Venrick: PhytoPlankton SPecieS in the california current SyStem off Southern california: the SPatial DimenSionS

this paper examines 16 years of microscopic information about phytoplankton taxonomic composition and abundances from each of four regions in the calcOFi area. the nE region is approximately the region of the Southern california bight; the SE region is the lower edge of the bight; the Offshore is the westernmost region; and the alley is the path between the nE and the Offshore through which the california current meanders. the nE region and the alley consistently had the highest phytoplankton abundances, dominated by diatoms. these two areas were most similar with respect to abundance fluctuations and species composition. the Offshore had the lowest abundances, dominated by coccolithophores. the SE region was intermediate with respect to both abundance and composition. temporal patterns of abundance and composition differed among regions. an increase in phytoplankton abundance was centered in the Offshore region, but was not accompanied by a change in phytoplankton composition. the only detectable effect of the EnSO cycle on phytoplankton abundance was an increase in abundance during la niña events in the Offshore. however, a cycle of Offshore species composition with a period of five to eight years did not appear to be related to EnSO. Seasonal cycles were strongest in the nE and alley. in both these regions, high abundances in spring during the early years of this study decreased and the annual abundance maximum appeared to migrate to summer and fall. these shifts may have been driven by decreases in the abundances of the diatom phytoplankton in the spring or by an interaction between the present cruise schedule and a gradual delay in the spring bloom. introDuction the region of the california current off southern california is one of the best studied ocean areas in the world. the california cooperative Oceanic Fisheries investigations (calcOFi) has accumulated 65-year data sets of temperature, salinity, ichthyoplankton, and macrozooplankton and more than 30 years of data for nutrients and chlorophyll a. based on these data, several temporal cycles have been defined and studied, including the seasonal cycle (lynn and Simpson 1987; legaard and thomas 2006); the El niño–Southern Oscillation (EnSO; chelton et al. 1982; bograd and lynn 2001); and two decadal cycles, the Pacific Decadal Oscillation (Ebesmeyer et al. 1991; roemmich and McGowan 1995a, b; Mantua et al. 1997); and the north Pacific Gyre Oscillation (bond et al. 2003; Di lorenzo et al. 2008). information on these scales of variability provides a background against which to evaluate longer period changes. routine sampling for phytoplankton species on calcOFi cruises began in 1990. Prior to this, most phytoplankton studies in the california current that were conducted on a taxonomic level had been relatively short term and/or local studies, difficult to extrapolate to a scale comparable to that of the calcOFi studies (allen, 1936, 1941, 1945a,b; Sverdrup and allen 1939; balech 1960; beers 1986; Matrai 1986; busse et al. 2006). Using the routine calcOFi samples, Venrick (2012) examined the influence of temporal scales of variability upon the abundance and taxonomic composition of phytoplankton collected from the mixed layer on calcOFi cruises between 1990 and 2009. Spatial variability was removed by combining all counts for each taxon into a single mean value for each cruise. in addition to seasonal cycles, this time span encompassed three El niño events and two la niña events. it also included a period near the year 2000 in which a shift in the signs of the Pacific Decadal Oscillation (PDO) and the north Pacific Gyre Oscillation (nPGO) may have occurred (Ebbesmeyer et al. 1991; bond et al. 2003; lavaniegos and Ohman 2003; Dilorenzo et al. 2008). the exact natures of the interdecadal climate events are still open to debate (bjorkstedt et al. 2012). these three scales and their influences upon the regional hydrography, chemistry and biology are summarized in some detail in Venrick (2012). During the 1990–2009 study, there was a slow increase in both chlorophyll a and phytoplankton abundance, accompanied by a decrease in the number of phytoplankton species per cruise. there was no evidence Venrick: PhytoPlankton SPecieS in the california current SyStem off Southern california: the SPatial DimenSionS calcofi rep., Vol. 56, 2015 169 a weakness of this first study is the lack of spatial resolution. the calcOFi area is a dynamic, heterogeneous system and it is not known whether the temporal patterns revealed in Venrick (2012) reflect the entire region or whether they are merely a composite of several dissimilar, smaller-scale patterns. the present paper extends the observations of temporal scales to the spatial dimension, using the four regions defined by hayward and Venrick (1998; fig. 1). the northeast region (nE) is approximately the region of the Southern california bight (Scb). the Southeast (SE) region often contains the southern, shoreward-flowing limb of the quasi-permanent Southern california Eddy, which entrains both california current water and Offshore water, moving them toward the coast at the southern boundary of the Scb. the Offshore region is the easternmost central Pacific, which is influenced by the california current. the alley is the region between the nE region and the Offshore through which the southward-flowing california current meanders and generates eddies; where it approaches the coast, as at Point conception, it may entrain locally upwelled water. these four regions were initially defined on the basis of the similarity of temporal patterns of chlorophyll a for a major change in biomass or composition near the year 2000; however there was an unexplained increase in the temporal heterogeneity of phytoplankton composition after 2000. During the 1990–2009 study, the response of phytoplankton abundance to EnSO events was similar to that of zooplankton, with reduced chlorophyll a and abundance during El niño events and increased values during la niña events. however, the relative magnitudes of changes were smaller than those of zooplankton. the effect of the EnSO cycle on phytoplankton abundance was not statistically significant. During the 1990–2009 study, there was a clear seasonal cycle in both chlorophyll a and phytoplankton abundance. however, during the study period, the annual peak appeared to shift from spring to summer; this was accompanied by a change in composition of the peak, especially by a decrease in the abundance of hyalochaete species, a large subgroup of the diatom genus Chaetoceros. Venrick (2012) hypothesized that the shift reflected the change in nutrient source from winddriven coastal upwelling in the spring to wind-stresscurl-driven upwelling in the summer (rykaczewski and checkley 2008). POINT CONCEPTION