Two phases of the phytoplankton community in the Baja California upwelling

The method of principal component analysis has been applied to quantitative data on 60 species of phytoplankton from the cruises MESCAL I and MESCAL II. Dinoflagellate dom-inance in the phytoplankton was associated with upwelling of water of relatively low salinity and lower nutrients from below the minimum salinity core of the California Current. Diatom dominance was related to upwelling of deeper salinc and high nlltrient water from the Cal-ifornia Undercurrent. Nutrient enrichment following upwell-ing along the California coast may induce the growth of dense diatom- or dinofla-gellate-dominated populations. Dinofla-gellate blooms are often intense enough to deserve the designation of red tides. 11espite work by Holmes et al. (1967) and Blasco (1975, 1977, 1978), many ques-tions concerning the relationship be-tween hydrographic conditions and phy-toplankton population changes remain unanswered. Generally, diatom blooms h ave been associated with upwelling conditions that lead to low temperatures and high salinity at the surface (Sverdrup and Allen 1939; Bolin and Abbott 1963; Tont 1976). Eppley and IIarrison (1975) related dinoflagellate blooms to the oc-currence of a nutrient-depleted, shallow mixed layer overlying a steep, shallow thermocline with nutrient-rich water be-low it. Our hypothesis is that the com-position of the upwelling phytoplankton is related to the origin of the enriched waters. To evaluate this relationship fur-ther, we applied a multivariate statistical technique to data collected during the cruises MESCAL I and 1MESCAL II, car-ried out off Punta San IIipolito, Baja Cal