PHOSPHATE AND IRON LIMITATION OF PHYTOPLANKTON BIOMASS IN LAKE TAHOE

Field measurements and bioassay experiments were coupled to investigate the interdependent processes affecting phytoplankton biomass at Lake Tahoe using a trace metal protocol. Water samples were analyzed for suspended particulate matter, dissolved organic carbon, major ions and macronutrients, adenosine triphosphate, and phytoplankton abundance. Concentrations of total Cd (less-than-or-equal-to 18 pM), Cu (2.25-8.85 nM), and Fe (22-49 nM) were similar to or lower than those reported for other oligotrophic lakes. Bioassays were carried out to assess the response of inoculated, single-species diatom populations (Cyclotella meneghiniana and Aulocosiera italica) to additions of synthetic chelators (EDTA, EDDHA) and phosphate. A chemical speciation model along with the field data was also used to predict how trace metal speciation, and hence bioavailability, was affected by the chelator additions. Results suggest that phosphate was limiting to phytoplankton biomass. Other solutes, Fe in particular, may also exert controls on biomass. Nitrate limitation seems less likely, although Fe-limiting conditions, as suggested by the bioassays, may have led to an effective N limitation because algae require Fe to carry out nitrate reduction. Small perturbations in water chemistry may have pronounced effects on phytoplankton biomass in oligotrophic systems where essential nutrients are at low concentrations.