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Seasonally varying nitrogen isotope biogeochemistry of particulate organic matter in Lake Kinneret, Israel
Author(s) -
Hadas Ora,
Altabet Mark A.,
Agnihotri Rajesh
Publication year - 2009
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2009.54.1.0075
Subject(s) - nitrification , denitrification , hypolimnion , environmental chemistry , nitrate , biogeochemistry , phytoplankton , δ15n , epilimnion , isotopes of nitrogen , nitrogen cycle , water column , isotope fractionation , spring bloom , organic matter , bloom , stable isotope ratio , oceanography , chemistry , δ13c , nitrogen , fractionation , ecology , eutrophication , nutrient , geology , biology , organic chemistry , physics , quantum mechanics
Large temporal variations in the nitrogen isotopic composition (δ 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 δ 15 N POM values (25.5‰) were observed in early winter, a consequence of isotopic fractionation by chemoautotrophic microbial NH 4 + assimilation, and during the first phase of the nitrification period, by seasonally dominant NH 4 + oxidizers. Nitrification itself was strongly fractionating, producing 15 N‐depleted NO 3 − and strongly enriching water‐column NH 4 + in 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‰ jump in POM δ 15 N because of assimilation of high‐δ 15 N NH 4 + . Maximal δ 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 δ 15 N values decreased to values similar to δ 15 N for NO 3 − (8‐12‰), indicating a switch to this DIN source. NO 3 − assimilation into POM appeared to occur without isotopic fractionation. Late spring was also the denitrification period with NO 3 − removal in the newly suboxic hypolimnion. Though δ 15 NO 3 − increased in the residual NO 3 − , complete consumption by denitrification suggests no net effect, as supported by a lack of increase in δ 15 N POM at this time. Oligotrophic conditions are found in the epilimnion in late summer and early autumn and low (3.3‰) δ 15 N POM was found during the bloom of the filamentous N 2 ‐fixing cyanobacteria Aphanizomenon ovalisporum and Cylindrospermopsis cuspis. The δ 15 N‐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.