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

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.

Effects of finfish farms on pelagic protist communities in a semi-closed stratified embayment

“…Also, stratification reduces microbial dispersion among water layers separating microbial communities according to physiochemical conditions. (Costa, Santana & Neumann-Leitao 2018; DeLong et al 2006; Mena et al 2019). This changes in the water column influence both alpha and beta diversities.…”

Metalimnion as an ecotone in stratified freshwater lakes: A case study on benthic testate amoebae in Lake Valdayskoe

“…Many studies have been devoted to determining the influence of the thermocline on the species structure, species richness and spatial distribution of pelagic animals, such as fish and zooplankton (Cantin et al, 2011;Gray, 1996;Odum & Barret, 1971;Onda et al, 2017), and planktonic protists in marine (Costa et al, 2018) and freshwater bodies (Ueno et al, 2005). The influence of the thermocline on the benthic assemblages of Chironomidae, Crustacea and Foraminifera (Cantin et al, 2011;Corrège, 1993;Jiménez & Springer, 1996;Odum & Barret, 1971) has also been described, but the testate amoebae assemblages inside the metalimnion and the influence of the thermocline on benthic testaceans have not yet been well-studied.…”

“…At present, the important role of the metalimnion (which contains the thermocline), the layer of water in which temperature changes more rapidly with depth than it does in the layers above (surface or mixed layer) or below (deep water) (Fiedler, 2010), in the formation of the thermal regime and the transition of oxygen and OC in stratified freshwater lakes has been established, and the environmental conditions within the metalimnion, such as temperature and oxygen gradients and underwater currents, have been studied (Boehrer & Schultze, 2008; Nedogarko, 2021; Skowron, 2007). Many studies have been devoted to determining the influence of the thermocline on the species structure, species richness and spatial distribution of pelagic animals, such as fish and zooplankton (Cantin et al., 2011; Gray, 1996; Odum & Barret, 1971; Onda et al., 2017), and planktonic protists in marine (Costa et al., 2018) and freshwater bodies (Ueno et al., 2005). The influence of the thermocline on the benthic assemblages of Chironomidae, Crustacea and Foraminifera (Cantin et al., 2011; Corrège, 1993; Jiménez & Springer, 1996; Odum & Barret, 1971) has also been described, but the testate amoebae assemblages inside the metalimnion and the influence of the thermocline on benthic testaceans have not yet been well‐studied.…”