Catchability of pelagic trawls for sampling deep-living nekton in the mid-North Atlantic

Material collected in summer 2004 from the Mid-Atlantic Ridge between Iceland and the Azores with three pelagic trawls was used to estimate relative catchabilities of common fish, cephalopod, decapod, and jellyfish species. Catchability is defined as the ratio of numbers caught between two trawls, standardized for towed distance. Taxon-specific catchability coefficients were estimated for two large pelagic trawls with graded meshes, using a smaller pelagic trawl with a uniform mesh size as the reference trawl. Two of the trawls were equipped with multiple opening–closing codends that allowed sampling of different depth layers. Generalized linear and mixed models suggest that most of the taxa have catchabilities much lower than expected from the area of opening alone, indicating that only a few species are herded by the large mesh at the mouth of larger trawls. Catchability coefficients across taxa show a very large spread, indicating that the sampled volume for the larger trawls with graded meshes was highly taxon-specific. Part of this variability can be explained by body size and taxonomic group, the latter probably reflecting differences in body form and behaviour. The catchability estimates presented here form the basis for combining data for quantitative analyses of community structure

Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota

Bottom trawling accounts for almost one quarter of global fish landings but may also have significant and unwanted impacts on seabed habitats and biota. Management measures and voluntary industry actions can reduce these impacts, helping to meet sustainability objectives for fisheries, conservation and environmental management. These include changes in gear design and operation of trawls, spatial controls, impact quotas and effort controls. We review nine different measures and actions and use published studies anda simple conceptual model to evaluate and compare their performance. The risks and benefits of these management measures depend on the extent to which the fishery is already achieving management objectives for target stocks and the characteristics of the management system that is already in place. We offer guidance on identifying best practices for trawl-fisheries management and show that best practices and their likelihood of reducing trawling impacts depend on local, national and regional management objectives and priorities, societal values and resources for implementation. There is no universalbest practice, and multiple management measures and industry actions are required to meet sustainability objectives and improve trade-offs between food production and environmental protection

Short-term fate of seagrass and macroalgal detritus in Arenicola marina bioturbated sediments

© The authors 2020. Seagrass meadows are globally important ecosystems for carbon (C) sequestration. However, bioturbation by benthic fauna can alter the distribution, degradation and overall preservation of C in the sediment. We performed a 4 wk laboratory experiment to investigate the short-term degradation and burial of 2 major C sources in bare sediments associated with seagrass ecosystems. Eelgrass Zostera marina and macroalgal (Fucus vesiculosus) detritus were amended in sediment with and without bioturbation by the common polychaete Arenicola marina. Bioturbation did not significantly affect the loss of eelgrass detritus (>0.5 mm), but caused a rapid burial of this material as a discrete layer (55% recovery) at sediment depths ranging from 8 to 14 cm. A. marina effects on macroalgal detritus were more pronounced, resulting, in total, in an 80% loss of macroalgal detritus by microbial degradation and worm ingestion. We conclude that A. marina bioturbation effectively buries eelgrass detritus into deep anoxic sediments, but we cannot confirm that this leads to enhanced C preservation in coastal ecosystems. In contrast, A. marina bioturbation significantly increases the degradation of macroalgal tissue, and it is unlikely that this detritus is a major source for permanent C burial

Carbon degradation in agricultural soils flooded with seawater after managed coastal realignment

Permanent flooding of low-lying coastal areas is a growing threat due to climate change and related sea-level rise. An increasingly common solution to protect coastal areas lying below sea level is intentional flooding by "managed coastal realignment". However, the biogeochemical implications of flooding agricultural soils with seawater are still not well understood. We conducted a 1-year mesocosm experiment to investigate microbial carbon degradation processes in soils flooded with seawater. Agricultural soils were sampled on the northern coast of the island Fyn (Denmark) at Gyldensteen Strand, an area that was subsequently flooded in a coastal realignment project. We found rapid carbon degradation to TCO2 1 day after experimental flooding and onwards and microbial sulfate reduction established quickly as an important mineralization pathway. Nevertheless, no free sulfide was observed as it precipitated as Fe–S compounds with Fe acting as a natural buffer, preventing toxic effects of free sulfide in soils flooded with seawater. Organic carbon degradation decreased significantly after 6 months, indicating that most of the soil organic carbon was refractory towards microbial degradation under the anoxic conditions created in the soil after flooding. During the experiment only 6–7 % of the initial soil organic carbon pools were degraded. On this basis we suggest that most of the organic carbon present in coastal soils exposed to flooding through sea-level rise or managed coastal realignment will be permanently preserved

Organic N and P in eutrophic fjord sediments – rates of mineralization and consequences for internal nutrient loading

Nutrient release from the sediments in shallow eutrophic estuaries may counteract reductions of the external nutrient load and prevent or prolong ecosystem recovery. The magnitude and temporal dynamics of this potential source, termed internal nutrient loading, is poorly under\-stood. We quantified the internal nutrient loading driven by microbial mineralization of accumulated organic N (ON) and P (OP) in sediments from a shallow eutrophic estuary (Odense Fjord, Denmark). Sediments were collected from eight stations within the system and nutrient production and effluxes were measured over a period of ~ 2 years. Dissolved inorganic nitrogen (DIN) effluxes were high initially but quickly faded to low and stable levels after 50–200 days, whereas PO₄³⁻ effluxes were highly variable in the different sediments. Mineralization patterns suggested that internal N loading would quickly (< 200 days) fade to insignificant levels, whereas internal PO₄³⁻ loading could be sustained for extended time (years). When results from all stations were combined, internal N loading and P loading from the fjord bottom was up to 121 × 10³ kg N yr⁻¹ (20 kg N ha⁻¹ yr⁻¹) and 22 × 10³ kg P yr⁻¹ (3.6 kg P ha⁻¹ yr⁻¹) corresponding to 6 (N) and 36% (P) of the external nutrient loading to the system. We conclude that the internal N loading resulting from degradation of accumulated ON is low in shallow eutrophic estuaries, whereas microbial mineralization of accumulated OP is a potential source of P. Overall it appears that, in N-limited eutrophic systems, internal nutrient resulting from mineralization of ON and OP in sediments is of minor importance