Pacific Coastal Biome
401
it, and also near the surface than deeper. As has been found elsewhere, cell types that may
be distinguished by flow cytometry appear to be differentially distributed in relation to
irradiance and nutrients. Here, the principal change in cell type occurs at the thermocline.
A recent study (Bruland et al., 2001) suggests why blooms in some upwelling centers on
the Californian coast (and, by extension, in other similar systems) should be dominated
by small cells, and others by larger diatoms: it is a question of the relative supply of Fe
in the upwelled water. Where the shelf is relatively wide and Fe is regenerated in the
benthic ecosystem, as from Monterey Bay to Point Reyes, upwelled water is Fe-replete
(>10 nM), whereas in an upwelling center, such as the Big Sur coast, where the shelf is
very narrow, upwelled water is generally Fe-deplete (<1 nM). Consequently, the response
of the autotrophic cells is very different in the two regions: off Point Reyes, extensive
blooms of large diatoms deplete the upwelled water of macronutrients (NO 3 and SiO 3 ),
whereas off Big Sur the bloom that occurs after an upwelling event has a low abundance
of diatoms, so that the macronutrients remain largely unused. This accounts for some of
the differences in observed concentrations of macronutrients in upwelling cells.
The decadal-scale changes in the ocean conditions discussed earlier have been associated with appropriate changes in the response of the biota in this province; Hernández
de la Torre et al. (2003) show that integrated nitrate-based production in the California
Current 1970–2002 followed the environmental forcing pattern very closely. From 1970 to
1976, mean new production, integrated along CalCOFI lines 90, 107, and 120 (26–34
N),
was 0186 g C m
−2 , but for 1977–1998 it was only 0.085 g C m
−2 . Between 1999 and
2002, high values (0148 g C m
−2 ) typical of the earlier period returned. Anomalies from
the long-term mean values responded appropriately not only to these decadal changes of
state but also to individual ENSO-scale events.
This result recalls the relationship between zooplankton biomass and 10-m temperature that was identified as a proxy for relative upwelling strength from 1950 to 1980;
zooplankton biomass responds rapidly and positively to periods, and even individual
years, of temperature anomalies of both signs. Thus, from 1958 to 1961 there was a
positive temperature anomaly of >1
C, and a simultaneous negative anomaly for zooplankton biomass; the opposite condition occurred in 1955–1956. Seasonal patterns of
change in zooplankton biomass also respond to temperature anomalies; in the post-1977
warm period, the spring biomass increase was earlier and sharper than was normal during
the pre-1977 cool period (McGowan et al., 2003).
Studies off Oregon, in the northern California Current, show that such responses in
overall zooplankton biomass reflect changes in community composition (Petersen and
Keister, 2003); analysis of 206 serial samples from a midshelf station here showed that
strong upwelling was associated with the presence of Centropages abdominalis, Acartia
longiremus, and Microcalanus pusillus, and El Niño conditions with Calanus pacificus,
Corycaeus anglicus, and Ctenocalanus vanus. Comparable changes in distribution of
euphausiids, tunicates, and fish larvae have also been recorded.
The upwelling cells themselves are inhabited by diatom-copepod assemblages of
remarkably low diversity: in repeated net tows in one such cell a few kilometers off Baja
California (25
N), I could find no more than 29 species of mesozooplankton of all groups
(and no more than 20 in any one sample), or about one-quarter the number taken 25 km
farther offshore and examined with equivalent attention. Large, filter-feeding copepods,
especially C. pacificus (which you will find described as C. helgolandicus in much of
the Californian literature) at 115 ind m
−3 comprised 77% of zooplankton dry weight at
the coast. The seasonal ontogenetic migrations of this species cause the deep basins on the
continental shelf to trap large concentrations of overwintering stage 5 copepodites; early
in the winter these aggregate near the bottom, but the layers progressively shoal as oxygen
concentration in the bottom water declines, eventually forcing them over the sill depth of
401
it, and also near the surface than deeper. As has been found elsewhere, cell types that may
be distinguished by flow cytometry appear to be differentially distributed in relation to
irradiance and nutrients. Here, the principal change in cell type occurs at the thermocline.
A recent study (Bruland et al., 2001) suggests why blooms in some upwelling centers on
the Californian coast (and, by extension, in other similar systems) should be dominated
by small cells, and others by larger diatoms: it is a question of the relative supply of Fe
in the upwelled water. Where the shelf is relatively wide and Fe is regenerated in the
benthic ecosystem, as from Monterey Bay to Point Reyes, upwelled water is Fe-replete
(>10 nM), whereas in an upwelling center, such as the Big Sur coast, where the shelf is
very narrow, upwelled water is generally Fe-deplete (<1 nM). Consequently, the response
of the autotrophic cells is very different in the two regions: off Point Reyes, extensive
blooms of large diatoms deplete the upwelled water of macronutrients (NO 3 and SiO 3 ),
whereas off Big Sur the bloom that occurs after an upwelling event has a low abundance
of diatoms, so that the macronutrients remain largely unused. This accounts for some of
the differences in observed concentrations of macronutrients in upwelling cells.
The decadal-scale changes in the ocean conditions discussed earlier have been associated with appropriate changes in the response of the biota in this province; Hernández
de la Torre et al. (2003) show that integrated nitrate-based production in the California
Current 1970–2002 followed the environmental forcing pattern very closely. From 1970 to
1976, mean new production, integrated along CalCOFI lines 90, 107, and 120 (26–34
N),
was 0186 g C m
−2 , but for 1977–1998 it was only 0.085 g C m
−2 . Between 1999 and
2002, high values (0148 g C m
−2 ) typical of the earlier period returned. Anomalies from
the long-term mean values responded appropriately not only to these decadal changes of
state but also to individual ENSO-scale events.
This result recalls the relationship between zooplankton biomass and 10-m temperature that was identified as a proxy for relative upwelling strength from 1950 to 1980;
zooplankton biomass responds rapidly and positively to periods, and even individual
years, of temperature anomalies of both signs. Thus, from 1958 to 1961 there was a
positive temperature anomaly of >1
C, and a simultaneous negative anomaly for zooplankton biomass; the opposite condition occurred in 1955–1956. Seasonal patterns of
change in zooplankton biomass also respond to temperature anomalies; in the post-1977
warm period, the spring biomass increase was earlier and sharper than was normal during
the pre-1977 cool period (McGowan et al., 2003).
Studies off Oregon, in the northern California Current, show that such responses in
overall zooplankton biomass reflect changes in community composition (Petersen and
Keister, 2003); analysis of 206 serial samples from a midshelf station here showed that
strong upwelling was associated with the presence of Centropages abdominalis, Acartia
longiremus, and Microcalanus pusillus, and El Niño conditions with Calanus pacificus,
Corycaeus anglicus, and Ctenocalanus vanus. Comparable changes in distribution of
euphausiids, tunicates, and fish larvae have also been recorded.
The upwelling cells themselves are inhabited by diatom-copepod assemblages of
remarkably low diversity: in repeated net tows in one such cell a few kilometers off Baja
California (25
N), I could find no more than 29 species of mesozooplankton of all groups
(and no more than 20 in any one sample), or about one-quarter the number taken 25 km
farther offshore and examined with equivalent attention. Large, filter-feeding copepods,
especially C. pacificus (which you will find described as C. helgolandicus in much of
the Californian literature) at 115 ind m
−3 comprised 77% of zooplankton dry weight at
the coast. The seasonal ontogenetic migrations of this species cause the deep basins on the
continental shelf to trap large concentrations of overwintering stage 5 copepodites; early
in the winter these aggregate near the bottom, but the layers progressively shoal as oxygen
concentration in the bottom water declines, eventually forcing them over the sill depth of