Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 401

intensive irrigated crop farming. This surface water for irrigation lessened the aquifer's
overexploitation, thus raising the phreatic levels 33 and helping the main watercourse on
the watershed maintain a continuous flow (about 24 l/s) fed by ground water with high
nitrate levels to the lagoon. Due to overfertilization with nitrogen and pesticides used
in agriculture, this flow is at present the main way nitrate enters the lagoon and pesticides
enter the trophic food web. 34
Not all human activity on the coastline has had negative effects on the biological
assemblages. Some activities, such as the construction of small piers made of wood,
sometimes on concrete pillars—a component of the traditional Mar Menor land-
scape—have provided hard bottoms and shaded habitats that favor the settlement
and development of sciaphilic assemblages, thus increasing the lagoon biodiversity.
Such communities consist mainly of suspension feeders such as sponges, cnidarians,
briozoans, and ascidians, 9 which actively contribute to maintaining the water quality.

9.3.3 M AIN C HANGES A FFECTING THE L AGOON ' S E COLOGY

Two of the above-described human-induced changes have had, and continue to have,
significant impact on the transformation of the lagoon dynamic. On the one hand,
changes in hydrodynamics caused by the enlargement of the El Estacio Channel in
1972 produced an increase in the water renewal rates, decreasing salinity and lower
extreme temperature, thus permitting access to new, mainly benthic and nectonic
colonizers in the process of mediterranization of the lagoon ( Figure 9.3.7, Table
9.3.3). Decreases in salinity values were observed from then until 1988 with an
increase in colonizers such as the algae Caulerpa prolifera . On the other hand,
changes in the nutrient input regimen are, at present, producing a chain of changes
affecting mainly water quality, benthic vegetation, phytoplankton, and gelatinous
plankton. A more detailed description of these changes is provided in the following
section.

9.3.3.1 Changes Induced by Water Renewal Rates

As mentioned previously, the hydrographic conditions of the Mar Menor have
changed in the course of its geologic history because of the sea level fluctuations and
the development of the sand barrier and communication channels with the open sea.

TABLE 9.3.3
Influence of the Enlargement of the El Estacio Channel on Some
Hydrographical Features of the Mar Menor 32

1970 1980 1988

Outflow of water to the Mediterranean (m 3 ) 3.6 × 10 8 6.1 × 10 8 6.4 × 10 8

Inflow of water from the Mediterranean (m 3 ) 4.5 × 10 8 7.2 × 10 8 7.3 × 10 8

Residence time (years) 1.28 0.81 0.79

Temperature range (°C) 7.5-29 12-27.5 12-30.5

Salinity range 48.5-53.4 43-46 42-45