LAUSR

Simple Summary Coastal areas, especially river plumes, are very diverse and dynamic zones where numerous geological, chemical and biological processes take place. This is because fresh water from the river with all substances, including pollutants from the land, mixes with salt water from the sea, creating specific living conditions for the organisms that inhabit the area. These organisms, e.g., macroscopic invertebrates such as mussels or worms, live in the sediment where their movement and feeding activities cause the sediment to mix and allow water to flow through it—these activities are called bioturbation and bioirrigation. Our research aimed to investigate how the structure and functioning of benthic marine ecosystems change with distance from the river mouth. We found that coastal areas are very diverse and host a wide range of organisms that bioturbate and bioirrigate and support sediment transformations relatively deep (up to 15 cm) into the sediment. Farther away from the river mouth, organisms were very scarce and occurred only on the sediment surface and did not burrow into the sediment, so bioturbation and bioirrigation did not take place. The coastal zone is like a hotspot where ecosystem processes and services are intensively reflected, and this is especially important when deeper areas are not functioning properly, as in the Baltic Sea. For this reason, we should consider how we can support the protection and recovery of marine ecosystems. Abstract Macrozoobenthos plays a key role in the transformation of inputs from rivers to the sea, such as nutrients, organic matter, or pollutants, and influences biogeochemical processes in the sediments through bioturbation and bioirrigation activity. The purpose of our study was to determine the structure of benthic communities, their bioturbation (BPC) and bioirrigation potential (IPC), and the vertical distribution of macrofauna in the Gulf of Gdańsk. The study revealed changes in the structure of benthic communities and, consequently, in the bioturbation and bioirrigation potential in the study area. Despite the presence of diverse and rich communities in the coastal zone, BPC and IPC values, although high, were formed by a few species. Both indices were formed mainly by the clam Macoma balthica and polychaetes, although the proportion of polychaetes in IPC was higher than in BPC. In the deepest zones, the communities became poorer until they eventually disappeared, along with all macrofaunal functions. Both indices changed similarly with distance from the Vistula River mouth, and there was a very strong correlation between them. We also demonstrated that the highest diversity of the macrofauna was observed in the upper first cm of the sediment, but the highest biomass was observed in deeper layers—at a depth of up to 6 cm, and single individuals occurred even below 10 cm.