The effect of a transient frontal zone on the spatial distribution of extant coccolithophores around the Madeira archipelago (Northeast Atlantic)

摘要

In order to characterize the coccolithophore community around the Madeira archipelago and to understand the effect of a transient frontal zone on its distribution, 149 seawater samples from the first 150 m were collected in 37 stations, during the research cruise POS466 of RV Poseidon. The present study revealed the occurrence of two biogeographic domains, NE and SW, during the late winter of 2014, with distinct physical-chemical and calcareous nannoplankton characteristics. The NE sector was characterized by higher coccolithophore cell densities (mean of 56 x 10(3) cell L-1) and a slightly lower diversity (Margalef diversity index of 1.80) when compared with the SW sector (mean of 47 x 10(3) cell L-1; Md index of 1.86). The more productive sector, NE, was associated with colder, less saline and higher nutrient content water masses, linked to the injection of a westerly flow with origin in the Azores frontal system.Total cell densities ranged between 12 x 10(3) and 112 x 10(3) cell L-1, being Emiliana huxleyi the dominant species followed by small Gephyrocapsa. The most common and subordinate taxa, in order of decreasing abundance, were: Gephyrocapsa oceanica, Michaelsarsia spp., Syracosphaera spp., Umbilicosphaera spp. and Algirosphaera robusta. Relationships between environmental conditions and spatial and vertical variability in coccosphere abundance, associated E. huxleyi and small Gephyrocapsa with the more productive water mass conditions, linked to the thermohaline transient front. G. oceanica distribution indicated its preference for warmer and less turbulent coastal waters, when compared to E. huxleyi and small Gephyrocapsa. Michaelsarsia spp. and A. robusta revealed preference for the northwest coast of Madeira, associated with mesotrophic conditions of the water column. Syracosphaera spp. and Umbilicosphaera spp. were well distributed throughout the archipelago and along the sampled depths, from the coast to open ocean conditions, as well as from lower to higher productive zones, displaying affinities for the meso to oligotrophic conditions, typical of these subtropical waters. The broad depth range of several taxa and the nonexistence of the coccolithophore vertical succession were the result of homogeneous and generalized well-mixed surface layer during the present survey.