Cultural eutrophication shifts the relative importance of energy channels in seagrass food webs

Background/Question/Methods

Food webs are composed of networks of interconnected food chains, each originating from a unique basal resource (e.g. phytoplankton, seagrass, detritus).  These energy channels are coupled by higher-order consumers, with channels often becoming increasingly coupled at progressively higher trophic levels.  Recent work suggests that the capacity of energy channels to support top predators can vary with the shifting dominance (production:biomass ratio) of basal resources.  Notably, some of the most significant and ubiquitous anthropogenic agents of environmental change are known to cause such shifts.  Here we show that this ‘energy channel capacity’ framework can be used to understand and predict how food web structure responds to cultural eutrophication.

Results/Conclusions

Among seagrass ecosystems, eutrophication is associated with a shift from seagrass- to epiphyte- and phytoplankton-dominated communities.  Using data from seagrass communities situated across a gradient of oligotrophic to eutrophic conditions, we employ a stable isotope (δ¹³C, δ¹⁵N) approach to estimate the fraction of consumer diets that are ultimately derived from each of multiple basal resources, and identify the attendant energy channels.  Within sites, we find that energy channels based on dominant basal resources are characterized by elevated biomass of primary consumers.  Between sites, we find that these biomass aggregations shift from seagrass- to epiphyte- and phytoplankton-based energy channels under increasingly eutrophic conditions.  Next, we show that consumers at uppermost trophic levels rely on energy channels in proportion to the relative dominance of each channel’s basal resource.  Moreover, we demonstrate that higher-order consumers derive a greater fraction of their energy from epiphyte- and phytoplankton- based channels under increasingly eutrophic conditions.  Finally, we document greater omnivory and a narrowing of effective food web width (the breadth of basal resources used to support biomass at upper trophic levels) at progressively more degraded sites.  Our results suggest that food web structure can vary across a gradient of anthropogenic influence, and that the nature of this variation can be predicted from an understanding of what drives the relative importance of energy channels in food webs.