SEDIMENTARY PHOSPHORUS CYCLING AND A PHOSPHORUS MASS-BALANCE FOR THE GREEN BAY (LAKE MICHIGAN) ECOSYSTEM

The tributaries of Green Bay have long been recognized as major source s of phosphorus in the Lake Michigan basin. The status of Green Bay as a sink or source of phosphorus for Lake Michigan proper has been less well defined. The bay receives nearly 70% of its annual load of phosp horus (700 metric tons (t).year(-1)) from a single source: the Fox Riv er. Most of this phosphorus is deposited in sediments accumulating at rates that reach 160 mg.cm(-2).year(-1) with an average of 20 mg.cm(-2 ).year(-1). The phosphorus content of these sediments varies from <5 t o >70 mu mol.g(-1). Deposition is highly focused, with similar to 70% of the total sediment accumulation and at least 80% of the phosphorus burial occurring within 20% of the surface area of the bay. Diagenetic and stoichiometric models of phosphorus cycling imply that >80% of th e phosphorus deposited is permanently buried. External phosphorus load ing to the bay is combined with sediment fluxes of phophorus to arrive at a simple phosphorus budget. Green Bay acts as an efficient nutrien t trap, with the sediments retaining an estimated 70-90% of the extern al phosphorus inputs before flowing into Lake Michigan.