Hypereutrophic waters, which are characterized by nutrient inputs exceeding phytoplankton nutrient requirements, are often sites of chronic nuisance algal blooms and associated water quality deterioration problems . In order to restore such systems to acceptable water quality standards, identification of growth-limiting nutrients is of central importance . Conventional nutrient addition bioassay techniques are often ineffective in identifying potentially limiting nutrients, due to persistent nutrient excesses in hypereutrophic systems . Accordingly, we have developed a nutrient dilution bioassay, in which stepwise dilutions of phytoplankton nutrients (nitrogen, phosphorus, iron, trace metals) with a nutrient-free major ion solution are capable of ; 1) identifying those nutrients potentially most limiting, and 2) establishing magnitudes of respective nutrient input cutbacks required to bring about nutrient-limited control of phytoplankton growth . In situ deployment of dilution bioassays should help establish criteria governing minimal nutrient inputs required to arrest undesirable impacts of hypereutrophy . We have evaluated the field applicability of dilution bioassays, during a 2 year trial in the periodically hypereutrophic Neuse River, North Carolina .

Natural populations of the nuisance bloom cyanobacterium Microcystis aeruginosa obtained from the eutrophic Neuse River, N.C., revealed optimal chlorophyll a -normalized photosynthetic rates and resistance to photoinhibition at surface photosynthetically active radiation (PAR) intensities. At saturating PAR levels these populations exhibited higher photosynthetic rates in quartz than in Pyrex vessels. Eucaryotic algal populations obtained from the same river failed to counteract photoinhibition. At saturating PAR levels, such populations generally yielded lower photosynthetic rates in quartz containers than they did in Pyrex containers. Cultivation of natural Microcystis populations under laboratory conditions led to physiologically distinct populations which had photoinhibitory characteristics similar to those of other cultured cyanobacterial and eucaryotic algae. Our findings indicate that (i) photosynthetic production among natural surface populations is best characterized and quantified in quartz rather than Pyrex incubation vessels; (ii) extrapolation of natural photoinhibitory trends from laboratory populations is highly subjective to culture and PAR histories and may yield contradictory results; and (iii) buoyant surface-dwelling populations, rather than exhibiting senescence, are poised at optimizing PAR utilization, thereby maintaining numerical dominance in eutrophic waters when physico-chemical conditions favor bloom formation.

Using high-performance liquid chromatography (HPLC), we showed that spectrophotometric determinations of algal lipophilic pigments are often biased by the choice of extraction solvent. Several commonly used extraction solvents and extraction periods were tested on diverse phytoplankton species isolated from the Neuse River, NC. Extraction solvents included 100% acetone, 90% acetone – 10% distilled deionized water (DDW), 50% acetone – 50% methanol, and 90% methanol – 10% DDW. Extraction efficiencies varied significantly among genera. For the diatom Cyclotella meneghiniana, at least three solvents extracted all lipophilic pigments with equal efficiency, but 50:50 acetone–methanol was clearly superior for the chlorophycean Chlorella vulgaris. A host of differences in extraction efficiencies and extraction times were observed when we compared eukaryotic with prokaryotic (blue-green algae) phytoplankton. We were unable to find a single extraction technique that could consistently provide quantitative and uniform pigment yields from diverse phytoplankton communities. Nevertheless, HPLC techniques can be rapidly and routinely employed to determine appropriate extraction protocols for lipophilic pigments of interest in specific aquatic ecosystems.