Seasonally varying nitrogen isotope biogeochemistry of particulate organic matter in Lake Kinneret, Israel

Abstract

Large temporal variations in the nitrogen isotopic composition (δ15N) of particulate organic matter (POM) and dissolved inorganic nitrogen (DIN) species in Lake Kinneret occurred in response to seasonal phasing of dominant nitrogen cycle processes. The lowest δ15N POM values (25.5‰) were observed in early winter, a consequence of isotopic fractionation by chemoautotrophic microbial NH4+ assimilation, and during the first phase of the nitrification period, by seasonally dominant NH4+ oxidizers. Nitrification itself was strongly fractionating, producing 15N-depleted NO3− and strongly enriching water-column NH4+ in 15N. Toward the end of nitrification and into the phytoplankton bloom period in later winter and early spring, there was a corresponding 15-30‰ jump in POM δ15N because of assimilation of high-δ15N NH4+. Maximal δ15N POM values were thus measured during the algal blooms of Peridinium gatunense in 2004 and Debarya sp. and Microcystis sp. in 2005. Toward the end of the blooms in mid- to late spring, POM δ15N values decreased to values similar to δ15N for NO3− (8-12‰), indicating a switch to this DIN source. NO3− assimilation into POM appeared to occur without isotopic fractionation. Late spring was also the denitrification period with NO3− removal in the newly suboxic hypolimnion. Though δ15NO3− increased in the residual NO3−, complete consumption by denitrification suggests no net effect, as supported by a lack of increase in δ15N POM at this time. Oligotrophic conditions are found in the epilimnion in late summer and early autumn and low (3.3‰) δ15N POM was found during the bloom of the filamentous N2-fixing cyanobacteria Aphanizomenon ovalisporum and Cylindrospermopsis cuspis. The δ15N-dissolved organic nitrogen isotopic signature at this time was significantly higher, 8–12‰, suggesting a phytoplankton source from earlier in the seasonal cycle. Overall, the largest known seasonal variations in N isotope composition have been observed in Lake Kinneret, providing a uniquely useful biogeochemical tool for studying N cycling in lakes as well as detecting long-term changes in N source and cycling in response to watershed land use changes and climate change.