Intertidal benthic communities of two Chilean coastal islands (Santa María and Mocha, Southeastern Pacific)

Six macrotidal beaches located on two Chilean coastal islands (Santa Maria and Mocha, SE Pacific Ocean) were investigated in spring 2001 (29 October-3 November) to verify the role of the food supply (in terms of quantity and biochemical composition of the organic matter) on the abundance and diversity of the macro- and meiobenthic communities inhabiting the beaches. Samples of sediment were collected from the intertidal zone of the three beaches of each island investigated. The total organic matter content of the sediment did not differ between the islands, whereas the phytopigment and protein contents were significantly higher on Mocha (2.3 ± 0.9 and 75 ± 21 μg g-1, mean values of the three beaches ± SE, respectively) than on Santa Maria (0.5 ± 0.2 and 50 ± 16 μg g-1, mean values of the three beaches ± SE). The macro- and meiofaunal assemblages displayed the highest abundances on Mocha (821 ± 223 and 561 ± 194 ind. 10 ± cm-2, respectively). The abundance of both assemblages was correlated with the quality of the food supply, and significant correlations were observed with the phytopigment content (Spearman-rank R = 0.645 p < 0.01 and R = 0.934 p < 0.001 for the macro- and meiofauna, respectively). The results of the present study suggest that food availability may play a key role in structuring benthic communities of the oceanic beaches of Chilean coastal islands. A single observation is not enough to fully understand the real mechanisms that shape the beach communities, however, the snapshot image we collected during this study suggests that the role of food availability in shaping benthic beach communities may be as important as that played by the hydrodynamic conditions.