Contrasting Patterns of Net- and Nanoplankton Production and Biomass among Lakes

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Abstract
The relative contribution of nanoplankton to total algal biomass was shown to be negatively correlated with total phosphorus during summer over a wide range of lakes. We hypothesize that this pattern can be explained either by changes in the relative growth rates of net- or nanoplankton with increased nutrient levels, or through the regulation of the nanoplankton biomass by herbivores, allowing disproportionate increases in netplankton biomass in more eutrophic systems. These explanations were tested by examining the relationships between net primary production (grams of carbon per cubic metre per year) and biomass (micrograms per litre) of these size fractions and total phosphorus concentrations (micrograms per litre) using literature data from lakes through-out the world. The production of the net- and nanoplankton vary similarly with total phosphorus, while the biomasses (and thus the ratios of production to biomass) of the two groups show significantly different relationships. Our results support the hypothesis that changes in the size distribution of summer phytoplankton with enrichment seem to be influenced more by herbivores than by changes in the relative growth rates of the two size fractions.