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

Abstract

Large temporal variations in the nitrogen isotopic composition (delta sup(15) N) 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 delta sup(15) N POM values (- 5.5 ppt) were observed in early winter, a consequence of isotopic fractionation by chemoautotrophic microbial NH sup(+) sub(4) assimilation, and during the first phase of the nitrification period, by seasonally dominant NH sup(+) sub(4) oxidizers. Nitrification itself was strongly fractionating, producing sup(15) N-depleted NO sup(-) sub(3) and strongly enriching water-column NH sup(+) sub(4) in sup(15) N. Toward the end of nitrification and into the phytoplankton bloom period in later winter and early spring, there was a corresponding 15-30 ppt jump in POM delta sup(15) N because of assimilation of high –delta sup(15) N NH sup(+) sub(4) . Maximal delta sup(15) N 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 delta sup(15) N values decreased to values similar to delta sup(15) N for NO sup (-) sub(3) (8-12 ppt), indicating a switch to this DIN source. NO su(-) sub(3) assimilation into POM appeared to occur without isotopic fractionation. Late spring was also the denitrification period with NO sup(-) sub(3) removal in the newly suboxic hypolimnion. Though delta sup(15) NO sup(-) sub(3) increased in the residual NO sup(-) sub(3), complete consumption by denitrification suggests no net effect, as supported by a lack of increase in delta sup(15) N POM at this time. Oligotrophic conditions are found in the epilimnion in late summer and early autumn and low (3.3 ppt) delta sup(15) N POM was found during the bloom of the filamentous N sub(2) - fixing cyanobacteria Aphanizomenon ovalisporum and Cylindrospermopsis cuspis. The delta sup(15) N-dissolved organic nitrogen isotopic signature at this time was significantly higher, 8-12 ppt, 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.