Identification of factors constraining nitrate assimilation in Lake Superior, Laurentian Great Lakes

Despite a well-documented rise in nitrate concentration over the past century, Lake Superior has retained an oligotrophic character. In part, this status results from physical attributes of the lake including low temperatures and prolonged isothermy, resulting in deep-mixing and light limitation which constrain primary production. Lake Superior is also phosphorus deficient which limits phytoplankton growth. We conducted large (20 l) volume factorial bioassay experiments to assess the influence of light and nutrients (P, Fe) on nitrate assimilation by a Lake Superior chlorophyte alga. Bioassays seeded with the chlorophyte yielded a strong response to light resulting in the rapid depletion of nitrate. High light resulted in higher activities of the key N-assimilation enzyme nitrate reductase (NR) and increased algal biomass compared to low light treatments. NR activity was highly correlated with rates of nitrate incorporation in bioassays and field surveys suggesting that NR occupies a critical place in nitrate metabolism. In bioassays, the addition of nutrients (P, Fe) only slightly increased the rate at which nitrate became depleted. Parallel trials using a luminescent cyanobacterial bioreporter confirmed the lack of response by added nutrients supporting light as an important factor in constraining nitrate assimilation by phytoplankton in the lake.