Pond Food Webs Research Articles

Ephemeral piscivory in a mesopredator sunfish: Implications for pond food webs.

Ephemeral piscivory in a mesopredator sunfish: Implications for pond food webs.

Effect of pellet-size on fish growth, feeding behaviour and natural food web in pond polyculture

In carp-tilapia pond polyculture, carps are stocked once and usually harvested each 12 to 24 months, while, tilapia is stocked and harvested each 4 to 6 months. Therefore, per carp cycle there are 3 to 6 tilapia cycles. With time, carps grow bigger while tilapia reach only 10 to 50% the mass of carps. Pellet size can offer a feeding management tool in the ponds with large carp and small tilapia. This experiment investigated whether pellet-size can steer the growth of large carps (∼ 500 g) and small tilapias (∼30 g) in polyculture ponds. Four pellet-size treatments were applied in this study, each with a different combination of small (2 mm) and large (6 mm) pellets (on weight basis): S100L0 (100% small, 0% large), S50L50 (50% small, 50% large), S25L75 (25% small, 75% large) and S0L100 (0% small, 100% large). Each treatment was applied to six replicate ponds. The experiment lasted for 8 weeks. The treatments did not alter carp production, but tilapia production was highest in S100L0 treatment ponds (P < 0.001). The fish always needed more time to consume large pellets than the small pellets. Small pellets were consumed much quicker when the portion fed was small, indicating strong competition. A similar trend was observed with large pellets but the difference in feed intake time across the treatments was smaller than with small pellets. This is because tilapia ate the majority of both small and large pellets while carps ate less pellets. Tilapia at treatment S0L100 had a lower protein and fat content and a higher ash content than at the other pellet-size treatments. Carp's body composition remain unaffected by pellet-size treatment except the ash content of rohu which was highest at S100L0 and lowest at S0L100. By volume, tilapia had a higher amount of food in the stomach in treatments S100 L0 and S50L50 than in the other treatments. In contrast, more food was found in the gut of rohu in treatments S0L100 and S25L75 than in the other treatments. Pellet-size treatments did not affect water quality and the pond food web (P > 0.05) except phytoplankton diversity which was positively influenced when feeding more large pellets. The plankton population increased with the progression of the experiment, suggesting the food web was not fully exploited. In conclusion, pellet-size proves to be a feeding management tool to steer the production of tilapia as well as of total fish biomass, but not of carp's biomass in carp-tilapia pond polyculture.

Food web collapse and regime shift following goldfish introduction in permanent ponds.

In a global context of invasive alien species (IAS), native predators are often eradicated by functionally different IAS, which may induce complex cascading consequences on ecosystem functioning because of the key role predators play in structuring communities and stabilizing food webs. In permanent ponds, the most abundant freshwater systems on Earth, global human-mediated introductions of alien omnivores such as the pet trade goldfish are driving broad-scale patterns of native predators' exclusion, but cascading consequences on food web structure and functioning are critically understudied. We compared food webs of naturally fishless ponds versus ponds where dominant native predators (newts) had been extirpated by invasive goldfish within the last decade. Integrating community-wide isotopic, taxonomic and functional traits approaches, our study reveals that pond food webs collapsed in both vertical and horizontal dimensions following goldfish introduction and the associated exclusion of native predators. Consumer taxonomic diversity was drastically reduced, essentially deprived of amphibians as well as predatory and mobile macroinvertebrates to the profit of burrowing, lower trophic level consumers (detritivores). Changes in community structure and function underlined a regime shift from a macrophyte-dominated system mainly characterized by benthic primary production (periphyton), to a macrophyte-depleted state of ponds hosting communities mainly associated with phytoplankton primary production and detritus accumulation, with higher tolerance to eutrophication and low dissolved oxygen concentration. Results underline major impacts of widely introduced omnivores such as the goldfish on the functioning of pond ecosystems with potentially dramatic consequences on the key ecosystem services they deliver, such as global biodiversity support or water quality improvement. They also shed light on the key role of submerged aquatic vegetation in supporting diverse communities and complex food webs in shallow lentic systems and call for urgent consideration of threats posed by IAS on ponds' ecosystems by managers and policymakers.

Regional and fine-scale local adaptation in salinity tolerance in Daphnia inhabiting contrasting clusters of inland saline waters.

Understanding the spatial scales at which organisms can adapt to strong natural and human-induced environmental gradients is important. Salinization is a key threat to biodiversity, ecosystem functioning and the provision of ecosystem services of freshwater systems. Clusters of naturally saline habitats represent ideal test cases to study the extent and scale of local adaptation to salinization. We studied local adaptation of the water flea Daphnia magna, a key component of pond food webs, to salinity in two contrasting landscapes-a dense cluster of sodic bomb crater ponds and a larger-scale cluster of soda pans. We show regional differentiation in salinity tolerance reflecting the higher salinity levels of soda pans versus bomb crater ponds. We found local adaptation to differences in salinity levels at the scale of tens of metres among bomb crater pond populations but not among geographically more distant soda pan populations. More saline bomb crater ponds showed an upward shift of the minimum salt tolerance observed across clones and a consequent gradual loss of less tolerant clones in a nested pattern. Our results show evolutionary adaptation to salinity gradients at different spatial scales, including fine-tuned local adaptation in neighbouring habitat patches in a natural landscape.

Nutrient footprint versus EPA + DHA security in land-locked regions—more of local pond farmed, imported marine fish or fish oil capsules?

EPA + DHA intake in land-locked central Europe (CE) is barely fulfilled. Imported marine fish/farmed salmonids are likely the backbone of an ailing EPA + DHA security. Supplementing with captured marine fish oil capsules (~0.5 g up to 1.6 g CO2-eq. mg EPA + DHA−1) could be comparable in GHG emissions with fish consumption itself (~1 g to as low as 0.6 g CO2-eq. mg EPA + DHA−1). But synergistic benefits of EPA + DHA intake by consuming fish protein need consideration too. Taking semi-intensive pond carp and intensively farmed salmon as models, we analyzed footprint, eco-services, and resource use efficiency perspectives of achieving EPA + DHA security in a CE region. Despite a lower production footprint, pond-farmed fish greatly lag in EPA + DHA supply (carp 101–181 mg 100 g−1 < salmon 750–1300 mg 100 g−1). It doubles-to-quadruples footprint ‘per mg’ of EPA + DHA: nitrogen (carp 18.3 > salmon 8.7 mg N), phosphorus (carp 6.8 > salmon 1.6 mg P), and climate change (carp 1.84 > salmon 0.8 g CO2-eq.). With enhancements in pond carp (>300 mg EPA + DHA 100 g−1), these differences may cease to exist. Harnessing EPA + DHA bioaccumulation pathways active in ponds, finishing feeding strategies, and polyculture, the EPA + DHA content in pond fish may be increased. Ecosystem services with EPA + DHA mining from pond food web or high EPA + DHA output-to-input ratio (pond carp 1–200 > RAS salmon 0.75) make ponds an eco-efficient system. As fish consumption in CE must improve, pond-farmed fish would be needed to complement (but not substitute) salmonid/marine fish/oil capsules consumption. Achieving EPA + DHA security with minimum pressure on the environment or global resources.

Limited population and community effects of hatching asynchrony in a pond‐breeding salamander

AbstractUnderstanding attributes of phenology beyond the mean date of a life history event, such as variability among individuals within a population, is critical to predict how climate‐induced phenological shifts may alter population dynamics. Identifying how phenological variability impacts organisms is especially needed to better understand how phenological shifts affect trophic dynamics (e.g., shifts in variability of top predators affecting primary production). To better understand the effects of phenological variability on both populations and communities, we examined how variation in egg hatching synchrony of predatory marbled salamanders (Ambystoma opacum) impacted intraspecific interactions at the larval stage, ultimately affecting demographic traits and survival through metamorphosis. We also examined how hatching synchrony affected overall trophic dynamics (e.g., primary consumers and producers) in pond food webs. We experimentally manipulated the degree of hatching synchrony of embryonicA. opacumand subsequently reared larvae in outdoor mesocosms. We monitored demographic traits such as larval growth, size at and time to metamorphosis, and survival. To assess trophic dynamics, we monitored zooplankton abundance and phytoplankton biomass during the experiment. Larvae exhibited greater variability in body size in medium and low hatching synchrony treatments compared to high synchrony treatments. Larval body size variation diminished over time to ultimately result in no differences in most life history traits at metamorphosis or survival among hatching synchrony treatments. We also found no differences among treatments in zooplankton abundance or phytoplankton biomass, likely because of minimal variation inA. opacumsurvival among treatments that would induce top‐down changes. Overall, we found that phenological variation may be context dependent in its influence on demography and overall community structure. Because of concerns for how phenological shifts will affect species interactions, greater scrutiny into conditions that would promote changes in population and community dynamics is needed.

Direct effects influence larval salamander size and density more than indirect effects.

Direct and indirect effects both influence population and community dynamics. The relative strengths of these pathways are often compared using experimental approaches, but their evaluation in situ has been less frequent. We examined how individual and aggregate impacts of direct and indirect effects of species densities, proxies for competition and predation pressure, and habitat variables influenced patterns of larval density and body size of ringed (Ambystoma annulatum) and spotted salamanders (A. maculatum). We surveyed > 150 ponds in Missouri, USA, from 2012 to 2014 to measure the density and body size of each focal species, the density of co-occurring pond food web members, and select habitat features. We used structural equation modeling to quantify the relative importance of direct and indirect pathways on both body size and larval density. Overall, both responses were explained through a combination of direct and indirect effects. However, the magnitudes of direct effects were often greater than indirect effects. Some of the direct and indirect relationships with larval salamander size and density were also consistent with results from experimental studies. Finally, total direct and indirect effects were often weaker due to habitat and density variables negating each other's impacts. Overall, our study shows that direct effects were equivalent to, or more important than, indirect effects. We also demonstrate that the effects stemming from individual relationships can sum to produce net patterns that are negligible in magnitude. Further work on direct and indirect effects with observational data are needed to examine their magnitudes in natural communities.

Macrogeographic variation of a pond predator's top‐down effects in a common garden environment

AbstractGeographic variation in species behavior and life history has been well documented in biology. Species with wide geographic distributions (i.e., across a continent) but small home ranges (i.e., &lt;1 km2) likely experience wide variability in abiotic environments across the entirety of their range, possibly exhibiting strong local adaptation. Understanding variation across a large geographic scale is especially important when considering species that have strong ecological importance, such as keystone species. Yet, few studies have compared the potential cascading ecological effects of a predator with a keystone role in at least part of its range. To understand how keystone ability in pond food webs can vary across a large geographic range, we conducted an artificial pond experiment with a known keystone predator in at least part of its range, the marbled salamander (Ambystoma opacum). To do so, we collected size‐matched salamander larvae from three geographically distant populations (&gt;650 km apart) in Ohio, Mississippi, and North Carolina and placed them in mesocosms with a suite of spring breeding amphibian prey species. We observed differential survival of some prey species leading to differences in spring‐breeding amphibian diversity among the three predator populations, indicating that keystone predation may vary at a geographic scale. Prey diversity was lowest with predators from northern (Ohio) populations of salamanders. Further understanding of large‐scale variability in ecologically important predators and the potential effects of translocating wide‐ranging ambystomatid species is needed to direct future conservation efforts and preserve biodiversity.

The role of temporary ponds as foraging sites for glossy ibis (Plegadis falcinellus) during the non-breeding season in Numidia, Northeast Algeria

Aquatic macroinvertebrates are a major link in food webs in temporary ponds and useful indicators of wetlands’ecological integrity. We investigated the available trophic resources and the physicochemical quality of 14 wetlands frequented by glossy ibis (Plegadis falcinellus), a resident species with an increasing population in Numidia, northeast Algeria. Sampling of temporary ponds was carried out in winter and early spring, prior to the Glossy Ibis’ breeding period. Analyses indicated that the studied waterbodies contained a rich and diversified aquatic community, dominated mainly by insects and molluscs and characterized by a spatio-temporal change in composition and abundance, reflecting a spatial heterogeneity and a temporal niche partitioning of various members of the aquatic community. Furthermore, the density of Glossy Ibis in foraging areas changed seasonally and declined significantly with the distance to the nearest roost. In the context of climate change and human encroachment, the close dependence of Glossy Ibis and other waders on a matrix of secure roosts and temporary ponds during the nonbreeding season puts them in jeopardy unless these imperilled but generally unprotected habitats are actively managed.

Impacts of phenological variability in a predatory larval salamander on pond food webs

AbstractPhenological shifts are expected to affect species interactions, in part by influencing which size classes, life stages or species occur simultaneously in a community. Yet, changes in phenology beyond shifts to the first, mean or median date of an ontogenetic event are underexplored in their importance to community dynamics. Using outdoor mesocosms, we experimentally mimicked increasing variability in breeding phenology of a top predatory salamander in pond food webs (Ambystoma annulatum) to assess its impacts on its own demographic traits and survival. We also tested whether variability in predator breeding would cascade to impact survival and diversity of lower trophic levels (intermediate salamander predators, anuran tadpoles, zooplankton and periphyton). We found that only variability in body size at metamorphosis of A. annulatum was impacted by phenological manipulations, with size variability being greater at higher levels of phenological variability. Because size at metamorphosis is often correlated with adult fitness, covarying variability in body size and phenology may lead to altered population dynamics. We also found that the density and size of A. annulatum were better predictors of overall survival and diversity of amphibian prey compared to phenological variability. We speculate that overwintering mortality of A. annulatum due to pond freezing modulated the impacts of phenological variability, such that changes in demographic traits and cascading effects throughout the food web were mollified.

Ecological intensification in multi-trophic aquaculture ponds: an experimental approach

As aquaculture production is increasing considerably, it needs to become more environment-friendly. Based on a participatory process, an ecologically intensive pond system was designed to test three hypotheses: a combination of intensive and extensive areas provides more ecosystem services than an intensive or extensive area alone; coupling a planted lagoon with an intensive pond decreases the latter's environmental impacts and maintains or increases its fish productivity; and using formulated feed in polyculture increases growth of all fish species. To test these hypotheses, we designed a specific integrated multi-trophic aquaculture system composed of a polyculture of common carp (Cyprinus carpio), roach (Rutilus rutilus) and tench (Tinca tinca) and a lagoon planted with macrophytes to filter the water. This pond system was compared with “extensive” (unfed) and “semi-intensive” fishpond systems without a planted lagoon. We measured fish growth performances, water quality, chlorophyll concentrations and water and sediment nutrient contents. We also calculated the mass balance of nutrients. Concentrations of total nitrogen and phosphorus increased in sediments, indicating that nutrients were stored in the ponds, especially in planted lagoon; the planted lagoon decreased phytoplankton development, and limited blooms, but it slightly decreased fish growth performances compared to those in semi-intensive fishponds. The formulated feed supplied clearly increased fish growth performances and the survival rate, and seemed also to increase the use of available nutrients in ponds, which improved the production of overall fish biomass. In conclusion, the planted lagoon provides ecosystem services on nutrients cycling and habitat for natural biodiversity. Improving knowledge about nutrient cycles from formulated feed within the pond food web would be useful to increase the amounts of nutrients converted into fish biomass.

Trophic plasticity, environmental gradients and food‐web structure of tropical pond communities

Summary Freshwater pond communities exhibit strong patterns in species composition in response to environmental gradients such as ecosystem size, disturbance and productivity, serving as excellent systems for studies of food‐web structure. We surveyed 13 ponds that varied along environmental gradients of canopy cover, pond size and hydroperiod at the beginning and end of the rainy season in the semi‐arid thorn forests of the Gran Chaco ecoregion of Bolivia. We collected basal resources and consumers (tadpoles, macroinvertebrates and fishes) from these ponds and used stable isotope ratios of carbon (13C/12C) and nitrogen (15N/14N), to quantify the spatiotemporal dynamics of the food webs in these tropical ponds. There were no relationships between vertical structure of the food webs and the environmental gradients associated with the ponds. Consumers within these ponds exhibited high trophic variability, with multiple taxa occupying more than one trophic position across space or time. Consumers from larger ponds had a greater degree of trophic redundancy. More shaded ponds supported food webs that had lower trophic diversity. More permanent ponds had significantly greater trophic diversity as well as a greater range of basal resource diversity. The breeding ponds utilised by tadpoles and macroinvertebrates are patchily distributed across space and time. In these dynamic habitats, a feeding strategy of trophic generalism and plasticity enables consumers to exploit a broad range of resources and promote species coexistence. These results suggest that high diversity in tropical ponds does not necessarily translate into specialisation of trophic function.

Carbon sources and trophic structure in a macrophyte‐dominated polyculture pond assessed by stable‐isotope analysis

Summary Freshwater polyculture systems should efficiently utilise natural food resources and pond habitats with the consequent effects of reducing costs and increasing productivity. However, understanding of the primary carbon sources and food web within polyculture ponds, and especially macrophyte‐dominated ponds, remains limited. Stable‐isotope analysis was used to identify the main carbon sources and describe the trophic patterns of freshwater fauna in a polyculture pond around Lake Gucheng, China. An isotope mixing model (SIAR) was also used to estimate the relative contribution of four basal sources (suspended particulate organic matter [SPOM], sedimentary organic matter [SOM], macrophytes and epiphytes) to invertebrates and different fish trophic groups. Basal resources showed distinct δ13C values, ranging from −24.3 to −20.5‰, which allowed for a powerful discrimination of carbon sources. Consumers were also well separated in δ13C (−24.0 to −20.0‰ for invertebrates and −24.4 to −18.9‰ for fish), suggesting considerable differences in ultimate sources of carbon. Results from the SIAR model indicated that epiphytes were the most important carbon sources for crustaceans, herbivorous fish, omnivorous fish and piscivorous fish, while SPOM made the highest contribution to zooplankton, chironomids, molluscs, annelids, planktivorous fish and benthivorous fish. Mean δ15N values, combined with calculated food source contributions, were used to elucidate the food‐web structure of this polyculture pond, which suggested that carbon fixed by primary producers could be transported up the food chain to piscivorous fish through four main trophic pathways: one based mainly on SPOM, one on both SPOM and SOM, one on SPOM, SOM, macrophytes and epiphytes, and one based on both macrophytes and epiphytes. Our results emphasised the trophic importance of epiphytes on submerged macrophytes, a key component in the polyculture ponds, for the main culture species: Chinese mitten crab (Eriocheir sinensis: Varunidae), freshwater shrimp (Macrobrachium nipponense: Palaemonidae), and high‐value piscivorous fish. This study also revealed the high level of complexity of polyculture pond food webs, and suggests that a diversity of major trophic pathways may be a characteristic of macrophyte‐dominated aquatic ecosystems.

Reconciling the role of terrestrial leaves in pond food webs: a whole-ecosystem experiment.

Terrestrial carbon and nutrients can subsidize the detrital pool of freshwater ecosystems; yet, the importance of terrestrial subsidies to lake and pond food webs is uncertain and debated. Terrestrial detritus is expected to have the greatest impact on food webs when water bodies are small and shallow with low levels of incident light. Temporary forested ponds fit this description and are often assumed to have a leaf detritus-based food web, but this has not been quantified. In a whole-ecosystem experiment, we traced the flow of isotopically enriched leaf litter to primary producers and consumers in a small, forested pond. We found that terrestrial leaves provided nutrients to algae, offering an indirect pathway in which leaf litter can enter the food web. Terrestrial leaves were also consumed directly, and larval caddisfly (Limnephilus sp.) shredders likely mobilized leaf nutrients to other consumers, a process overlooked in many previous small-scale experiments that did not incorporate shredders. Unexpectedly, most consumers relied heavily upon algal food pathways despite low light and net heterotrophic conditions. Overall, our study highlights the interconnectedness of algal and leaf litter pathways in small pond food webs, and emphasizes that algal pathways are prevalent and important even in small, shaded ponds with high loads of terrestrial leaf litter.

Climate alters intraspecific variation in copepod effect traits through pond food webs.

Essential fatty acids (EFAs) are primarily generated by phytoplankton in aquatic ecosystems, and can limit the growth, development, and reproduction of higher consumers. Among the most critical of the EFAs are highly unsaturated fatty acids (HUFAs), which are only produced by certain groups of phytoplankton. Changing environmental conditions can alter phytoplankton community and fatty acid composition and affect the HUFA content of higher trophic levels. Almost no research has addressed intraspecific variation in HUFAs in zooplankton, nor intraspecific relationships of HUFAs with body size and fecundity. This is despite that intraspecific variation in HUFAs can exceed interspecific variation and that intraspecific trait variation in body size and fecundity is increasingly recognized to have an important role in food web ecology (effect traits). Our study addressed the relative influences of abiotic selection and food web effects associated with climate change on intraspecific differences and interrelationships between HUFA content, body size, and fecundity of freshwater copepods. We applied structural equation modeling and regression analyses to intraspecific variation in a dominant calanoid copepod, Leptodiatomus minutus, among a series of shallow north-temperate ponds. Climate-driven diurnal temperature fluctuations favored the coexistence of diversity of phytoplankton groups with different temperature optima and nutritive quality. This resulted in unexpected positive relationships between temperature, copepod DHA content and body size. Temperature correlated positively with diatom biovolume, and mediated relationships between copepod HUFA content and body size, and between copepod body size and fecundity. The presence of brook trout further accentuated these positive effects in warm ponds, likely through nutrient cycling and stimulation of phytoplankton resources. Climate change may have previously unrecognized positive effects on freshwater copepod DHA content, body size, and fecundity in the small, shallow bodies of inland waters that are commonly found in north-temperate landscapes.

Seasonal Changes in Zooplankton and Macro-fauna Populations of the East Calcutta Wetland Fish Ponds

The current study was to explore the seasonal variation in the zooplankton and aquatic macroinvertebrate population in India’s oldest and largest sewage fed fish ponds of East Kolkata Wetlands and whether they show different trends from the natural ponds. Zooplankton diversity was significantly higher in fish ponds whereas macroinvertebrate diversity was higher in natural ponds. Zooplanktons showed significant seasonal variation in the natural ponds but did not show any variation throughout the year in sewage fed fisheries. Among zooplankton species Brachionus spp., Keratella tropica which are pollution (eutrophy) indicator were found in greater abundance in EKW fisheries than natural ponds. Chironomid larvae which thrive and flourish in highly organically polluted or eutrophicated waters were dominant in EKW. The regular input of sewage provides a certain high level of nutrient to the fisheries which help to maintain greater and consistent population density and diversity of zooplanktons forming a strong base for the food webs in the EKW ponds.

Climate alters intraspecific variation in copepod effect traits through pond food webs

Climate alters intraspecific variation in copepod effect traits through pond food webs

Trophic interactions in an austral temperate ephemeral pond inferred using stable isotope analysis

Ephemeral ponds are vulnerable aquatic habitats which are difficult to protect given their dynamic nature and sensitivity to degradation during dry periods. Little information is available on these habitats in austral regions, with almost no information on food-web structure and complexity. The study aimed to assess trophic interactions among dominant organisms in an ephemeral pond food web, and investigate the importance of autochthonous and allochthonous carbon, using 13C and 15N isotopes. Results of the investigation suggest that the food web comprised four trophic levels, with the top predators being Notonectids (Notonecta sp.) and diving beetles (Cybister tripunctatus (Olivier)). Intermediary trophic levels comprised zooplankton (daphniids and copepodids), macroinvertebrates (e.g. micronectids and molluscs) and tadpoles. Generalist feeders dominated the higher trophic levels (>3) with specialists comprising the lower trophic levels (≤3). The consumers preferred autochthonous fine particulate organic matter, epiphyton and submerged macrophyte organic matter sources over allochthonous sources. Autochthonous organic matter was transferred to the food web via zooplankton and select macroinvertebrates including Micronecta sp. and Physa sp. The food-web structure within the pond appeared to reflect the secondary stage of trophic structural complexity in the evolution of ephemeral ponds over the course of their hydro-period.

Autumn leaf subsidies influence spring dynamics of freshwater plankton communities.

While ecologists primarily focus on the immediate impact of ecological subsidies, understanding the importance of ecological subsidies requires quantifying the long-term temporal dynamics of subsidies on recipient ecosystems. Deciduous leaf litter transferred from terrestrial to aquatic ecosystems exerts both immediate and lasting effects on stream food webs. Recently, deciduous leaf additions have also been shown to be important subsidies for planktonic food webs in ponds during autumn; however, the inter-seasonal effects of autumn leaf subsidies on planktonic food webs have not been studied. We hypothesized that autumn leaf drop will affect the spring dynamics of freshwater pond food webs by altering the availability of resources, water transparency, and the metabolic state of ponds. We created leaf-added and no-leaf-added field mesocosms in autumn 2012, allowed mesocosms to ice-over for the winter, and began sampling the physical, chemical, and biological properties of mesocosms immediately following ice-off in spring 2013. At ice-off, leaf additions reduced dissolved oxygen, elevated total phosphorus concentrations and dissolved materials, and did not alter temperature or total nitrogen. These initial abiotic effects contributed to higher bacterial densities and lower chlorophyll concentrations, but by the end of spring, the abiotic environment, chlorophyll and bacterial densities converged. By contrast, zooplankton densities diverged between treatments during the spring, with leaf additions stimulating copepods but inhibiting cladocerans. We hypothesized that these differences between zooplankton orders resulted from resource shifts following leaf additions. These results suggest that leaf subsidies can alter both the short- and long-term dynamics of planktonic food webs, and highlight the importance of fully understanding how ecological subsidies are integrated into recipient food webs.

Recognizing cross‐ecosystem responses to changing temperatures: soil warming impacts pelagic food webs

The energy and materials that move across ecosystem boundaries influence food web structure and key ecosystem functions. Despite the acknowledged importance of such ecological subsidies, surprisingly little information is available regarding the role of environmental temperature in influencing subsidy quality and the response of the recipient ecosystem. We evaluated the impacts of temperature‐mediated changes in leaves from deciduous trees, an important subsidy from terrestrial to freshwater ecosystems, on both the producer‐based and detritivore‐based components of a pelagic pond food web in a field mesocosm experiment. We hypothesized that variation in leaf chemistry driven by increased soil temperature would alter both the quality of leaf subsidies and the pond response. We collected red maple Acer rubrum leaves from heated and ambient temperature plots from the long‐term soil warming experiment at the Harvard Experimental Forest and added them to 167‐l field mesocosms containing established plankton communities, creating ‘no leaf’, ‘ambient leaf’ and ‘heated leaf’ treatments during autumn 2012. We then monitored physical, chemical, and biological responses to treatments until the mesocosms froze six weeks later. Experimental soil warming altered the chemical composition of deciduous leaves, the physical and chemical environment of the aquatic ecosystems to which leaves were added, and the pelagic pond food webs as measured by community composition. Compared to leaves from ambient‐temperature soils, leaves from warmed soils initially resulted in lower water column phosphorus and dissolved organic carbon, reducing bacterial densities. However, the diminished carbon and phosphorus resulting from soil warming also increased light availability that ultimately stimulated cladoceran zooplankton relative to ambient‐temperature leaves. Our results suggest that changes in temperature can alter ecological subsidies in unanticipated ways, and suggest that accurately predicting the potential consequences of climate change will require conducting research across ecosystem boundaries.