Fauna associated with detached kelp in different types of subtidal habitats of the White Sea

Abstract

The fauna, associated with Laminaria and other large brown macroalgae was studied by using SCUBA and dredging in two different types of underwater habitats of the White Sea.

In shallow water fjords and bays, with a depth of no more than 30–40 m, detached kelp (mainly Laminaria saccharina, L. digitata and Alaria esculenta) formed large accumulations. One of these benthic accumulations, which has existed more than 20 years, was studied. It covers about 2000 m², and is about 2 m thick. The upper layer of the accumulation of fronds is characterized by high turbulence and is well aerated. The lower layer is characterized by anoxic conditions. Mats of sulphur bacteria were not observed, although fronds in the middle layer were covered by layers of cyanobacteria. About 50 species of macroinvertebrates were found, mainly species that are normally associated with living kelp, such as the detritivorous species Ophiura robusta and Gammarus oceanicus, and few species that are specific inhabitants of organic-rich biotopes in the White Sea such as Capitella capitata, Ophryotrocha irinae and Nebalia bipes. It was remarkable that in the shallow water basins of the White Sea, the process of decomposition of brown algae in the sublittoral takes place without sea urchins, and no other macrofaunal form plays an ecological role in the mechanical breakdown of the plant substratum, even not in the large accumulations of detached kelp.

Along the open rocky shoreline, communities associated with dead detached kelp were situated at a depth of 60–90 m, 40–50 m below the belt of living kelp. In this deep zone, no macroinvertebrates typical of the kelp community in the photic zone were found. During the passage from the shoreline to the deeper benthic community, where sea urchins were dominant, all plant debris became fragmented. These deeper benthic communities appeared to be the zone for decomposition of the detached kelp.