174
U. Seeliger
12.4 Energy Flow
12.4.1 Primary Production Cycles
The unpredictable amplitude and timing and pronounced inter-annual
variability of physico-chemical conditions in the estuary strongly influence primary production cycles of microalgae, benthic macroalgae, and
submersed and emersed macrophytes. Phytoplankton biomass and production cycles are controlled by irradiance and temperature regimes and
dinoflagellates (i.e., Prorocentrum minimum, Peridinium quinquecorne),
diatoms (i.e., Chaetoceros, Rhizosolenia, Coscinodiscus, Odontella), and
the autotrophic ciliate Mesodinium rubrum tend to reach maximum abundance (104_ 10 5 cells 1- 1 ) during elevated temperatures and salinity (>20) in
the summer/fall. Following frequent seawater intrusion in spring/summer,
euryhaline diatoms (i.e., Skeletonema costatum, Cerataulina daemon)
reach peak production (160-350 mg C m- 3 h- 1 ) and high biomass accumulation (10 6 -10 9 cells 1- 1 ) during prolonged water residence. Lowest primary
production (2-5 mg C m- 3 h- 1 ) coincides with water temperatures below
20 °C. Especially in winter, when the mean water column irradiance of highly turbid waters falls below the critical light intensity ( 40 gcal cm- 2 day- 1 ),
net production becomes negative and heterotrophic processes prevail. The
mean annual biomass values (0.07 g C m- 2 ,mean 0.03-7.0 g C m- 2 ) and production rates (72 mg C m- 2 h- 1 , mean 4-320 mg C m- 2 h- 1 ) vary significantly between years, owing to differences in runoff volume (Abreu et al. 1994;
Seeliger et al. 1997).
In the shoals, the frequent resuspension of benthic centric (Melosira
spp., Pleurosira laevis) and pennate diatoms ( Cylindrotheca closterium,
Bacillaria paradoxa, Terpsinoe americana, Surirella spp.) by currents and
winds leads to biomass concentrations ( 6-22 g C m - 2 ) which substantially
exceed those of phytoplankton in deeper water. The peak abundance of
cells occurs during the winter, because benthic microalgae appear to be
inhibited by high incident light after resuspension in the summer. Benthic
macroalgal biomass formation in the estuary is limited by the lack of suitable substrate; however, as long as suspended drift-algae remain in the
photic zone of shallow waters, elevated biomass might be formed. The biomass and production peaks of the most prominent green (Enteromorpha
sp., Rhizoclonium riparium, Ulothrix flacca) and blue-green (Lyngbya confervoides, Microcoleus chthomoplastes) algal producers and the Xantophyta Vaucheria longicaulis are linked to light and temperature. The
highest mean monthly biomass of Enteromorpha sp. (34.3 g C m- 2 ), Rhizoclonium riparium (308 g C m- 2 ), Ulothrix flacca (2.2 g C m- 2 ), and Vau-
U. Seeliger
12.4 Energy Flow
12.4.1 Primary Production Cycles
The unpredictable amplitude and timing and pronounced inter-annual
variability of physico-chemical conditions in the estuary strongly influence primary production cycles of microalgae, benthic macroalgae, and
submersed and emersed macrophytes. Phytoplankton biomass and production cycles are controlled by irradiance and temperature regimes and
dinoflagellates (i.e., Prorocentrum minimum, Peridinium quinquecorne),
diatoms (i.e., Chaetoceros, Rhizosolenia, Coscinodiscus, Odontella), and
the autotrophic ciliate Mesodinium rubrum tend to reach maximum abundance (104_ 10 5 cells 1- 1 ) during elevated temperatures and salinity (>20) in
the summer/fall. Following frequent seawater intrusion in spring/summer,
euryhaline diatoms (i.e., Skeletonema costatum, Cerataulina daemon)
reach peak production (160-350 mg C m- 3 h- 1 ) and high biomass accumulation (10 6 -10 9 cells 1- 1 ) during prolonged water residence. Lowest primary
production (2-5 mg C m- 3 h- 1 ) coincides with water temperatures below
20 °C. Especially in winter, when the mean water column irradiance of highly turbid waters falls below the critical light intensity ( 40 gcal cm- 2 day- 1 ),
net production becomes negative and heterotrophic processes prevail. The
mean annual biomass values (0.07 g C m- 2 ,mean 0.03-7.0 g C m- 2 ) and production rates (72 mg C m- 2 h- 1 , mean 4-320 mg C m- 2 h- 1 ) vary significantly between years, owing to differences in runoff volume (Abreu et al. 1994;
Seeliger et al. 1997).
In the shoals, the frequent resuspension of benthic centric (Melosira
spp., Pleurosira laevis) and pennate diatoms ( Cylindrotheca closterium,
Bacillaria paradoxa, Terpsinoe americana, Surirella spp.) by currents and
winds leads to biomass concentrations ( 6-22 g C m - 2 ) which substantially
exceed those of phytoplankton in deeper water. The peak abundance of
cells occurs during the winter, because benthic microalgae appear to be
inhibited by high incident light after resuspension in the summer. Benthic
macroalgal biomass formation in the estuary is limited by the lack of suitable substrate; however, as long as suspended drift-algae remain in the
photic zone of shallow waters, elevated biomass might be formed. The biomass and production peaks of the most prominent green (Enteromorpha
sp., Rhizoclonium riparium, Ulothrix flacca) and blue-green (Lyngbya confervoides, Microcoleus chthomoplastes) algal producers and the Xantophyta Vaucheria longicaulis are linked to light and temperature. The
highest mean monthly biomass of Enteromorpha sp. (34.3 g C m- 2 ), Rhizoclonium riparium (308 g C m- 2 ), Ulothrix flacca (2.2 g C m- 2 ), and Vau-