Factors controlling seasonal variations in stable isotope composition of particulate organic matter in a softwater eutrophic lake
Gu, Binhe, Andrew D. Chapman, and Claire L. Schelske
Limnol. Oceanogr., 51(6), 2006, 2837–2848

Weekly water samples were taken to measure stable isotope composition (d¹³C and d¹⁵N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average d¹³C 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 d¹³C 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 d¹⁵N of POM was 1.3% and varied within a narrow range (20.1% to 2.5%). No significant correlation between phytoplankton biovolume and the d¹⁵N of POM was found. The low d¹⁵N 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.