Larval abundance, recruitment and early mortality in Paracentrotus lividus (Echinoidea). Interannual variability and plankton-benthos coupling

Abstract

The temporal patterns of larval abundance, recruitment and early mortality in the sea urchin Paracentrotus lividus (Lamarck) in the northwestern Mediterranean were studied by combining planktonic and benthic surveys. The abundance of planktonic stages was monitored by weekly plankton hauls from 1992 to 1996. The presence of juveniles was studied by collecting epibenthic samples at monthly intervals from 1992 to 1995. The size distribution of the same population was studied in 1993. A main recruitment episode in late spring-summer occurred every year, although small recruitment events had been observed in autumn and winter. Interannual variability in larval abundance and recruitment was high, both being most intense in 1992, when up to 33.47 larvae m'3 were recorded in plankton samples, and up to 23000 recruits (sea urchins <2 mm) m-' were found in the benthic samples. We estimated that only about 12.7% of larvae (from the 8-armed stage onwards) survived the planktonic life, and that 0.5 to 0.7% of the settlers survived until they reached a diameter of 2 mm, while 0.040% of settlers survived the first year of benthic life and only 0.028% of settlers may attain reproductive size. The main bottlenecks in the dynamics of this species occurred, therefore, during the planktonic phase and the first year of benthic existence, particularly during the early post-settlement phase. The benthic samples provided a more robust depiction of events than planktonic data, which featured a higher variability. There was a significant relationship between the interannual variation in the late winter phytoplankton bloom and the spring peak of larvae of this species, indicating that changes in larval abundance are coherent with changes in planktonic primary production. There was also an exponential negative relationship between the abundance of larvae and abundance of recruits relative to larvae over the years studied, indicating density-dependent mortality rates in the planktonic and early benthic stages. Successful cohorts of this species can be traced to years with high larval abundance, which in turn are related to episodes of high planktonic primary production.