Climate change consequences cross ecosystem boundaries: increases in soil temperature impact lake plankton communities through modifying terrestrial vegetation subsidies

Background/Question/Methods

The energy and materials provided by cross-ecosystem subsidies critically shape food web structure and ecosystem functioning.  Despite the acknowledged importance of subsidies, surprisingly little information is available regarding the effects of climate change on the availability and quality of ecological subsidies and the resulting biological response in the recipient system. Leaves from terrestrial vegetation are an important subsidy for aquatic ecosystems, especially during autumn leaf drop. Leaf chemistry is highly variable and depends on many environmental factors, including climate. We hypothesized that climate-mediated alterations in leaf chemistry are biologically meaningful for plankton food webs during autumn leaf drop, including both the photoautotroph-based “green” and detrivivore-based “brown” food webs. To test this hypothesis, we collected leaves from heated and ambient temperature forest plots from a long-term soil warming experiment at the Harvard Experimental Forest in autumn 2010 and 2011.  In October 2012, we added these leaves to field mesocosms containing established lake plankton communities, thus creating “no added leaves”, “ambient leaves” and “heated leaves” treatments (each replicated 5 times for 15 total mesocosms). We then monitored physical, chemical, and biological responses to leaves in each mesocosm for eight weeks.

Results/Conclusions

We found that soil warming influenced the quality of leaf subsides to aquatic ecosystems and altered both green and brown pelagic food webs. Soil warming in forest plots modified the chemical composition of leaf subsidies. Most notably, phosphorus concentrations in leaves collected from ambient temperature plots were twice as high as those collected from heated plots. In the mesocosms, nutrients leached out of the leaves in proportion to their abundance, elevating concentrations relative to the no-leaf controls; total and soluble reactive phosphorus were elevated in the “ambient leaves” treatment, while total nitrogen concentrations did not differ between leaf treatments. Water pH, specific conductance, and dissolved oxygen all decreased following leaf additions relative to no-leaf controls, but did not differ between leaf treatments.  Bacterial densities increased immediately following leaf additions, while chlorophyll-a declined, most likely as a result of decreased light availability. On the last sampling date, the heated leaf treatment contained more cladocerans (especially Daphnia and Chydorus) than ambient leaf treatments, but there were no differences in copepod densities. Taken together, our results highlight the need for continued research across ecosystem boundaries in order to accurately predict the broad-scale ecological consequences of climate change.