Spatial trends and environmental drivers of epibenthic shelf community structure across the Aleutian Islands

Abstract

The continental shelf around the Aleutian Islands supports important commercial and subsistence fisheries as well as multiple seabird and marine mammal populations. To sustainably manage these populations, more information is needed on the distribution of the benthic communities that support some of the top level consumers. Given the vast size and highly variable physical environment of the Aleutian Islands, it is likely that epibenthic community structure on the continental shelf will vary by geographic area and physical and oceanographic conditions. This project examined spatial patterns in Aleutian epibenthic shelf communities among oceanographic regions (island groups separated by major oceanographic passes) and islands within these regions and identified environmental drivers responsible for important community divisions. Benthic trawls were conducted at 12 Aleutian islands across four oceanographic regions to characterize epibenthic shelf community structure along the island chain. It was tested whether the spatial variability in shelf community structure among regions and islands was correlated to multiple environmental variables including bottom water temperature, water depth, distance from shore, exposure, bottom rugosity, sediment grain size, sediment chlorophyll content, and drift algal food subsidies. Overall, communities differed both among regions and among islands within regions. Communities in the Far Western region (Near Strait to Buldir Strait) differed from communities in other regions, largely due to a high density of sand dollars in the Far West. However, none of the measured environmental characteristics explained this difference. Additionally, there was no evidence for a break in epibenthic shelf community structure across Samalga Pass between the East and the Central regions, even though Samalga represents a biogeographic break for many other Aleutian community types, including zooplankton, fish, and kelp forest communities. Within the Central region, a characteristic soft-sediment community (including the flatfish Atheresthes spp. and the crabs Labidochirus splendescens and Chionoecetes bairdi) distinguished Adak Island from other Central islands. Compared with groundfish trawl surveys conducted by the Alaska Fisheries Science Center (AFSC), this study captured less fish but more invertebrates by biomass, which is likely related to different gear selectivity used by the two studies. These findings provide information on the distribution of Aleutian shelf communities that complement existing information from AFSC surveys. In particular, it is shown that there is potentially an important division in epibenthic shelf communities across Buldir Strait, in agreement with the literature identifying this pass as an important biogeographic break. Furthermore, it is suggested that future assessments of Aleutian epibenthic communities should employ a combination of sampling gear types to better represent various epibenthic taxa.