Marino, R. Cornell University, rmm3@cornell.edu
Howarth, R. Cornell University, rwh2@cornell.edu
Chan, F. Cornell University, fc19@cornell.edu
Pelletier, D. M.. Cornell University, dmp25@cornell.edu
Pace, M. L.. Institute of Ecosystem Studies, pacem@ecostudies.org

 
AN EXPERIMENTAL TEST OF THE EFFECT OF LOW MOLYBDENUM AVAILABILITY IN SEAWATER AS A CONTROL ON PLANKTONIC N FIXATION IN ESTUARIES
 
In sharp contrast to lakes, N fixation by planktonic cyanobacteria rarely occurs in estuaries even when strongly N limited. We have hypothesized that this results from an interaction of bottom-up and top-down controls: low availabilities in seawater of micronutrients required for N fixation (Mo, Fe) lead to slow growth of heterocystic, planktonic cyanobacteria, thus making them highly vulnerable to top-down control by grazers. In a mesocosm experiment using seawater from Narragansett Bay, RI, we tested our hypothesis using a 2 X 2 treatment matrix with Mo and grazing pressure. We found that both Mo availability and grazing pressure affected rates of planktonic N fixation, and that these two controls interacted so that highest rates of N fixation occurred when Mo availability was high and grazing pressure was low. We also observed major effects on the stoichiometric relationship between N and P. Both high Mo availability and low grazing pressure led to significant decreases in SRP and to significant increases in dissolved and total N. Consequently, these factors, through regulation of N fixation, may have an important influence on whether an estuary is N or P limited.
 
Day: Tuesday, Feb. 2
Time: 02:45 - 03:00pm
Location: Sweeney Center
 
Code: SS49TU0245S