RELATIVE IMPORTANCE OF IRON AND MOLYBDENUM IN RESTRICTING PHYTOPLANKTON BIOMASS IN HIGH PHOSPHORUS SALINE LAKES

Eleven prairie saline (conductivity 1.8-58.8 mS cm(-1)) lakes were exa mined over the 1994 growing season to determine what salinity-related factor or factors were responsible for controlling phytoplankton stand ing crops. The study lakes were characterized by high total P (0.15-24 .2 mg liter(-1)), total N (3.75-12.35 mg liter(-1)), total Fe (55-2,80 0 mu g liter(-1)), dissolved organic C (40-195 mg liter(-1)), pH and a lkalinity, but comparatively low (usually <100 mu g liter(-1)) dissolv ed inorganic N. Chlorophyll a (Chi a) concentrations in all but the tw o least saline lakes were relatively low (20 mu g liter(-1)), up to th ree orders of magnitude below those predicted by freshwater P-based mo dels. High alkaline phosphatase activities (APA) and rapid (PO4)-P-32 (orthophosphate) uptake indicated that the two least saline lakes were P limited these lakes had seston deficient in P, N, and protein. APA and (PO4)-P-32 uptake were below detection in the more saline lakes (c onductivity >3 mS cm(-1)), indicating P sufficiency; seston from these lakes was deficient in N but not protein. Nitrogen-fixing cyanophytes were important only in one of the lakes examined. Nutrient addition b ioassays indicated that phytoplankton biomass was not limited exclusiv ely by inorganic N availability, nor by a combination of Mo and N. For water from all but one of the P-sufficient lakes, addition of Fe to b ioassays resulted in a remarkable increase in Chi a concentrations. Ad dition of Fe and Mo had the same effect as that of Fe alone, while the most saline lake appeared to be limited by one or more additional tra ce elements (but not Mo). Reducing the alkalinity of the bioassay wate r stimulated growth in the same manner as the Fe additions, suggesting that the bioavailability of the (largely particulate) Fe already pres ent was severely restricted by lake-water alkalinity. Some component o f lake-water alkalinity (which increased with conductivity in these la kes) appears to be the key factor limiting Fe bioavailability and rest ricting phytoplankton standing crops in the higher salinity lakes.