zooplankton native to Eurasia known for its large caudal spine. This species can decrease Daphnia abundance and may decrease growth rates of native planktivorous fish including yellow perch (Perca flavescens). Bythotrephes was first documented in Oneida Lake, USA in 2019 and reached high densities in 2020 and 2021. This recent invasion created an opportunity to investigate at what length yellow perch feed on Bythotrephes, if age‐0 yellow perch select for Bythotrephes, and if yellow perch first‐year growth decreased. We found Bythotrephes in the stomachs of 23% (2020) and 35% (2021) of age‐1 and older yellow perch caught from June through September and 54% of the age‐0 yellow perch caught from June through October (2021). Age‐0 yellow perch started feeding on Bythotrephes at 27 mm and selected for Bythotrephes from June 24 through the autumn of 2021. Bythotrephes accounted for 54% of age‐0 diets by dry weight over the summer and fall. We used the extensive water quality and biological monitoring data from Oneida Lake to develop a multivariate linear regression model for age‐0 yellow perch October length from 1993 to 2021. Significant variables included were age‐0 yellow perch abundance (negative effect) and temperature, Daphnia biomass, and Bythotrephes presence, all positive effects. Contrary to our expectations, Bythotrephes were selected by age‐0 yellow perch from end of June through October, and contributed a substantial proportion of ingested mass by age‐0 perch, without negatively affecting age‐0 yellow perch growth.

A Non-Stressful Temperature Rise and Greater Food Availability Could Increase Tolerance to Calcium Limitation of Daphnia cf. pulex (Sensu Hebert, 1995) Populations in Cold Soft-Water Lakes.

Simple SummaryClimate warming is evident at many cold sites, including high-altitude and high-latitude lakes. The cladoceran Daphnia pulex, with high calcium (Ca) demands for its heavily calcified exoskeleton, is an ecologically important zooplankton species inhabiting these lakes, many of which are Ca-limited. Other stressors, such as warming and food limitation, may interact with Ca limitation and affect Daphnia populations adapted to these cold environments. In a study of factorial design, a clone of North American Daphnia cf. pulex was exposed to a low-Ca gradient, to temperature rises within its preferred range and expected in these lakes, and to conditions of low and high food quantity. Results suggest that climate warming and higher food availability will make D. pulex populations more tolerant to Ca limitation. Changes in the abundance of Daphnia may be relevant to ecological processes and the functioning of aquatic ecosystems.Calcium (Ca) is an important driver of community structure in freshwaters. We examined the combined effects of increased temperatures and variations in food quantity on the tolerance to low Ca of Daphnia pulex. The aim was to predict the impact of climate warming on this keystone zooplanktonic species in cold-climate lakes. We conducted a factorial life-history experiment in a clone of North American Daphnia cf. pulex to analyse the interaction effects of a temperature increase (17.5 °C–21 °C) within their physiological preferred range and expected by climate warming over the next few decades and a narrow Ca gradient (0.25–1.74 mg Ca L−1) under stressful vs. abundant food conditions. We found a striking positive synergistic effect of Ca and temperature on D. pulex reproduction at high food conditions. Although the increase in temperature to 21 °C greatly reduced survival, high energy allocation to reproduction at high food levels allowed the population to succeed in poor Ca (<0.25 mg Ca L−1). Results suggest that climate warming and higher food availability will make the populations of many cold and Ca-limited lakes more tolerant to low Ca levels with higher growth population rates, thereby altering zooplanktonic community structures and inducing potential cascading effects on the food web.

Short Communication: Utilization of Vegetables Waste as Carbon Source in Cultivation Medium for Daphnia sp.

Daphnia sp. is one of the most important natural feeds in freshwater fish farming sector. Daphnia contain considerably high protein, easy to be cultivated, short harvest time, the size is appropriate to the fish's mouth size, its movement can stimulate fish larvae to eat it, does not pollute the aquatic environment or larval rearing media and can be enriched with certain ingredients. Carbon is a source of energy for the growth of Daphnia directly or through microorganisms that become feed for Daphnia. This study aimed to analyze the effect of providing vegetable waste as a carbon source on the abundance of Daphnia and determine the best type of vegetable waste for the cultivation of Daphnia. The research method was an experimental method using 3 (three) different vegetables, namely spinach, mustard greens, and cabbage. The study was conducted using a completely randomized design with 4 (four) treatments in the form of different vegetables and control and the test sample was repeated 3 (three) times. The growth of Daphnia was observed for 16 days of cultivation time. The results showed that the provision of vegetables affected the population growth of Daphnia. Spinach (P3) was the treatment with the highest growth value, which average growth was sebanyak 96,67 individual (ind) of Daphnia with a specific growth rate of 0.055%. The temperature in Daphnia culture media generally ranged from 25-27℃, while the pH in Daphnia culture media showed a value of 7.4-9.5. The results of PCA analysis showed that water temperature was a variable that was strongly correlated with the growth of Daphnia.

Implementation of a microalgae and live prey production system (daphnia) to improve African catfish Clarias gariepinus fry growth

Fish farming in Senegal encounters many difficulties, especially during the first life stages, which are more demanding for food and more sensitive to variations in the physicochemical parameters. Most of the species of aquaculture interest have offspring whose size at birth is very small, thus requiring a particular rich food adapted to the size of the mouth (e.g. live preys which are very rich in nutritive reserves). The objective of this study was to set up a production system of live prey, daphnia for the feeding of fish fry. It consisted of culturing microalgae that were used to fed daphnia reared to fed the fry of the African catfish, Clarias gariepinus. The temperature and dissolved oxygen content in the culture and rearing media were monitored throughout the experiment. The results on species identification showed that the cultured microalgae and reared daphnia species are Microcystis sp and Daphnia magna, respectively. Analyses of algal biomass and daphnia abundance show an increase in these parameters with time. Temperature variations do not affect algal biomass, daphnia abundance and fry growth. On the other hand, a decrease in dissolved oxygen content below a certain threshold (6mg/l) leads to a decrease in algal biomass and daphnia abundance. Among the three feeding rates 1.5g, 3g, 4.5g (the equivalent of 0.5L, 1L, and 1.5L of microalgae, respectively) that were used to feed daphnia, the feeding rate of 3g of biomass corresponding to 1L is more adequate as it gave the best production. The comparison of the effects of the types of feed shows that live daphnia give better growths of C. gariepinus fry than the artificial feed, which may be due to their adequate size and nutrient richness. Thus, this study contributes to the establishment of live prey production systems to boost local aquaculture development and demonstrates the importance of daphnia for the feeding of early fish life stages.

A Trophic Cascade Following the Introduction of an Omnivorous Fish, Dorosoma cepedianum, in a Mid-continental Reservoir (Mozingo Studies IV)

Mozingo Lake, a reservoir in the mid-continental United States, was sampled 63 times between January 1999 and October 2015. Prior to 2005, phytoplankton biovolume was largely composed of Cryptomonas and cyanobacteria (mostly Aphanizomenon); these taxa correlated with the abundance of Daphnia and juvenile copepods, and were also influenced by competition between cyanobacteria and bacillariophytes. These relationships suggest the zooplankton community was primarily controlled by bottom-up processes during this time. In 2005, gizzard shad (Dorosoma cepedianum) first appeared in the lake, coinciding with the first spined and helmeted forms of Daphnia as well as their decline from 39% to 3% of zooplankton; the relative abundance of juvenile copepods increased concurrently. By 2007 phytoplankton biovolume had decreased by 88%. Although all phytoplankton phyla declined in absolute abundance, some were more heavily impacted. The relative abundance of Aphanizomenon declined from 50% to 6% of the phytoplankton biovolume; Cryptomonas remained common, but bacillariophytes became codominant. Despite the steep decline in phytoplankton biovolume, several important trophic interactions did not change: phytoplankton biovolume continued to control juvenile copepods, and competition continued between cyanobacteria and bacillariophytes. Although juvenile copepods continued to compete with Daphnia, the scarcity of the latter allowed a relative expansion of juvenile copepods and bacillariophytes. Because very few of the standard parameters correlated with phytoplankton, it seems likely that most of the changes were caused by the introduction of gizzard shad, which is an intense consumer of cyanobacteria and Daphnia. Gizzard shad likely initiated a trophic cascade both directly (through consumption) and indirectly (by shifting competition). Although other factors may have played a role, it seems clear that the introduction of gizzard shad can cause dramatic changes in both the zooplankton and phytoplankton communities by altering species relationships in top-down, bottom-up, as well as lateral control processes.

Climate warming restructures an aquatic food web over 28years.

Climate warming can restructure lake food webs if trophic levels differ in their thermal responses, but evidence for these changes and their underlying mechanisms remain scarce in nature. Here we document how warming lake temperatures by up to 2°C, rather than changes in trophic state or fishing effort, have restructured the pelagic food web of a large European lake (Lake Maggiore, Italy). Our approach exploited abundance and biomass data collected weekly to yearly across five trophic levels from 1981 to 2008. Temperature generally had stronger effects on taxa than changes in fish predation or trophic state mediated through primary productivity. Consequently, we found that, as the lake warmed, the food web shifted in numerical abundance towards predators occupying middle trophic positions. Of these taxa, the spiny water flea (Bythotrephes longimanus) most prospered. Bythotrephes strongly limited abundances of the keystone grazer Daphnia, strengthening top-down structuring of the food web. Warmer temperatures partly restructured the food web by advancing peak Bythotrephes densities by approximately 60days and extending periods of positive population growth by three times. Nonetheless, our results suggested that advances in the timing and size of peak Bythotrephes densities could not outpace changes in the timing and size of peak densities in their Daphnia prey. Our results provide rare evidence from nature as to how long-term warming can favour higher trophic levels, with the potential to strengthen top-down control of food webs.

Two Trophic Upsurges With Differing Responses During the Filling of a Mid-Continental Reservoir (Mozingo Studies III)

Mozingo Lake was sampled monthly for 4 y following its impoundment in order to further understand trophic upsurges and to refine the classic model of upsurges. Upsurge 1 occurred in Month 8, when lake area increased by 184% and inundated only floodplain with comparable coverages of forest, cropland, and grassland (30%–33% each). At this time nitrate increased dramatically and was related to both precipitation and lake area increase, likely due to ammonia-based fertilizers from the flooded cropland. Phytoplankton biovolume remained low, however, likely due to the abundance of Daphnia and other zooplankton grazers. After recovery, the lake expanded dramatically again in Month 21 (Upsurge 2); lake area increased by 36%, but this expansion flooded land with less floodplain (23%) and with different proportions of land use (45% grassland, 31% forest, 14% cropland). Upsurge 2 experienced initial increases in nitrate and zooplankton, but these were minor compared to Upsurge 1, and phytoplankton remained low. These initial responses were followed by substantial increases in phosphate, which related to lake area increase; cyanobacteria biovolume expanded, but zooplankton abundance declined. Therefore the two upsurges were driven by different factors: nitrate in Upsurge 1, but phosphate in Upsurge 2. These drivers, in turn, may reflect differences in the newly-flooded land, including differences in land use and in landscape position (i.e., slope). In addition to seasonal succession, the phytoplankton of Mozingo Lake exhibited longer-term primary succession that related first to nitrate, then to grazing, then to multiple factors, suggesting increases in food web complexity. In contrast to the classic model of upsurges, the biotic responses in Mozingo Lake were quite transitory, and the nature of the upsurges varied with the characteristics of the newly-inundated land. Therefore, it appears that trophic upsurge is more variable than the classic model suggests.

Lake restoration influences nutritional quality of algae and consequently Daphnia biomass

Food quality is one of the key factors influencing zooplankton population dynamics. Eutrophication drives phytoplankton communities toward the dominance of cyanobacteria, which means a decrease in the availability of sterols and long-chain polyunsaturated fatty acids (EPA and DHA). The effects of different restoration measures on the nutritional quality of the phytoplankton community and subsequent impacts on zooplankton biomass have rarely been considered. We analyzed the nutritional quality of phytoplankton in the eutrophic Lake Vesijärvi in southern Finland over a 37-year period, and studied the impacts of two restoration measures, biomanipulation and hypolimnetic aeration, on the abundance of high-quality phytoplankton. We found that biomanipulation had a positive impact on the abundance of taxa synthesizing sterols, EPA, and DHA and, concurrently, on the biomass of the keystone species Daphnia. In contrast, hypolimnetic aeration did not result in such a beneficial outcome, manifested as a decrease in the abundance of Daphnia and frequent phytoplankton blooms dominated by cyanobacteria suggesting reduction in the nutritional quality of food for Daphnia. Our analysis shows that the determination of the nutritional value of algae and the contribution of essential fatty acids and sterols is an effective method to evaluate the success of various restoration measures.

Top-down versus bottom-up control in planktonic systems: some case studies from the English Lake District

In this review, I use examples drawn from long-term studies in the English Lakes to highlight the limitations of top-down control in planktonic systems. These limitations are most obvious at the base of the food chain where there are large differences in the potential growth rates of the phytoplankton and zooplankton. Three of the examples are taken from a 40-year study of the impact of perch on Daphnia in the North Basin of Windermere, one on the rates of change measured in an experimental enclosure and one from an 18-year study of Daphnia dynamics in Esthwaite Water. The Windermere examples show that the commercial perch fishery established in 1941 had little effect on the abundance of Daphnia but the enrichment of the lake by war-time ploughing led to a short-lived increase in their reproductive rate. The other examples demonstrate that time plays an important part in all trophic interactions and shows that established patterns of phytoplankton succession are frequently disrupted by changes in the intensity of wind-induced mixing. The results are discussed in relation to recent reviews of the efficacy of biomanipulation and the challenges of analysing complex interactions in dynamic systems.

Modeling diel vertical migration with membrane computing

Diel vertical migration (DVM) is an important ecological phenomenon in which zooplankton migrate vertically to deal with trade-offs associated with greater food availability in shallow waters and lower predator risk in deep waters due to lower light availability. Because of these trade-offs, DVM dynamics are particularly sensitive to changes in light intensity at the water surface. Therefore, changes in the proportion of cloudy and sunny days have the potential to disrupt DVM dynamics. We propose a new membrane computing model that captures the effect of cloud cover on DVM in Daphnia, and we use it to explore the impacts of an increased proportion of cloudy days that are predicted to occur with climate change. Our 2-dimensional, spatially explicit model integrates multiple trophic levels from abiotic nutrients to Daphnia predators. We analyzed the effect that different proportions of cloudy and sunny days throughout the summer have on our model. The model simulations suggest that an increase in sunny days promotes a high phytoplankton concentration near the surface but does not necessarily promote an increased abundance of Daphnia. Our model also suggests that a higher proportion of cloudy days would increase Daphnia abundance due to a shift in the vertical distribution of Daphnia populations towards superficial waters. Our results highlight that climate changes in multiple regions will affect animal migrations leading to altered food web dynamics in freshwater ecosystems, and emphasize the potential of membrane computing as a modeling framework for spatially and temporally explicit ecological processes.

Episodic zooplanktivory by largemouth bass (Micropterus salmoides) on Daphnia: a 25-year natural history record from a small northern temperate lake

We observed episodic zooplanktivory by largemouth bass (Micropterus salmoides; LMB) ≥ 150 mm total length (TL) on the cladoceran Daphnia spp. over two decades in a small oligotrophic lake. While zooplanktivory is beneficial for young-of-year and juvenile LMB, dietary utilization of Daphnia by adults is unusual because zooplankton are not energetically favorable and the LMB buccal cavity is poorly adapted for filter feeding. We classified the threshold of ≥ 38 Daphnia per feeding event as an episode of zooplanktivory based on data from a zooplankton specialist, the bluegill sunfish (Lepomis macrochirus). In this study, 1718 LMB were caught, marked, sampled, and released. Gut contents at the time of capture were collected via gastric lavage. We captured the same 612 LMB 3 or more times. Of these, approximately 26% had at least one episode of zooplanktivory and approximately 6% had multiple episodes. There was no relationship between the population sizes of Daphnia and LMB, but the yearly percentage of individual LMB with at least one episode of zooplanktivory significantly increased with LMB population density. Episodes of zooplanktivory were more likely to occur later in the season when Daphnia were more abundant in the lake. The probability of an individual LMB having a repeat episode of zooplanktivory was not significantly different from expected; however, the probability of a LMB not having a single episode was significantly greater than expected by random chance signifying that some LMB were better than others at eliminating Daphnia from their diets. The probability of episodic zooplanktivory significantly decreased with increasing fish TL and weight but significantly increased with increasing Daphnia abundance. LMB without episodes of zooplanktivory achieved significantly greater maximum weight and body condition factor than those with them, but there was no statistical difference in their maximum TL or longevity (duration between first and final capture of the same LMB). In contrast, LMB with episodes of zooplanktivory had significantly greater instantaneous growth rates than those without. These results suggest that LMB within the resource-limited lake partitioned prey resources when some LMB sporadically targeted under-utilized, energetically unfavorable prey.

Causes of Daphnia midsummer decline in two deep meromictic subalpine lakes

Abstract Daphnia are key organisms in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamic follows regular seasonal patterns, with spring peaks followed by summer population declines (midsummer declines, MSDs). Midsummer declines show high inter‐annual variation, which has been attributed to different causes. However, the mechanisms controlling the MSD remain poorly understood, especially in deep stratified lakes. We tried to disentangle the factors causing Daphnia MSDs in Lake Lugano and Lake Iseo (in Switzerland and Italy), two deep peri‐alpine lakes with similar trophic status and vertical mixing dynamics, characterised by phosphorus accumulation in the hypolimnion and variable mixing during late‐winter turnovers. Specifically, we assessed the effects of three different hypothetical pathways according to which: (1) winter air temperature controls MSDs by influencing mixing depth during turnovers and epilimnetic phosphorus replenishment; (2) vernal air temperature influences MSD by accelerating the timing of spring population peak; and (3) summer temperature influences MSDs by increasing fish predation. We assessed the relative strength of these pathways using structural equation modelling on long‐term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo). Between the hypothesised pathways, the one driven by winter air temperature (through P replenishment) influenced Daphnia abundance in spring in both lakes, but the effects propagated to summer Daphnia abundance only in Lake Lugano. Additionally, summer Daphnia abundance was influenced by the summer air temperature through a positive (although weak) effect. By comparison, vernal air temperature had no detectable effects on summer Daphnia abundance. The results revealed marked differences between the meromictic study lakes and the shallow hypertrophic water bodies that were the focus of previous research on Daphnia MSD, and also between the two study lakes. The influence of epilimnetic P replenishment on the summer Daphnia abundance in Lake Lugano, which was recovering from past eutrophication, may have reflected the greater susceptibility of deep, stratified lakes to P depletion after spring compared to shallow hypertrophic lakes or reservoirs. This effect might not have been detected in Lake Iseo because P was more consistently depleted during the study period (i.e. variance in the predictor was too low to detect an effect). This study highlighted the complexity of the effects of climate variability on Daphnia MSD in deep lakes, showing that the responses can differ even between two neighbouring lakes with similar vertical mixing dynamics and trophic status. At the same time, the results suggest that future increases in winter air temperature, caused by global warming, may cause critically low densities of Daphnia during spring and summer and compromise the ability of zooplankton to control phytoplankton biomass.

Diversity and distribution of Daphnia across space and time in Danube Delta lakes explained by food quality and abundance

Eutrophication of shallow lakes often triggers a series of cascading ecological effects. Among these are shifts in the zooplankton community structure due to phytoplankton changes, or shifts in the fish community reducing size-selective feeding of planktivorous fish. In such conditions, larger zooplankton (e.g. Daphnia) can have a selective advantage. Re-oligotrophication can reverse such food-web interactions. Europe’s largest wetland system (the Danube Delta) went through a period of eutrophication and is now slowly recovering again. However, changes in the Daphnia populations triggered by eutrophication in this system have remained unstudied. We used different sampling strategies to screen 24 lakes (which differ in their ecological state) for the presence of Daphnia as well as for biotic and abiotic parameters potentially explaining Daphnia abundance. Daphnia densities were surprisingly low. We found D. magna ephippia in the sediment, but no D. magna in the water column. Microsatellite analyses on pelagic Daphnia populations confirmed the presence of the D. longispina complex and provided evidence for significant hybridisation events. FluoroProbe data showed that Daphnia was mainly present in lakes with a higher phytoplankton production. Our study provides insights into the spatial and temporal distribution of Daphnia in a very dynamic wetland system.

Freshwater Bacterioplankton Metacommunity Structure Along Urbanization Gradients in Belgium.

Urbanization is transforming and fragmenting natural environments worldwide, driving changes in biological communities through alterations in local environmental conditions as well as by changing the capacity of species to reach specific habitats. While the majority of earlier studies have been performed on higher plants and animals, it is crucial to increase our insight on microbial responses to urbanization across different spatial scales. Here, using a metacommunity approach, we evaluated the effects of urbanization on bacterioplankton communities in 50 shallow ponds in Belgium (Flanders region), one of the most urbanized areas in Northwest Europe. We estimated the relative importance of local environmental factors (35 abiotic and biotic variables), regional spatial factors and urbanization (built-up area) quantified at two spatial scales (200 m × 200 m and 3 km × 3 km). We show that urbanization at local or regional scales did not lead to strong changes in community composition and taxon diversity of bacterioplankton. Urbanization at regional scale (3 km × 3 km) explained only 2% of community composition variation while at local scale (200 m × 200 m), no effect was detected. Local environmental factors explained 13% (OTUs with relative abundance ≥ 0.1%) to 24% (12 dominant OTUs -≥ 1%) of community variation. Six local environmental variables significantly explained variation in bacterioplankton community composition: pH, alkalinity, conductivity, total phosphorus, abundance of Daphnia and concentration of copper (Cu), of which pH was partly mediated by urbanization. Our results indicate that environmental rather than spatial factors accounted for the variation in bacterioplankton community structure, suggesting that species sorting is the main process explaining bacterioplankton community assembly. Apparently, urbanization does not have a direct and strong effect on bacterioplankton metacommunity structure, probably due to the capacity of these organisms to adapt toward and colonize habitats with different environmental conditions and due to their fast adaptation and metabolic versatility. Thus, bacterioplankton communities inhabiting shallow ponds may be less affected by environmental conditions resulting from urbanization as compared to the impacts previously described for other taxa.

Spatial distribution and filtering efficiency of Daphnia in a deep subtropical reservoir

Studies of Daphnia distribution and function could help people manage and protect water quality. We investigated how spatial distribution and filtering efficiency of Daphnia in the transition and lacustrine zones of the Nanwan Reservoir (China). Samplings were conducted seasonally for 2 years from six sites in the reservoir. Daphnia abundance and biomass were significantly higher in the lacustrine zone than in the transition zone. Similar composition and biomass of edible phytoplankton were found in the two zones, suggesting that food quantity could not explain high Daphnia distribution in the lacustrine zone. The variations of water velocity and food quality could help explaining Daphnia patchy distribution in the reservoir. On the one hand, rapid water velocity can cause the Daphnia decrement in the transition zone. On the other hand, the ratio of particulate organic carbon (POC) to chlorophyll-a (chl-a ) concentration was significantly higher in the transition zone, indicating more allochthonous material constituted the food source for Daphnia. The lower quality food likely suppressed Daphnia development in the transition zone. A linear regression between Daphnia abundance and Secchi depth (SD) may suggest a cause-effect relationship where increased filtering efficiency was responsible for increased water clarity to some extent.

Nutrients mediate the effects of temperature on methylmercury concentrations in freshwater zooplankton

Methylmercury (MeHg) bioaccumulation in freshwater aquatic systems is impacted by anthropogenic stressors, including climate change and nutrient enrichment. The goal of this study was to determine how warmer water temperatures and excess nutrients would alter zooplankton communities and phytoplankton concentrations, and whether those changes would in turn increase or decrease MeHg concentrations in freshwater zooplankton. To test this, we employed a 2 × 2 factorial experimental design with nutrient and temperature treatments. Mesocosms were filled with ambient water and plankton from Cottage Grove Reservoir, Oregon, U.S.A., a waterbody that has experienced decades of elevated MeHg concentrations and corresponding fish consumption advisories due to run-off from Black Butte Mine tailings, located within the watershed. Treatment combinations of warmer temperature (increased by 0.7 °C), nutrient addition (a single pulse of 10× ambient concentrations of nitrogen and phosphorous), control, and a combination of temperature and nutrients were applied to mesocosms. The individual treatments altered phytoplankton densities and community structure, but alone the effects on MeHg concentrations were muted. Importantly, we found a significant interactive effect of nutrients and temperature: the nutrient addition appeared to buffer against increased MeHg concentrations associated with elevated temperature. However, there was variability in this response, which seems to be related to the abundance of Daphnia and edible phytoplankton. Nutrients at low temperature were associated with marginal increases (1.1×) in zooplankton MeHg. Our findings suggest that global change drivers that influence community composition and ecosystem energetics of both zooplankton and phytoplankton can alter MeHg pathways through food webs.

A changing menu in a changing climate: Using experimental and long‐term data to predict invertebrate prey biomass and availability in lakes of arctic Alaska

Abstract Changes in seasonality associated with climate warming (e.g. temperature, growing season duration) are likely to alter invertebrate prey biomass and availability in aquatic ecosystems through direct and indirect influences on physiology and phenology, particularly in arctic lakes. However, despite warmer thermal regimes, photoperiod will remain unchanged such that potential shifts resulting from longer and warmer growing seasons could be limited by availability of sunlight, especially at lower trophic levels. Thus, a better understanding of warming effects on invertebrate prey throughout the growing season (e.g. early, peak, late) is important to understand arctic lake food‐web dynamics in a changing climate. Here, we use a multifaceted approach to evaluate prey availability to predators in lakes of arctic Alaska. In a laboratory mesocosm experiment, we measured different metrics of abundance for snails (Lymnaea elodes) and zooplankton (Daphnia middendorffiana) across three time periods (early, mid‐ and late growing season) and across three temperature and photoperiod treatments (control, increased temperature and increased temperature × photoperiod). Additionally, we used generalised additive models and generalised additive mixed‐effects models to relate long‐term empirical observations of zooplankton biomass (1983–2015) to observed temperature regimes in an arctic lake. We then simulated zooplankton biomass for the warmest temperature observations across the growing season to inform likely zooplankton biomass regimes under future change. We observed variable responses by snails and zooplankton across experiments and treatments. Early in the growing season, snail development was accelerated at multiple life stages (e.g. egg and juvenile). In mid‐season, in accordance with warmer temperatures, we observed significantly increased Daphnia abundances. However, in the late season, Daphnia appeared to be limited by photoperiod. Confirming our experimental results, our models of zooplankton biomass showed an increase of nearly 20% in warmer years. Further, these model estimates could be conservative as the consumptive demand of fishes may increase in warmer years as well. Overall, our results highlight the importance of interactive effects of temperature and seasonality. Based primarily on temperature, we can readily predict the response of fish metabolism in warmer temperatures. However, in this context, we generally require a better understanding of climate‐driven responses of important invertebrate prey resources. Our results suggest invertebrate prey biomass and availability are likely to respond positively with climate change based on temperature and seasonality, as well as proportionally to the metabolic requirements of fish predators. While further research is necessary to understand how other food‐web components will respond climate change, our findings suggest that the fish community at the top of arctic lake food webs will have adequate prey base in a warming climate.

Fish-mediated plankton responses to increased temperature in subtropical aquatic mesocosm ecosystems: Implications for lake management

Although it is well established that climate warming can reinforce eutrophication in shallow lakes by altering top-down and bottom-up processes in the food web and biogeochemical cycling, recent studies in temperate zones have also shown that adverse effects of rising temperature are diminished in fishless systems. Whereas the removal of zooplanktivorous fish may be useful in attempts to mitigate eutrophication in temperate shallow lakes, it is uncertain whether similar mitigation might be achieved in warmer climates. We compared the responses of zooplankton and phytoplankton communities to climate warming in the presence and absence of fish (Aristichthys nobilis) in a 4-month mesocosm experiment at subtropical temperatures. We hypothesized that 1) fish and phytoplankton would benefit from warming, while zooplankton would suffer in fish-present mesocosms and 2) warming would favor zooplankton growth but reduce phytoplankton biomass in fish-absent mesocosms. Our results showed significant interacting effects of warming and fish presence on both phytoplankton and zooplankton. In mesocosms with fish, biomasses of fish and phytoplankton increased in heated treatments, while biomasses of Daphnia and total zooplankton declined. Warming reduced the proportion of large Daphnia in total zooplankton biomass, and reduced the zooplankton to phytoplankton biomass ratio, but increased the ratio of chlorophyll a to total phosphorus, indicating a relaxation of zooplankton grazing pressure on phytoplankton. Meanwhile, warming resulted in a 3-fold increase in TP concentrations in the mesocosms with fish present. The results suggest that climate warming has the potential to boost eutrophication in shallow lakes via both top-down (loss of herbivores) and bottom-up (elevated nutrient) effects. However, in the mesocosms without fish, there was no decline in large Daphnia or in total zooplankton biomass, supporting the conclusion that fish predation is the major driver of low large Daphnia abundance in warm lakes. In the fishless mesocosms, phytoplankton biomass and nutrient levels were not affected by temperature. Our study suggests that removing fish to mitigate warming effects on eutrophication may be potentially beneficial in subtropical lakes, though the rapid recruitment of fish in such lakes may present a challenge to success in the long-term.