Aarhus University Seal / Aarhus Universitets segl

No. 405: Immigration and biodiversity on the new rock reef at Livø

Dahl, K., Buur, H., Andersen, O.N., Göke, C. & Tonetta, D. 2020. Indvandring og biodiversitet på det nye stenrev ved Livø. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 60 s. - Videnskabelig rapport nr. 405. http://dce2.au.dk/pub/SR405.pdf

Summary

New reef structures were laid out near Livø in March 2017 and these, as well as the surrounding seabed, were investigated with a large quantitative field collection program in June 2019.

The study showed that the area of ​​the new reef structures contained both existing dense pebble and small stone reef areas and areas exclusively with sand.

Species diversity on the surrounding seabed was relatively high and comparable to the fauna in terms of species numbers with similar studies from reef areas in the Kattegat. The slightly more than two year old reef structures contained approx. 11 times higher biomass than the surrounding seabed and also significantly higher individual numbers of bottom fauna. The establishment of reefs was at the expense of particular mussel species and some species of burrowing Polychaeta worms. The red-listed species Modiolus modiolus (Horse clam) was also found around the reefs and some are believed to be lost at the time of establishment. One species, the trapped limestone worm Spirobranchus triqueter, was particularly dominant both on hard substrate on the surrounding seabed, but in particular on the new reef structures. On the new reef structures, the limestone worm grew "shoulder by shoulder" and perpendicular to the solid substrate.

There were large variations in both biomass and individual densities between the individual samples. The same large variation was seen in similar studies on reefs in the Kattegat. Despite the variations, we found a significant positive effect of the existing hard bottom on biomasses of fauna on the surrounding seabed around the new reef structures. Furthermore, biomass and species numbers decreased significantly with the depth of the new reef structures.

Macroalgae were not found at all in the suction samples on the surrounding seabed. However, algae were observed and documented at about 4 m depth. Macroalgae were present on the new reef structures, but all together they accounted for only between 3 and 7% of the average biomass in the three depth intervals studied (2.4-3m, 3.1-4m and 4.1-5m).

The sea urchin Psammechinus miliaris (green urchin) was present in large numbers both on the surrounding seabed and on the new reef structures. The high number of sea urchins is likely to cause considerable grazing pressure on macroalgae vegetation. The sparse macroalgae occurrences observed therefore result from a combination of poor lighting conditions and high grazing pressure from sea urchins.