Effects of bottom trawling and environmental factors on benthic bacteria, meiofauna and macrofauna communities and benthic ecosystem processes.

Soft sediment marine benthic ecosystems comprise a diverse community of bacteria, meiofauna and macrofauna, which together support a range of ecosystem processes such as biogeochemical cycling. These ecosystems are also fishing grounds for demersal species that are often caught using bottom trawling. This fishing method can have deleterious effects on benthic communities by causing injury or mortality, and through alteration of sediment properties that in turn influence community structure. Although the impacts of bottom trawling on macrofauna are relatively well studied, less is known about the responses of meiofauna and bacteria to such disturbances, or how bottom trawling impacts benthic ecosystem processes. Quantifying trawling impacts against a background of natural environmental variability is also a challenge. To address these questions, we examined effects of bottom trawling and a range of environmental variables (e.g. water chemistry and physical and biochemical surface sediment properties) on a) bacterial, meiofaunal and macrofaunal community structure and b) benthic ecosystem processes (nutrient fluxes, extracellular enzyme activities and carbon turnover and degradation rates). We also investigated the link between the benthic macrofauna community and the same ecosystem processes. While there was a significant effect of bottom trawling intensity on macrofaunal community structure, the same was not seen for bacterial or meiofaunal community composition, which were more affected by environmental factors, such as surface sediment properties. The labile component of the surface sediment carbon pool was higher at highly trawled sites. Carbon degradation rates, extracellular enzyme activities, oxygen fluxes and some nutrient fluxes were significantly affected by trawling, but ecosystem processes were also strongly linked to the abundance of key bioturbators (Macoma balthica, Halicryptus spinulosus, Scoloplos armiger and Pontoporeia femorata). Although benthic ecosystems were affected by a combination of trawling and natural variability, disentangling these showed that the anthropogenic effects were clearest on the larger component of the community, i.e. macrofauna composition, and on ecosystem processes related to sedimentary carbon.

Effects of Recreational Boating on Microbial and Meiofauna Diversity in Coastal Shallow Ecosystems of the Baltic Sea.

Recreational boating can impact benthic ecosystems in coastal waters. Reduced height and cover of aquatic vegetation in shallow Baltic Sea inlets with high boat traffic have raised concerns about cascading effects on benthic communities in these ecosystems. Here, we characterized the diversity and composition of sediment-associated microbial and meiofaunal communities across five bays subjected to low and high degrees of boating activity and examined the community-environment relationships and association with bay morphometry. We found that recreational boating activity altered meiofauna alpha diversity and the composition of both micro- and meiobenthic communities, and there were strong correlations between community structure and morphometric variables like topographic openness, wave exposure, water surface area, and total phosphorous concentrations. Inlets with high boat traffic showed an increase of bacterial taxa like Hydrogenophilaceae and Burkholderiaceae. Several meiofauna taxa previously reported to respond positively to high levels of suspended organic matter were found in higher relative abundances in the bays with high boat traffic. Overall, our results show that morphometric characteristics of inlets are the strongest drivers of benthic diversity in shallow coastal environments. However, while the effects were small, we found significant effects of recreational boating on benthic community structure that should be considered when evaluating the new mooring projects. IMPORTANCE With the increase of recreational boating activity and development of boating infrastructure in shallow, wave-protected areas, there is growing concern for their impact on coastal ecosystems. In order to properly assess the effects and consider the potential for recovery, it is important to investigate microbial and meiofaunal communities that underpin the functioning of these ecosystems. Here, we present the first study that uses DNA metabarcoding to assess how benthic biodiversity in shallow coastal areas is impacted by recreational boating. Our study shows a relatively small, but significant, effect of recreational boating both on meiofauna alpha diversity and meiofauna and bacterial community composition. However, both meiofauna and bacterial community composition in shallow benthic habitats is mediated to a higher degree by abiotic variables, such as topographic openness, area or size of the inlets, and wave exposure. Despite the fact that the effects were small, such impacts on benthic biodiversity should be considered in the management of coastal shallow habitats.

Meiofauna improve oxygenation and accelerate sulfide removal in the seasonally hypoxic seabed.

Oxygen depleted areas are widespread in the marine realm. Unlike macrofauna, meiofauna are abundant in hypoxic sediments. We studied to what extent meiofauna affect oxygen availability, sulfide removal and microbial communities. Meiofauna were extracted alive and added to intact sediments simulating abundance gradients previously reported in the area. A total of 324 porewater microprofiles were recorded over a 3-week incubation period and microbial community structure and cable bacteria densities were determined at the end of the experiment. At high abundances meiofauna activity deepened oxygen penetration by 85%, 59%, and 62% after 5, 14, and 22 days, respectively, compared to control sediment with scarce meiofauna. After 6 days, meiofauna increased the volume of oxidized, sulfide-free sediment by 68% and reduced sulfide fluxes from 8.8 to 0.4 mmol m-2 d-1. After 15 days, the difference with the control attenuated due to the presence of a cable bacteria population, which facilitated sulfides oxidation in all treatments. 16S rRNA gene analysis revealed that meiofauna affected microbial community structure (beta diversity). Thus, meiofauna bioturbation plays an important role in deepening oxygen penetration, counteracting euxinia and in structuring microbial diversity of hypoxic sediments. Co-existence with cable bacteria demonstrates neutralism interaction between these two ecosystem engineers.