Eutrophication and self-purification: counteractions forced by large-scale cycles and hydrodynamic processes.

During the 1990s nitrogen reserves have shown rising trends in the northern Baltic Sea. Changes in denitrification explain some of the fluctuations observed in nitrogen reserves. Although denitrification is an anaerobic process, the most efficient removal of nitrogen by denitrification occurs where the sediment is moderately well oxidized. A dramatic decrease in the ratio of SiO4 to DIN (DIN = NO3 + NH4) in the northern Baltic proper during the period 1973-1999 was recorded. If dissolved silica limits phytoplankton spring bloom, diatom blooms fade and become replaced by flagellates leading to changes in summer phosphate reserves and sedimentation. Seven years have elapsed from the previous strong saltwater inflow and anoxia has spread over large areas. Deep bottom phosphorus levels have started to increase and denitrification capacity is weakening. Thus, all efforts to minimize N- and P-loading will, in the long run, help the Baltic Sea to recover from the unacceptable status of eutrophication.

Horizontal variation of bacterioplankton in the baltic sea.

Variations in the thymidine incorporation rate, bacterial abundance, and mean cell volumes in the surface water (0.5 m) of the Baltic Sea in spring and summer were compared in studies with different spatial scales (570 nautical miles [nmi] [ca. 1056 km], 220 nmi [ca. 407 km], 24 nmi [ca. 44 km], 12 nmi [ca. 22 km], and 200 m). The objective of the comparison was to investigate whether a single sample taken at one sampling point is representative enough for researchers to make generalizations about a larger water area. Bacterioplankton variation was connected more to seasonal characteristics than to the spatial scale of sampling. Variation was greater and more random in spring than in summer. The state variables (bacterial abundance and mean cell volume) varied less than the rate variable (thymidine incorporation). The results suggest that the sampling design for bacterioplankton studies in northern temperate seas should be planned primarily according to the season and that more stress should be put on rate variable measurements than on those of state variables.

Assessing phytoplankton and bacterioplankton production during early spring in lake erken, sweden.

The spring development of both phytoplankton and bacterioplankton was investigated between 18 April and 7 May 1983 in mesotrophic Lake Erken, Sweden. By using the lake as a batch culture, our aim was to estimate, via different methods, the production of phytoplankton and bacterioplankton in the lake and to compare these production estimates with the actual increase in phytoplankton and bacterioplankton biomass. The average water temperature was 3.5 degrees C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The C and O(2) methods of estimating primary production gave equivalent results (r = 0.999) with a photosynthetic quotient of 1.63. The theoretical photosynthetic quotient predicted from the C/NO(3) N assimilation ratio was 1.57. The total integrated incorporation of [C]bicarbonate into particulate material (>1 mum) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 x 10 to 1.52 x 10 cells liter ( approximately 0.5 mug of C liter day). Estimates of bacterioplankton production from rates of [H]thymidine incorporation were ca. 1.2 to 1.7 mug of C liter day. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 mug of C liter day, indicating a bacterial growth yield of 25%. The bulk of the bacterioplankton production was accounted for by algal extracellular products. Gross bacterioplankton production (production plus respiration) was 20% of gross primary production, per square meter of surface area. We found no indication that bacterioplankton production was underestimated by the [H]thymidine incorporation method.

Respiration of tritiated substrates in heterotrophic activity assays.

The respiration percentage of H-labeled glucose by heterotrophic microorganisms in water samples was determined by a method based on equimolar addition of [H]- and [C]glucose to samples. The respiration percentage of [H]glucose exceeded that of [C]glucose by 10% over the assayed range of respiration rates. Respiration percentages diminished with decreasing temperature, the relation between respiration of [H]- and [C]glucose remaining the same. The significance of tritiated water respiration with regard to experimental design is emphasized.