AN ALGAL CARBON BUDGET FOR PELAGIC-BENTHIC COUPLING IN LAKE-MICHIGAN

A budget for algal carbon was constructed to quantify the magnitude and major pathways of pelagic-benthic coupling at a site in southeastern Lake Michigan. The flux of algal C to the benthos and the rate of carbon burial were estimated from sediment traps and dated sediment cores, respectively. Assimilation and respiration rates of Diporeia sp., an abundant benthic amphipod, and of sediment microheterotrophs were measured in a microcosm study with C-14-labeled algae (Melosira italica). Melosira (italica and islandica) accounted for 53% of the algal C flux to the sediments. Radionuclide concentrations indicated no net sediment burial of organic C. Of the total C assimilated by Diporeia, 60% was respired, 35% was incorporated into biomass, and 5% was accounted for as soluble dissolved organic compounds. The areal rate of Diporeia respiration (29 nmol C cm-2 d-1) was 23 times greater than that for sediment bacteria (1.3 nmol C cm-2 d-1). Release of radioisotope in the form of dissolved organic compounds was much lower than that incorporated and respired for both Diporeia and sediment bacteria. Of the 61 mmol C M-2 of algal C estimated to be deposited during the spring bloom, Diporeia assimilation accounted for 61%, significantly more than the 2% observed for microbially mediated algal decomposition. These observations support the hypothesis of a strong pelagic-benthic energy coupling between the spring diatom bloom and Diporeia in Lake Michigan.