Shallow coastal ecosystems, particularly estuaries,
are highly productive and important nursery
grounds for marine species (Costa 1988; Elliot et
al. 1990; Drake and Arias 1991a). Many estuaries
are highly stressed by anthropogenic inputs and
human activities (Clark 1992). One example is the
Guadalquivir estuary, the largest estuary in the
Gulf of Ca´diz (southwest Iberian Peninsula). Originally
the Guadalquivir marshlands covered an
area of about 136,000 ha, but during the 20th century
there was a significant transformation of the
marshlands for agricultural purposes; about 92,000