Warming, eutrophication, and predator loss amplify subsidies between aquatic and terrestrial ecosystems

Abstract

The exchange of organisms and energy among ecosystems has major impacts on food web structure and dynamics, yet little is known about how climate warming combines with other pervasive anthropogenic perturbations to affect such exchanges. We used an outdoor freshwater mesocosm experiment to investigate the interactive effects of warming, eutrophication, and changes in top predators on the flux of biomass between aquatic and terrestrial ecosystems. We demonstrated that predatory fish decoupled aquatic and terrestrial ecosystems by reducing the emergence of aquatic organisms and suppressing the decomposition of terrestrial plant detritus. In contrast, warming and nutrients enhanced cross-ecosystem exchanges by increasing emergence and decomposition, and these effects were strongest in the absence of predators. Furthermore, we found that warming advanced while predators delayed the phenology of insect emergence. Our results demonstrate that anthropogenic perturbations may extend well beyond ecosystem boundaries by influencing cross-ecosystem subsidies. We find that these changes are sufficient to substantially impact recipient communities and potentially alter the carbon balance between aquatic and terrestrial ecosystems and the atmosphere.

Spatially cascading effect of perturbations in experimental meta-ecosystems

“…Subsidy quality can also have important impacts on recipient ecosystems [44][45][46]. In our experiment, the autotrophic protist species was not only a key species because of its inability to recover from perturbation over time, but also because it could increase the total level of resource in each ecosystem through photosynthetic activities.…”

Warming shifts top-down and bottom-up control of pond food web structure and function

“…To show how the effects of fish predators are altered by warming in the benthos versus pelagic, we present the direct and indirect interaction strengths of fish with all six functional groups, as well as litter decomposition rates (from Greig et al [33]) from two time periods when all dependent variables were sampled (figure 3). Interaction strength is measured as the log e of the ratio of biomass of each group in the presence versus the absence of fish.…”

“…This choice was made for simplicity of interpretation; owing to the seven dependent variables and many sampling periods, we focus only on those effects that were apparent across time rather than those that emerged at particular points during the experiment. We presented the results of different time-series analyses using time as a fixed variable and demonstrating the seasonally varying effects of treatments and their interactions elsewhere [32,33]. All data were log e -transformed prior the analyses to achieve normality of residuals and to improve homoscedasticity of variances.…”

“…Benthic macroinvertebrates were sampled in three microhabitats: (i) on bottom of mesocosms with standard sweeps within a 0.02 m 2 cylinder placed in two different areas, (ii) on tank walls with two sweeps of a 12 cm wide, 0.5 mm mesh net from the tank bottom to the water's surface and (iii) in the water column with three sweeps of a 25 cm net across the mesocosm. Invertebrates were identified and biomass calculated from length-mass regressions following Greig et al [33]. The measurements were aggregated into a single biomass estimate after adjusting for the relative area of each habitat type.…”

“…We previously described the responses of the mean and temporal variability of phytoplankton biomass [32], showing that warming dampened the response of phytoplankton to fertilization but magnified the effects of trophic cascades. We also examined the effects on exchange of biomass between terrestrial and aquatic environments in terms of incorporation of terrestrial plant detritus into aquatic food webs, and the emergence of insect and amphibian biomass from the ponds [33]. The goal of the present study is to compare the direct and indirect effects of predators, temperature and eutrophication on the structure and function of aquatic food webs and to test for interactive or non-additive responses to the three manipulated factors.…”