1Department of Biological Sciences, Dartmouth College, Hanover, NH 03755
2Present address: Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520
3Departments of Civil and Environmental Engineering and Bacteriology, University of Wisconsin, Madison, Wisconsin 53706
The export of nutrients, detritus, and living organisms from marine, terrestrial, and freshwater ecosystems can critically shape food web structure, food web stability, and ecosystem functioning in the recipient ecosystem (Polis et al. 1997, Gratton et al. 2008, Takimoto et al. 2002). Despite the acknowledged importance of subsidies and the potential for both organisms and detritus to be affected by temperature (Kovach et al. 2013), little information is available regarding how temperature could alter the quality of ecological subsidies or the response of the recipient system to such alterations (Greig et al. 2012). This paucity of information reflects a general lack of integrative research across ecosystems and hinders current predictions of the ecological consequences of climate change (Woodward et al. 2010).
Leaves from deciduous terrestrial vegetation (Figure 1) provide an ideal opportunity for investigating how changes in temperature might alter ecological subsidies due to their importance for aquatic ecosystems (France and Peters 1995) and sensitivity to local environmental conditions (Melillo et al. 2011). The dissolved organic carbon (DOC), nitrogen (N) and phosphorus (P) leached from leaves can stimulate both detritivore-based “brown” food webs, including bacteria and their consumers (Lennon and Pfaff 2005), and producer-based “green” food webs, including phytoplankton (Figure 2). Additionally, the biogeochemical and physiological processes that allow leaves to become important subsidies for lakes and ponds also create the potential for local environmental conditions to alter the quality of these subsidies (Butler et al. 2012, Sardans and Peñuelas 2012). As such, we hypothesized that variation in leaf chemistry driven by increased soil temperature would alter the quality of leaf subsidies for freshwater pond pelagic food webs.
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. We then added leaves to 167 L field mesocosms containing established plankton communities (containing bacteria, phytoplankton, and zooplankton assemblages), creating “no leaf”, “ambient leaf” and “heated leaf” treatments. We monitored physical, chemical, and biological responses prior to and following the imposition of treatments until the mesocosms froze 6 weeks later. This experiment was conducted in Hanover, NH, USA in autumn 2012.
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 had two fold lower foliar phosphorus concentrations. Leaf additions to mesocosms initially resulted in lower water column phosphorus and dissolved organic carbon (Figure 3), which reduced 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 (Figure 4).
This experiment demonstrates that variation in soil temperature is capable of impacting pelagic pond food webs during the period of autumn leaf drop by altering the quality of leaf subsidies. The response of different components of the pond food webs to leaves grown under different environmental conditions was unexpectedly complex, with leaves from experimentally warmed soils ultimately producing lower bacterial densities but higher cladoceran zooplankton densities relative to leaves from unwarmed soils. As such the potential for terrestrial temperatures to indirectly affect pond ecosystems by altering cross-ecosystem leaf subsidies may be an important and underappreciated ecological process. More generally, our results suggest that accurately predicting the potential consequences of climate change will require conducting research across ecosystem boundaries.
Full study published in Oikos’ Synthesising Ecology, Feb. 25, 2015.
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Featured Image: Lake Sunapee. (Credit: Flickr User Rick Kloeppel via Creative Commons 2.0)