Mesoscale and sub-mesoscale variability in phytoplankton community composition in the Sargasso Sea

Abstract

The Sargasso Sea is a dynamic physical environment in which strong seasonal variability combines with forcing by mesoscale (~ 100 km) eddies. These drivers determine nutrient, light, and temperature regimes and, ultimately, the composition and productivity of the phytoplankton community. On four cruises (2011 and 2012; one eddy per cruise), we investigated links between water column structure and phytoplankton community composition in the Sargasso at a range of time and space scales. On all cruises, cyanobacteria (Prochlorococcus and Synechococcus) dominated the phytoplankton numerically, while haptophytes were the dominant eukaryotes (up to 60% of total chl-a). There were substantial effects of mesoscale and sub-mesoscale forcing on phytoplankton community composition in both spring and summer. Downwelling (in anticyclones) resulted in Prochlorococcus abundances that were 22-66% higher than at ‗outside' stations. Upwelling (in cyclones) was associated with significantly higher abundances and POC biomass of nanoeukaryotes. In general, however, each eddy had its own unique characteristics. The center of anticyclone AC1 (spring 2011) had the lowest phytoplankton biomass (chl-a) of any eddy we studied and had lower nitrate + nitrite (N+N <5 mmol m-2) and eukaryote chl-a biomass as compared to its edge and to the Bermuda Atlantic Time-Series station (BATS). At the center of cyclone C1 (summer 2011), we observed uplift of the 26.5 kg m-3 isopycnal and high nutrient inventories (N+N = 74 ± 46 mmol m-2). We also observed significantly higher haptophyte chl-a (noncoccolithophores) and lower cyanobacterial chl-a at the center and edge of C1 as compared to outside the eddy at BATS. Cyclone C2 (spring 2012) exhibited a deep mixed layer, yet had relatively low nutrient concentrations. We observed a shift in the 2 taxonomic composition of haptophytes between a coccolithophore-dominated community in C2 (98% of total haptophyte chl-a) and a non-coccolithophore community at BATS. In summer 2012, downwelling associated with anticyclone AC2 occurred at the edge of the eddy (not at the center), where AC2 interacted with a nearby cyclone. At the edge, we found significantly lower Synechococcus abundances and higher eukaryote chl-a compared to the center of AC2 and BATS. These along-transect nuances demonstrate the significance of small-scale perturbations that substantially alter phytoplankton community structure. Therefore, while seasonality in the North Atlantic is the primary driver of broad-scale trends in phytoplankton community composition, the effects of transient events must be considered when studying planktonic food webs and biogeochemical cycling in the Sargasso Sea.

Clade and strain specific contributions of Synechococcus and Prochlorococcus to carbon export in the Sargasso Sea

“…Our objective here was to focus on the cyanobacteria community and their cell and carbon export during the winter and summer cruises of 2012. Cotti‐Rausch et al () and De Martini () showed that seasonality was the main driver of the phytoplankton community composition during our study, even though some variability could be observed in response to the mesoscale eddies. Detailed information on hydrography, nutrients and phytoplankton composition at each station can be found in Cotti‐Rausch et al () and De Martini ().…”

“…This study is part of the Trophic‐BATS project, which investigated the plankton community in the Sargasso Sea, their trophic dynamics and relationship with carbon export in 2011 and 2012 (Cotti‐Rausch et al ; De Martini ). Our objective here was to focus on the cyanobacteria community and their cell and carbon export during the winter and summer cruises of 2012.…”

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

“…In the case of sAC and ssAC2, similar or lower microplankton fraction values were found in comparison with those in the CZ during the first weeks of tracking. For sAC, this could be explained as a response to the physical dynamics of this eddy type, usually characterized by a downward displacement of isopycnals, which forces a flush of nutrients below the euphotic zone and the consequent decrease in primary production [3,[9][10][11]. In the case of ssAC2, Morales et al [33] have previously described that this eddy was interacting with a coastal front during a relaxation phase of upwelling, time at which the nanoplankton made the largest contributions to total Chl-a.…”

“…In the case of mesoscale structures (10-100 km, day-months), such as fronts and eddies, their movement through the ocean causes modifications in the physical properties (i.e., light, temperature) of the water column, changing nutrient conditions and, consequently, the phytoplankton community structure [3][4][5]. Mesoscale eddies occupy 25-30% of the surface ocean, contain more than 80% of the kinetic energy of oceanic circulation, and can generate different biological responses depending on eddy type (i.e., surface cyclonic/anticyclonic or subsurface anticyclonic eddy) [6][7][8].…”

Biogeochemical versus ecological consequences of modeled ocean physics

“…Observations have shown that phytoplankton biomass and community structure have characteristic temporal and spatial scales corresponding most closely with the oceanic mesoscale and submesoscale (Platt, 1972;Strass, 1992;Abbott and Letelier, 1998;Doney et al, 2003;Cotti-Rausch et al, 2016). However, most global ocean models incorporating biogeochemical and ecological processes (especially those coupled in climate studies) do not resolve scales less than ∼ 1 • .…”