Incorporation rates and biomass content of C and P in phytoplankton and bacteria in the Bay of Aarhus (Denmark) June 1992

Biomass content and incorporation rates for carbon (C) and phosphorus (P) in microbial plankton in the Bay of Aarhus were investigated during a 1 week period. Samples taken daily at a fixed station, as well as during a diel cycle, allowed comparison of the water masses above, in and below the pycnocline, and in influxes of different water masses. The coupling between the activity of phytoplankton and bacteria was examined by the stoichiometric composition in pools and fluxes. From microscope-based estimates of bacterial C and chemical determination of phosphate in the size fraction 1-0.2 μm, the mean C:P ratio (molar) for bacterial biomass was 78, while the mean C:P ratio for incorporation was 96 or 63 from thymidine- and leucine-based estimates of bacterial C production. Thymidine-based estimates of mean bactenal generation time varied from 2.5 to 4.1 days with a minimum in the interface between water of Baltic and Skagerrak origin. These values are in good agreement with P-based generation time estimates. 14C measurements of primary production and orthophosphate incorporation into the size fraction >1 μm gave a mean C:P ratio of 118, similar to the Redfield ratio (106), but about twice the estimated mean C:P of 52 in protist biomass. The combined methodological concept can be used to examine the simultaneous role of microorganisms in the C and P budgets.