Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 404

120 80

100 70

80 60

60 50

40 40

# of species of molluscs
# of species of fishes
Salinity

20 30

0 20

1650 B.C . 1315 A.D. 1880s 1890 -1953 1972-1982 1988 1991

FIGURE 9.3.11 Relation between the number of species of molluscs and fishes and the
salinity of the waters in the Mar Menor Lagoon. Mollusc data before 1950 correspond to
stratigraphical study of sediments by Simonneau. 28 Data for fishes for the same period come
from works on fisheries 35,36 and probably underestimate benthic cryptic species, mainly for
median age. Estimation of salinity in ancient time was based on historical data for the open
conditions of the lagoon, which maintained a higher water surface level and active commercial
navigation through its golas . 62

overenrichment of nutrients in the water, in a second phase, acted in a synergistic
way on the colonization rate of the C. prolifera up to the point that it now occupies
most of the lagoon bottoms, including rocky substrates. Instability of sediment was
mainly caused by dredging of parts of the lagoon bottoms, the fill-in of some beaches,
and the input of terrigenous allochthonous materials from torrential rains. Nutrient
enrichment mainly resulted from urban and agricultural wastewater.
Despite, or probably because of, the increased biodiversity in the lagoon there was
a decrease in some fish production and catch. The colonization increase at the end of
the 19th century resulted in a decrease in grey mullet. The enlargement of the El
Estacio Channel in 1972 caused a decrease in Mugilidae and Sparus aurata (Figure
9.3.12) . These changes have been related to the increase in interspecific competition
and to changes in sediment properties and the bottom environment. 32,44

9.3.3.2 Changes Related to Nutrient Inputs

Recent history of nutrient inputs into the lagoon has been closely related to urban,
industrial, and agricultural development, either on the coastline or in the watershed,
respectively. The eutrophication process, which has been described in many systems
with observational rather than experimental data in coastal lagoons, starts with the
increase in nutrients following a general trend in which seagrasses are replaced by
macroalgae as the first step. Later small phytoplankton cells are replaced by larger
ones that shade the bottom and hinder submerged vegetation growth by the decom-
position of benthic organic matter and subsequent production of anoxia at the
sediment level and in the water column afterward ( see Chapter 5).