Total phosphorus — chlorophyll relationships in tropical coastal lagoons in Mexico

Summary

The hydrobiological conditions and the trophic state, as indicated by total phosphorus and chlorophyll a, of 24 tropical coastal lagoons (area = 150 to 200,000 ha) along the Atlantic and Pacific coasts of Mexico were investigated between 1980 and 1991. These waterbodies differ in many respects from those used to develop the OECD total phosphorus — chlorophyll a model.

These coastal waterbodies were experiencing permanent or temporary sea intrusion. The majority of them exhibited great seasonal extremes of salinity, from freshwater phase (rainy season) to euryhaline (dry season). Two lagoons were hyperhaline throughout the year. In several lagoons extremely low values of chlorophyll and net primary production rates were observed seasonally.

The lagoons were eutrophic or hypertrophic with respect to mean annual total phosphorus concentration, which exceeded the highest total phosphorus measured in the OECD data set in 13 of the lagoons studied. The total phosphorus — chlorophyll relationship in 10 lagoons fell within “average” OECD lines. These lagoons were either freshwater types or had relatively low seasonal change in salinity. The two hypersaline lagoons fell well below the “normal” curve as well as the remaining six lagoons which had both severe nitrogen limitation and extreme seasonal changes in salinity. The hypertrophic lagoons, outside the range of the OECD data set, all had lower chlorophyll response than would have been expected from the extension of the OECD line. The lower chlorophyll response to phosphorus in these waterbodies is caused by the severe nitrogen limitation and not the self-shading of algae at high densities.