TITLE: Mo & Fe Influences on Nitrate Assimilation in Lake Ontario and the St Lawrence River. AUTHORS (FIRST NAME ... Nathaniel E Ostrom2. INSTITUTIONS ...
TITLE: Mo & Fe Influences on Nitrate Assimilation in Lake Ontario and the St Lawrence River AUTHORS (FIRST NAME, LAST NAME): Michael R Twiss1, Kateri Salk2, Lindsay N Avolio1, Anthony Chappaz3, Nathaniel E Ostrom2 INSTITUTIONS (ALL): 1. Dept Biology, Clarkson Univ., Potsdam, NY, United States. 2. Dept Zoology, Michigan State Univ., East Lansing, MI, United States. 3. Inst for Great Lakes Res, Central Michigan Univ, Mount Pleasant, MI, United States. ABSTRACT BODY: Lake Ontario has undergone a steady increase in nitrate since the early 1970s, a phenomenon also occurring in other large lakes. Possible causes of this increase include rising urban and agricultural runoff, atmospheric deposition, less demand for N due to effective point source P control, and trace metal-N co-limitation as observed in Lake Erie. Despite the abundance of nitrate in Lake Ontario, heterocystous cyanobacteria have been detected here setting up the paradoxical situation wherein some cyanobacteria are investing in the more costly diazotrophy whilst surrounded by a form of N that requires less energy and metal quota to assimilate. Mo and Fe are involved in reductive nitrate assimilation making it possible that reductive nitrate assimilation in Lake Ontario is limited in phytoplankton by low trace metal bioavailability. To test this hypothesis, 1-d enrcihment experiments were conducted using trace metal clean techniques in June 2013 at two coastal sites in Lake Ontario, and 4-d enrichment experiments were conducted in July 2013 on main channel waters of the St. Lawrence River, the outflow of Lake Ontario. Water was sampled from the metalimnion of Lake Ontario and from surface water of the main channel of the river. Water was enriched with the the following treatments in triplicate: control, 100 nM KH2PO4, 50 nM FeCl3, 50 nM Na2MoO4, and a mix of P, Fe & Mo. Experiments in the river showed significant effects due to P (increase in Chl-a, NO3 and SiO2 drawdown, changes in phytoplankton community, increase in photosynthetic efficiency [Fv/Fm]) but less impact of trace metals relative to control, presumably due to greater ambient trace metal bioavailability. As measured using FluoroProbe, the phytoplankton community changed very little (over 1 d) in lake waters; there was no significant change in total chl-a. However, as in the river, Fv/Fm revealed significant metal and P effects with the P, Fe & Mo mix being significantly greater than control (Fig. 1; we will present data relating to the response of PON and POC pools to the enrichment experiments). The results suggest that P was limiting phytoplankton growth and that Mo and Fe were more co-limiting in the lake than in the river. This information builds on prior work demonstrating Fe/N colimitation and is the first study to investigate the role of Mo in the N cycle in the Laurentian Great Lakes.
