Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 96

An observed release of Si from sediment in Thau Lagoon, France, was attributed
mainly to the dissolution of silica that was directly controlled by temperature. In
contrast to NH 4 + and soluble reactive phosphorus, anaerobic conditions are not
supposed to enhance benthic Si fluxes but rather should decrease the release of Si
by cessation of bioturbation. However, this behavior of Si, based on experimental
results, was not confirmed by the observations in the Thau Lagoon. 28

4.1.4 D ISSOLVED O XYGEN

Dissolved oxygen (DO) is considered to be a very important and sensitive indicator
of the health of aquatic systems. DO is necessary to support the life functions of
higher organisms and to drive many redox reactions. 56 DO also provides key infor-
mation about the system state, 57, 58 e.g., insight into algal blooms, oxygen depletion
rates, and zones of oxygen depletion. Changes in the shape of the DO depth curve,
as well as the oxygen deficiency in bottom waters, are meaningful eutrophication
indices. 59
Low dissolved oxygen can cause the loss of aquatic animals. Most estuarine
populations can tolerate short exposure to low dissolved oxygen concentrations
without adverse effects. Extended exposures to dissolved oxygen concentrations less
than 60% oxygen saturation can result in modified behavior, reduced abundance and
productivity, adverse reproductive effects, and mortality. 60 For example, Thau
Lagoon, France, has lost part of its shellfish production over the past decades due
to mass mortality caused by the diffusion of hydrogen sulfide into the water column
during anoxic periods. 28,38
Many aquatic animals have adapted to a short period of hypoxia (dissolved
oxygen concentrations below 3 mg l −1 ) by taking up more oxygen and transporting
it more effectively to their cells and mitochondria, that is, by ventilating their
respiratory surfaces more intensely and increasing their heart rate. If these responses
are insufficient to maintain the blood's pH, then the oxygen-carrying capacity will
decrease. 61 An early behavioral response is the locomotory, i.e., the moving of
organisms toward better-oxygenated waters even when other conditions there may
be unfavorable. 61,62 Under hypoxic conditions, the animal may also reduce its
swimming and feeding, which will reduce its need for energy and hence oxygen.
Although reduced activity may make the animal more hypoxia tolerant for a short
period, reduced swimming activity makes the animal more vulnerable to predators,
and reduced feeding decreases its growth. If oxygen insufficiency persists, deaths
will ultimately occur. 61
Dissolved oxygen criteria developed by the U.S. EPA 61 for coastal waters apply
to both continuous and cyclic DO conditions. If the DO conditions are always above
the chronic criterion for growth (4.8 mg l −1 ), most of the animals and plants can
grow and reproduce unimpaired. DO conditions below the juvenile/adult survival
criterion (2.3 mg l −1 ) mean that there is not enough DO to protect aquatic life. When
DO conditions are persistent between these two values, living organisms often
become stressed and require further evaluation of duration and intensity of low DO
events to determine whether the available levels of oxygen can support a healthy
aquatic community. 61,63