Comparative study of two maerl beds with different otter trawling history, southeast Iberian Peninsula

1. A comparative study of the characteristics of sediment structure and associated assemblages was conducted in two areas with different trawling histories, one with low trawling pressure and protected since 1989 (Marine Reserve of Tabarca), and another with a high frequency of trawling (Benidorm). 2. The study was based on seasonal quantitative and qualitative sampling of macrofauna, megafauna and algae, over a period of 1 year, using scuba diving and dredging. 3. Sediment composition differed between sites. Overall Tabarca had a coarser sediment than Benidorm where there was a higher mud content and no vertical stratification. At Tabarca there was a differential vertical profile, with a higher percentage of coarse sediment in the surficial horizon, which corresponded mostly to rhodoliths. Benidorm. had more mineral gravel, and Tabarca a higher percentage of biogenic gravel. 4. There were no differences between sites in the percentages of live and dead rhodolitbs in the sediment. Cover of rhodoliths, however, was four times greater at Tabarca. Maximum size of rhodoliths was greater at Tabarca ( mean 16.18mm S.D. 5.73) than at Benidorm (mean 7.64 mm S.D. 2.16). 5. The number of species of algae did not differ significantly between sites: 155 species were found at Tabarca, and 153 at Benidorm, with 13 and 10 exclusive species respectively. Although both algal assemblages were qualitatively similar, there were significant differences in the cover of the main species. At Tabarca, the cover of Corallinales was around 50% of total algal cover, while at Benidorm approximately 90% of the cover was due to non-Corallinales algae, mainly species from the genus Peysonnelia. 6. The number of macrofaunal species at Tabarca was greater than at Benidorm, with 293 (144 exclusive spp.) and 204 (53 exclusive spp.) species, respectively. Density and biomass of macrofauna was also greater at Tabarca. 7. Based on our observations, well preserved Mediterranean maerl grounds are sites with a high diversity and also support a high macrobenthic secondary production which may be important for species of commercial interest. High trawling pressure on maerl areas may affect assemblages negatively by breaking up rhodoliths, diminishing their cover and hence affecting the associated biota. Copyright (C) 2003 John Wiley Sons, Ltd.