Response of diatoms and silicoflagellates to climate change and warming in the California Current during the past 250 years and the recent rise of the toxic diatom Pseudo-nitzschia australis

Abstract

Diatoms and silicoflagellate assemblages were examined in two year-increments of varved samples spanning the interval from 1748 through 2007 in Santa Barbara Basin (SBB) box core SBBC0806 to determine the timing and impact of possible 20th century warming on several different components of the plankton. Diatoms (Thalassionema nitzschioides = TN) and silicoflagellates (Distephanus speculum s.l. = DS) indicative of cooler waters and a shallow thermocline begin to decline in the 1920s and persistently compose a lower percentage of the assemblage in the SBB by about 1940. Prior to 1940, TN constituted on average ∼30% of the Chaetoceros-free diatom sediment assemblage and DS on average ∼36% of the silicoflagellate assemblage. Between 1940 and 1996, these relative abundances were ∼20% (TN) and ∼8% (DS). These results are consistent with results from planktonic foraminifera and radiolarians that indicate an influence of 20th century warming on marine ecosystems before most scientific observations began. Cooling of surface waters coincident with the one of the strongest La Niña events of the 20th century (and a return to negative PDO conditions) in late 1998 brought about a return to pre-1940 values of these cool water taxa (TN ∼31%, DS ∼25%). However, this recent regional cooling appears to have been accompanied by profound changes in the diatom assemblage. Pseudo-nitzschia australis, and Pseudo-nitzschia multiseries, diatom species associated with domoic acid, a neurotoxin that causes shellfish poisoning and marine mammal deaths, rapidly became dominant in the SBB sediment record at the time of the regional cooling (1999) and increased substantially in numbers as a bloom-forming taxon (relative to Chaetoceros spores) in 2003. Prior to 2003, diatom blooms recorded in the SBB sediment record consisted predominantly of Chaetoceros spores and less commonly of Rhizosolenia-related species (Neocalyptrella robusta and Rhizosolenia setigera). Fecal pellets dominated by valves of P. australis, however, were particularly abundant in both the 2003 and 2006 samples, coincident with recorded incidents of domoic acid increase and widespread shellfish poisoning in the SBB.