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Abstract

Size-fractionated 32PO4-uptake experiments were done with mixed laboratory cultures of P-limited algae and their bacterial contaminants and with field samples from Lake Constance and three small lakes. In the mixed culture samples, tracer uptake was dominated by bacteria, but increasingly shifted toward algae when unlabeled orthophosphate was added. In samples from Lake Constance (2-m depth), the bacterial size fraction (<1 µm) on average accounted for ∼ 50% of the tracer uptake. Small additions of unlabeled phosphate (0.016–0.16 µM) resulted in significant shifts of tracer uptake toward larger size classes (> 1, >3, > 12 µm). We tested some possible causes for nutrient inhomogeneities.32P-labeled Daphnia appeared to release tracer in patchy fashion, favoring algal uptake in mixed culture samples, as well as in field samples from Lake Constance. In Lake Constance, the proportion of 32PO4 uptake by larger size classes (> 3, > 12, > 30 µm) was significantly higher in water from depths of 6 and 10 m than from 2 m. In samples from three small lakes, mixing with nutrient-rich hypolimnetic water favored 32PO4 uptake by algal size classes.