Salinity Intrusion and Seasonal Water Quality Variations in the Tidal ...

Nutrients are generally more abundant in river water than in sea water due partly to
their removal by organisms from oceanic water and partly to the input to rivers from
pollutants and land weathering. Rivers serve as nutrient sources for estuaries and here
biogenic removal is often found. For phytoplankton blooms to occur in estuaries the
surface water must be fairly clear so that the light necessary for photosynthesis can pass
though, large amounts of suspended sediments tend to inhibit organic growth. In
temperate estuaries blooms are seasonal occurring in the spring and summer when there is
more light and wanner water temperatures. Another factor that influences bloom in an
estuary is flushing time for fresh water. Longer flushing time favors greater growth and
more biogenic nutrient removal. The flushing time for fresh, nutrient rich water is longer
in a wellmixed estuary than in a stratified estuary where the river water rapidly flows out
in the surface layer. Krone and Bemer (1981) found that there was a considerable flux
from sediments to the water column ofdissolved P04 which had been adsorbed on ferric
oxyhydroxides. This P release accompanied the reduction ofthe iron minerals by H2S in
anoxic portionsofthe sediment. Thus the flux ofP04 from the bottom sediment was due
to both the microbial breakdown of organic P and the release of adsorbed inorganic P.
Some planktonic organisms remove silica from solution as well as P and N. Diatoms use
Si in forming their opaline-silica shells. Silica removal associated with diatom blooms
has been observed in the Amazon River estuary (Milliman & Boyle 1975). However
because of the redissolution of diatom shells on the bottom the net removal of dissolved
river silica by deposition ofbiogenic silica in natural estuarine sediments is probably far
less than 20% and in some well mixed estuaries at least silica is conservative. In addition
.to dissolved nutrientscarried in fresh water river run offinto estuaries, there are a number
ofother nutrient sources for phytoplankton in the surface water (Bemer and Bemer, 1987)
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