BIOGEOCHEMICAL CONTROL OF PHOSPHORUS CYCLING AND PRIMARY PRODUCTION IN LAKE-MICHIGAN

A 3-yr study in Lake Michigan has shown a 27 mmol P m-2 increase in th e mass of total P (TP) in the water during spring when the lake is mix ed from surface to sediment. This value is an order of magnitude great er than the annual P input from external sources. TP changed in concer t with increases in chlorophyll a and organic N and decreases in nitra te and soluble Si. The concentration of soluble reactive PO43- (SRP) r emained relatively constant throughout the study. We hypothesize that the SRP concentration is maintained by a chemical equilibrium with cal cium-phosphate species. The increased mass of TP arises from the seque stering of P by algae which displaces the chemical equilibrium and all ows more P to be released to the water from the sediments. Solar irrad iance and the duration of mixing determine the magnitude of the spring bloom and the demand for P that must be supplied through the flux of P from the sediments to the overlying water.