Seasonal variation in the biogeochemical cycling of seston in Grand Traverse Bay, Lake Michigan

This study describes the biogeochemical cycling of seston in Grand Traverse Bay, Lake Michigan. Seston was characterized by carbon and nitrogen elemental and isotopic abundances. Fluorescence, temperature, light transmittance, and concentrations of dissolved inorganic nitrogen were also determined. PCBs were analyzed from surface (10 m) seston and ΣPCB was calculated by summing all of the congeners quantified in each sample. The vertical and seasonal trends in the δ13C values of seston exhibited a broad range from −30.7 to −23.9‰. Low δ13C values that occur concurrently with a peak in fluorescence below the thermocline reflect uptake of 13C depleted respiratory CO2 and/or the accumulation of 13C depleted lipids by phytoplankton. High δ13C values late in the season likely result from a reduction in photosynthetic fractionation associated with a decrease in the CO2 pool. Seasonal δ15N values of seston were high in the spring and declined through August. The δ15N values of seston reflect a balance between fractionation during assimilation of NH4+ or NO3− and degradative processes. The seston ΣPCB and fluorescence were both high in the spring and subsequently declined, suggesting that the concentrations of PCBs in seston were associated with labile material derived from primary productivity. The strong seasonal trends in the organic geochemical characteristics of seston and concentrations of PCBs emphasize the complex nature of particle cycling in aquatic environments.