Projecting responses of aquatic food webs to climate change: Synthesizing ecological data and models with a new paleoproductivity indicator

Background/Question/Methods Despite the vulnerability of aquatic food webs to global warming (Naiman et al. 2005) and their importance as integrators and sentinels of environmental change across landscapes (Williamson et al. 2008), there are few studies that span the required temporal and spatial scales to assess long-term effects of global climate changes at the watershed scale. Logistical constraints, as well as poor preservational environments, limit paleoecological data for many watersheds. A new paleo-productivity indicator is proposed based on a common freshwater diatom genus, Epithemia. As a dominant, late-successional epiphyte, Epithemia closely tracks the bloom size for its host Cladophora, and serves as a proxy for productivity. An 18-year record has recorded regime shifts between “low algae” and “high algae” years that involve the climate-driven discharge interacting with food web controls on algae (Power et al. 2008). Due to its exclusively freshwater habit, robust frustule and abundance in western North American watersheds where nitrogen is limiting, Epithemia is a good candidate for preservation in offshore sedimentary cores in sufficient numbers to track changes in productivity regimes on interannual and decadal scales. Sedimentary cores from the submarine Eel River canyon, collected as part of the STRATAFORM project in 2001, have been selected for analysis due to the high sediment accumulation rate, low bioturbation and direct influx of ~50% of the sediment from the Eel freshwater plume. Exploratory diatom counts have been performed for three STRATAFORM cores in the northernmost canyon entrance at several depths, coinciding with deposits from flood and drought years in the Eel basin. Results/Conclusions Epithemia frustules have been found in each sample examined. The frustule accumulation rate varied over 1-2 orders of magnitude, and included two species of Epithemia commonly found well upstream, two species of Cocconeis, and several other common freshwater epiphytic diatoms. Further isotopic analysis is underway to more precisely date the originating strata, but strata with higher Epithemia counts have age ranges overlapping flood years, while strata with lower Epithemia counts have age ranges overlapping drought years. The presence of Epithemia in offshore cores provides a new method to estimate algal productivity in the entire Eel watershed over the length of the marine sedimentary record. Additional cores covering 2000 years of sedimentation are being analyzed for interannual, decadal and centennial patterns in diatom accumulation. As Cladophora and Epithemia are widespread in temperate freshwater ecosystems, this method holds promise for estimating paleoproductivity across a wide range of watersheds and timescales.