A budget for algal carbon was constructed to quantify the magnitude and major pathways of pelagicbenthic 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 14C-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-l) was 23 times greater than that for sediment bacteria (1.3 nmol C cm--2 d-I). 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 6 l%, 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.

Green Bay, while representing only ,7% of the surface area and ,1.4% of the volume of Lake Michigan, contains one-third of the watershed of the lake, and receives approximately one-third of the total nutrient loading to the Lake Michigan basin, largely from the Fox River at the southern end of the bay. With a history of eutrophic conditions dating back nearly a century, the southern portion of the bay behaves as an efficient nutrient and sediment trap, sequestering much of the annual carbon and nitrogen input within sediments accumulating at up to 1 cm per year. Depositional fluxes of organic matter varied from ,0.1 mol C m 22 yr 21 to .10 mol C m 22 yr 21 and were both fairly uniform in stoichiometric composition and relatively labile. Estimates of benthic recycling derived from pore-water concentration gradients, whole-sediment incubation experiments, and deposition-burial models of early diagenesis yielded an estimated 40% of the carbon and 50% of the nitrogen recycled back into the overlying water. Remineralization was relatively rapid with ,50% of the carbon remineralized within ,15 yr of deposition, and a mean residence time for metabolizable carbon and nitrogen in the sediments of 20 yr. On average, organic carbon regeneration occurred as 75% CO 2 , 15% CH 4 , and 10% dissolved organic carbon (DOC). Carbon and nitrogen budgets for the southern bay were based upon direct measurements of inputs and burial and upon estimates of export and production derived stoichiometrically from a coupled phosphorus budget. Loadings of organic carbon from rivers were ,3.7 mol m 22 yr 21 , 80% in the form of DOC and 20% as particulate organic carbon. These inputs were lost through export to northern Green Bay and Lake Michigan (39%), through sediment burial (26%), and net CO 2 release to the atmosphere (35%). Total carbon input, including new production, was 4.54 mol m 22 C yr 21 , equivalent to ,10% of the gross annual primary production. Nitrogen budget terms were less well quantified, with nitrogen export ,54% of total inputs and burial ,24%, leaving an unquantified residual loss term in the nitrogen budget of ,22%.

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Short-term (approximately monthly) sediment deposition and resuspension rates of surficial bed sediments in two PCB-laden impoundments on the Fox River, WI, were determined in the summer and fall of 1998 using 7Be, a naturally occurring radioisotope produced in the atmosphere. Decay-corrected activities and inventories of 7Be were measured in bed sediment and in suspended particles. Beryllium-7 activities generally decreased with depth in the top 5-10 cm of sediments and ranged from undetectable to approximately 0.9 pCi cm(-3). Inventories of 7Be, calculated from the sum of activities from all depths, ranged from 0.87 to 3.74 pCi cm(-2), and the values covaried between sites likely reflecting a common atmospheric input signal. Activities of 7Be did not correlate directly with rainfall. Partitioning the 7Be flux into "new" and "residual" components indicated that net deposition was occurring most of the time during the summer. Net erosion, however, was observed at the upstream site from the final collection in the fall. This erosion event was estimated to have removed 0.10 g (cm of sediment)(-2), corresponding to approximately 0.5 cm of sediment depth, and approximately 6-10 kg of polychlorinated biphenyls (PCBs) over the whole deposit. Short-term accumulation rates were up to approximately 130 times higher than the long-term rates calculated from 137Cs profiles, suggesting an extremely dynamic sediment transport environment, even within an impounded river system.