Phytoplankton Community Structure Is Driven by Stratification in the Oligotrophic Mediterranean Sea

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.

Seasonal and Spatial Variation of Pelagic Microbial Food Web Structure in a Semi-Enclosed Temperate Bay

“…Previous studies on picoplankton (picophytoplankton) community structure in coastal area were focused on spatial variation, but seasonal change was very seldom investigated. The dominance of PEUK in Sanggou Bay is consistent with previous studies in coastal areas (Pan et al, 2006;Linacre et al, 2010;Bock et al, 2018;Patten et al, 2018;Otero-Ferrer et al, 2018;Mena et al, 2019). In the deep waters of Adriatic Sea, SYN FIGURE 6 | Microbial food web structures in Sanggou Bay and Arabian Sea in four successive seasons and Monsoon stages, respectively.…”

“…At smaller scale, upwelling conditions influenced the picoplankton community structure in oceans south of Australia (Van Dongen-Vogels et al, 2012) and in coastal China Sea (Wu et al, 2014). Stratification (Bouman et al, 2011;Mena et al, 2019) and fronts (Pan et al, 2006) also separated water masses with different Microbial Structure. The coast geomorphology increases the diversity of habitat (Pierrot-Bults and Angel, 2012).…”

Seasonal Niche Partitioning of Surface Temperate Open Ocean Prokaryotic Communities

“…At the North Pacific Subtropical Gyre (station ALOHA), Prochlorococcus dominates year-round without remarkable seasonal changes ( Bryant et al, 2016 ). Considering that Synechococcus and Prochlorococcus dominate the photosynthetic community in the oligotrophic Mediterranean Sea ( Mena et al, 2019 ), the results suggest an important role of these groups in the strong primary production seasonality of the open ocean Mediterranean Sea ( Marty and Chiavérini, 2002 ), with reportedly higher new primary production at the DCM during summer stratification and in the upper layers during mixing conditions ( Estrada, 1996 ; Pedrós-Alió et al, 1999 ). Based on ocean color and a phytoplankton class-specific bio-optical model, Uitz et al (2012) estimated an annual contribution of picophytoplankton of ~31% in the western basin of the Mediterranean Sea, smaller than nanophytoplankton (~48%) and larger than microphytoplankton (~21%).…”

Deep maxima of phytoplankton biomass, primary production and bacterial production in the Mediterranean Sea during late spring

“…During PEACETIME, the contribution of cells > 2 µm in diameter and cells > 5 µm in diameter to total phytoplankton biomass increased with depth, and this trend was associated with a significant increase in the contribution of diatoms to total phytoplankton biomass, which reached at least 30 % in the DCM of all stations, and was particularly high (nearly 50 %) in the most stratified station, located in the Ionian Sea. Deep maxima in diatom abundance are common in the Mediterranean Sea during stratified conditions (Ignatiades et al, 2009;Siokou-Frangou et al, 2010;Mena et al, 2019) and are often associated with peaks in biogenic silica (Crombet et al, 2011). The increased prevalence of diatoms at the base of the euphotic layer, which illustrates the ecological diversity of this group (Kemp and Villareal, 2018), is likely a result of multiple adaptations and mechanisms, including high growth efficiency under low light conditions (Fisher and Halsey, 2016), buoyancy regulation (Villareal et al, 1996), the ability to exploit transient nutrient pulses through luxury uptake and storage (Kemp and Villareal, 2013;Cermeño et al, 2011) and the enhanced ammonium assimilation mediated by microbial interactions in the phycosphere (Olofsson et al, 2019).…”