| Summary: | Many nutrient amendment studies in lakes have disregarded the importance of links between benthic and pelagic food webs (Vadeboncouer et al. 2002). We used whole-lake and in situ mesocosm experiments to examine indirect and direct linkages between pelagic (water-column nutrients, primary productivity, zooplankton, fish, and settling particulate matter) and benthic (primary productivity and chironomid growth) habitats in oligotrophic arctic lakes at the Toolik Lake LTER station. Data from three years of 15N-tracer, NH4NO3 and PO4 additions to the surface of shallow and deep lakes indicate phytoplankton outcompete benthic algae for added NH4; the rate of sedimentation of particulate matter from pelagic to benthic habitats increases with fertilization and is higher in shallow lakes compared to deep lakes; pelagic fish derive 90% of their diet from benthic sources; and in deep lakes, profundal grazers derive a significant portion of their diet from pelagic subsidies (settling particulate matter). We used in situ mesocosm experiments to observe whether nutrient amendments (6μmols N + 0.375μmols P over one month), zooplankton biomass (low and high) and pelagic subsidies (settling particulate matter) affect growth of individual chironomids at 2.5 meters. Water-column nutrient additions did not affect benthic chlorophyll-a concentrations or chironomid growth. When zooplankton biomass was high, pelagic chlorophyll-a concentrations decreased; however, benthic chlorophyll-a and chironomid growth rates increased, suggesting zooplankton may mediate competition between pelagic and benthic algae for nutrients. Pelagic subsidies increased chironomid growth rates. Our study emphasizes the importance of studying benthic-pelagic links in lake ecosystems.
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