Primary productivity in the water column of Estero Morales, a mangrove system in the Gulf of Nicoya, Costa Rica

Primary productivity was measured in the estuary Estero Morales during 15 m onths (1990-1991). Water column primary productivity was determined using t he dissolved oxygen method at two different depths (1=50% and 1=10%) at eac h research site and during two consecutive days of sampling. Rosettes were incubated for five hours with three clear bottles and two dark bottles. The Winkler method was used to determine dissolved oxygen. Primary productivit y was also measured with lower sampling frequency at the Cortezas Islands t o obtain a point of comparison. These sites are all located in the Gulf of Nicoya, Costa Rica (10 degrees N, 85 degrees W). Water column net primary p roductivity in Estero Morales presented average values from 1.20 +/- 0.70 g C/m(2) d (around 450g C/m(2) year). Values obtained from Islas Cortezas we re around 1.69 gC / m(2) d. There was a significant difference between the dry and rainy seasons with respect to net primary productivity (z=2.196, p< 0.03). Another significant difference was determined between the depths 1=5 0% and 1=10% in terms of net primary productivity (z=2.51, p<0.006). The va lues obtained for primary productivity establish Nicoya Gulf as one of the most productive estuaries in the world. A follow-up study is recommended fo r this area, consisting of simultaneous sampling at different points of the Gulf of Nicoya. These samples should be taken at a minimum of five differe nt incubation depths for each sample in order to obtain more detailed infor mation in calculating productivity by layers in water column. It is also re commended that frequency of sampling be increased for greater assurance tha t results are not produced by the distribution of patches of phytoplankton.