Stefan Forster

Although marine sediments are recognized as a sink for marine microplastic particles, investigations into the fate of once deposited particles have received relatively little interest so far. Through interactions with sediment-dwelling organisms, these particles are subject to potential burial in the wake of bioturbation. In this study, we analyzed the effect of sediment reworking by the polychaete Hediste diversicolor O.F. Müller, 1776, using different particle tracers—luminophores, polyethylene (PE), and polyamide (PA). Transport induced by H. diversicolor was observed to extend to sediment depths up to 14 cm with similar biodiffusive and advective transport rates for all tracer types. Local transport (Db) was 1.09 cm² yr⁻¹ for luminophores; microplastics showed slightly lower local transport with Db of 0.81 cm² yr⁻¹ (PA) and 0.57 cm² yr⁻¹ (PE). Non-local transport (r) was comparably low for all particle types, ranging from 0.69 yr⁻¹ for luminophores and 0.95 yr⁻¹ (PA) and 1.27 yr⁻¹ (PE) for microplastics. Food addition did not significantly increase bioturbation activity of H. diversicolor. Despite differences in size and density, the particle tracer types in this study showed no significant differences in modeled bioturbation coefficients or burial depth, emphasizing that bioturbation of microplastics is not governed by particle properties alone but also by the functional ecology of the bioturbator.

Coastal sediments are frequently permeable due to their relatively large grains size. Knowledge on exchange processes across the sediment-water interface and metabolism in these sediments is limited however. We characterize permeable sediments at about 5 m water depth in a coastal stretch of ~3.5 km² at the southern Baltic coast off Germany. Permeability ranged from 1.4 .10-12 m² to 11.3 .10-11 m² (organic content: 0.1% - 0.2% dry mass). We determined total oxygen uptake, TOU, of 10 – 28 mmol O2 m² d-1 from in situ measurements in the dark. Benthic net primary production determined in situ varied between 1 - 14 mmol O2 m² d-1. We observed an increase in volumetric oxygen uptake rates in flow-through experiments when highly reactive glucose was supplied as substrate, pointing to the pivotal role of reactive organic substrate availability. However, we could detect only marginally enhanced TOU (uptake doubled at one out of three locations) when applying stirring rates inducing pore water flow in benthic chambers under natural conditions. We conclude that stimulating effects of permeability associated with pore water flow are not detectable in benthic exchange rates below a threshold of 7 .10-11 m² under field conditions. This threshold is higher than previously reported. Ex situ experiments demonstrated that the distribution of oxygen in the sediment was affected by photosynthetic activity of microphytobenthos and by pore water flow. Benthic primary production determined by the dark-light shift method exceeded the summed fluxes of oxygen into the water and into the sediment driven by concertation gradients, and increased with light intensity as well as with organic substrate availability. These findings indicate that calculated net ecosystem metabolism can shift from autotrophy to heterotrophy owed to an increased consumption within the sediment during advection. We argue that under advective conditions the export flux of photosynthetically produced oxygen may differ from the flux under diffusive conditions. This may seriously impair photosynthesis rate determinations obtained from incubation experiments.

The response of benthic habitats and organisms to bottom-contact fishing intensity is investigated in marine protected areas (MPAs) of the German EEZ in the North and Baltic Seas. We examined the current state of macrofauna biodiversity in 2020–2022. Comparative analysis for macrofauna (in- and epifauna) inhabiting nine Natura 2000 MPAs constitutes a baseline to assess the effects of bottom-contact fishing exclusion in the future. Aspects of spatial and temporal variability are briefly summarized and discussed. We provide a species list for each region, including 481 taxa, of which 79 were found in both regions, 183 only in the North Sea, and 219 only in the Baltic Sea. The Baltic Sea dataset surprisingly included higher numbers of taxa and revealed more Red List species. The share of major taxonomic groups (polychaetes, bivalves and amphipods) in species richness showed peculiar commonalities between the two regions. In the North Sea, multivariate analysis of community structure revealed significantly higher within-similarity and stronger separation between the considered MPAs compared to the Baltic MPAs. Salinity, temperature and sediment fractions of sand were responsible for over 60% of the variation in the North Sea macrofauna occurrence data. Salinity, mud fraction and bottom-contact fishing were the most important drivers in the Baltic Sea and, together with other considered environmental drivers, were responsible for 53% of the variation. This study identifies aspects of macrofauna occurrence that may be used to assess (causes of) future changes.

In the process of reworking sediments and thus shaping biogeochemical processes, marine bottom-dwelling animals are thought to play a pivotal role in many benthic environments. Bioturbation (particle reworking) includes the downward transport of particles into the sediment as a major process and is sometimes detected as subsurface maxima (peaks) of specific particulate substances (tracers). Here, we document the fact that subsurface peaks, such as those typically attributed to biological particle transport in sediments, may equally be generated by otter boards in bottom-trawling fishery. Boards can generate tracer peaks whereby they scoop sediment from the surface, flip it over, and deposit it onto the adjacent seafloor. These peaks are indistinguishable from those generated by benthic fauna burying surface material at sediment depth. We demonstrate this for the particle tracer chlorophyll a in silty sand from the western Baltic Sea with fauna that generally do not burrow deep in a global comparison. Our inability to distinguish the driving processes generating the peaks indicates limits to our understanding of the magnitude and spatial extent of bioturbation traces in this environment. It also poses a problem for the assessment of fishery resource use and benthic processes. However, based on natural fauna abundance, behavioral information, and fishery intensity data, we identify macrofauna and not otter boards as the dominant cause of peaks at the sites investigated here.

In the process of reworking sediments and thus shaping biogeochemical processes, marine bottom dwelling animals are thought to play a pivotal role in many benthic environments. This aspect of bioturbation (particle reworking) is often partitioned into short distance local and non-local transport acting over relatively longer distances. Here we document that subsurface peaks, such as those typically attributed to biological particle transport in sediments, may equally be generated by otter boards in bottom trawling fishery. Boards can generate tracer peaks whereby they scoop sediment from the surface, flip it over and deposit it onto the adjacent sea floor. These peaks are indistinguishable from those (presumably) generated by benthic fauna in a process whereby. We demonstrate this for the particle tracer chlorophyll a in silty sand from the Western Baltic Sea with fauna that generally does not burrow deep in a global comparison. Our inability to distinguish the driving processes generating the peaks indicates limits to our understanding of magnitude and spatial extend of bioturbation traces in this environment. It also poses a problem to the assessment of fishery resource use and benthic ecosystem services. However, we can clearly identify macrofauna and not otter boards as the cause for peaks at the sites investigated here.

The information on the biodiversity of crustaceans, especially brachyuran crabs on the northern shore of the Gulf of Aden is even more limited and almost completely unknown. The coasts in the south of the Arabian Peninsula at the Gulf of Aden have not received much attention, with few studies in the field of marine biodiversity in general and brachyuran crabs in particular. There is a large gap in knowledge on the fauna of brachyuran crabs inhabiting the coastline in this region, so; this study aims to fill the gap in the data in this part of the Arabian Peninsula. In this study, 1750 brachyuran crab specimens were collected from 36 sites along the Yemeni coastal waters in the Gulf of Aden. These specimens were collected by hand, hand nets, snorkeling up to 2 m depth, SCUBA diving. Moreover, trawl nets were used up to 300 meters and small fish traps. All these specimens were transferred to ZSM for identification (by morphological characteristics and morphological characteristics of the gonopods) and putting together all information such as sex, measurements, locations, collection dates, habitats, and collectors of the specimens. This study recorded 163 brachyuran crabs’ species belonging to 17 super families, 28 families and 110 genera. Of the 163 species, 53 species represent the first record for Yemeni coastal waters at the Gulf of Aden. Moreover, 8 species were considered the first recorded in the marine waters of the Arabian Peninsula is given.