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 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 dI). 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. The magnitude of pelagic-benthic coupling in aquatic environments is constrained by how efficiently energy, in the form of biochemical compounds such as lipids, carbohydrates, and proteins, is transferred spatially from the pelagic to the benthic zone and between trophic levels. In Lake Michigan, average particle residence times in the water have been estimated with radionuclides to be on the order of a few weeks (Eadie et al. 1984). However, large algal inputs during the spring bloom may be important in rapidly supplying high-energy, easI Present address: U.S. Geological Survey, mandy Lane, Madison, Wisconsin 53719. 6417 NorAcknowledgments We thank Gary Fahnenstiel for advice on algal labeling and enumeration and for providing the labeled bicarbonate, Gerald Bell for assistance in construction of the sediment traps, Peter Landrum and his students for assistance with animal collection, John Robbins for use of the well detector and comments on the manuscript, Brian Eadie, Glenn Lopez, and James Cotner for comments on the manuscript, Rick Carlton for unpublished dissolved oxygen data and comments, John Lehman for ship time, and the captains and crew of the RV Shenehon and the RV Laurentian for field assistance. This work was supported by the National Research Council through a postdoctoral fellowship to S.A.F. The U.S. Geological Survey provided logistical support during the final preparation of the manuscript. GLERL contribution 783. ily metabolizable material to even the deepest depositional basins in a matter of hours or days. The fate of this material is determined largely by uptake rates and assimilation efficiencies of benthic macroinvertebrates, meiofauna, and microbial heterotrophs. The extent to which bacteria remineralize fresh organic detritus, thereby acting as an energy sink, necessarily constrains the amount of energy that can be directly transferred to benthic invertebrates and hence to higher trophic levels. Also, if sufficiently abundant and metabolically active, benthic invertebrates may play a significant role in organic matter mineralization through respiration and excretion (Gardner et al. 1987). Therefore, to understand the fate and effects of particles settling from the spring diatom bloom, it is important to determine both the amount of metabolizable organic matter reaching the sediment-water interface and the pathways of organic matter assimilation and mineralization. Diporeia sp. (formerly called Pontoporeia hoyi; see Bousfield 1989) is an important component of benthic-pelagic coupling in Lake Michigan for several reasons. This amphipod accounts for 65% of total benthic biomass at depths > 30 m in southern Lake Michigan (Nalepa 1989). It is a favored prey item for most Great Lakes forage fish including rainbow smelt