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

The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant to our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both the biomass and primary production but not in the growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the ratio of carbon to chlorophyll a (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mg C m−3 at the surface to 10–15 mg C m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The ratio of fucoxanthin to total chlorophyll a increased markedly with depth, suggesting an increased contribution of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fuelled by new nutrients but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the peaks of deep primary production associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

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

The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant for our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both biomass and primary production but not in growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the carbon to chlorophyll a ratio (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mgC m−3 at the surface to 10–15 mgC m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The fucoxanthin to total chlorophyll a ratio increased markedly with depth, as did the biomass contribution of large cells, suggesting a dominance of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fueled by new nutrients, but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the deep primary production peaks associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

Picophytoplankton distribution at the Ob section and in western part of the Kara sea

The spatial distribution of picophytoplankton abundance, biomass, chlorophyll a and contribution of picoalgae to total chlorophyll a was studied in the outer Ob estuary with an adjacent shelf and in the western part of the Kara Sea. In August-September picophytoplankton abundance and biomass varied from 0.1 to 17.3106 cell/l and from 0.06 to 9.20 mg С/m3, respectively. Cyanobacteria dominated in plankton picofraction, its contribution to total picophytoplankton biomass did not exceed 11%. The highest contribution of picophytoplankton to the total phytoplankton abundance was observed at a lower (11 mg/m2) chlorophyll a concentration in the euphotic layer. The spatial heterogeneity of picoforms contribution was determined by the silicon concentration.

Evaluation of primary production in the northeastern Japan Sea on the base of shipboard and satellite data

Satellite data on chlorophyll concentration from ESA (CCI-OC) and Goddard Space Flight Center, NASA and shipboard observations of CTD, P, N, Si, inorganic carbon, DCI, and Chlaat 38 stations in the northeastern Japan Sea (46th cruise of RV Academik M.A. Lavrentyev on July 9–19, 2009) are analyzed. The highest chlorophyll concentrations were found in the subsurface layer (depth 20–40 m) or even deeper in the Polar Front zone, so they were not reflected in the satellite data. The minimal depths of the subsurface maximum were observed northward from the Polar Front where the estimations of chlorophyll concentration in the upper optical layer (Zd= 1/kd) were similar for the shipboard and satellite measurements (on average 0.384 ± 0.160 mg/m3 and 0.406 ± 0.120 mg/m3, respectively). Primary production was calculated using the assimilation number 4.46 mgC/mgChl per hour. Depth of euphotic layer was estimated using the vertical profles of nutrients and Chla. Within this layer, the primary production in the northeastern Japan Sea was evaluated for the shipboard stations as 895–2275 mgС.m–2.day–1, on average 1450 ± 430 mgС.m–2.day–1, and for the satellite data on average 770 ± 190 mgС.m–2.day–1. The estimations based on the shipboard and satellite data were weakly correlated. The shipboard estimations exceed considerably the results obtained by Koblents-Mishke et al. (1956, 1970) and Yamada et al. (2005). Poor accuracy of satellite estimations of primary production is concluded because the deeper part of the euphotic layer with the maximum concentration of chlorophyll is in shadow for satellite sensors.

The Relationship between POC Export Efficiency and Primary Production: Opposite on the Shelf and Basin of the Northern South China Sea

Accurate estimation of particulate organic carbon (POC) export efficiency in the euphotic layer is essential to understand the efficiency of the ocean’s biological carbon pump, but field measurements are difficult to conduct and data are sparse. In this study, we investigated the relationship between POC sinking export efficiency and ocean net primary production (NPP) in the euphotic layer of the northern South China Sea (NSCS), with the help of high spatiotemporal coverage satellite-derived NPP. Annual mean POC export efficiency in euphotic zone is 34% for the shelf areas and 24% for the basin of the NSCS in the context of satellite-derived 16-day-composited NPP. Similar to what is generally observed in the global ocean, the POC export efficiency on the shelf areas appears to be strengthened with the increase of NPP. However, in the basin areas, the opposite relationship is observed. That is, the POC export efficiency significantly decreases with the increase of NPP. Seasonal decoupling between NPP and POC export, phytoplankton size structure, grazing by zooplankton, and dissolved organic carbon export might account for the observed negative relationship between the POC export efficiency and NPP in the euphotic layer of basin region. System comparison between shelf and basin would be helpful to promote understanding of the regulation mechanism of POC export in the tropical marginal seas.