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 (d13C and d15N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average d13C of POM was 219.3%, 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. Stable isotope analysis is an excellent tool to study carbon (C) and nitrogen (N) biogeochemical cycles in lacustrine systems. Measurements of stable C isotope ratios (d13C) of particulate organic matter (POM) have helped gain important insights into sources of organic matter, primary productivity, and CO2 concentration in the surface water (Quay et al. 1986; Hodell and Schelske 1998; Lehmann et al. 2004). Stable N isotope ratios (d15N) of POM have been used to understand various N cycling processes (Owens 1987; Teranes and Bernasconi 2000; Lehmann et al. 2004). The d13C and d15N of POM in lacustrine systems vary over time (Zohary et al. 1994; Gu and Schelske 1996; Lehmann et al. 2004), and these variations are typically related to external loadings, phytoplankton species composition, and primary productivity, as well as source, isotope signature, and concentrations of dissolved inorganic C and N (DIC and DIN) (Rau et al. 1989; Falkowski 1991; Grey et al. 2001). Temporal variations in the d13C and d15N of POM and their controls must be fully understood before these isotope proxies can be used with confidence. However, few studies have adopted a sampling frequency adequate to capture the detailed seasonal patterns of the d13C and d15N of POM. Furthermore, compared to the marine system, stable N isotope composition of organic matter in freshwaters has received less attention, although it could be a powerful tool to study the sources and fates of anthropogenic pollution, nitrate uptake, and N2 fixation (Owens 1987; Van Dover et al. 1992; Montoya et al. 2002). This study addressed the seasonal variations in d13C and d15N of POM in a shallow soft water and eutrophic lake in Florida. We employed a weekly sampling scheme over a full annual cycle to obtain 52 weekly samples at two central lake stations from June 1994 to May 1995. To our knowledge, our data represent the most frequent collection of POM for d13C and d15N determinations in natural waters over an annual cycle reported to date. We also took weekly samples to identify major phytoplankton species composition, measure algal biomass, and describe several physical and chemical variables to aid data interpretation. 1 To whom correspondence should be addressed. Present address: Everglades Division, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, Florida 33406