Size fractionation of algal chlorophyll, carbon fixation and phosphatase activity: relationships with species-specific size distributions and zooplankton community structure

Algal chlorophyll, carbon fixation and alkaline phosphataseactivity were net-fractionated using 22-µm, and 75-µmscreens in three lakes with contrasting zooplankton communities.Size distributions of algal biovolume were also determined throughmicroscopic examination. Relatively good correspondence wasfound between size distributions obtained through net fractionationand those determined by microscopic examination. Biovolume-specificcarbon fixation and chlorophyll decreased with increasing fractionsize but no differences were observed among fractions for chlorophyll-specificcarbon fixation. High algal standing stocks and low phosphorusdeficiency in Tuesday Lake were attributed to low grazing pressureby small, inefficient zooplankton and possible limitation bynutrients other than phosphorus. Algal standing stocks werelow and phosphorus deficiency was high in Peter and Paul Lakes,in which the zooplankton was dominated by large grazers. Differentalgal size fractions experienced differing degrees of phosphorusdeficiency. These size fraction differences in P-deficiencyin Peter and Paul Lakes were attributed to differences in algalspecies composition and to differing levels of zooplankton grazingpressure and nutrient regeneration. A unimodal relationshipbetween relative nanoplankton biovolume and zooplankton biomasswas found and reflects the positive (nutrient regeneration)and negative (grazing mortality) effects of zooplankton on thealgal community.