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

Factorial nutrient enrichment experiments were conducted with water containing natural phytoplankton assemblages collected from Grand Traverse Bay. Levels of phosphorus (phosphate) were 5, 15, 25, 40, and 60 µ g P liter −1 and levels of nitrogen (nitrate) were 0.23, 0.84, and 1.12 mg N liter −1 . Nutrients were maintained at these levels by additions at 2‐ or 3‐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 highest levels of phosphorus was independent of concentration and could be predicted by urn:x-wiley:00243590:media:lno19741930409:lno19741930409-math-0001 where XXX was the ln of Chl a at day 4 and t was time in days. Phytoplankton growth, measured as chlorophyll production, fitted a Michaelis‐Menten 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 enrichments.