dominated by large cells, especially chain-forming diatoms. During the upwelling
season, chlorophyll levels range from 1 to 10 mg Chl m−3, and most of the primary
production is in the upper 10–20 m of the water column. Small phytoplankton (<10
μm) can be abundant, but at chlorophyll levels above ∼2 mg m−3, large cells account
for 60–90% of the phytoplankton biomass (Fig. 11.40). Initially, EBCS were thought
to have simple and short food-chains: primarily diatoms grazed by copepods eaten by
small pelagic fish. However, microzooplankton (small heterotrophic protists) are
abundant and can account for a large portion of the grazing on both small and large
phytoplankton (Sherr & Sherr 2007, 2009). Although microzooplankton can account
for ∼60% of the total grazing pressure, the herbivory of mesozooplankton (especially
copepods) is at times more important in nearshore waters (Fig. 11.41).
Microzooplankton are an important component of the mesozooplankton diet, just as in
oceanic biomes. The recurring emphasis on copepods in upwelling ecosystems may
be justified, as both large species (particularly Calanus spp.) and small species (of
Acartia, Pseudocalanus, Paracalanus, Oithona, Oncaea, and more) are consistently
present and active, often dominating net samples. However, euphausiids can be very
abundant, especially in the outer reaches of upwelling ecosystems. In the CCS, there
are two dominant euphausiids. Thysanoessa spinifera is restricted to shelf waters,
while Euphausia pacifica resides chiefly in slope waters (Feinberg & Peterson 2003).
Fig. 11.40 Percent chlorophyll > 10 μm plotted as a function of total chlorophyll.
Filled symbols are post-El Niño cruises, open symbols are El Niño cruises. Circles =
shelf stations, triangles = slope stations, and squares = off-shelf stations.
(^) (After Corwith & Wheeler 2002.)
Fig. 11.41 Ratios of depth-integrated grazing rates of mesozooplankton relative to
distance offshore for drifter experiments in May 2006 and April 2007.
(After Landry et al. 2009.)