Seasonal variability in phosphorus speciation and deposition in a calcareous, eutrophic lake

Abstract

Settling particulate matter was collected in sediment traps from the major depositional basin of Onondaga Lake, a calcareous, eutrophic lake in Syracuse, New York, U.S.A. Sediment traps were deployed at three depths from July to November and for a brief period under ice cover in February to investigate both vertical and seasonal variations in sedimentation. 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, CaCO 3 associated-P, and Fe and Al bound-P), extractable biogenic-P (easily degradable organic-P and biogenic inorganic poly phosphates), calcium mineral-P (e.g., apatite), and refractory organic-P. Extractable biogenic-P and refractory organic-P were strongly correlated, suggesting a common, autochthonous origin in plankton. Calcium mineral-P appears to be of terrigenous origin. A dramatic increase in the loosely bound-P content of the particulate matter was observed during fall turnover when dissolved Fe 2+, which had accumulated in the anoxic hypolimnion, mixed with the surface waters and oxidized with attendant adsorption of P. We estimate that the labile-, or exchangeable-P (i.e. loosely bound- and extractable biogenic-P forms) content of the settling particulates constitutes ~ 50% of the total P influx to the sediments over the period 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 traps deployed under ice cover was 6 mg P m −2 day −1. From these values 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 indicates that the sediments are approximately at steady state with respect to current external P loadings to the lake.