Fractionated phytoplankton primary production, exudate release and bacterial production in a Baltic eutrophication gradient

Abstract

The distribution of phytoplankton primary production into four size fractions (>10 μm, 10-3 μm, 3-0.2 μm and <0.2 μm), the utilization of algal exudates by bacteria and the bacterial production were studied in a eutrophication gradient in the northern Baltic proper. The polluted area exhibits substantially increased nutrient, especially nitrogen, levels while only minor differences occur in salinity and temperature regimes. Total primary production was 160 g C · m-2 · yr-1 at the control station and about 275 g C · m-2 · yr-1 at the eutrophicated stations. The estimated total exudate release was 16% of the totally fixed 14CO2 in the control area and 12% in the eutrophicated area (including the estimated bacterial uptake of exudates). The difference in14CO2 uptake rates between incubation of previously filtered water (<3, <2, <1 μm) and unfiltered water was used to estimate bacterial uptake of phytoplankton exudates which were found to contribute about half of the estimated bacterial carbon requirement in both areas. Bacterial production was estimated by the frequency of dividing cells (FDC) method as being 38 g C · m-2 · yr-1 at the control station and 50 g C · m-2 · yr-1 at the eutrophicated stations. To estimate the mean in situ bacterial cell volume a correlation between FDC and cell volume was used. The increased annual primary production in the eutrophicated area was due mainly to higher production during spring and autumn, largely by phytoplankton cells (mainly diatoms) retained by a 10 μm filter. Primary production duringsummer was similarin the two areas, as was the distribution on different size fractions. This could possibly explain the similar bacterial production in the trophic layers at all stations since the bulk of bacterial production occurs during summer. It was demonstrated that selective filtration does not quantitatively separate photoautotrophs and bacteria. A substantial fraction of the primary production occurs in the size fraction <3 μm. The primary production encountered in the 3-0.2 μm fraction was due to abundant picoplankton (0.5 to 8 · 107 ind · l-1), easily passing a 3 μm filter. The picoplankton was estimated to constitute up to 25% of the total phytoplankton biomass in the control area and up to 10% in the eutrophicated area.