Factors controlling seasonal variations in stable isotope composition of particulate organic matter in a soft water eutrophic lake

Weekly water samples were taken to measure stable isotope composition (delta C-13 and delta N-15) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average delta C-13 of POM was -19.3 parts per thousand, consistent with an autochthonous origin from phytoplankton production, and exhibited a seasonal pattern that coincided with changes in water temperature, pH, CO2 concentration, and phytoplankton biomass in the surface water. The C-13 enrichment in POM was attributed to reduced isotope fractionation due to carbon (C) limitation and the use of an isotopically heavy dissolved inorganic carbon pool supported mainly by atmospheric invasion and anaerobic respiration. Intermittent declines in delta C-13 of POM were related to the frequent collapses of phytoplankton blooms and increases in CO2 concentration resulting from both increased community respiration and terrestrial loading. Average delta N-15 of POM was 1.3 parts per thousand and varied within a narrow range (-0.1 parts per thousand to 2.5 parts per thousand). No significant correlation between phytoplankton biovolume and the delta N-15 of POM was found. The low delta N-15 is indicative of strong N-2 fixation, which is in line with the low concentration of dissolved inorganic nitrogen and the presence of high biovolume of N-2-fixing cyanobacteria in the surface water. This study suggests that stable C isotopes are good proxies for surface water CO2 concentration and primary production, while stable N isotopes can be used to indicate N-2 fixation.