Phytoplankton is considered to be responsible for approximately 50% of the planetary primary production and at the basis of the trophic chain. Large scale factors such as climate, ocean circulation, and mostly anthropogenic activities, affect phytoplankton biomass and distribution. For all of these reasons, in the ocean, phytoplankton is defined as a sort of sentinel of changes in the ecosystem, because it rapidly responds to environment perturbations. Light, nutrients and temperature are the most important environmental variables that influence phytoplankton production.
Phytoplankton cells respond to changes in light and nutrients with physiological strategies that enhance the efficiency of light capturing, photosynthetic capacity, growth and persistence. There are two different kinds of phytoplankton responses to light: photoadaptation and photoacclimation. The photoadaptation describes changes that might happen at genotype level, and are expected to occur at a long evolutionary time-scale. The photoacclimation is a cellular process that allows phytoplankton to change the intracellular chlorophyll-a concentration (Chl) in relation to environmental factors and it includes, among the others, regulation of the pigment amount and other components of the photosynthetic machinery.
The temperature is the other main environmental agent that affects phytoplankton. It has been proved that ocean warming, mostly due to anthropogenic activities, causes an expansion of the low-Chl and low-productivity areas impacting strongly on marine ecosystem.
The most important and easily observable mechanism due to photoacclimation is variation of the photosynthetic pigment concentration at the cellular scale, which can be observed and quantified using space-borne observations. Photoacclimation can be described in terms of variation of the ratio between chlorophyll-a and carbon (Chl:C ratio). Unfortunately, this process is currently overlooked by standard operational ocean colour algorithms used to retrieve information about both the phytoplankton standing stock and production.
PhysioGlob project, funded by the ESA Living Planet Fellowship 2018, wants to study the inter-annual physiological response of phytoplankton to the global warming using long-term satellite observations (starting from the entire ESA OC-CCI time-series) through the Chl:C ratio. Phytoplankton carbon (Cphyto) can be estimated from the particle backscattering, bbp(λ). For this reason, the first and fundamental step is to infer the possible sources of errors and to define an updated version of the algorithm currently used for bbp(λ) in the ESA OC-CCI context: Quasi Analytical Algorithm (QAA). To do it, we use a large field dataset of remote-sensing reflectance (Rrs) and inherent optical properties (IOP) (e.g., buoy data or BGC-Argo floats). Afterwards, we compare the bbp-based phytoplankton carbon with Chl to estimate the physiological signal. In order to study the trend and oscillations of this process, we plan also to: i) study each time series using the Multichannel Singular Spectral Analysis (M-SSA) to evaluate similarities among the inter-annual variabilities of the Chl:Cphyto ratio, Sea Surface Temperature (SST), and phytoplankton indices, also highlighting possible differences; ii) proceed with a joint M-SSA analysis of the time series to better understand the spatio-temporal structure associated with inter-annual variability in the Chl:Cphyto ratio or phytoplankton indices and global ocean temperature field. The physiological footprint of phytoplankton is well recognized on a global scale (e.g. in the gyres and low productivity areas), but its evolution (i.e. where, how, when) is not widely known using long-term observations. PhysioGlob can shad light to which extent the inter-annual oscillatory modes found in the Chl:Cphyto ratio, or phytoplankton indices, can be attributed to its response to inter-annual variability in SST field.

It is well known that mesoscale eddies in the oceans contribute to intensification of primary producing processes and as a consequence to increasing of phytoplankton biomass which can be estimated remotely from space using ocean colour information on chlorophyll-a. Dynamic factors such as divergence and convergence lead to upflow of nutrients and accumulation of biomass respectively. Usually increasing of phytoplankton biomass is observed at edges of mesoscale eddies. The aim of this study is to investigate the pattern of chlorophyll-a distribution in cyclones and anticyclones for better understanding of principle difference between their influences on productivity in terms of phytomass.
For this, we analyzed chlorophyll-a anomalies in cyclones and anticyclones for 18 years (1998 – 2015) in the Nordic Seas (62o – 82 o N, 22 o W – 20 o E). Eddies were identified using the method by Nencioli et al. (2010) from weekly 0.25o х0.25o AVISO satellite altimetry data-set. Merged eight-day 4x4 km chlorophyll-a composites were taken from Climate Change Initiative Ocean Colour data base, version 3.1. Chlorophyll-a anomalies represent a difference between each part of an eddy: central (0.5 of its radius), middle (0.5-1 of its radius), skirt (1-1.5 of its radius), and the eddy periphery (1.5-2 of its radius).
Results showed negative anomalies in central pats of eddies both in cyclones and anticyclones meaning that its periphery regions are more productive in terms of chlorophyll-a. This tendency overall remains for four seasons of chlorophyll-seasonal cycle in the region (February-April, May-June, July-August and September-October). It was found that on average middle and skirt parts of anticyclones had the same chlorophyll-a content as over the periphery, while in cyclones these areas still showed negative anomalies. Chlorophyll-a concentration increased in the skirts of anticyclones in comparison with their periphery from May to September. Thus, these results suggest that anticyclones have vaster areas of positive or/and null anomalies of chlorophyll-a, than cyclones. This could be explained by differences in chlorophyll-a increasing/transport in mesoscale eddies with different polarity

Adopted in June 2008, the aim of the European Union’s (EU) Marine Strategy Framework Directive (MSFD) is to protect the marine environment across Europe. The goal is to achieve a Good Environmental Status (GES) of the EU’s marine waters by 2020. To this end, each Member State (MS) is required to develop a strategy for its marine waters, that must be kept up-to-date and reviewed every 6 years according to the Directive adaptive management approach. This approach foresees cyclical assessments that started in 2012 with the first, initial assessment of the environmental status of national marine waters and had the first revision in 2018. This paper describes how ocean colour products were used in the assessments for the continental Portugal subdivision in 2012 and 2018, the results obtained, and discusses future routes for the application of remote sensing data in MFSD implementation.
In the initial 2012 assessment, in situ and case 1 waters chlorophyll concentration (Chl1) data for the period 1998 - 2008 were used to assess the direct effects of nutrient enrichment (MFSD Descriptor 5 - “Eutrophication”). Chl1 data at 8-day and 4 km resolution, for the same time span as the in situ data, were used to compute statistical measures following the MSFD recommendations. Reference and assessment values were established for six areas off continental Portugal, based on the analysis of the seasonal evolution of grand median values. Categorical assessment maps were produced from the difference between the assessment values and the 90% percentile of the Chl1 time series for each pixel.
In the first revision, carried out in 2018, a similar approach was followed using the ESA Ocean Colour CCI surface (OC-CCI v3.1), conducted as a preliminar coordination effort in the frame of the INTERREG Atlantic area Project - iFADO (Innovation in the Framework of the Atlantic Deep Ocean). The OC-CCI chlorophyll data, for the period under assessment (2012 - 2017), was obtained from the Copernicus Marine Environment Monitoring Service (CMEMS) . These multi-mission data were selected given their temporal consistency and the strategy of weighting the outputs of the best performing algorithms based on the water types present, improving the performance in Case-2 waters. The categorical maps using the new data set and evaluation period led to the proposal that, as a whole, the six areas should keep their classification as non-problem areas. However, it is recognized that high phytoplankton biomass levels are occasionally recorded in areas under the influence of major river plumes, prompting the need for improvement in the data and analysis methods in those regions.

Natural waters contain optically active components that determine their inherent optical properties, namely, the absorption, scattering and light attenuation coefficients. The total light absorption coefficient in the visible spectral domain of radiation is determined by phytoplankton, non-algal particles (NAP), colored dissolved organic matter (CDOM) and water. Quantitative and qualitative characteristics of these optical components significantly affect the amount and spectral distribution of light in the sea. The Black Sea belongs to the Case 2 waters, where CDOM dominates in light absorption and determines mainly spectral features downwelling radiance. The research was aimed to analyze an effect of CDOM on downwelling radiance and on species composition and absorption capacity of phytoplankton within euphotic zone (Zeu). For this research bio-optical data, obtained during several cruises in the Black Sea carried out during periods of seasonal stratification in years between 1996 and 2016, were used.
It was obtained that downwelling radiance penetrated to the bottom of Zeu in the range ~490 – 590 nm, which was related to that CDOM contribution to the total light absorption at ~440 nm (maximum phytoplankton absorption) was 50-75%. Both low levels of irradiance, and its spectral quality near the bottom of Zeu are likely to be key factors associated with the increased prevalence of the picocyanobacteria Synechococcus spp in deep chlorophyll maximum (DCM) phytoplankton communities near the bottom of Zeu. In the DCM layer, low values of chlorophyll a specific light absorption coefficients by phytoplankton and the appearance of a shoulder at ~490 nm and local maximum at ~550 nm corresponding to the absorption bands of phycourobilin and phycoerythrobilin are caused by changes in the abundance of cyanobacteria. It was revealed that shifts to Synechococcus dominance are associated with seasonal water stratification when the thermocline "locks" the phytoplankton near the bottom of the euphotic zone.
The parameterization of light absorption by phytoplankton in the DCM layer resulted from this study will make it possible to refine spectral models for assessment of downwelling radiance and primary production in the Black Sea based on remote sensing data. This parameterization is a noteworthy improvement because it accounts for environment-specific acclimative and adaptive responses of phytoplankton communities, including changes in intracellular pigment concentrations, species composition, and size structure.
RAS funded this research [grant numbers АААА-А18-118020890112-1, АААА-А18-118020790229-7 and АААА-А18-118012690119-7]. The research was partly supported by the Russian Foundation for Basic Research, [projects numbers 17-05-00113 and 18-45-920070].

In this work we investigated the spatial-temporal variability of the phytoplankton community structure in the Mediterranean Sea. A regional ocean-colour product of four phytoplankton functional types (PFTs; diatoms, dinoflagellates, nanophytoplankton, picophytoplankton, in chlorophyll concentrations, retrieved from the ESA’s Climate Change Initiative Ocean Colour product) was assimilated for the first time into a coupled physical-biogeochemical model of the region (POLCOMS-ERSEM) by using the Ensemble Kalman filter (EnKF). The stochastic EnKF variant was applied here with 100 ensemble members, log-transformation of states and observations, and spatially variable localization of the analysis radius. Per-pixel estimates of the ocean-colour PFT uncertainty were applied to define the observational error in the EnKF scheme. We performed an assimilative reanalysis simulation for years 1998-2014. We found that the reanalysis outperformed the reference model simulation in forecasting the ocean-colour PFT distributions. This was demonstrated by increased spatial correlations between EnKF forecasts and the satellite product after PFT assimilation. Crucially, the reanalysis was skilled in reproducing a large in situ dataset of ten physical and biogeochemical variables, including pigment-based PFT data and biogeochemical-ARGO observations (fluorescence chlorophyll, oxygen, nitrate). The skill was evaluated by calculating robust skill metrics for the reanalysis-to-data match-ups. Annual PFT climatologies were obtained from the reanalysis product and their spatial variability was investigated by using a self-organizing-map (SOM) algorithm. We found that the Mediterranean Sea can be subdivided in five regions based on the relative abundance and phenology of the PFTs. The mean values and variability of the carbon export and trophic transfer efficiency were sensibly different in each region, in relation to the most relevant PFTs. The reanalysis and regional mapping presented in this paper offer new insights on the variability of the structure and functioning of the plankton community structure and related biogeochemical fluxes in the Mediterranean Sea, with possible applications in climate studies, as well as for aquaculture and ecosystem health management.

This study aims to analyze the relationship between the concentration of Chlorophyll-a (Chl-a) and the position of fishing boats using light for fishing operations derived from the night band (DNB) Visible Infrared Imaging Radiometer Suite (VIIRS) sensor in Indonesian waters. The distribution of fishing vessels derived by VIIRS sensor was downloaded from the National Oceanic and Atmospheric Administration (NOAA) Center for Environment Information (https://ngdc.noaa.gov/eog/services.html). Fishing boats that use light for fishing operations in Indonesian waters are Purse seine and Lift net boats. DNB VIIRS data shows that the fishing boats were concentrated in the Java Sea around 250-1,000 boats per day. The number of ships detected during the Southeast Monsoon period is higher than during the Northwest Monsoon. During the Southeast Monsoon period the concentration of Chl-a concentration is higher than in the Northwest Monsoon period. Overlayed between the position of the fishing boats and the Chl-a concentration shows that fishing boats are concentrated in 0.1 to 0.5 mg/m³3 which is estimated to be a fishing ground.