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

Abstract

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.

What has Changed in 20 Years? Structure and Function of Soft-sediment Macrofauna in a Subarctic Embayment, Newfoundland (Canada)

“…Overall, 14 species/taxa were only observed in the historical community, whereas 10 species/taxa were unique to the contemporary. While 63% of species/taxa making up the major taxonomic groups were shared between these two-time points, this contrasts with 86% shared between the two contemporary years (2019 vs. 2020) which were also shown by Komendić (2023) to be similar in community structure. Herein, the contemporary community exhibited a lower density of macrofauna but had similar richness, resulting in higher evenness and diversity compared to the historical one.…”

“…It has freshwater inputs at the head of the bay including the Come by Chance River (mean discharge ~ 1.95 m 3 s −1 ) (MacIsaac et al 2023) and Swift Current (Ramey et al 2003). Moreover, sedimentary TOM has been shown to be highest at the head of the bay compared to other areas (Ramey et al 2003;Komendić 2023). Changes in SST (Han et al 2015) may also play a role as it can influence the timing and amount of food supply (e.g., primary production) to the benthos.…”

“…Variation in macrofaunal community structure and environmental variables between the historical (1998) and contemporary years (this group includes individual samples taken in 2019 and 2020; see Table 1) were examined by conducting Principal Coordinates Ordination (PCO) analysis in PRIMER (v7 +PERMANOVA) (Anderson 2001(Anderson , 2017Clarke and Gorley 2015). Samples collected in 2019 and 2020 were grouped as "contemporary" since community metrics and multivariate analysis of species/taxon data for 2019 vs. 2020 were not different (see Komendić 2023). For the macrofaunal data, the PCO was conducted on a Bray-Curtis dissimilarity matrix generated from species/taxon composition and abundance (at the level of sample; n total = 77).…”

“…This ensured complete independence of each term in the model (Anderson et al 2008) as we were focused on examining the main effect of time (historical vs. contemporary) independent of site differences in the present study. Ramey and Snelgrove (2003) and Komendić (2023) examined community structure in relation to spatial scale differences among sites in the bay). Similarity percentage (SIMPER, Clarke 1993) was used to identify the species/taxa contributing to 50% of the dissimilarity between years.…”