Relative importance of iron and molybdenum in restricting phytoplankton biomass in high phosphorus saline lakes

Abstract

Eleven prairie saline (conductivity 1.8–58.8 mS cm−1) lakes were examined over the 1994 growing season to determine what salinity-related factor or factors were responsible for controlling phytoplankton standing 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 (55–2,800 µg liter−1), dissolved organic C (40–195 mg liter−1), pH and alkalinity, but comparatively low (usually <100 µg liter−1) dissolved inorganic N. Chlorophyll a (Chl a) concentrations in all but the two least saline lakes were relatively low (20 µg liter−1), up to three orders of magnitude below those predicted by freshwater P-based models. High alkaline phosphatase activities (APA) and rapid 32PO4 (orthophosphate) uptake indicated that the two least saline lakes were P limited; these lakes had seston deficient in P, N, and protein. APA and 32PO4 uptake were below detection in the more saline lakes (conductivity >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 bioassays indicated that phytoplankton biomass was not limited exclusively 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 bioassays resulted in a remarkable increase in Chl a concentrations. Addition 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 trace elements (but not Mo). Reducing the alkalinity of the bioassay water stimulated growth in the same manner as the Fe additions, suggesting that the bioavailability of the (largely particulate) Fe already present was severely restricted by lake-water alkalinity. Some component of lake-water alkalinity (which increased with conductivity in these lakes) appears to be the key factor limiting Fe bioavailability and restricting phytoplankton standing crops in the higher salinity lakes.