Deep-sea benthic communities and oxygen fluxes in the Arctic Fram Strait controlled by sea-ice cover and water depth

Arctic Ocean surface sea-ice conditions are linked with the deep sea benthic oxygen fluxes via a cascade of interdependencies across ecosystem components such as primary production, food supply, activity of the benthic community, and their functions. Additionally, each ecosystem component is influenced by abiotic factors such as light availability, temperature, water depth, and grain size structure. In this study, we investigated the coupling between surface sea-ice conditions and deep-sea benthic remineralization processes through a cascade of interdependencies in the Fram Strait. We measured sea-ice concentrations, a variety of different sediment characteristics, benthic community parameters, and oxygen fluxes at 12 stations of the LTER HAUSGARTEN observatory, Fram Strait, at water depths of 275–2500&thinsp;m. Our investigations reveal that the Fram Strait is bisected into two long-lasting and stable regions: (i) a permanently and highly sea-ice-covered area and (ii) a seasonally and low sea-ice-covered area. Within the Fram Strait ecosystem, sea-ice concentration and water depth are two independent abiotic factors, controlling the deep-sea benthos. Sea-ice concentration correlated with the available food and water depth with the oxygen flux. In addition, both abiotic factors sea-ice concentration and water depth correlate with the macrofauna biomass. However, at water depths &gt;&thinsp;1500&thinsp;m the influence of the surface sea-ice cover is minimal with water depth becoming more dominant. Benthic remineralization across the Fram Strait on average is  ∼ 1&thinsp;mmol C&thinsp;m−2&thinsp;d−1. Our data indicate that the portion of newly produced carbon that is remineralized by the benthos is 5&thinsp;% in the seasonally low sea-ice-covered eastern part of Fram Strait but can be 14&thinsp;% in the permanently high sea-ice-covered western part of Fram Strait. Here, by comparing a permanently sea-ice-covered area with a seasonally sea-ice-covered area, we discuss a potential scenario for the deep-sea benthic ecosystem in the future Arctic Ocean, in which an increased surface primary production may lead to increasing benthic remineralization at water depths &lt;&thinsp;1500&thinsp;m.</p

Experimental beam-trawling in <i>Lanice conchilega</i> reefs: impact on the associated fauna

To study fisheries impact at the species level in temperate sandy bottom areas, a controlled field manipulation experiment was designed focusing on areas with high densities of the habitat-structuring, tube-dwelling polychaete Lanice conchilega (i.e. L. conchilega reefs). The hypothesis was that the impact on L. conchilega would be minimal, but that the fauna benefiting from the biogenically structured habitat would be impacted by beam-trawling. In this study, the impact of beam-trawl passage on intertidal L. conchilega reefs and its associated fauna was quantified. A treatment zone was exposed to a one-off experimental trawling. Subsequently, the impact on and recovery of the associated fauna was investigated for a period of 9 days post-impact. Community analysis showed a clear impact followed by a relatively quick recovery as apparent through MDS analysis (stress 0.06), a significant (p p = 0.001) dissimilarities between treatment and control and through SIMPER analysis (decreasing dissimilarities over time). This impact and subsequent recovery was largely explained by two species: Eumida sanguinea and Urothoe poseidonis. Species analysis confirmed the beam-trawl passage significantly (p = 0.001) impacted E. sanguinea for the whole period of the experiment. The experiment confirmed that closely associated species of L. conchilega reefs are impacted by beam-trawl fisheries. This small-scale intertidal study provides some pointers which indicate that the tightly associated species will be impacted significantly when beam-trawling L. conchilega reefs in subtidal areas

Meiofauna metabolism in suboxic sediments: currently overestimated

Oxygen is recognized as a structuring factor of metazoan communities in marine sediments. The importance of oxygen as a controlling factor on meiofauna (32 µm-1 mm in size) respiration rates is however less clear. Typically, respiration rates are measured under oxic conditions, after which these rates are used in food web studies to quantify the role of meiofauna in sediment carbon turnover. Sediment oxygen concentration ([O2]) is generally far from saturated, implying that (1) current estimates of the role of meiofauna in carbon cycling may be biased and (2) meiofaunal organisms need strategies to survive in oxygen-stressed environments. Two main survival strategies are often hypothesized: 1) frequent migration to oxic layers and 2) morphological adaptation. To evaluate these hypotheses, we (1) used a model of oxygen turnover in the meiofauna body as a function of ambient [O2], and (2) performed respiration measurements at a range of [O2] conditions. The oxygen turnover model predicts a tight coupling between ambient [O2] and meiofauna body [O2] with oxygen within the body being consumed in seconds. This fast turnover favors long and slender organisms in sediments with low ambient [O2] but even then frequent migration between suboxic and oxic layers is for most organisms not a viable strategy to alleviate oxygen limitation. Respiration rates of all measured meiofauna organisms slowed down in response to decreasing ambient [O2], with Nematoda displaying the highest metabolic sensitivity for declining [O2] followed by Foraminifera and juvenile Gastropoda. Ostracoda showed a behavioral stress response when ambient [O2] reached a critical level. Reduced respiration at low ambient [O2] implies that meiofauna in natural, i.e. suboxic, sediments must have a lower metabolism than inferred from earlier respiration rates conducted under oxic conditions. The implications of these findings are discussed for the contribution of meiofauna to carbon cycling in marine sediments

The early life history of the clam <i>Macoma balthica</i> in a high CO₂ world

This study investigated the effects of experimentally manipulated seawater carbonate chemistry on several early life history processes of the Baltic tellin (Macoma balthica), a widely distributed bivalve that plays a critical role in the functioning of many coastal habitats. We demonstrate that ocean acidification significantly depresses fertilization, embryogenesis, larval development and survival during the pelagic phase. Fertilization and the formation of a D-shaped shell during embryogenesis were severely diminished: successful fertilization was reduced by 11% at a 0.6 pH unit decrease from present (pH 8.1) conditions, while hatching success was depressed by 34 and 87%, respectively at a 0.3 and 0.6 pH unit decrease. Under acidified conditions, larvae were still able to develop a shell during the post-embryonic phase, but higher larval mortality rates indicate that fewer larvae may metamorphose and settle in an acidified ocean. The cumulative impact of decreasing seawater pH on fertilization, embryogenesis and survival to the benthic stage is estimated to reduce the number of competent settlers by 38% for a 0.3 pH unit decrease, and by 89% for a 0.6 pH unit decrease from present conditions. Additionally, slower growth rates and a delayed metamorphosis at a smaller size were indicative for larvae developed under acidified conditions. This may further decline the recruit population size due to a longer subjection to perturbations, such as predation, during the pelagic phase. In general, early life history processes were most severely compromised at ,pH 7.5, which corresponds to seawater undersaturated with respect to aragonite. Since recent models predict a comparable decrease in pH in coastal waters in the near future, this study indicates that future populations of Macoma balthica are likely to decline as a consequence of ongoing ocean acidification