Geology Reference
In-Depth Information
6
Taieri estuary
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4
3
2
1
10
20
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Salinity (‰)
Figure 14 The distribution of dissolved Fe vs. Salinity in the Taieri Estuary, New
Zealand;
21 October 1980, 4 December 1980
(Adapted from Hunter, 1983.
27
)
Whereas some of the biological material (either transported via the fresh-
waterorproducedin situ within the estuarine zone) can be flushed out to
sea, the remainder settles out of the surface water to be deposited onto the
floor of the estuary. Respiration of this debris by benthic organisms
regenerates nutrients and any contaminants that have been accumulated,
releasing them into the bottom water. In estuaries where a two-layer flow
is well defined, these nutrients are transported upriver in the salt wedge
and entrained into the exiting river water, thereby adding to the available
nutrient pool. Polluted rivers having a high nutrient loading are subject to
eutrophication due to over-stimulation of biological activity. Anoxic
conditions within the bottom waters and/or underlying sediments can
result, depending upon the organic loading. Aeolian transport of nitrate to
Chesapeake Bay can lead to low O
2
conditions. As mentioned previously,
some fjords develop anoxic conditions when bottom waters stagnate due
to limited mixing.
