Monitoring of Eutrophication in the Outer Oslofjord 2019-2023. Benthos surveys in 2019. Technical report

Summary

Benthic investigations in the outer Oslofjord in 2019 included surveys of hard bottom flora and fauna in the littoral zone. Quantitative quadrat registrations were conducted at 4 sites and beach transects at 14 sites. The investigation also included determination of lower growth limit (depth) for a selection of up to 9 species of macroalgae at 8 sites. In Norway we have two indices for algal vegetation on marine hard bottom: RSLA/RSL (Reduced Species List with Abundance) and MSMDI (Multi Species Macroalgae Depth Index). 48 taxa were registered at the 4 sites in the Quantitative quadrat registrations. Opportunistic green algae were registered with relatively low abundance. The introduced species Crassostrea gigas was registered at site G6, G7 and G19. Relatively high abundance of juvenile Mytilus edulis was observed at the sites. A total of 78 taxa were registered during the beach transect surveys. Site G8 had the highest number of registered taxa (43 taxa) and the lowest number of taxa (17) were registered at site G10. The opportunistic green algae C. albida were registered with high abundance at site G10 and Ulva spp. were registered with high abundance at site G9. Such fast-growing species can indicate eutrophication. High abundance of juvenile M. edulis were registered at sites G3, G15 and G17. The introduced species C. gigas, D. baillouviana and S. muticum were registered during this survey. Using the MSMDI index, 1 site was classified as having “good” ecological condition, 6 sites were classified as “moderate” and 1 site was classified as having “poor” ecological condition.