VENRICK: PHYTOPLANKTON SPECIES IN THE CALIFORNIA CURRENT SYSTEM OFF SOUTHERN CALIFORNIA: THE SPATIAL DIMENSIONS CalCOFI Rep., Vol. 56, 2015 PHYTOPLANKTON SPECIES IN THE CALIFORNIA CURRENT SYSTEM OFF SOUTHERN CALIFORNIA: THE SPATIAL DIMENSIONS ELIZABETH L. VENRICK Integrative Oceanography Division Scripps Institution of Oceanography University of California, San Diego La Jolla, CA 92093-0227 ph: (858) 534-2068 fax: (858) 534-6500 [email protected] ABSTRACT temporal cycles have been defined and studied, includ- This paper examines 16 years of microscopic infor- ing the seasonal cycle (Lynn and Simpson 1987; Legaard mation about phytoplankton taxonomic composition and Thomas 2006); the El Niño–Southern Oscillation and abundances from each of four regions in the Cal- (ENSO; Chelton et al. 1982; Bograd and Lynn 2001); COFI area. The NE region is approximately the region and two decadal cycles, the Pacific Decadal Oscilla- of the Southern California Bight; the SE region is the tion (Ebesmeyer et al. 1991; Roemmich and McGowan lower edge of the bight; the Offshore is the western- 1995a, b; Mantua et al. 1997); and the North Pacific most region; and the Alley is the path between the NE Gyre Oscillation (Bond et al. 2003; Di Lorenzo et al. and the Offshore through which the California Cur- 2008). Information on these scales of variability pro- rent meanders. vides a background against which to evaluate longer The NE region and the Alley consistently had the period changes. highest phytoplankton abundances, dominated by dia- Routine sampling for phytoplankton species on toms. These two areas were most similar with respect ­CalCOFI cruises began in 1990. Prior to this, most to abundance fluctuations and species composition.T he phytoplankton studies in the California Current that Offshore had the lowest abundances, dominated by coc- were conducted on a taxonomic level had been rela- colithophores. The SE region was intermediate with tively short term and/or local studies, difficult to extrap- respect to both abundance and composition. olate to a scale comparable to that of the CalCOFI Temporal patterns of abundance and composition studies (Allen, 1936, 1941, 1945a,b; Sverdrup and Allen differed among regions.A n increase in phytoplankton 1939; Balech 1960; Beers 1986; Matrai 1986; Busse abundance was centered in the Offshore region, but was et al. 2006). not accompanied by a change in phytoplankton compo- Using the routine CalCOFI samples, Venrick (2012) sition. The only detectable effect of the ENSO cycle on examined the influence of temporal scales of variabil- phytoplankton abundance was an increase in abundance ity upon the abundance and taxonomic composition during La Niña events in the Offshore. However, a cycle of phytoplankton collected from the mixed layer on of Offshore species composition with a period of five to ­CalCOFI cruises between 1990 and 2009. Spatial vari- eight years did not appear to be related to ENSO. Sea- ability was removed by combining all counts for each sonal cycles were strongest in the NE and Alley. In both taxon into a single mean value for each cruise. In addi- these regions, high abundances in spring during the early tion to seasonal cycles, this time span encompassed three years of this study decreased and the annual abundance El Niño events and two La Niña events. It also included maximum appeared to migrate to summer and fall. These a period near the year 2000 in which a shift in the signs shifts may have been driven by decreases in the abun- of the Pacific Decadal Oscillation (PDO) and the North dances of the diatom phytoplankton in the spring or by Pacific Gyre Oscillation (NPGO) may have occurred an interaction between the present cruise schedule and (Ebbesmeyer et al. 1991; Bond et al. 2003; Lavaniegos a gradual delay in the spring bloom. and Ohman 2003; DiLorenzo et al. 2008). The exact natures of the interdecadal climate events are still open INTRODUCTION to debate (Bjorkstedt et al. 2012). These three scales and The region of the California Current off southern their influences upon the regional hydrography, chem- California is one of the best studied ocean areas in the istry and biology are summarized in some detail in world. The California Cooperative Oceanic Fisher- Venrick (2012). ies Investigations (CalCOFI) has accumulated 65-year During the 1990–2009 study, there was a slow data sets of temperature, salinity, ichthyoplankton, and increase in both chlorophyll a and phytoplankton abun- macrozooplankton and more than 30 years of data for dance, accompanied by a decrease in the number of phy- nutrients and chlorophyll a. Based on these data, several toplankton species per cruise. There was no evidence 168 VENRICK: PHYTOPLANKTON SPECIES IN THE CALIFORNIA CURRENT SYSTEM OFF SOUTHERN CALIFORNIA: THE SPATIAL DIMENSIONS CalCOFI Rep., Vol. 56, 2015 49 51 55 Alley60 POINT PHYTOPLANKTON STATION CONCEPTION DIRECTION OF TRAVEL 70 55 40.6 60 51 80 82.47 42 NE 34°N 70 33 90 55 35 Offshore 40 60 51 80 45 28 30 76.7 100 50 70 90 55 37 60 35 80 45 28 26.7 SAN 70 35 90 53 30 DIEGO 40 80.0 100 60 80 45 100 50 70 90 55 83.3 110 60 SE 80 100 70 90 80 100 CALCOFI 86.7 110 90 110 STATION POSITIONS 100 100 km 90.0 120 110 30°N Offshore 93.3 120 122°W 118°W Figure 1. The basic CalCOFI station plan. Solid circles are stations from which phytoplankton samples were collected. Also indicated are the four environmental regions compared in this study. for a major change in biomass or composition near the A weakness of this first study is the lack of spatial year 2000; however there was an unexplained increase resolution. The CalCOFI area is a dynamic, heteroge- in the temporal heterogeneity of phytoplankton com- neous system and it is not known whether the tempo- position after 2000. ral patterns revealed in Venrick (2012) reflect the entire During the 1990–2009 study, the response of phyto- region or whether they are merely a composite of sev- plankton abundance to ENSO events was similar to that eral dissimilar, smaller-scale patterns. The present paper of zooplankton, with reduced chlorophyll a and abun- extends the observations of temporal scales to the spatial dance during El Niño events and increased values dur- dimension, using the four regions defined by Hayward ing La Niña events. However, the relative magnitudes and Venrick (1998; fig. 1).T he Northeast region (NE) of changes were smaller than those of zooplankton. The is approximately the region of the Southern California effect of the ENSO cycle on phytoplankton abundance Bight (SCB). The Southeast (SE) region often contains was not statistically significant. the southern, shoreward-flowing limb of the quasi-per- During the 1990–2009 study, there was a clear sea- manent Southern California Eddy, which entrains both sonal cycle in both chlorophyll a and phytoplankton California Current water and Offshore water, moving abundance. However, during the study period, the annual them toward the coast at the southern boundary of the peak appeared to shift from spring to summer; this was SCB. The Offshore region is the easternmost Central accompanied by a change in composition of the peak, Pacific, which is influenced by the California Current. especially by a decrease in the abundance of hyalo- The Alley is the region between the NE region and the chaete species, a large subgroup of the diatom genus Offshore through which the southward-flowing Cali- Chaetoceros. Venrick (2012) hypothesized that the shift fornia Current meanders and generates eddies; where reflected the change in nutrient source from wind- it approaches the coast, as at Point Conception, it may driven coastal upwelling in the spring to wind-stress- entrain locally upwelled water. curl-driven upwelling in the summer (Rykaczewski and These four regions were initially defined on the basis Checkley 2008). of the similarity of temporal patterns of chlorophyll a 169 VENRICK: PHYTOPLANKTON SPECIES IN THE CALIFORNIA CURRENT SYSTEM OFF SOUTHERN CALIFORNIA: THE SPATIAL DIMENSIONS CalCOFI Rep., Vol. 56, 2015 Cruise Dates 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 o Jan o o Feb Mar o o o o o o o Apr o May June o o July o o Aug Sept o Oct o o o o Nov o Dec Figure 2. Cruise schedules, 1996–2012. Black bars indicate cruise dates. at groups of stations between 1984 and 1996 (­Hayward METHODS and Venrick 1998). Groups of stations with similar tem- poral patterns of chlorophyll were associated with simi- CalCOFI Protocol lar physical forcing factors. Later, these groups of stations CalCOFI surveys have been conducted since 1949. were shown to be characterized by different phytoplank- Since 1984, cruises have occurred quarterly, and the basic ton assemblages, with the exception of the Alley, which sampling grid has consisted of 66 stations arranged in was characterized by the lack of a stable association six lines spaced 40 nm apart (fig. 1). Within the quar- (Venrick 1998a; Venrick 2009). The Alley appears to terly framework, cruise dates and durations vary (fig. 2). represent a meandering boundary between the NE and In this study, seasons are defined as winter (Dec–Feb), the Offshore.T he phytoplankton assemblage in the Off- spring (Mar–May), summer (June–Aug) and fall (Sept– shore, as defined here, has been shown to be more simi- Nov). A cruise is assigned to a season on the basis of its lar to that of the Central Pacific to the west than to that mid-date. Between 1996 and 2012, there were no cruises of the rest of the CalCOFI area (Venrick 1992, 2009). in June, or Dec, and only one in May.