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

Abstract

Weekly water samples were taken to measure stable isotope composition (δ13C and δ15N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average δ13C of POM was ‐19.3%, 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 13C 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 δ13C 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 δ15N of POM was 1.3% and varied within a narrow range (20.1% to 2.5%). No significant correlation between phytoplankton biovolume and the δ15N of POM was found. The low δ15N is indicative of strong N2 fixation, which is in line with the low concentration of dissolved inorganic nitrogen and the presence of high biovolume of N2‐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 N2 fixation.