Carbon export in the seasonal sea ice zone north of Svalbard from winter to late summer

Phytoplankton blooms in the Arctic Ocean's seasonal sea ice zone are expected to start earlier and occur further north with retreating and thinning sea ice cover. The current study is the first compilation of phytoplankton bloom development and fate in the seasonally variable sea ice zone north of Svalbard from winter to late summer, using short-term sediment trap deployments. Clear seasonal patterns were discovered, with low winter and pre-bloom phytoplankton standing stocks and export fluxes, a short and intense productive season in May and June, and low Chl a standing stocks but moderate carbon export fluxes in the autumn post-bloom conditions. We observed intense phytoplankton blooms with Chl a standing stocks of >350 mg m−2 below consolidated sea ice cover, dominated by the prymnesiophyte Phaeocystis pouchetii. The largest vertical organic carbon export fluxes to 100 m, of up to 513 mg C m−2 day−1, were recorded at stations dominated by diatoms, while those dominated by P. pouchetii recorded carbon export fluxes up to 310 mg C m−2 day−1. Fecal pellets from krill and copepods contributed a substantial fraction to carbon export in certain areas, especially where blooms of P. pouchetii dominated and Atlantic water advection was prominent. The interplay between the taxonomic composition of protist assemblages, large grazers, distance to open water, and Atlantic water advection was found to be crucial in determining the fate of the blooms and the magnitude of organic carbon exported out of the surface water column. Previously, the marginal ice zone was considered the most productive region in the area, but our study reveals intense blooms and high export events in ice-covered waters. This is the first comprehensive study on carbon export fluxes for under-ice phytoplankton blooms, a phenomenon suggested to have increased in importance under the new Arctic sea ice regime

The summer bacterial and archaeal community composition of the northern Barents Sea

Climate change related alterations in the Arctic have influences on the marine ecosystems, in particular on phytoplankton bloom dynamics. Since phytoplankton blooms are the main provider of carbon sources to the microbial loop, the bacterial and archaeal community are affected by the changes as well. Warmer water and less sea ice can lead to an earlier onset of phytoplankton blooms and consequently also to changes in the bacterial and archaeal community dynamics throughout Arctic summers. Here, we compared the bacterial and archaeal community composition during three summers (2018, 2019, and 2021) along a transect from the Barents Sea to the Arctic Ocean north of Svalbard. We used 16S rRNA gene sequencing to investigate changes in the communities in time and space. The main results showed that, Gammaproteobacteria (Nitrincolaceae), Bacteroidia (Polaribacter), and Alphaproteobacteria (SAR11 clade 1a members) dominated the bacterial and archaeal community in the surface waters but varied in abundance patterns between the years. The variations are potentially a result of different phytoplankton bloom stages and consequently differences in the availability of carbon sources. The distinctly different deep water communities were dominated by Candidatus Nitrosopumilus, Marinimicrobia, and members of the SAR324 clade in all years. The results indicate that changes in phytoplankton bloom dynamics can influence bacterial and archaeal community and thereby marine carbon cycling in surface waters, although direct links to the effects of global warming remain uncertain.publishedVersio

A red tide in the pack ice of the Arctic Ocean

Source at https://doi.org/10.1038/s41598-019-45935-0. In the Arctic Ocean ice algae constitute a key ecosystem component and the ice algal spring bloom a critical event in the annual production cycle. The bulk of ice algal biomass is usually found in the bottom few cm of the sea ice and dominated by pennate diatoms attached to the ice matrix. Here we report a red tide of the phototrophic ciliate Mesodinium rubrum located at the ice-water interface of newly formed pack ice of the high Arctic in early spring. These planktonic ciliates are not able to attach to the ice. Based on observations and theory of fluid dynamics, we propose that convection caused by brine rejection in growing sea ice enabled M. rubrum to bloom at the ice-water interface despite the relative flow between water and ice. We argue that red tides of M. rubrum are more likely to occur under the thinning Arctic sea ice regime

The seeding of ice algal blooms in Arctic pack ice: The multiyear ice seed repository hypothesis

Source at http://dx.doi.org/10.1002/2016JG003668 During the Norwegian young sea ICE expedition (N-ICE2015) from January to June 2015 the pack ice in the Arctic Ocean north of Svalbard was studied during four drifts between 83° and 80°N. This pack ice consisted of a mix of second year, fi rst year, and young ice. The physical properties and ice algal community composition was investigated in the three different ice types during the winter-spring-summer transition. Our results indicate that algae remaining in sea ice that survived the summer melt season are subsequently trapped in the upper layers of the ice column during winter and may function as an algal seed repository. Once the connectivity in the entire ice column is established, as a result of temperature-driven increase in ice porosity during spring, algae in the upper parts of the ice are able to migrate toward the bottom and initiate the ice algal spring bloom. Furthermore, this algal repository might seed the bloom in younger ice formed in adjacent leads. This mechanism was studied in detail for the dominant ice diatom Nitzschia frigida . The proposed seeding mechanism may be compromised due to the disappearance of older ice in the anticipated regime shift toward a seasonally ice-free Arctic Ocean

Earlier sea-ice melt extends the oligotrophic summer period in the Barents Sea with low algal biomass and associated low vertical flux

The decrease in Arctic sea-ice extent and thickness as a result of global warming will impact the timing, duration, magnitude and composition of phytoplankton production with cascading effects on Arctic marine food-webs and biogeochemical cycles. Here, we elucidate the environmental drivers shaping the composition, abundance, biomass, trophic state and vertical flux of protists (unicellular eukaryotes), including phytoplankton, in the Barents Sea in late August 2018 and 2019. The two years were characterized by contrasting sea-ice conditions. In August 2018, the sea-ice edge had retreated well beyond the shelf break into the Nansen Basin (>82°N), while in 2019, extensive areas of the northwestern Barents Sea shelf (>79°N) were still ice-covered. These contrasting sea-ice conditions resulted in marked interannual differences in the pelagic protist community structure in this area. In August 2018, the protist community was in a post-bloom stage of seasonal succession characterized by oligotrophic surface waters and dominance of small-sized phytoplankton and heterotrophic protists (predominantly flagellates and ciliates) at most stations. In 2019, a higher contribution of autotrophs and large-celled phytoplankton, particularly diatoms, to total protist biomass compared to 2018 was reflected in higher chlorophyll a concentrations and suggested that the protist community was still in a late bloom stage at some stations. It is noteworthy that particularly diatoms contributed a considerably higher proportion to the protist biomass at the ice-covered stations in both years compared to the open-water stations. This pattern was also evident in the higher vertical protist biomass flux in 2019, dominated by dinoflagellates and diatoms, compared to 2018. Our results suggest that the predicted transition toward an ice-free Barents Sea will lengthen the oligotrophic summer period with low algal biomass and associated low vertical flux.publishedVersio

Discharge of nutrient wastes from salmon farms: environmental effects, and potential for integrated multi-trophic aquaculture

We quantified release rates of carbon (C), nitrogen (N) and phosphorus (P) waste from Norwegian salmon farms in 2009 in order to evaluate the theoretical environmental influence on surrounding waters and the potential for integrated multi-trophic aquaculture (IMTA) driven by salmon aquaculture. Of the total feed input, 70% C, 62% N and 70% P were released into the environment, equivalent to an annual discharge of about 404000, 50600 and 9400 t of C, N and P, respectively, based on total salmon production of 1.02 × 106 t. We predicted that 48% of feed C was respired as CO2, 45% of feed N was excreted as dissolved inorganic N (DIN), and 18% of feed P was excreted as dissolved inorganic P (DIP). Approximately 44% of feed P was released as particles, dominating solid wastes. The mean food conversion ratio (feed supplied per fish produced) of Norwegian salmon farms was 1.16 ± 0.08 SE in 2009. Estimates of the potential for IMTA driven by salmon farming showed a far higher potential for seaweed production based on the released DIN than for mussel production based on released appropriately sized particulate organic carbon (POC). The daily volumetric loading rates of DIN from salmon farms (range for counties: 40 to 501 µg N m−3 d−1) were <15% of the natural loading rate of nitrate from deep water, suggesting that the nutrient loading rate is within safe limits

Responses in the bacterial community structure to waste nutrients from aquaculture: an in situ microcosm experiment in a Chilean fjord

Chilean salmon farms release inorganic nutrients excreted by the fish into the surrounding water in Patagonian fjords. The objective of this experiment from the Comau Fjord (42.2° S) in southern Chile was to study how increased input of ammonium (NH4) and phosphate (PO4) from salmon farms might affect the community structure of bacteria in surface waters where fish farms are located. We used microcosms (35 l) with NH4-N and PO4-P added to the natural seawater in a gradient of nutrient-loading rates, with the same N:P ratio as in salmon aquaculture effluents. Additionally, we measured bacterial community structure at different depths in the Comau Fjord to assess the natural variation to compare with our experiment. We used denaturing gradient gel electrophoresis (DGGE) to create 16S rDNA fingerprints of the bacterial communities and monitored biological and environmental variables (chlorophyll a, inorganic nutrients, pH, microbial abundance). The nutrient-loading rate had a significant impact on the bacterial community structure, and the community dissimilarity between low and high nutrient additions was up to 78%. Of the measured environmental variables, phytoplankton abundance and increased pH from photosynthesis had a significant effect. We observed no significant changes in bacterial diversity, which remained at the same level as in the unmanipulated community. Thus, the bacterial community of the fjord was not resistant, but resilient within the time frame and nutrient gradient of our experiment

Iron cycling in a mesocosm experiment in a north Patagonian fjord: Potential effect of ammonium addition by salmon aquaculture

Salmon aquaculture in Chile has been a rapidly growing industry, generating increasing inputs of organic matter and inorganic nutrients into the ecosystem. We studied the potential impacts of ammonium input by this industry on the cycling of iron (Fe) in a Chilean fjord. The distribution of different Fe fractions at varying ammonium concentrations was monitored in a twenty-two day mesocosm experiment. The setup involved brackish water and seawater; each had a control and four ammonium concentrations. Measurements were performed for total (TFeCh) and dissolved (DFeCh) chelex labile Fe fractions, and particulate Fe (PFe). Results for both brackish and seawater showed similar trends but differences in magnitude. Over time, DFeCh decreased with increasing ammonium concentration, while TFeCh showed up to a three-fold increase positively correlated with ammonium addition, chlorophyll and particulate organic carbon. Overall, PFe values increased over time with 37%–89% of this fraction estimated to be of lithogenic origin. When normalized to particulate organic carbon and chlorophyll, PFe was negatively correlated with ammonium showing an exponential decrease. The PFe measured in the 20–140 μm fraction, showed a hyperbolic relationship with particulate phosphorus, suggesting a change in the ratio for these elements in this size fraction. The increase and dominance of diatoms over time in both water types, together with the observed PFe trend, suggest that large phytoplankton potentially act as the main carrier phase of potential scavenged Fe via the available surfaces of sinking cells. Positive correlations between changes in TFeCh and changes in chlorophyll and particulate organic carbon suggest a biological role in controlling the particulate labile Fe fraction, hence resulting in a potential increase of bioavailable Fe. Increasing ammonium addition in the fjords of Chile caused by salmon aquaculture may affect the phytoplankton assemblage composition and therefore the PFe to organic carbon ratio. Possible changes in biogeochemical Fe cycling may result from nutrient enhanced diatom-dominated blooms acting as more efficient vectors for downward export of organic matter

Responses in the microbial food web to increased rates of nutrient supply in a southern Chilean fjord: Possible implications of cage aquaculture

ABSTRACT: Cage fish farms release the inorganic nutrients ammonium (NH4) and phosphate (PO4) into the surrounding water. The objectives of this experiment from the Comau fjord (42.2° S) in southern Chile was to study how increased input of NH4 and PO4 to pelagic waters affects the biomass of defined functional groups of the microbial food web and the community composition of the micro-autotrophs. We used microcosms with NH4 and PO4 added in a gradient of concentrations, with the same N:P ratio as in aquaculture effluent. In addition, silicic acid was added in a 1:1 ratio with nitrogen to mimic the rich supply of silicon from both deep water and river water entering this fjord. A positive biomass response to nutrient loading rate was observed in the microautotroph, micro-heterotroph, and meso-heterotroph functional groups dominated by microphytoplankton, ciliates, and copepods, respectively. Silicon concentration was reduced to a low level, but the Si supply ratio maintained a dominance of diatoms. Biomass increase was accompanied by a succession in the diatom-dominated community towards large and elongated, possibly grazer-resistant species and some small elongated species adapted to silicon-limited conditions. Grazers had a role in the succession by removing successful non-elongated competitors. Silicon, which is not released by fish farms, played a crucial role in the phytoplankton response. With reduced freshwater input in Patagonia as predicted due to climate change, the supply of silicon to the productive zone below the brackish layer might be reduced, which could shift the phytoplankton community more in favor of dinoflagellates or other non-silicified species