Changes in macrophytes distribution in a hypersaline coastal lagoon associated with the development of intensively irrigated agriculture

In the Mar Menor lagoon (SE Spain) recent changes in the nearby agricultural practices, from extensive dry crop farming to intensively irrigated crops, have increased nutrient and particulate inputs through the principal watercourse that flows into the lagoon, the Albujَn wadi. The concentration of nutrients and suspended sediments in the waters has risen, increasing phytoplankton densities and diminishing water column transparency. As a result of these environmental changes, the traditional main macrophyte of the Mar Menor, Cymodocea nodosa (Ucria) Ascherson, has been replaced by the macroalga Caulerpa prolifera (Forsskal) Lamouroux creating changes in the sediment characteristics with profound consequences to benthic communities and also to local fishing and tourism. In the study area, seagrass respiration exceeded photosynthesis while the macroalga presented positive rates of net photosynthesis. C. prolifera is better adapted to these new conditions of light and nutrient concentration and has expanded its distribution, occupying most of the bottom of the lagoon and restricting C. nodosa to small patches in very shallow areas, where plant growth is not limited by light and no C. prolifera can be found because of photoinhibition and emersion stress.