Phytoplankton distribution and its ecological and hydrographic controls in two contrasting areas of a stratified oligotrophic system

In order to analyse the relationship between oceanographic factors and the distribution of marine phytoplankton in oligotrophic conditions, an oceanographic cruise was carried out in the waters surrounding Cozumel Island, Mexico. Due to the contrasting bathymetric characteristics around Cozumel, and the possible influence of such feature on the stratification and phytoplankton ecology, physico-chemical and biological variables were measured on both sides of the island in the surface, fluorescence maximum and the halocline. Our results show that there is a strong stratification in the water column at the east side of the island, unlike the Cozumel Channel on the west, where stratification is milder and there is an uplift of the picnocline associated with the powerful Yucatan current and the reduced bathymetry of the channel. The phytoplankton community, mainly composed of diatoms, dinoflagellates and the dominant cyanobacteria Trichodesmium , showed high dissimilarity between sides of Cozumel and presented low richness, diversity and cell density. The difference between the density and species composition of the surface phytoplankton (related with high temperature), compared to the halocline layer (related with high nutrient concentrations), suggests that the effect of dynamic uplifting on stratification is a key factor that controls the phytoplankton community structure in oligotrophic waters.