Changes in microplanktonic protists assemblages promoted by the thermocline induced stratification around an oceanic archipelago.

Alejandro E.S.F Da Costa,Sigrid Neumann-Leitão,Jana R De Santana

doi:10.1590/0001-3765201820170607

Alejandro E.S.F Da Costa, Sigrid Neumann-Leitão + Show 1 more

Open Access

https://doi.org/10.1590/0001-3765201820170607

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Abstract

In the area around the archipelago of São Pedro e São Paulo (ASPSP), a set of small rocky islands in the Equatorial Atlantic, the thermocline begins at a more superficial depth than in the surrounding waters. This raises the question if there is any change in heterotrophic microplanktonic protists assemblages occurring in the surface mixed layer and in the subsurface layer. To answer this question, we sampled seawater in seven depths (between 1-100 m) with a 10L Niskin bottle and obtained temperature and salinity profiles with a CTD at eight sites around the ASPSP. The depth of the surface mixed layer varied between 18-63 meters. Dinoflagellates dominated in terms of abundance representing over 80% and 65% of the individuals found in the surface and subsurface layers, respectively. A PERMANOVA test revealed significant differences (p < 0.001) among the dinoflagellate and tintinnid assemblages occurring in both layers, but no evidence for difference in the radiolarian assemblage was detected. Good ecological indicators were found mainly within the dinoflagellate assemblage. The thermocline induced stratification allowed the development of different microplanktonic protists assemblages above and under the depth of the surface mixed layer in the upper 100 m of the water column.

Highlights

Tropical oceans are characterized by a strong and permanent thermocline, which inhibits the enrichment of the upper layers of the water column by nutrient-rich deeper waters
Diverse heterotrophic protists assemblages have been suggested as important consumers of the primary production, performing a very important role in the flow of energy within marine ecosystems, since they belong to a size class that is capable of feeding on pico- and nanoplanktonic cells and is still available to a wide range of metazoans (Sanders and Wickham 1993, Sherr and Sherr 1994, Calbet 2008) – characterizing the so called microbial loop (Azam et al 1983)
Two water masses were sampled during the study, the Tropical Surface Water (TSW) and the South Atlantic Central Water (SACW) (Figure 2)

Summary

INTRODUCTION

Tropical oceans are characterized by a strong and permanent thermocline, which inhibits the enrichment of the upper layers of the water column. Diverse heterotrophic protists assemblages have been suggested as important consumers of the primary production, performing a very important role in the flow of energy within marine ecosystems, since they belong to a size class that is capable of feeding on pico- and nanoplanktonic cells and is still available to a wide range of metazoans (Sanders and Wickham 1993, Sherr and Sherr 1994, Calbet 2008) – characterizing the so called microbial loop (Azam et al 1983) These protists assemblages are commonly associated with the primary producers and their vertical distribution is not uniform as well. The waters surrounding the archipelago are strongly stratified, with a surface mixed layer (SML) separated by a permanent thermocline – and associated pycnocline – from deeper nutrient-rich waters. This is of great importance since studies related to the vertical distribution of microplanktonic protists assemblages in the tropical Atlantic are scarce (Dworetzky and Morley 1987, Boltovskoy et al 1996)

MATERIALS AND METHODS

RESULTS

A B stat p