Modeling Carbon Budgets and Acidification in the Mediterranean Sea Ecosystem Under Contemporary and Future Climate

We simulate and analyze the effects of a high CO2 emission scenario on the Mediterranean Sea biogeochemical state at the end of the XXI century, with a focus on carbon cycling, budgets and fluxes, within and between the Mediterranean sub-basins, and on ocean acidification. As a result of the overall warming of surface water and exchanges at the boundaries, the model results project an increment in both the plankton primary production and the system total respiration. However, productivity increases less than respiration, so these changes yield to a decreament in the concentrations of total living carbon, chlorophyll, particulate organic carbon and oxygen in the epipelagic layer, and to an increment in the DIC pool all over the basin. In terms of mass budgets, the large increment in the dissolution of atmospheric CO2 results in an increment of most carbon fluxes, including the horizontal exchanges between eastern and western sub-basins, in a reduction of the organic carbon component, and in an increament of the inorganic one. The eastern sub-basin accumulates more than 85% of the absorbed atmospheric CO2. A clear ocean acidification signal is observed all over the basin, quantitatively similar to those projected in most oceans, and well detectable also down to the mesopelagic and bathypelagic layers.

Main characteristics of arrowworms in the Bering Sea (species composition, distribution, biomass, production)

Zooplankton was sampled from the epipelagic layer (0–200 m) in the Bering Sea using Jedae net (mouth 0.1 m2, mesh size 0.168 mm) in 1986–2018. Arrowworms were the most numerous predators, represented with 3 species: Parasagitta elegans, Eukrohnia hamata, and Pseudosagitta maxima. Their summary biomass was 215.7 mg/m3, on average (26.3 % of the total zooplankton biomass), and varied seasonally from 105.9 mg/m3 in winter to 311.8 mg/m3 in autumn. Parasagitta elegans dominated absolutely (> 99 % WW). Mean stock of this species was 64.36. 106 t; its seasonal yield was estimated as 22.1. 106 t in winter, 78.5. 106 t in spring, 191.9. 106 t in summer, and 130.3. 106 t in autumn, so its mean production was 422.8. 106 t WW per year.

Organic matter transport by cyclonic eddies formed in eastern boundary upwelling system supports heterotrophy in the open oligotrophic ocean

<p>Mesoscale eddies formed in Eastern boundary upwelling systems are elementary components of ocean circulation and play important roles in the offshore transport of organic carbon and nutrients. Yet, most of our knowledge about this lateral transport and its influence on biogeochemical cycles relies on modelling studies and satellite observations, while in situ measurements of biogeochemical parameters are scarce. For example, little is known about the effects of mesoscale eddies on organic carbon distribution, microbial activity, and organic matter (OM) turnover in the open oligotrophic ocean. To address this gap, we investigated the horizontal and vertical variability of phytoplankton and bacterial activity as well as dissolved organic carbon along a zonal corridor of the westward propagation of eddies between the Cape Verde Islands and Mauretania in the Eastern Tropical North Atlantic (ETNA). We additionally collected samples from a cyclonic eddy along this transect at high spatial resolution. Our results indicate a strong impact of cyclonic eddies on both microbial abundance and metabolic activity in the epipelagic layer (0–200 m). Generally, all determined parameters (bacterial abundance, heterotrophic respiration rates, bacterial biomass production, bacterial growth efficiency, bacterial carbon demand and net primary production) were higher in the eddy than in the stations along the meridional transect. Along the transect, microbial biomass and activity rates were gradually decreasing from the coast to the open ocean. We further observed high variability of biogeochemical parameters within the eddy with elevates microbial abundances as well as process rates in the south-western periphery. This can be explained by the rotational flow of the cyclonic eddy, which perturbs local OM and nutrient distribution via azimuthal advection. The local positive anomaly of microbial activity in the cyclonic eddy compared to all other stations including the near coast ones results from eddy pumping of nutrient into the epipelagic layer that promotes growth of phytoplankton. Overall, our study supports that cyclonic eddies are important vehicles for the transport of fresh OM that fuel heterotrophic activity the open ocean, highlighting the coupling between productive EBUS and the adjacent oligotrophic ETNA.</p>

Large-scale metabarcoding analysis of epipelagic and mesopelagic copepods in the Pacific

A clear insight into large-scale community structure of planktonic copepods is critical to understanding mechanisms controlling diversity and biogeography of marine taxa, owing to their high abundance, ubiquity, and sensitivity to environmental changes. Here, we applied a 28S metabarcoding approach to large-scale communities of epipelagic and mesopelagic copepods at 70 stations across the Pacific Ocean and three stations in the Arctic Ocean. Major patterns of community structure and diversity, influenced by water mass structures, agreed with results from previous morphology-based studies. However, large-scale metabarcoding approach could detected community changes even under stable environmental conditions, including changes in the north/south subtropical gyres and east/west areas within each subtropical gyre. There were strong effects of epipelagic environment on mesopelagic communities, and community subdivisions were observed in the environmentally-stable mesopelagic layer. In each sampling station, higher operational taxonomic unit (OTU) numbers and lower phylogenetic diversity were observed in the mesopelagic layer than in the epipelagic layer, indicating a recent rapid increase of species numbers in the mesopelagic layer. The phylogenetic analysis utilizing representative sequences of OTUs revealed trends of recent emergence of cold-water OTUs mainly distributed at high latitudes with low water temperatures. Conversely, high diversity of copepods at low latitudes was suggested to have been formed through long evolutionary history under high water temperature. The metabarcoding results suggest that evolutionary processes have strong impacts on current patterns of copepod diversity, and support the “out of the tropics” theory explaining latitudinal diversity gradients of copepods. Both diversity patterns in epipelagic and mesopelagic showed high correlations to sea surface temperature; thus, predicted global warming may have a significant impact on copepod diversity in both layers.Author SummaryMarine planktonic copepods are highly dominant and diverse, and revealing their community structure and diversity is important to understanding marine ecosystems. We used molecular-based metabarcoding to reveal a total of 205 copepod communities in the ‘sunlight’ or epipelagic layer (0– 200 m) and the ‘twilight’ or mesopelagic layer (200–500 m and 500–1,000 m), mainly in the Pacific Ocean (data for 70 stations), but also in the Arctic Ocean (data for three stations). Different copepod communities were found in each geographical region with different environmental conditions, including tropical, subtropical, transition, Kuroshio Current, California Current, subarctic and arctic areas. The metabarcoding method sensitively detected small changes of copepod community even in environmentally-stable subtropical ocean systems and the mesopelagic layer. A high diversity of copepods was detected at low latitudes, and copepod diversity was higher in the mesopelagic layer than in the epipelagic layer in each area. These diversity patterns were influenced by both evolutionary history and present environmental conditions. The copepod community in the mesopelagic layer was strongly influenced by environmental conditions in the epipelagic layer. Thus, predicted climate changes may affect marine ecosystems not only in the epipelagic layer but also in the mesopelagic layer.

TROPHODYNAMICS OF MARINE ORGANISMS IN THE EPIPELAGIC LAYER OF THE OKHOTSK SEA IN 2000S

New data on matter and energy transfer between major components of the Okhotsk Sea ecosystem are obtained on the base of trophodynamic modeling, taking into consideration their production and food consumption rates. The main trophodynamic relationships in the pelagic and bottom communities are determined from observations on zooplankton and nekton abundance, organic carbon content, food habits of marine organisms, and their isotope composition in 2000–2014. The total zooplankton production in the entire Okhotsk Sea in these years is assessed as 2616 . 106 t in raw weight, including 2275 . 106 t for non-predatory plankton, and 341 . 106 t for predatory plankton. So high total production of zooplankton is conditioned by favorable environmental conditions and dominance of high-productive species. Taking into account the rate of zooplankton consumption by predators, only 22.4 % of the total annual zooplankton production was consumed annually, with 16.2 % grazed by predatory plankton and 6.2 % by nekton. In carbon units, 831.0 . 106 tC was produced annually in the Okhotsk Sea at the first trophic level, 177.4 . 106 tC at the second trophic level, 18.1 . 106 tC at the third trophic level, 0.74 . 106 tC at the fourth trophic level, and 0.016 . 106 tC at the fifth trophic level. Pelagic nekton consumed 159 . 106 tC annually. The nekton prey included 85.5 % of zooplankton, 12.8 % of nekton, and 1.7 % of zoobenthos, by biomass. The main part of zooplankton consumed by nekton (50.7 %) was grazed by walleye pollock, 18.9 % by herring, 16.6 % by squids, 7.6 % by capelin, 5.3 % by deep-sea smelt, and 0.9 % by salmons. The total annual production of organisms in the epipelagic layer of the Okhotsk Sea exceeded 109 tons of C (1027.4 . 106 tC/year equal to the biomass of 17.85 . 109 t in wet weight). Primary production is estimated as 67.60 % of gross production in carbon units, microheterotrophic organisms produce 13.30 %, dominant zooplankton groups — 18.60 % (copepods 11.40 %, euphausiids 5.50 %, sagittas 1.20 %, and hyperiids 0.50 %), the portion of nekton production is estimated as 0.13 % of gross production.