Seasonality of recruitment in Antarctic sessile marine benthos

Recruitment is a principal factor determining the establishment, diversity and persistence of assemblages in marine benthic ecosystems. Despite considerable research in temperate and tropical latitudes, however, almost nothing is known of recruitment processes in polar regions. This study presents the first assessment of short-term recruitment of sessile epibenthos at a location within the Antarctic Circle (66.5°S). Recruitment was measured using acrylic panels immersed at 2 depths (8 and 20 m) at each of 3 locations in Ryder Bay, SW Antarctic Peninsula (67°35’S, 68°10’W). Recruitment to upward- and downward-facing panel surfaces was monitored at monthly intervals from March to August 2001, and from April 2002 to February 2003. A total of 41 taxa from 9 phyla were recorded. Bryozoans and spirorbid polychaetes were the most abundant groups, and cheilostome bryozoans were the most speciose. Recruitment occurred throughout the year and average assemblage composition followed a cyclical pattern, suggesting annual reproduction in a majority of the taxa recorded. Within this pattern, most species exhibited pronounced seasonality. In contrast to the general pattern of summer recruitment in temperate assemblages, however, a peak in the number of taxa recruiting occurred in late winter and for cheilostome bryozoans the timing of recruitment showed a correlation with competitive ability: weaker competitors recruiting earlier in the year than stronger competitors. It is suggested that the apparent trend for winter recruitment may be an adaptive response to an environment in which disturbance peaks during summer. Total recruitment to upward-facing surfaces during the study was comparable with that to downward-facing surfaces but seasonal reductions in the relative number of recruits on upper surfaces suggest that post-settlement mortality may be important.