in the south-western part of the Netherlands. Freshwater discharge into the Oosterschelde
basin is regulated by sluices at the land-side, while at the sea-side the construction of a
storm surge barrier is going on. Lake Grevelingen, is a former basin now surrounded by
dikes. However its saline character is maintained by sluices at the sea-side. Sampling
stations LGPK ans Kr5 are situated in tidal channels in shallow parts of the Oosterschelde
basin at mean depths of 19 and 8 m, respectively. At low tides, these stations are
surrounded by intertidal mud-flats. Sampling stations Kats and KtA are located in the
main tidal channels halfway the basin at mean depths of 34 and 22 m, respectively.
Sampling station R14 is situated at the mouth of the basin at a mean depth of 35 m.
Sampling station G17 is located in Lake Grevelingen at a depth of 20 m.
METHODS
From October 1983 till October 1984, samples were taken every three weeks at each
sampling station from 1.00 m below the surface of the water. Sampling was done
independently from the tide, since preliminary experiments did not show a correlation
between the tidal period and the amount of oxygen consumed. Reverse filtration, described by Williams (1981 b), started immediately after sampling. Only Nuclepore polycarbonate filters with a pore size of 3 fim and a diameter of 142 mm were used. The filters were
used once, since multiple utilization led to a manifold increase in the oxygen consumption
rates measured. The hydrostatic pressure was always kept below 10 cm water, yet
filtration for respiration studies was always finished within 15 minutes. Filtration of
larger volumes of samples for chlorophyll a analyses took more time. Samples for
determination of oxygen comsumption rates were incubated in the dark at in situ
temperatures for 24 and 48 hours in summer and in winter, respectively. Oxygen concentrations were measured according to the method of Bryan et al. (1976). The number of
free-living as well as attached bacteria and the mean cell volumes were determined by
epifluorescence microscopy as described by Hobbie et al. (1977). The amount of bacterial
cell-carbon was determined by multiplying the measured mean cell-volume by a conversion factor of 121 fgC.µUm -3 given by Watson et al. (1977). Chlorophyll a was determined
by HPLC.
Bacterial contribution to overall pelagic oxygen consumption rates (B c ) was calculated
according to the formula:
R
C - A
b c =
B - A
where A, B and C are the percentages of chlorophyll a, bacterial cells and oxygen
consumption rates that passed a 3 µm polycarbonate filter respectively.
RESULTS AND DISCUSSION
Overall pelagic oxygen consumption rates were measured regularly every three weeks in
water samples taken from 1 m below the surface of the water in the marine Oosterschelde
basin and the saline lake Grevelingen. Oxygen uptake rates varied between 20 and 1000µg
l -1 d -1 in winter and summer, respectively. No significant differences were observed
between the data from the basin and the lake (manuscript in preparation). Reversed
filtration was applied to elucidate the role of bacteria in overall oxygen consumption
rates. The percentages of overall oxygen consumption rates recovered in the 3 µm
fractions varied between 10% and more than 100% in the individual samples, but the
averages were rather constant over the year for both the marine basin and the saline lake
studied (Tab 1).
192
basin is regulated by sluices at the land-side, while at the sea-side the construction of a
storm surge barrier is going on. Lake Grevelingen, is a former basin now surrounded by
dikes. However its saline character is maintained by sluices at the sea-side. Sampling
stations LGPK ans Kr5 are situated in tidal channels in shallow parts of the Oosterschelde
basin at mean depths of 19 and 8 m, respectively. At low tides, these stations are
surrounded by intertidal mud-flats. Sampling stations Kats and KtA are located in the
main tidal channels halfway the basin at mean depths of 34 and 22 m, respectively.
Sampling station R14 is situated at the mouth of the basin at a mean depth of 35 m.
Sampling station G17 is located in Lake Grevelingen at a depth of 20 m.
METHODS
From October 1983 till October 1984, samples were taken every three weeks at each
sampling station from 1.00 m below the surface of the water. Sampling was done
independently from the tide, since preliminary experiments did not show a correlation
between the tidal period and the amount of oxygen consumed. Reverse filtration, described by Williams (1981 b), started immediately after sampling. Only Nuclepore polycarbonate filters with a pore size of 3 fim and a diameter of 142 mm were used. The filters were
used once, since multiple utilization led to a manifold increase in the oxygen consumption
rates measured. The hydrostatic pressure was always kept below 10 cm water, yet
filtration for respiration studies was always finished within 15 minutes. Filtration of
larger volumes of samples for chlorophyll a analyses took more time. Samples for
determination of oxygen comsumption rates were incubated in the dark at in situ
temperatures for 24 and 48 hours in summer and in winter, respectively. Oxygen concentrations were measured according to the method of Bryan et al. (1976). The number of
free-living as well as attached bacteria and the mean cell volumes were determined by
epifluorescence microscopy as described by Hobbie et al. (1977). The amount of bacterial
cell-carbon was determined by multiplying the measured mean cell-volume by a conversion factor of 121 fgC.µUm -3 given by Watson et al. (1977). Chlorophyll a was determined
by HPLC.
Bacterial contribution to overall pelagic oxygen consumption rates (B c ) was calculated
according to the formula:
R
C - A
b c =
B - A
where A, B and C are the percentages of chlorophyll a, bacterial cells and oxygen
consumption rates that passed a 3 µm polycarbonate filter respectively.
RESULTS AND DISCUSSION
Overall pelagic oxygen consumption rates were measured regularly every three weeks in
water samples taken from 1 m below the surface of the water in the marine Oosterschelde
basin and the saline lake Grevelingen. Oxygen uptake rates varied between 20 and 1000µg
l -1 d -1 in winter and summer, respectively. No significant differences were observed
between the data from the basin and the lake (manuscript in preparation). Reversed
filtration was applied to elucidate the role of bacteria in overall oxygen consumption
rates. The percentages of overall oxygen consumption rates recovered in the 3 µm
fractions varied between 10% and more than 100% in the individual samples, but the
averages were rather constant over the year for both the marine basin and the saline lake
studied (Tab 1).
192