Microbial phosphorus-cycling, with particular emphasis on algal/bacterial competition, was studied in depth profiles through the halocline separating a brackish top layer, rich in nitrate and poor in phosphate, from the underlying coastal water, poor in both nitrate and phosphate, in the Sandsfjord area, western Norway. At 2 stations along the axis of natural freshwater outflow, physiological P-deficiency of algae and bacteria in the brackish layer was inferred from rapid luxury consumption of added PO: -by organisms in size fractions > l pm and 1-0.2 pm, respectively. In a branch of the fjord without natural freshwater outlets, luxury consumption in the brackish layer was less, and without a clear difference between the 2 water layers. High luxury consumption coincided with short (< 30 min) turnover times for ~0 2 -a n d strong bacterial dominance of 32P0,"--uptake, suggesting bacterial superiority as competitors during P-limiting conditions. Estimation of P-specific maximum uptake rate and affinity for PO:--uptake from isotope dilution experiments indicated bacterial superiority at low and algal superiority at high concentrations. Although most of the 32P hydrolyzed from added y -~T 3 2 P was initially liberated as free 32P04, partitioning of incorporated 32P between size fractions > l pm and 1-0.2 pm was found to depend on whether label was added as y -~T " P or as 3 2~0 , , -, indicating that coupling of uptake to hydrolysis by cell-bound enzymes could modify the outcome of algalbacterial phosphorus competition. Disappearance rate of "P from the 1-0.2 pm size fraction following initial labeling and a subsequent cold chase with PO:-, was used to estimate the flow-rate of phosphorus through the microbial food web. Combined with measured kinetic constants for PO: -uptake. alkaline phosphatases and 5'nucleotidases, a coherent flow-scheme could only be obtained assuming very low (< 1 nmol I-') concentrations of PO;'-and nucleotides. Chemically measured concentrations of dissolved organic phosphorus (DOP) more than 2 orders of magnitude above the estimated nucleotide level and with an estimated turnover time of ca 500 h, are consistent with the view that this large Preservoir consists mainly of slowly hydrolyzable polymers.