Seasonal change of community structure and size spectra of zooplankton in the Kaw River estuary (French Guiana)

The zooplankton community of the Kaw River estuary in French Guiana and its relationship to its physical and chemical environment are described. Sampling was carried out in November 1998, June 1999, and November 2001, corresponding to the end of the dry season in 1998 and 2001 and to the end of the rainy season in 1999. The Kaw is a small coastal river that drains a vast swamp and flows into the Atlantic Ocean, close to the equator and to the mouth of the Amazon. The hydrodynamic and hydrochemical functioning of the estuary, and the development of littoral mangroves, is strongly constrained by the transport of sediments from the Amazon.During the rainy season the zooplankton, originating from the highly oligotrophic upstream sectors of the Kaw River, was of a strictly freshwater type. It was highly dispersed within the flooding water and dominated (in terms of density) by highly diversified rotifer taxa and (in terms of biomass) by cladocerans. During the dry season the estuary, turbid but enriched by exports from adjacent mudflats and mangroves, was colonized by a large and abundant zooplankton community that was dominated by tintinnids, with copepods as their main associated taxa. Exploiting the rich autochthonous phytoplankton and allochthonous phytobenthos, the microzooplankton components were only constrained when strong hydrodynamic exchanges allowed mixing between the opportunistic estuarine community and coastal mesozooplankton (copepods, chaetognaths, bivalve veligers). Between 1998 and 2001, these two communities were partially isolated from each other due to the gradual arrival of a mud bank, causing the blockage of the estuary. This isolation resulted in the under-exploitation of the microzooplankton. The intensification of tidal currents (spring tide) that occurred during the subsequent stabilization phase of the mudflat induced a more balanced zooplankton community. While the “estuarine” zooplankton of the Kaw River estuary therefore relies on the relative isolation of its water mass, its contribution to the coastal ecosystem also implies the existence of strong tidal currents that temporarily break this isolation.