Seasonal changes of the microplankton community along a tropical estuary

Microplankton organisms are an important link in the transfer of matter and energy between the benthic-pelagic microbial food web and higher trophic levels in estuaries. Although tropical estuaries are among the most productive aquatic systems globally, information on the spatial and seasonal dynamics of microplankton in such systems is scarce. In order to identify which variables control microplankton abundance and community structure a number of environmental variables were measured along the tropical Gulf of Nicoya (Costa Rica) during the rainy and dry seasons (2011-2012). The Tempisque River was a major source of nutrients and turbidity and thus imposing a clear gradient along the estuary. Chlorophyll a (chl a) concentration was highest in the middle of the estuary (2.7-20 mg m(-3)), where turbidity decreased. The microplankton comprised mainly diatoms (88%) and dinoflagellates (8%). Multivariate analysis revealed five different microplankton assemblages associated with a seasonal and riverine-marine gradient, and supporting an ecotone model at the estuary head that shifts to an ecocline model for the rest of the estuary. Our results suggest that primary producers in the estuary were mainly limited by light rather than nutrients. (C) 2015 Elsevier B.V. All rights reserved.