W. Admiraal et al., SEASONAL-VARIATION IN COMPOSITION AND PRODUCTION OF PLANKTONIC COMMUNITIES IN THE LOWER RIVER RHINE, Freshwater Biology, 32(3), 1994, pp. 519-531
1. The composition and activity of phytoplankton, zooplankton and bact
erioplankton in the lower River Rhine were measured in 1990 as part of
an international biological inventory of the river. A seasonal study
was carried out on two stations: one in the river mouth (km 1019) and
one at the German/Dutch border (km 863). 2. High densities of phytopla
nkton (with up to 140 mug chlorophyll a l-1) and occasional depletion
of dissolved silicate were observed at the upstream station. Phosphate
concentrations were also lowered during blooms. 3. Phytoplankton bloo
ms, dominated by a few species of centric diatoms, declined one order
of magnitude during downstream transport. During non-bloom conditions
(low) algal densities were maintained during transport, or increased s
lightly, indicating the suitability of the river reach for algal growt
h. 4. Bacterial cell number and production (measured by the H-3-thymid
ine method) showed a broad summer maximum with activity peaks (0.5 nm
thymidine h-1) coincident with declining phytoplankton blooms. Winter
values of bacterial production (0.02 - 0.05 nm h-1) were substantial,
probably as a result of allochthonous input of organic matter. 5. Roti
fers and crustaceans made up the greater part of the zooplankton biovo
lume, but at the upstream station the contribution of Dreissena larvae
and rhizopods was also substantial. High zooplankton biovolumes, of o
ver 500 x 10(6) mum3 l-1, were observed only during the phytoplankton
spring bloom. 6. Quantitative relationships between the high phytoplan
kton production (2.1 - 3.4 g C m-2 day-1), the high bacterial substrat
e uptake (0.5 -1 g C m-2 day-1), and grazing were analysed for the gro
wing season 1990. Algal grazing by metazoan herbivores was substantial
only during spring, while the role of phagotrophic microplankton and
cell lysis were indicated as major factors responsible for the downstr
eam decline of phytoplankton blooms in the lower Rhine.