Comparative study of Two Rocky Intertiday Communities in Las Grutas, Río Negro, Argentina

BOTTA, V.; CASAVECCHIA, C; DILLON, E.; DOMININO, J.; ESTALLO, E.; SOLEDAD, L.; PORCASI,X.; RODRIGUEZ. P;TARANTELLI.M.S.

Maó, España

Congreso; 36th European Marine Biology Symposium; 2001

Intertidal communities are sensible to different physical and biological disturbances, which determine a tipical zonation pattern, originated by exposition, wave action, substrate type, predation, etc. This study was carried out in Las Grutas, (40º 49´ S, 65º 05´O), Golfo San Matías, Río Negro, Argentina. The locality is a touristic beach, where the attached fauna were removed by artificial mechanical action in some areas, approximately seven years ago. The objetive of this study was to identify zonation patterns and to compare two different areas characterized by adscence (station A) and prescence (station B) of the refered human disturbance. Sampling were performed in the two areas along 5 transects (100m each one) paralels to the coast line and distributed through the intertidal area at intervals of  150m. Raw data were obtained by samplig 20 cuadrats (100-point, 0.25 m2)along each transect. In order to analyze the communities structure, percentage cover, abundance, diversity by Shannon-Wiener index and k-dominance curves were performed. Multivariate analysis, classification (cluster) and ordination (MDS), were also used. A marked zonation was observed along a tidal  vertical gradient in accordance with general patterns described in intertidal systems. Species composition are discussed. Both infralittoral communities were characterized by the dominance of macroalgal assemblages while mesolittoral were dominated by faunal organisms, like Brachydontes sp. with scarce macroalgae. Cluster analyzes showed that the infralittoral community structure of the station B is more similar to mesolittoral of station A than the rest intertidal zone at its own station. This pattern appears to be due to the lack of  artificial remotion in the infralittoral of station B. Diversity gradient was different in the two stations; a higher diversity was observed in the infralittoral and in the high mesolittoral in station A, while a diversity decrese was recorded in station B, with the highest diversity in the infralittoral. Exposition-inmersion time and artificial remotion appear to be the main structural forces that determine the observed patterns. The disturbed area at the the station B has not recovered its original structure (particularly the reestablishment of the bivalve comunity) at the time of this study. Therefore, we can infer that the resilience time of this system in longer than seven years.