A comparative study of the subtidal meiobenthic community of Mallorca and Cabrera (Balearic Islands, Western Mediterranean) has been carried out in order to investigate the taxon diversity of the meiofaunal components with special emphasis on the nematode associations. Due to the lack of previous studies this preliminary work characterizes the different biotopes and identifies the major spatial distribution patterns within the meiofauna taxa. Samples were collected during September 1992 at 7 stations: 3 sites located on Mallorca and 4 at Cabrera. Differences in diversity between sites are used towards a better understanding concerning the detection of the effect of disturbances caused by anthropogenic influence (Mallorca, highly touristic place, is compared with the protected area of the National Park of Cabrera).The aim was to examine the structure of shallow subtidal meiobenthic communities and their dependence on the physical environment and/or the degree of disturbance or anthropogenic effects. A special attempt was intended to explore the community structure and the spatial distribution in overall population density of the meiofauna in general, and in the genus composition in the population density of the nematode fauna in particular. Nematodes were studied apart from the other meiobenthic taxa as they are abundant with high species diversity, making them suitable for ecological and statistical analysis. They are ubiquitious and persistent as a taxon and are found in all environmental conditions that can support metazoan organisms. The spatial distribution of the meiobenthic communities seems to reflect the environmental variables. Primarily, sediment granulometry seem to be the main factor determining aggregations. Secondly, factors affecting the 'architecture' of the sediment, such as biogenic structures (Posidonia), together with disturbance effects (predation, organic enrichment, sediment reworking due to anchoring activites), are likely expected to play an important role in the infauna distribution in the sediments of Mallorca and Cabrera. Considering the high similarity between replicates, microspatial heterogeneity seems not to affect the infaunal distribution. Assuming that taxonomy is not a problem, the meiofauna is an excellent group to study community structure mechanisms. However, before anthiopogenic influence can be detected there must be a fundamental pool of knowledge relating to the effects of natural factors in the environment on the communities studied so that these effects can be distinguished from pollution effects (Ferris and Ferris, 1979). Therefore, in order to discuss the structural attributes of the marine nematode assemblages changes in physical and biological environmental factors - e.g., sediment granulometry, depth, water temperature, available food resources, biogenic structures (like Posidonia shoots)- should be taken into consideration.The studied meiobenthic communities exhibit average densities ranging between 1000 and 4000 ind./10 cm², similar values to the ones previously reported from other shallow subtidal areas. Nematode are the most dominant taxa in terms of abundance (40-75%); densities vary between 400 and 2800 ind./10cm². Turbellaria is especially abundant at some stations, reaching nearly 50%. Copepoda is following with 2 to 14% of the overall meiobenthic density.Diversity is not greatly differing between the stations. The k-dominance curves are rather similar for all of them. Results from the multivariata analysis show that replicability among cores of the same station is perfect for the nematode taxocenosis, and nearly perfect for the whole meiobenthic community. |
