Responses of phosphorus limited Lake Michigan phytoplankton to factorial enrichments with nitrogen and phosphorus1

Factorial nutrient enrichment expcrimcnts were conducted with water containing natural phytoplankton assemblages collected from Grand Traverse Bay. Levels of phosphorus (phosphate) were 5, 15, 25, 40, and 60 pg P liter4 and levels of nitrogen (nitrate) were 0.23, 0.84, and 1.12 mg N liter-l. Nutrients were maintained at these levels by additions at 2or S-day intervals. Two different analyses of variance tests indicated that the responses were due to effects of phosphorus and not nitrogen. Production of chlorophyll a at the four highcst levels of phosphorus was indcpcndent of concentration and could bc predicted by In Chl a = XXX -t 0.153t whcrc XXX was the In of Chl a at day 4 and t was time in days. Phytoplankton growth, measured as chlorophyll production, fitted a Michaclis-Mentcn model when phosphorus concentrations were adjusted to correct for chemically measurable phosphorus apparently not available to phytoplankton. Assimilation numbers and species composition were not affected greatly by the nitrogen and phosphorus cnrichmcnts. Phosphorus limits the growth of phytoplankton in Lake Michigan (Schclske and Stoermcr 1972). Inputs of phosphorus have increased in the past 30 years, producing larger standing crops of algae and depleting the supply of silica required by phytoplankton assemblages dominated by diatoms (Schelskc and Stoermer 1971). We predicted that continued silica depletion would limit the growth of diatoms and gradually cause a shift from assemblages dominated by diatoms to those dominated by blue-green and green algae, assuming that the effects of nitrogen were small, since silica was depleted by small additions of phosphorus. Since the effects of nitrogen on growth of phytoplankton in Lake Michigan had not been investigated directly, we designed a factorial experiment to study the effects and ’ Contribution No. 175 of the Great Lakes Rcsearch Division, The University of Michigan. Support provided by The University of Michigan Sea Grant Program and United States Atomic Energy Commission Contract COO-2003-21. interactions of nitrogen and phosphorus. Because the effects of nitrogen additions were small relative to those for phosphorus it was possible to USC a Michaelis-Menten model to calculate growth kinetics as a function of phosphorus levels. Although the Michaelis-Menten model has been used in kinetic studies of nitrogen limited natural phytoplankton assemblages in the oceans (Thomas 1970; MacIsaac and Dugdale 1969; Epplcy and Thomas 1969), which have recently been extended to interactions of light and nitrogen (MacIsaac and Dugdale 1972)) we have not seen published studies on growth kinetics for natural assemblagcs limited by phosphorus. We would like to acknowledge C. Matthews, T. Ladewski, W. Moore, L. Feldt, S. Fielck, N. McLean, K. White, L.-F. Ilwang, T. Scullin, and P. Gimotty for their assistance.