Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 218

the intensive farming and the monocultures may diminish genetic and species diver-
sity, increasing their vulnerability to infections and invasion. 132
Mussel rearing represents a more or less self-regulated extensive aquaculture system
that is, to a large extent, integrated with the natural marine ecosystem. Cage-farming
of fishes adds to eutrophication due to nutrient input through the supplied food, while
mussel culturing counteracts eutrophication since no feed is added and nutrients are
removed when mussels are harvested. Since mussels belong to a low trophic level, and
therefore demand less primary production, it is possible to harvest a much larger biomass
of mussels per unit of water area when compared with high trophic level fish cultures. 133
In systems with abundant bivalve populations, top-down control (grazing) will
dominate, which means that increasing nutrient loading may result in only a limited
response of phytoplankton. 83 From an aquaculture point of view, this also means that
the effect of fertilization of lagoons will be limited. From a water management point
of view, this implies that bivalves could be used for eutrophication control. 134,135
However, cultured molluscs are highly susceptible to the effects of eutrophication
and pollution in lagoons, as they will rapidly accumulate pathogens, toxins, heavy
metals, and other contaminants, thereby posing a potential risk to human health. 103

5.5 REMARKS

For modeling biological assemblages and communities of lagoons it is necessary to
avoid single-factor approaches and work with multifactorial models (Figure 5.10)
where ionic composition of the waters, trophic status, and water renewal rates are

Ionic composition

Oligohaline
Mesohaline

Thalasogenic vs. Limnogenic

Mixoeuhaline

Metahaline

I

Hyperhaline

II

III

IV

V

VI

Salinity

Confinement

α -hypersaline

β -hypersaline

FIGURE 5.10 Diagram showing the possible classification of coastal lagoons and lagoon
water bodies in function of the three main parameters (ionic composition of salts, salinity,
and confinement) that affect biological communities. (Redrawn from Pérez-Ruzafa, A. and
Marcos, C., Rapp. Comm. Int. Mer Mediter ., 33, 100, 1992. With permission.)