Phosphorus Deficiency of Lake Ontario Plankton

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Abstract
The phosphate deficiency index (PDI), the ratio of photosynthetic carbon assimilation at optimal light to the maximum uptake velocity for phosphate, showed that P deficiency occurred in the near-shore plankton of Lake Ontario during spring thermal bar conditions and in the midlake plankton immediately after stratification. This condition persisted until October, when deep mixing of the water column occurred. Carbon and phosphate assimilation were by similar size classes, and as the ratio of two rate processes, PDI was independent of both temperature and biomass. Turnover time and alkaline phosphatase activity (APA) correlated with PDI. Short-term photosynthetic depression with phosphate enrichment occurred in only 7 of 23 experiments and may either reflect P-sufficient conditions or high protein to carbohydrate ratios due to physical events common to large lakes. PDI, protein to carbohydrate ratios, and photosynthetic depression respond principally to algal activity. Although turnover time and APA reflect the activity of smaller microorganisms, primarily heterotrophic bacteria, all indices were consistent.

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