Seaweed communities in four subtidal habitats within the Great Bay estuary, New Hampshire: Oyster farm gear, oyster reef, eelgrass bed, and mudflat

Abstract

The seaweed communities that developed on oyster farm gear in the Great Bay Estuary in New Hampshire (NH) were compared to three adjacent natural subtidal habitats: an oyster reef, eelgrass bed, and a mudflat. Both farm gear and oyster reefs have received little attention with respect to associated seaweeds. Comparisons were based upon replicate quadrat samples taken during August, and October 2014, plus August 2015. Mean species richness (all dates combined, N = 12) was significantly and substantially lower on the mudflat (2.86 ± 0.56 SE taxa/0.25 m2), but not different among the other three habitats (range: 9.00 ± 0.97 to 11.00 ± 1.41 taxa/0.25 m2). Mean biomass was also statistically different across habitats (P < .0001), ranging from 5.6 ± 3.0 SE g/m2 on the mudflat to 409.9 ± 67.9 g/m2 on the farm gear. Multivariate (PRIMER) analysis showed each habitat pair had significantly different seaweed communities. Thirty-nine seaweed taxa were recorded from the four habitats over the three dates, plus June 2014 for all the habitats excluding Farm Gear: 22 red, 14 green, and 3 brown algae. Thirty-six of the 39 (92.3%) were native species, including several ulvoid green algae and the brown alga Pylaiella littoralis that has been associated with eutrophic habitats. Eight disjunct species that are more common south of Cape Cod were also collected. Three introduced Asiatic red algae were collected: Dasysiphonia japonica, Agarophyton vermiculophyllum and Melanothamnus harveyi. Non-native seaweeds represented 81% of the biomass on farm gear and 84% on mudflats, but lower fractions in other habitats. Overall, these data document the substantial value of the gear used on oyster farms in providing seaweed habitat in northern New England. Our findings for seaweeds are similar to previous research in the region and elsewhere, which documents the habitat value of oyster farm gear for fish and invertebrates. The artificial materials used for oyster farm gear can provide habitat for native as well as introduced species.