Cuhel, R. L.. Great Lakes Water Institute, rcuhel@uwm.edu
Aguilar, C. L.. Great Lakes Water Institute, aguilar@uwm.edu

 
CAUSES AND CONSEQUENCES OF PLANKTON PHOSPHORUS DEFICIENCY DURING THE 1998 EL NINO IN LAKE MICHIGAN
 
Phosphorus deficiency in phytoplankton is a widespread phenomenon following limitation of mixing brought about by density stratification in aquatic systems. Due to plasticity of algal phosphorus cell quotas, deficiency may not be expressed for several generations in environments with substantial winter-spring concentrations of reactive phosphorus. In Lake Michigan, phosphate is <50nM all year and deficiency becomes pronounced quickly when phytoplankton are denied access to the sediment-water interface by stratification. A depth-time sequence of hydrographic, nutrient, and productivity measurements confirmed onset of stratification more than three weeks earlier than normal, with subsequent evidence of phosphorus deficiency lagging by less than 10 days. Deficiency was greatest in the mixed layer and decreased progressively into the physically stable thermocline zone. Slower development of deficiency near the bottom of the thermocline is consistent with slower, light-limited growth rates. Available phosphate was absent throughout the water column. Sub-thermocline populations retained photosynthetic capacity without phosphorus deficiency for at least two months after stratification. In the laboratory, deep water held in light or dark conditions developed deficiency only in the light after a lag of similar scale to lake populations. Collectively these observations help to constrain the circulation rate for pre-stratified spring Lake Michigan.
 
Day: Thursday, Feb. 4
Time: 09:45 - 10:00am
Location: Sweeney Center
 
Code: SS39TH0945S