Population dynamics of the barnacle Balanus amphitrite in an intertidal area affected by sewage pollution

Recruitment and mortality patterns in an intertidal population of Balanus amphitrite were studied for 4.5 years close to a sewage outfall near Quequen, Argentina. Recruitment occurred mainly during summer. although a small proportion of the population recruited in autumn. As a consequence of the failure of 4 successive annual recruitments, population density decreased almost steadily from 130 to 6 ind.m(-2). With the exception of the postsettlement period, which was not considered in the present study, mortality was gradual during benthic life. The number of barnacles dying per unit time decreased as annual cohorts grew older. Mean longevity was estimated to be 22 months. Maximal longevity attained 5-6 years. Between 4 and 6 cohorts coexisted during the study period. The percentage of dead barnacles found with empty shells still attached to the substratum was higher in winter/spring than in summer/autumn, and increased with age. The causes of the extended life history of B. amphitrite at Quequen may be related to low temperature, low population density, and near absence of intraspecific competition and predation. The population studied seems to be regulated by recruitment and physical disturbance.