Why coastal lagoons are so productive? Physical bases of fishing productivity in coastal lagoons

Coastal lagoons are among the most productive marine ecosystems in the world. Annual primary production varies from 50 to > 500 g C m^-2 year^-1, being of the same order of magnitude as that of the upwelling areas. Many lagoons lie within the range of eutrophic (300-500 g C m^-2 year^-1) or hypereutrophic (> 500 g C m^-2 year^-1) conditions. The high productivity of coastal lagoons makes them subject of exploitation by many marine fishes and invertebrates, that use them as nursery areas and feeding grounds during their early life cycle phases, and most lagoons support important fisheries or maintain aquaculture exploitations. The high levels of their biological production can be explained by some of their common features as shallowness and the strong influence of terrestrial systems. Shallowness favors that the photic zone extends to the lagoon bottom and that wind can promote the resuspension of nutrients and organisms. The interaction with land also introduces significant amounts of nutrients. However, trophic variables can explain < 43 % of the fishing yields, and further than the trophic status of the lagoons, several works showed that the biological productivity of coastal lagoons can be explained by their geomorphological features such as the positive influence of shoreline development and the negative influence of depth. Using the Mar Menor lagoon as a case study, we propose that although nutrient inputs and light can be limiting factors for photosynthetic based productivity, increasing fishing yield up to a certain limit, the productivity of lagoons is mainly promoted by more general forces associated to physical and chemical gradients.