Nitrate uptake by phytoplankton and periphyton: Whole-lake enrichments and mesocosm 15N experiments in an oligotrophic lake

Periphyton nitrate uptake and denitrification, not phytoplankton assimilation, accounted for-90 % and 73 % of the depletion of- 35 pg N03-N liter-l added as whole-epilimnion enrichments of ammonium nitrate to Castle Lake, California, in midsummer 1980 and 198 1. The importance of benthic processes is suggested by similar rates of depletion for nitrate and ammonium in the whole-lake experiments; these rates are inconsistent with previous microcosm studies of phytoplankton that showed strong preferential uptake of ammonium and low uptake rates of nitrate. The importance of the benthic pathways was confirmed by an NH,15N03 mesocosm experiment, which simulated the whole-epilimnion experiments. Less than 9 % of the 15N transformed was associated with water-column pools; the remainder was incorporated into epipelic periphyton and sediments (56%) or attributed to 15Nz J5Nz0 loss via denitrification. Historical analysis of nitrate depletion in the epilimnion during spring suggests that periphyton may outcompete phytoplankton for water-column nutrients. Since about 1980 there has been an increasing appre-ciation of the potential role of nitrogen deficiency in reg-ulating phytoplankton productivity in many lakes (e.g. Axler et al. 1982; Elser et al. 1990). Unproductive tem-perate lakes, which have low external loading of both N and P, appear likely to exhibit either N deficiency or colimitation with P during much of the growing season (e.g. Axler et,al. 1994; Dodds et al. 1989; Morris and Lewis 1988). Productive lakes, such as those influenced by high inputs of natural or anthropogenic P, may often develop populations of N,-fixing cyanobacteria during the midsummer period of lowest external N availability (e.g. Schindler 1977). Various other cofactors have been invoked to explain why planktonic N2 fixation is generally insignificant in less productive systems (e.g. dissolved organic C, Fe, pH-CO,-alkalinity, temperature, etc.; cf