A correlation between the percentages of attached bacteria and the amounts of suspended
particles was also observed in the Humber estuary (Goulder, 1977: Bent and Goulder,
1981). The relatively low amounts of seston in the Oosterschelde basin in comparison to
estuaries might be explained by the low discharge of freshwater into the basin. The higher
percentages found in the winter period might reflect a larger freshwater discharge as well
as more rough weather conditions at this time of the year.
The relatively high percentages of attached bacteria both in the Oosterschelde basin as
well as in Lake Grevelingen during the winter period may also reflect poor growth
conditions in the water. For example, carbon limitation may promote adhesion of
bacterial cells to particles (Brown et al., 1977).
The presence of active small phytoplankton cells in the bacterial size fraction may
overestimate the bacterial contribution to overall pelagic oxygen consumption rates
calculated from size fractionation studies. For this reason analysis of chlorophyll a was
included in the studies. The percentages of the total amounts of chlorophyll a recovered in
the 3 µm fraction are summarised in Table 5.
Relatively high amounts of chlorophyll a passed the polycarbonate filter in water samples
of Lake Grevelingen during the summer period. At least a part of the chlorophyll-a in the
small, bacterial fraction belonged to active phytoplankton cells, since phytoplankton
production was observed in this fraction (de Visscher, unpublished results). The percentages of chlorophyll-a and bacterial cells that passed a 3 µm polycarbonate filter were used
to calculate the bacterial contribution to overall oxygen consumption rates (tab 6).
Respiration of other organisms such as zooplankton was neglected, since their biomasses
are small compared to those of bacteria and algae in both water bodies (C. Bakker, pers.
comm.). Since oxygen consumption rates were determined in five-fold in bottles, a
possible inclusion of zooplankton into these bottles could be observed and excluded from
calculations. The oxygen consumption rate per organism was assumed to be independent
upon free-living of attached, or upon size class.
Hence, without taking into account the activity of attached bacteria and the small algae,
the bacterial contribution to overall, pelagic oxygen consumption rates may be under- or
overestimated when using size fractionation alone. The presence of many bacteria attached to particles had a large impact on the calculated bacterial contribution to overall
pelagic activity in the Oosterschelde basin in the winter period. In Lake Grevelingen, the
measured bacterial activity was slightly overestimated by the presence of phytoplankton
cells in the small, bacterial fraction. Considering the corrected values for bacterial
contribution to overall oxygen consumption rates from Table 6 in more detail, the
difference between the marine Oosterschelde basin and the saline Lake Grevelingen
during the winter period is most striking. Since the levels of primary production are
almost the same in winter in both water bodies (de Visscher, unpublished results), the
difference in bacterial contribution to overall oxygen consumption rates could largely be
explained by resuspension of organic matter from the bottom by the tide in the open
Oosterschelde basin. The calculated bacterial contribution to overall pelagic activity may
have been overestimated due to growth of the bacteria in the oxygen bottles during the
incubations in the dark at in situ temperatures. Changes in bacterial biomass were
calculated from microscopic changes in cell numbers and mean cell volumes during these
incubations (Table 7).
In Lake Grevelingen, a large difference between unfiltered and filtered samples was never
observed with regard to increases of biomass. During the summer period, a large diffe195
particles was also observed in the Humber estuary (Goulder, 1977: Bent and Goulder,
1981). The relatively low amounts of seston in the Oosterschelde basin in comparison to
estuaries might be explained by the low discharge of freshwater into the basin. The higher
percentages found in the winter period might reflect a larger freshwater discharge as well
as more rough weather conditions at this time of the year.
The relatively high percentages of attached bacteria both in the Oosterschelde basin as
well as in Lake Grevelingen during the winter period may also reflect poor growth
conditions in the water. For example, carbon limitation may promote adhesion of
bacterial cells to particles (Brown et al., 1977).
The presence of active small phytoplankton cells in the bacterial size fraction may
overestimate the bacterial contribution to overall pelagic oxygen consumption rates
calculated from size fractionation studies. For this reason analysis of chlorophyll a was
included in the studies. The percentages of the total amounts of chlorophyll a recovered in
the 3 µm fraction are summarised in Table 5.
Relatively high amounts of chlorophyll a passed the polycarbonate filter in water samples
of Lake Grevelingen during the summer period. At least a part of the chlorophyll-a in the
small, bacterial fraction belonged to active phytoplankton cells, since phytoplankton
production was observed in this fraction (de Visscher, unpublished results). The percentages of chlorophyll-a and bacterial cells that passed a 3 µm polycarbonate filter were used
to calculate the bacterial contribution to overall oxygen consumption rates (tab 6).
Respiration of other organisms such as zooplankton was neglected, since their biomasses
are small compared to those of bacteria and algae in both water bodies (C. Bakker, pers.
comm.). Since oxygen consumption rates were determined in five-fold in bottles, a
possible inclusion of zooplankton into these bottles could be observed and excluded from
calculations. The oxygen consumption rate per organism was assumed to be independent
upon free-living of attached, or upon size class.
Hence, without taking into account the activity of attached bacteria and the small algae,
the bacterial contribution to overall, pelagic oxygen consumption rates may be under- or
overestimated when using size fractionation alone. The presence of many bacteria attached to particles had a large impact on the calculated bacterial contribution to overall
pelagic activity in the Oosterschelde basin in the winter period. In Lake Grevelingen, the
measured bacterial activity was slightly overestimated by the presence of phytoplankton
cells in the small, bacterial fraction. Considering the corrected values for bacterial
contribution to overall oxygen consumption rates from Table 6 in more detail, the
difference between the marine Oosterschelde basin and the saline Lake Grevelingen
during the winter period is most striking. Since the levels of primary production are
almost the same in winter in both water bodies (de Visscher, unpublished results), the
difference in bacterial contribution to overall oxygen consumption rates could largely be
explained by resuspension of organic matter from the bottom by the tide in the open
Oosterschelde basin. The calculated bacterial contribution to overall pelagic activity may
have been overestimated due to growth of the bacteria in the oxygen bottles during the
incubations in the dark at in situ temperatures. Changes in bacterial biomass were
calculated from microscopic changes in cell numbers and mean cell volumes during these
incubations (Table 7).
In Lake Grevelingen, a large difference between unfiltered and filtered samples was never
observed with regard to increases of biomass. During the summer period, a large diffe195