What has changed in 20 years? Structure and function of soft-sediment macrofauna in a subarctic embayment, Newfoundland (Canada)

Understanding how natural and anthropogenic disturbances affect the structure and functioning of marine ecosystems and their associated services is central to predicting future dynamics. Placentia Bay is an Ecologically and Biologically Significant Area (EBSA) in the north Atlantic exposed to multiple stressors. To investigate changes in the community and functional structure of soft-sediment macrofauna as well as environmental drivers of observed variation, we compared contemporary (2019-2020) and historical (1998) samples at eight stations (n=77) collected 21 years apart. Although community and functional structure significantly differed between these two time points, functional traits were maintained (i.e., no loss of 36 trait modalities). Thirty-seven percent of species/taxa where only observed in either the historical or contemporary community, and the contemporary community exhibited a lower density of macrofauna but had similar richness, resulting in higher evenness and diversity. Highly tolerant subsurface deposit feeders having small body sizes (<10mm) and direct development dominated the historical community, whereas the contemporary community had nearly equal proportions of surface and subsurface deposit feeders with small to medium body sizes (<10-50 mm) with pelagic larvae, and the proportion of highly tolerant species/taxa was reduced. These changes likely reflect the large reduction in polychaetes (91 vs. 58%) and increased bivalves (4 vs. 25%) relative to the historical time point. Community variation was driven by changes in the sedimentary habitat. Contemporary versus historical sediments were [~]4.5x coarser and had higher levels of sedimentary organic matter. This work contributes to advancing our understanding of relationships between benthic macrofauna, functional traits, and the sedimentary habitat in coastal environments.