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

We have examined the species composition of 127 sediment trapsamples collected sequentially from the Santa Barbara Basin,USA, during a 7-year period. The sampling period included twowarm-water periods (El Niños) and two cold-water periods(one La Niña and one more local). We examined changesin total species composition and changes in subsets of speciesaccompanying extreme environmental conditions, as well as changesin flux composition associated with periods of anomalous flux.Our goal was to improve the scale and precision of hindcastingpast conditions from the sedimentary record and, hence, predictionof the biological consequences of environmental change. Althoughlarge-scale climate events were accompanied by changes in theflux composition, there was little similarity in compositionduring environmentally similar periods, precluding generalization.Eighty-four percent of the flux (number of cells m^–2 d^–1)and 89% of its variability were due to changes in the flux offour dominant species. Anomalous flux events were due to changesin 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 environmentaltemperatures but little redundancy of species between similarperiods. Our data indicate that, at present, we can predictonly that the specific composition of flux will change in responseto extreme environmental conditions. It is premature to generalizeabout the specific composition of that response. Knowledge aboutmechanisms linking flux with ocean environment is currentlyinsufficient to permit the precision of hindcast and predictionthat we were seeking from our data. Nevertheless, species compositionremains a potentially important tool for interpreting past environmentalconditions on both the regional and the local scales.