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The ocean plays a central role in modulating the Earth’s carbon cycle. Monitoring how the ocean carbon cycle is changing is fundamental to managing climate change. Satellite remote sensing is currently our best tool for viewing the ocean surface globally and systematically, at high spatial and temporal resolutions, and the past few decades have seen an exponential growth in studies utilising satellite data for ocean carbon research. Satellite-based observations must be combined with in-situ observations and models, to obtain a comprehensive view of ocean carbon pools and fluxes. To help prioritise future research in this area, a workshop was organised that assembled leading experts working on the topic, from around the world, including remote-sensing scientists, field scientists and modellers, with the goal to articulate a collective view of the current status of ocean carbon research, identify gaps in knowledge, and formulate a scientific roadmap for the next decade, with an emphasis on evaluating where satellite remote sensing may contribute. A total of 449 scientists and stakeholders participated (with balanced gender representation), from North and South America, Europe, Asia, Africa, and Oceania. Sessions targeted both inorganic and organic pools of carbon in the ocean, in both dissolved and particulate form, as well as major fluxes of carbon between reservoirs (e.g., primary production) and at interfaces (e.g., air-sea and land–ocean). Extreme events, blue carbon and carbon budgeting were also key topics discussed. Emerging priorities identified include: expanding the networks and quality of in-situ observations; improved satellite retrievals; improved uncertainty quantification; improved understanding of vertical distributions; integration with models; improved techniques to bridge spatial and temporal scales of the different data sources; and improved fundamental understanding of the ocean carbon cycle, and of the interactions among pools of carbon and light

In 2018, a working group sponsored by the NASA Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) project, in conjunction with the International Ocean Colour Coordinating Group (IOCCG), European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), and Japan Aerospace Exploration Agency (JAXA), was assembled with the aim to develop community consensus on multiple methods for measuring aquatic primary productivity used for satellite validation and model synthesis. A workshop to commence the working group efforts was held December 5–7, 2018, at the University Space Research Association headquarters in Columbia, MD, USA, bringing together 26 active researchers from 16 institutions. In this document, we discuss and develop the workshop findings as they pertain to primary productivity measurements, including the essential issues, nuances, definitions, scales, uncertainties, and ultimately best practices for data collection across multiple methodologies.

© 2020 by the authors. Primary production by marine phytoplankton is one of the largest fluxes of carbon on our planet. In the past few decades, considerable progress has been made in estimating global primary production at high spatial and temporal scales by combining in situ measurements of primary production with remote-sensing observations of phytoplankton biomass. One of the major challenges in this approach lies in the assignment of the appropriate model parameters that define the photosynthetic response of phytoplankton to the light field. In the present study, a global database of in situ measurements of photosynthesis versus irradiance (P-I) parameters and a 20-year record of climate quality satellite observations were used to assess global primary production and its variability with seasons and locations as well as between years. In addition, the sensitivity of the computed primary production to potential changes in the photosynthetic response of phytoplankton cells under changing environmental conditions was investigated. Global annual primary production varied from 38.8 to 42.1 Gt C yr-1 over the period of 1998-2018. Inter-annual changes in global primary production did not follow a linear trend, and regional differences in the magnitude and direction of change in primary production were observed. Trends in primary production followed directly from changes in chlorophyll-a and were related to changes in the physico-chemical conditions of the water column due to inter-annual and multidecadal climate oscillations. Moreover, the sensitivity analysis in which P-I parameters were adjusted by ±1 standard deviation showed the importance of accurately assigning photosynthetic parameters in global and regional calculations of primary production. The assimilation number of the P-I curve showed strong relationships with environmental variables such as temperature and had a practically one-to-one relationship with the magnitude of change in primary production. In th

Glacial meltwater discharge in fjords on the west coast of Spitsbergen is increasing due to climate change. The influence of this discharge on phytoplankton nutrient limitation, composition, productivity and photophysiology was investigated in central (M) and inner (G) Kongsfjorden (79°N, 11°40’E). Freshwater influx intensified stratification during June 2015, coinciding with surface nutrient depletion. Surface nutrient concentrations were negatively correlated with stratification strength at station M. Here, nitrate addition assays revealed increasing N limitation of surface phytoplankton during the second half of June, which was followed by a pronounced compositional change within the flagellate-dominated phytoplankton community as dictyochophytes (85% of chl a) were replaced with smaller haptophytes (up to 60% of chlorophyll a) and prasinophytes (20% of chlorophyll a). These changes were less pronounced at station G, where surface phosphate, ammonium and nitrate concentrations were occasionally higher, and correlated with wind direction, suggesting wind-mediated transport of nutrient-enriched waters to this inner location. Therefore, glacial meltwater discharge mediated nutrient enrichment in the inner fjord, and enhanced stratification in inner and central Kongsfjorden. Surface chlorophyll a and water column productivity showed 3–4-fold variability, and did not correlate with nutrient limitation, euphotic zone depth, or changed taxonomic composition. However, the maximum carbon fixation rate and photosynthetic efficiency showed weak positive correlations to prasinophyte, cryptophyte, and haptophyte chlorophyll a. The present study documented relationships between stratification, N limitation, and changed phytoplankton composition, but surface chlorophyll a concentration, phytoplankton photosynthetic characteristics, and water column productivity in Kongsfjorden appeared to be driven by mechanisms other than N limitation.

The current study aimed to assess changes in phytoplankton composition and productivity along an oligotrophic gradient in relation to changes in sea surface temperature (SST). Phytoplankton pigments, nutrients, and physical water column properties were studied along a longitudinal transect in the Mediterranean Sea (MED) in May–June 2013, covering its western (wMED) and eastern (eMED) basins. Pigments were used to determine phytoplankton taxonomic composition and taxon specific carbon fixation, whereas beam attenuation was used to calculate particulate organic carbon (POC). Nitrate, phosphate, and silicate concentrations integrated over 0–125 m declined from wMED to eMED (on average 13, 9, and 2 fold lower in eMED compared with wMED, respectively) and correlated inversely with SST (16.2–23.0 °C) for the whole transect in the Mediterranean Sea.N:P ratios in the euphotic zone averaged 5.6 and 3.1 for wMED and eMED respectively, suggesting that phytoplankton was nitrate limited. Average phytoplankton productivity and biomass were 4 and 2.5 fold higher in the wMED than in the eMED, respectively. Relative abundances of diatoms, prasinophytes, dinoflagellates, and cryptophytes showed inverse correlations with SST and positive correlations with nutrient concentrations. In contrast, pelagophytes, chlorophytes, euglenophytes, Synechoccocus and Prochlorococcus showed positive correlations with SST and negative correlations with nutrient concentrations. Particulate organic carbon (POC) of the upper 200 m showed up to 3 fold higher concentrations in the wMED than in the eMED, and correlated positively with chlorophyll concentrations, productivity, and nutrient concentrations, and inversely with SST. Inverse correlations were observed for phytoplankton biomass and SST below 19 °C. SST above 19 °C as observed in the eMED did not correlate with phytoplankton biomass and productivity, showing that in this temperature range phytoplankton productivity is uncoupled from nutrient supply

We investigated the impacts of climate change-associated abiotic factors on the species composition and size structure of coastal phytoplankton communities. Surface coastal water collected off the coast of Málaga (Spain) was incubated outdoors during a 7 d microcosm experiment. The natural phytoplankton communities were exposed to high and low conditions of CO2, nutrients and light. During the first 2 d, a positive response to increased CO2 and nutrient concentration was observed in terms of abundance and chlorophyll in all size fractions (<2, 2 to 20, and >20 µm). After 2 d, a trophic cascade effect was observed within the phytoplankton communities for all treatments. The absence of mesozooplankton led to an increase in microzooplankton abundance,which coincided with a decrease in the abundance of phytoplankton <6 µm equivalent spherical dia meter (ESD). At the same time, an increased concentration of larger phytoplankton was observed. Consequently, a diatom bloom dominated by Leptocylindrus danicus and Chaetoceros sp. developed, peaking on Day 5 in the high-light treatment and on Day 6 in the low-light treatment.The cascade effect was evident in both the smaller and the larger ranges of the size-abundance spectra (SAS). Although this trophic interaction occurred in all treatments in a similar way, therewere still significant differences among treatments. Diatoms with cell sizes >20 µm ESD showed a positive response to the effects of increasing CO2 and nutrient concentration. These results high -light the importance of trophic interactions other than abiotic factors such as CO2 and nutrient availability in shaping the size structure of Mediterranean phytoplankton. More specifically, this work shows the importance of trophic cascade effects in scaling the plankton SAS and should be considered in both enclosure experiments and field measurements that deal with size distribution.

We studied the physiological response of phytoplankton to the interacting effects of 3 factors affected by global climate change: CO2, nutrient loading and irradiance. Treatments had a high and low level for each factor: CO2 was bubbled at 1000 ppm by volume versus present atmo spheric values; high nutrient treatments had a combination of inorganic and organic nutrients; and light treatments were obtained by covering the tanks with a single or double layer of screen. We measured esterase activity, oxidative stress (ROS), cell death, DNA damage, photosynthetic efficiency and 14C assimilation as particulate or dissolved organic material (POC and DOC respectively). Conditions simulating future global change scenarios showed similar chlorophyllnormalized primary productivity as present conditions. The main effect driving phytoplankton physiology was the downregulation of the photosynthetic apparatus by elevated CO2, which decreased esterase activity, ROS, cell death and DNA damage. Nutrient concentration and light acted as additional modulators, upregulating or contributing to downregulation. The percentage of DO14C extracellular release (PER) was low (0 to 27%), significantly lower under ultraviolet radiation (UVR) than under photosynthetically active radiation (PAR), and acted mainly to reequilibrate the internal balance when cells grown under UVR were exposed to PAR. PER was almost 3 times lower under high CO2, confirming a higher resource use efficiency of phytoplankton under future CO2 concentrations.

Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63 degrees N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0-50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13-23 degrees C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0-410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200m was weakest. This suggests that stronger stratification does not necessarily result in higher phytoplankton biomass standing stock in this region.

Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63 N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0-50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13-23 C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0-410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200 m was weakest. This suggests that stronger stratification does not necessarily result in higher phytoplankton biomass standing stock in this region. © Author(s) 2013.

It is expected that climate change will expand the open oligotrophic oceans by enhanced thermal stratification. Because temperature defines the geographic distribution of picophytoplankton in open-ocean ecosystems and regulates photophysiological responses, it is important to understand how temperature affects picophytoplankton growth and photophysiology. Two prokaryotic and 2 eukaryotic picophytoplankton strains were acclimated to 3 different temperatures, ranging from 16 to 24 degrees C. Temperature-dependent growth and photophysiology were assessed by measurements of specific growth rates, cell size, pigment composition, absorption and electron transport rates. Growth of Prochlorococcus marinus (eMED4), Prochlorococcus sp. (eMIT9313), Ostreococcus sp. (clade B) and Pelagomonas calceolata was positively related to temperature, especially in the prokaryotic strains. Changes in photophysiology included increased light harvesting, increased electron transport and reduced photoinhibition at elevated temperatures. However, the changes related to light harvesting and electron transport could not fully explain the observed difference in growth. This suggests that other processes, such as Calvin cycle activity, are likely to limit growth at sub-optimal temperatures in these picophytoplankton strains. The overall changes in photophysiology during temperature acclimation will possibly allow photosynthesis at higher irradiance intensities, but the genetically defined low temperature tolerances and photosynthetic characteristics of the different ecotypes will likely be more important in determining picophytoplankton (depth) distribution and community composition.

Six (pico)phytoplankton strains typical for the open oligotrophic oceans were acclimated to different irradiance regimes mimicking stable (ranging from 10 to 125 mu mol photons m(-2) s(-1)) and dynamic (averaging at 50 and 125 mu mol photons m(-2) S-1) water column conditions. Photoacclimation potential of the different phytoplankton species was assessed by analysis of specific growth rates, pigment composition, pigment absorption, elemental composition, and photosynthetic characteristics. Results showed distinct differences between the studied prokaryotic species, Prochlorococcus marinus and Synechococcus sp. (two strains), and the eukaryotic species. Ostreococcus sp., Emiliania huxleyi, and Thalassiosira oceanica. Based on growth and photosynthetic characteristics, the photoacclimation potential of the eukaryotic species was significantly higher compared to that of the prokaryotic species under high and dynamic irradiance conditions. Likewise, the eukaryotic species performed better than the prokaryotic species after photoacclimation to low irradiance conditions. No consistent differences between constant and dynamic irradiance treatments were found. Differences in pigment composition, for example the presence of a xanthophyll cycle, may have played an important role in the success of photoacclimation of the studied phytoplankton species. These results imply that the high photoacclimation potential of eukaryotic oceanic phytoplankton offers a selective advantage over prokaryotic phytoplankton, in both the upper mixed layer and the deep chlorophyll maximum. Thus, factors other than photoacclimation potential, for example low nutrient availability, are likely to explain the high abundance of prokaryotic picophytoplankton in the open oligotrophic oceans.