Temporal patterns of species composition of siliceous phytoplankton flux in the Santa Barbara Basin

Abstract

We have examined the species composition of 127 sediment trap samples collected sequentially from the Santa Barbara Basin, USA, during a 7-year period. The sampling period included two warm-water periods (El Ninos) and two cold-water periods (one La Nina and one more local). We examined changes in total species composition and changes in subsets of species accompanying extreme environmental conditions, as well as changes in flux composition associated with periods of anomalous flux. Our goal was to improve the scale and precision of hindcasting past conditions from the sedimentary record and, hence, prediction of the biological consequences of environmental change. Although large-scale climate events were accompanied by changes in the flux composition, there was little similarity in composition during environmentally similar periods, precluding gener- alization. Eighty-four percent of the flux (number of cells m 22 d 21 ) and 89% of its variability were due to changes in the flux of four dominant species. Anomalous flux events were due to changes in flux of these species rather than introduction of new flora. Rare species showed the same patterns as the flora as a whole: there were changes in composition associated with extreme environ- mental temperatures but little redundancy of species between similar periods. Our data indicate that, at present, we can predict only that the specific composition of flux will change in response to extreme environmental conditions. It is premature to generalize about the specific composition of that response. Knowledge about mechanisms linking flux with ocean environment is currently insuf- ficient to permit the precision of hindcast and prediction that we were seeking from our data. Nevertheless, species composition remains a potentially important tool for interpreting past environ- mental conditions on both the regional and the local scales.