Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 210

As discussed previously, the result of the competition between macroalgae and
rooted submerged vegetation seagrasses is a widespread and usually thick seaweed
layer, implying a large amount of organic matter retained at the sediment level.
Under hypereutrophic conditions, seaweed starts to decompose, as light limitation
by phytoplankton imposes a serious constraint on growth, liberating organic matter
both into the water column and into sediment, producing severe anoxia.

5.2.3.3 Benthic Fauna

As the degree of eutrophication increases, the effects of anoxia cause strong changes
in the benthic community by limiting the growth of benthic filter feeders and thus
causing providing a shift in species composition from bivalves toward polychaetes
and oligochaetes on sand flats and mudflats. 84,93 The benthic communities can be
perturbed by sediment organic enrichment to the point of disappearance. As men-
tioned above, it is generally assumed that the benthic communities subjected to an
increasing load of organic matter show a decrease in species richness, an increase
in the total number of individuals (due to high densities of a reduced number of
opportunistic species), a general reduction of the biomass, a decrease in the average
size of the species and individuals, and a decrease in the sediment thickness occupied
by the fauna. All of these alterations cause a change in the trophic web. 84,94
The depth to which benthic animal life occurs is limited by food availability.
In eutrophic sediment oxygen availability and sulfide concentrations also are
limiting factors. Animals feeding at depth must have access to oxygen, either in
the surface layer or from the overlying water. When oxygen availability is limited,
or when the sediment oxygen demand at the feeding depth is great, animals are
presumably no longer able to withstand this environmental stress. 8 In shelter areas
where large-celled phytoplankton assemblages are controlled by the top-down
mechanisms of benthic filter feeders, anoxia implies the loss of one of the important
control mechanisms of eutrophication at the entire lagoon level. 83 On the other
hand, in addition to moving around within sediment, the fauna build tubes, con-
struct burrows and feeding pits, and transport sediment. When the fauna disappear,
due to anoxia events, the physical and chemical characteristics of the top sediment
layers are greatly changed. 8 Increased organic loadings will lead to upward move-
ment of reducing conditions and anoxia in sediment and ultimately in the water
column. This will shift sediment from aerobic to anaerobic pathways, which may
ultimately lead to the disappearance of the fauna. 95

5.2.3.4 Fish and Bird Assemblages

There is no direct evidence that eutrophication has damaged the fisheries in some
eutrophic waters. This is largely due to the belief that the decline in commercially
important fish populations may be due to over-exploitation. 96 However, as the
increase in primary production due to eutrophication has not been followed by an
increase in decomposition, the net result has been the production of anoxic condi-
tions in deeper waters, either impoverishing or completely eliminating benthic
communities and fish populations. 97 Strongly eutrophic areas can be subjected to
algal blooms, including toxic dinoflagellates, resulting in anoxic conditions near