ASSESSMENT OF PHYTOPLANKTON NUTRIENT LIMITATION IN PRODUCTIVE WATERS - APPLICATION OF DILUTION BIOASSAYS

Excessive nutrient loads to aquatic systems can complicate otherwise p redictable relationships between nutrient concentrations and phytoplan kton biomass. We conducted six bioassays on surface phytoplankton asse mblages collected from productive Lake Apopka, Florida, to measure the effect of nutrient reduction on phytoplankton growth and nutritional state. Lake water was mixed with one of three diluents to create a gra dient of ambient nutrient concentrations; nitrogen (N) and phosphorus (P) limitation at each level of dilution was evaluated in a 2 x 2 fact orial design. While the addition of N clearly increased the growth of phytoplankton in undiluted Lake Apopka water, the phytoplankton became more P limited with the reduction of particles (30-60% dilution). Reg ression of algal yields onto total P concentrations from our bottle ex periments indicated that an 8 mu g.L(-1) change in P leads to only a 1 mu g.L(-1) change in chlorophyll yield, probably due to the high conc entration of P in the lake. Because dilution influences factors in add ition to ambient nutrient concentrations, results obtained with the te chnique must be carefully evaluated. Despite this, reduction of partic les to improve water quality may, in concept, be a reasonable manageme nt scheme in lakes where a large fraction of the nutrients is particul ate.