Experimental evidence that hemlock decline changes the role of detrital subsidies in freshwater food webs
Wholesale replacement of foundation tree species are a reoccurring feature of fossilized pollen records, and are also anticipated in contemporary ecosystems as a consequence of the direct and indirect effects of climate change. Although these changes are expected to alter forested freshwater ecosystems via changes in terrestrial subsidies, outcomes will likely depend whether the replacement of foundational species by local forest dominants increases or decreases subsidy quality. We investigated this hypothesis in a mesocosm experiment that tested the response of pond food webs to changing detrital resources as a consequence of eastern hemlock (Tsuga canadensis) decline. We added leaf-litter varying along a gradient of palatability that reflected a shift from hemlock to either a less-palatable species (Rhododendron maximum) that is becoming dominant in the southeast, or a more labile leaf replacement (red maple, Acer rubrumL.) that is anticipated in the northeast. Mesocosms were inoculated with microbes, plankton, and benthic invertebrates, and were left open to active colonization by winged adults. We measured a suite of ecosystem and food-web responses including leaf decomposition, DOC, dissolved nutrients, producers, and consumers.
The incorporation of detrital resources in pond food webs strongly depended on the palatability of hemlock replacement species. Large pulses of colored dissolved organic carbon (DOC) were released in mesocosms with a greater proportion of labile maple, resulting in thermal stratification and the deposition of fine particulate detritus. Associated with this gradient was a rapid increase in dissolved nutrients that were quickly assimilated by benthic and pelagic producers. In contrast, increasing proportions of Rhododendron had little influence on concentrations of nutrients and colored DOC. Although this shifting resource base with detritus quality influenced both producers and consumers, responses attenuated with increasing trophic position, and varied depending on the relative biomass of benthic vs pelagic producers. The rapid response of our experimental systems to simple resource replacement suggests the loss of hemlock will have important ecosystem-level consequences for forested freshwaters. However, the nature of these changes is dependent on replacement tree species and the relative importance benthic and pelagic production.