Abstract
The phytoplankton community composition, structure, and biomass were investigated under stratified and oligotrophic conditions during summer for three consecutive years in the Mediterranean Sea. Our results reveal that the phytoplankton community structure was strongly influenced by vertical stratification. The thermocline separated two different phytoplankton communities in the two layers of the euphotic zone, characterized by different nutrient and light availability. Picoplankton dominated in terms of abundance and biomass at all the stations sampled and throughout the photic zone. However, the structure of the picoplanktonic community changed with depth, with Synechococcus and heterotrophic prokaryotes dominating in surface waters down to the base of the thermocline, and Prochlorococcus and picoeukaryotes contributing relatively more to the community in the deep chlorophyll maximum (DCM). Light and nutrient availability also influenced the communities at the DCM layer. Prochlorococcus prevailed in deeper DCM waters characterized by lower light intensities and higher picophytoplankton abundance was related to lower nutrient concentrations at the DCM. Picoeukaryotes were the major phytoplankton contributors to carbon biomass at surface (up to 80%) and at DCM (more than 40%). Besides, contrarily to the other phytoplankton groups, picoeukaryotes cell size progressively decreased with depth. Our research shows that stratification is a major factor determining the phytoplankton community structure; and underlines the role that picoeukaryotes might play in the carbon flux through the marine food web, with implications for the community metabolism and carbon fate in the ecosystem.
Highlights
Oligotrophic oceanic regions comprise vast areas of the global ocean
We explored whether the two-layered euphotic zone holds two separated communities and whether these communities are influenced by the mesoscale front structure
vertical stratification index (VSI) was significantly correlated to surface salinity in the 3 years of study (Supplementary Figure S3), i.e., higher stratification values coincided with larger contributions of new Atlantic water (AW)
Summary
Oligotrophic oceanic regions comprise vast areas of the global ocean These regions are predicted to expand following the strengthening of the water column stratification with increasing temperatures (Polovina et al, 2008; Gruber, 2011; Capotondi et al, 2012), resulting in reduced nutrient fluxes and primary productivity (Falkowski and Oliver, 2007; Lozier et al, 2011). Sinking and grazing rates are size-dependent as larger cells sink more efficiently (Mackinson et al, 2015) and faster (Marañón, 2015) than smaller phytoplankton cells. It has been argued that larger phytoplankton cells can escape grazing more than smaller cells due to the different generation times of their main predators, metazoans and protists, respectively (Acevedo-Trejos et al, 2015; Marañón, 2015 and references therein). The size structure of the phytoplankton community is a critical issue to foresee future carbon cycling and trophic regimes, since it determines the carbon fate in the ecosystem toward higher trophic levels, export to the deep ocean or remineralization within the photic zone (Behrenfeld et al, 2015; Irwin et al, 2015)
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