PRIMARY RESEARCH PAPER A mass balance evaluation of the ecological significance of historical nitrogen fluxes in Lake Kinneret A. Nishri • D. P. Hamilton Received: 21 January 2010 / Revised: 28 July 2010 / Accepted: 3 August 2010 / Published online: 27 August 2010 Ó Springer Science+Business Media B.V. 2010 Abstract Lake Kinneret (LK) is a monomictic lake that has undergone significant biological and chem- ical changes over the last three decades of the twentieth century. The transition between the 1970s and the 1980s attracted a lot of scientific attention as it was marked by significant changes in the ecology of the lake. In the early 1980s, phytoplankton biomass increased, apparently in response to an increase in the external soluble reactive phosphorus (SRP) load. This period was marked by a rise in hypolimnetic levels of ammonium (NH 4 ) and SRP as well as surface water dissolved oxygen (DO) and pH. Cconcomitantly, in surface waters in winter levels of NH 4 increased and NO 3 decreased. In this study interrelationships amongst these observations were examined with a mass balance modelling approach, including simulation of individual nutrient sources and sinks, focusing on nitrogen fluxes in winter. The step-like rise in phytoplankton biomass in 1981 may have been triggered by the increase in winter external loads of SRP, as P is likely to be the growth-limiting nutrient during this season. The additional P load led to a sequence of changes including greater summer phytoplankton biomass, followed by enhanced sedi- mentation of organic matter. Furthermore, higher organic matter mineralization fluxes within the hypolimnion resulted in elevated levels of NH 4 and SRP in this layer through the 1980s, with a feedback to productivity in the trophogenic zone following seasonal destratification in early winter. In an appar- ent transition period (late 1970s to early 1980s), an increase in the modelled rate of nitrate (NO 3 ) production occurred via nitrification together with increased uptake of the additional nitrate by phyto- plankton. These results are consistent with increased phytoplankton abundance and elevated levels of surface water NH 4 and DO during this period. Through this period the increase in phytoplankton uptake of NO 3 predominated over the increase in nitrification, and NO 3 concentrations in the 1980s were reduced compared with the previous decade, with increased partitioning of N in biomass and NH 4 . Keywords Nitrate  Ammonium  Nitrification  Denitrification  Long-term data series Handling editor: Luigi Naselli-Flores A. Nishri (&) Israel Oceanographic & Limnological Research, The Yigal Allon Kinneret Limnological Laboratory, P.O. Box 447, 14950 Migdal, Israel e-mail: nishri@ocean.org.il D. P. Hamilton Centre for Biodiversity and Ecology Research, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand e-mail: d.hamilton@waikato.ac.nz 123 Hydrobiologia (2010) 655:109–119 DOI 10.1007/s10750-010-0408-3