Importance of bioturbators for biodiversity maintenance: indirect effects of fishing disturbance

Abstract

The relative impact on macrofaunal diversity of 7 naturally co-occurring bioturbators has been examined in a benthic mesocosm experiment. The bioturbators chosen were selected because of their potential vulnerability to the disturbances associated with demersal trawling. The experiment was conducted at the Solbergstrand mesocosm (Norwegian Institute for Water Research) using subtidal sediment collected from Bjornehodebukta, a small sheltered bay in the Oslofjord. For 3 of the bioturbating species (Brissopsis lyrifera, Aphrodita aculeata and Amphiura chiajei) a positive, linear relationship was demonstrated between bioturbator abundance and measures of species rich- ness (number of species and Margalef species richness). It was suggested that the presence of these bioturbating species increased oxygen penetration into the sediment, leading to an enhancement in the ability of benthic systems to process organic material. This had a beneficial effect on the associ- ated fauna in terms of maintaining levels of diversity. For Nephtys caeca, highest species diversity was seen in the low abundance treatments. Predation by N. caeca on small polychaetes is proposed as a possible explanation of the lower diversity in high-density treatments. It is concluded that B. lyrifera, A. aculeata, A. chiajei and N. caeca are functionally similar with respect to setting the diversity of the associated macrobenthic community and may therefore fulfill the same role within the benthic ecosystem. The presence of Calocaris macandreae, Nuculana minuta and Astarte sulcata had no detectable effect on diversity. Trawling has been shown to reduce the abundance of bioturbating species, and the current study demonstrates important implications of this loss on the maintenance of diversity. In addition, the interactions between bioturbation, diversity and organic enrichment high- light the need for management of anthropogenic impacts within the coastal environment in a holistic way rather than by isolation.