SEASONAL VARIABILITY IN PHOSPHORUS SPECIATION AND DEPOSITION IN A CALCAREOUS, EUTROPHIC LAKE

Settling particulate matter was collected in sediment traps from the m ajor depositional basin of Onondaga Lake, a calcareous, eutrophic lake in Syracuse, New York, U.S.A. Sediment traps were deployed at three d epths from July to November and for a brief period under ice cover in February to investigate both vertical and seasonal variations in sedim entation. A sequential chemical extraction (fractionation) method was applied to the collected material to quantify the following phosphorus (P) fractions, or forms: loosely bound-P (sorbed-P, CaCO3 associated- P, and Fe and Al bound-P), extractable biogenic-P (easily degradable o rganic-P and biogenic inorganic polyphosphates), calcium mineral-P (e. g., apatite), and refractory organic-P. Extractable biogenic-P and ref ractory organic-P were strongly correlated, suggesting a common, autoc hthonous origin in plankton. Calcium mineral-P appears to be of terrig enous origin. A dramatic increase in the loosely bound-P content of th e particulate matter was observed during fall turnover when dissolved Fe2+, which had accumulated in the anoxic hypolimnion, mixed with the surface waters and oxidized with attendant adsorption of P. We estimat e that the labile-, or exchangeable-P (i.e. loosely bound- and extract able biogenic-P forms) content of the settling particulates constitute s similar to 50% of the total P influx to the sediments over the perio d of study. The labile-P flux over the July-November study period was 11 mg P m(-2) day(-1) and the labile-P flux estimated from sediment tr aps deployed under ice cover was 6 mg P m(-2) day(-1). From these valu es the annual average labile-P flux to the sediments was estimated to be 9 mg P m(-2) day(-1), only slightly less than the estimated annual average sediment P release rate of 10 mg P m(-2) day(-1). This indicat es that the sediments are approximately at steady state with respect t o current external P loadings to the lake. (C) 1997 Elsevier Science B .V.