Predation in aquatic microbial food webs is dominated by phagotrophic protists, yet these microorganisms are still understudied compared to bacteria and phytoplankton. In pelagic ecosystems, predaceous protists are ubiquitous, range in size from 2 µm flagellates to >100 µm ciliates and dinoflagellates, and exhibit a wide array of feeding strategies. Their trophic states run the gamut from strictly phagotrophic, to mixotrophic: partly autotrophic and partly phagotrophic, to primarily autotrophic but capable of phagotrophy. Protists are a major source of mortality for both heterotrophic and autotrophic bacteria. They compete with herbivorous meso- and macro-zooplankton for all size classes of phytoplankton. Protist grazing may affect the rate of organic sinking flux from the euphotic zone. Protist excretions are an important source of remineralized nutrients, and of colloidal and dissolved trace metals such as iron, in aquatic systems. Work on predation by protists is being facilitated by methodological advances, e.g., molecular genetic analysis of protistan diversity and application of flow cytometry to study population growth and feeding rates. Examples of new research areas are studies of impact of protistan predation on the community structure of prey assemblages and of chemical communication between predator and prey in microbial food webs.

/pdf/significance-of-predation-by-protists-in-aquatic-microbial-3p9r3hwxki.pdf

Significance of predation by protists in aquatic microbial food webs.

Arctic sea-ice loss is a leading indicator of climate change and can be attributed, in large part, to atmospheric forcing. Here, we show that recent ice reductions, weakening of the halocline, and shoaling of the intermediate-depth Atlantic Water layer in the eastern Eurasian Basin have increased winter ventilation in the ocean interior, making this region structurally similar to that of the western Eurasian Basin. The associated enhanced release of oceanic heat has reduced winter sea-ice formation at a rate now comparable to losses from atmospheric thermodynamic forcing, thus explaining the recent reduction in sea-ice cover in the eastern Eurasian Basin. This encroaching "atlantification" of the Eurasian Basin represents an essential step toward a new Arctic climate state, with a substantially greater role for Atlantic inflows.

https://science.sciencemag.org/content/sci/356/6335/285.full.pdf

Greater role for Atlantic inflows on sea-ice loss in the Eurasian Basin of the Arctic Ocean

http://www.vliz.be/imisdocs/publications/129585.pdf

Phaeocystis blooms in the global ocean and their controlling mechanisms: a review

Seawater concentrations of the climate-cooling, volatile sulphur compound dimethylsulphide (DMS) are the result of numerous production and consumption processes within the marine ecosystem. Due to this complex nature, it is difficult to predict temporal and geographical distribution patterns of DMS concentrations and the inclusion of DMS into global ocean climate models has only been attempted recently. Comparisons between individual model predictions, and ground-truthing exercises revealed that information on the functional relationships between physical and chemical ecosystem parameters, biological productivity and the production and consumption of DMS and its precursor dimethylsulphoniopropionate (DMSP) is necessary to further refine future climate models. In this review an attempt is made to quantify these functional relationships. The description of processes includes: (1) parameters controlling DMSP production such as species composition and abiotic factors; (2) the conversion of DMSP to DMS by algal and bacterial enzymes; (3) the fate of DMSP-sulphur due to, e.g., grazing, microbial consumption and sedimentation and (4) factors controlling DMS removal from the water column such as microbial consumption, photo-oxidation and emission to the atmosphere. We recommend the differentiation of six phytoplankton groups for inclusion in future models: eukaryotic and prokaryotic picoplankton, diatoms, dinoflagellates, and other phytoflagellates with and without DMSP-lyase activity. These functional groups are characterised by their cell size, DMSP content, DMSP-lyase activity and interactions with herbivorous grazers. In this review, emphasis is given to ecosystems dominated by the globally relevant haptophytes Emiliania huxleyi and Phaeocystis sp., which are important DMS and DMSP producers.

/pdf/environmental-constraints-on-the-production-and-removal-of-1ex5ss0534.pdf

Environmental constraints on the production and removal of the climatically active gas dimethylsulphide (DMS) and implications for ecosystem modelling

Food webs and physical–biological coupling on pan-Arctic shelves: Unifying concepts and comprehensive perspectives ☆

http://www.vliz.be/imisdocs/publications/59196.pdf

Variations in hydrography, nutrients and chlorophyll a in the marginal ice-zone and the central Barents Sea

Investigations of the hydrography, nutrients, fluorescence, and suspended biomass in the basin of the north Norwegian fjord Balsfjord, revealed highly variable hydrographic features from March to early October 1996. Among those are (a) an inflow of more saline water in spring, probably of deep origin outside the fjord, which was slowly diluted until August, (b) an inflow of phytoplankton- and nutrient-rich, but nitrate-low surface water into the deep basin in March-early April and late April to June, respectively, and (c) the intrusion of phytoplankton-rich layers at variable times and depths throughout the water column, probably of surface origin. An extended spring bloom took place between early April to early June and resulted in biomass accumulation in the upper 30 m up to 23 g C m-2. The spring bloom was dominated by the haptophyte Phaeocystis pouchetii (Hariot), which is reflected in the depletion of nitrate and phosphate during the bloom, while silicate prevailed.

Seasonal variation in hydrography, nutrients, and suspended biomass in a subarctic fjord: Applying hydrographic features and biological markers to trace water masses and circulation significant for phytoplankton production

A comparison of vertical flux of biogenic matter, phytoplankton and faecal pellets was carried out in April 1997 in 3 fjords, Ullsfjord, Balsfjord and Malangen, which exhibit different hydro- graphic conditions in the inner northern Norwegian coastal zone. Vertical export of biogenic matter was high (200 to 850 mg particulate organoc carbon (POC) m -2 d -1 ) compared to previous years, par- ticularly that of chlorophyll a (0.9 to 22 mg m -2 d -1 ). Some differences in the timing and composition between the fjords were detected, but on average, vertical export in the upper 100 m was similar in all three. Average phytoplankton carbon (PPC) sedimentation was 136 mg C m -2 d -1 , and it con- tributed 35 ± 12% to the vertical export of POC. The diatoms Thalassiosira angulata, T. norden- skioeldii, Chaetoceros socialis and C. furcellatus were by far the most important taxa, contributing on average 68% to the PPC export. Another dominant spring bloom species in northern Norwegian fjords, the colony-forming prymnesiophyte Phaeocystis pouchetii, contributed on average only 12% to the vertical PPC flux, and it was of minor importance in Balsfjord. In contradiction to previous find- ings in the area, vertical POC export was not dominated by faecal pellet carbon (FPC) (125 mg C m -2 d -1 ). While a significant export of FPC and retention of PPC was observed during the vernal blooms in 1982, 1992 and 1996 (PPC/FPC = 0.23 ± 0.02), a mismatch scenario was encountered in 1997 (PPC/ FPC = 1.32 ± 0.31), resulting in large-scale vertical export of diatoms. The differences in vertical export of biogenic matter between fjords were much smaller than interannual differences. It is sug- gested that interannual differences in oceanic-coastal coupling, crucial for the advection of meso- and macrozooplankton from the shelf adjacent to the fjords and for grazing, are the cause for the recurrent match and infrequent mismatch scenarios.

/pdf/comparison-of-the-springtime-vertical-export-of-biogenic-2l49h8t59a.pdf

Comparison of the springtime vertical export of biogenic matter in three northern Norwegian fjords

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Variations in hydrography, nutrients and chlorophyll a in the marginal ice-zone and the central Barents Sea

Seasonal variation in hydrography, nutrients, and suspended biomass in a subarctic fjord: Applying hydrographic features and biological markers to trace water masses and circulation significant for phytoplankton production

Comparison of the springtime vertical export of biogenic matter in three northern Norwegian fjords