DIEL DISSOLVED-OXYGEN DYNAMICS AND EUTROPHICATION IN A SHALLOW, WELL-MIXED TROPICAL LAGOON (CANCUN, MEXICO)

Bojorquez Lagoon (BL), located on the Mexican Caribbean, has received sewage and dredging impacts as a result of tourism development. The lagoon supports a high diversity of primary producers compared to sheltered adjacent lagoons dominated by Thalassia testudinum communities. The Diurnal Curve Method (Odum and Hoskin 1958) was used to measure community metabolism and assess eutrophication in BL by comparing it to the nonimpacted lagoons and to other systems studied with this method. Dissolved oxygen community input to the water column in BL ranged between 8.3 g O2 m-2 d-1 and 41.5 g O2 m-2 d-1 during 1985 and 1986, and averaged 17.1, whereas dissolved oxygen community consumption ranged from 6.4 g O2 m-2 d-1 to 37.6 g O2 m-2 d-1 and averaged 15.2. These values are higher than those found for the adjacent lagoons and similar coastal lagoons, and are similar to results from other lagoons with sewage or seafood waste discharge. Net flux of oxygen from the community to the water column averaged 1.9 g O2 m-2 d-1 and ranged from -9.8 g O2 m-2 d-1 to 8.1 g O2 m-2 d-1. These values are low compared to the adjacent lagoons, and close to zero, as in dystrophic environments. Primary productivity, as estimated by oxygen input, increased in BL during the period of study, indicating that eutrophication is proceeding, but the lagoon has not reached yet a level of "critical eutrophication" as defined by Mee (1988).