| Abstract |
Sandy beaches are dynamic interfaces between sea and land, where nutrients are transferred in both directions. In recent years, an increasing interest for the study of these dynamic and continuously changing environments has arisen due to their significance both as buffer zones against the sea and as recreational assets. However, there is a limited number of studies focused on the rich and dominant meiofauna of the sandy beaches while, only few of them, mainly descriptive, are considering the unique, almost atidal environment of most mediterranean shores. Therefore, the present study is aiming at contributing to the knowledge of the ecology of sandy beach meiobenthos through the investigation of the seasonal and spatial dynamics of the meiofauna and the meiobenthic copepods in particular. A 13-month sampling design was applied to two geographically spaced sandy beaches in Crete, namely Elafonissi and Pahia Ammos (Greece, Eastern Mediterranean), which according to the exposure rating system of McLachlan are classified as sheltered and exposed respectively. At each beach, sediment samples were collected by means of an acrylic tube corer along a transect extending from the mediolittoral to the nearshore sublittoral zone. Three stations were located on the swash zone corresponding to the retention, resurgence and saturation zones of Salvats intertidal zonation scheme, while a fourth station at 1 m depth was sampled at the sublittoral. Triplicate cores of 30 cm sediment depth from each station were used for meiofaunal analyses while two additional cores were collected for measuring organic carbon and chloroplastic pigments concentrations as well as for grain size analysis. Each sediment core was subdivided in 6 equal fractions in order to investigate the vertical distribution of faunal and environmental parameters. Temperature, salinity and Eh were also measured for each sediment layer and the beach profile of each site was measured monthly. Several uni- and multi-variate statistical techniques were applied for the data analyses and Hills diversity numbers were calculated for estimating the diversity of meiobenthic copepods. The abundances of major taxa and dominant harpacticoid species as well as diversity indices were compared in space and time using ANOVA and the Kruskal-Wallis test. Bray-Curtis similarities were calculated between different stations and months in order to examine the faunal zonation patterns by non-metric Multi-Dimensional Scaling (nMDS) and Cluster Analysis. For defining the plausible environmental gradients Principal Component Analysis (PCA) was applied on the environmental data. Though the two study sites experience different degree of exposure, they share common features, namely, the well oxygenated substratum of medium sand, the steep slope of the beach face and the low level of organic carbon and chloroplastic pigments in the sediment. These factors are also accounting for the differentiation of the medio- and sub-littoral zones in the sheltered sandy beach at Elafonissi. Meiobenthic composition differs between the two beaches, as nematodes predominate in sheltered conditions and copepods in the exposed environment. Meiobenthic abundances were higher during warm months and seasonal differences were more pronounced in the retention zone. Hydrodynamism seems to be the key factor explaining the vertical meiofaunal distribution forcing the interstitial animals to move deeper into the sediment at the exposed beach of Pahia Ammos, while in the sheltered beach at Elafonissi, the meiofauna appears closer to the upper layers of the sediment. During this study, most of the dominant copepod species displayed several peaks of abundance as well as a trend towards avoidance of the surface sediment layer in both types of beaches as a result of the movement of the shoreline, the wave action and weather changes. Benthic copepod diversity is higher in the sheltered environment of Elafonissi beach, exhibiting a pattern similar to that described for macrofaunal communities. However, the across-shore diversity patterns differ with regard to the beach type. In the sheltered beach the sublittoral zone hosts a higher number of species in comparison with the mediolittoral zone, while at the exposed sandy beach the horizontal diversity patterns appear irregular and heterogeneous in time. Hydrodynamic conditions and food availability are always assumed to be the main control factors influencing the observed patterns. The meiobenthic copepod assemblages support the existence of two biological zones in both types of beaches, which correspond to the medio- and sub-littoral zones. However, the zonation patterns in the mediolittoral zone do not agree with the traditional macrofaunal schemes, especially with regard to the exposed beach, reinforcing thus the view that biological zonations are intensively and dynamically changing as a response to the continuously fluctuating environment of sandy beaches.
|
Οικολογία του μειοβενθούς σε αμμώδεις ακτές της Κρήτης: πρότυπα κατανομής των μειοβενθικών κωπηποδών στο χώρο και στο χρόνο
