Pacific Trade Winds Biome
365
enhancement from the coast out to 98
W. The SeaWiFS sequence of monthly chlorophyll
fields reveals the clear effects of the strong El Niño of 1997–98; surface chlorophyll
was significantly lower than in subsequent years, and seasonality was strongly depressed.
Recovery occurred at the end of 1998, and “normal” seasonality was re-established in
winter 1999–2000 and continued for at least the following two winters. These two events
remind us that investigations performed at sea in single seasons must be interpreted in
the light of the existing condition of the Southern Oscillation Index.
Seasonal images also show that to the east of the region investigated by Fiedler, the
NECC bloom exists in summer when it is absent farther to the west. In boreal summer
the NECC bloom is truncated at 120
W but extends westward to about 145
W.
During the period of greatest chlorophyll biomass there is a clear continuity between
enhancement in the PNEC and over the CRD, and there is clear separation between this
linear zone of higher chlorophyll than background, and the enhancement that occurs
within the PEQD province to the south. These observations are consistent with the
observations at sea of high chlorophyll both at the surface and in the DCM at 10–12
N
and 90–120
W during the EASTROPAC chlorophyll surveys in boreal winter. These
seasonal grids in the NECC also confirm a boreal winter primary production maximum
(300–400 mg C m
−2 d
−1 ) and a rather long period of high column-integrated chlorophyll
(February or March to June or July) reaching values of 25 mg chl m
−2 (Blackburn et al.,
1970). The divergent upwelling effect may be so strong as to remove the entire mixed
layer laterally and so expose the strong tropical thermocline (and its associated nutricline)
at the surface with significant biological consequences. The location of the CRD is shown
as values higher than background for nitrate, productivity, zooplankton volume in the
EASTROPAC Atlas, as it is in data for blue whale sightings maintained by the Southwest
Fishery Center, La Jolla, CA.
A typical expression of the ecology of the CRD was investigated in March and April
1981 (Herman, 1989; Longhurst, 1985a; Longhurst and Harrison, 1989), unfortunately
before the importance of autotrophic picoplankton was fully understood. The main
gradient of the thermocline mixed layer lay at 20–50 m and the DCM at 15–20 m with
a maximum concentration of 30 mg m
−3 , and primary production was maximal at
5–15 m with a maximal rate of about 80 mg C m
−3 hr
−1 . Copepods were the principal
mesozooplankton grazers, having their depth centroids closer to the depth of maximal
primary production than to the DCM. There was, however, a broad mesozooplankton
zone of high abundance from the surface down to about 50 m, below which layers of diel
migrants by day occurred at 200–250 m, well above the top of the deep oxygen minimum
zone. Repetitive profiles were much more variable within the CRD than beyond it. Below
250 m there was a remarkable association between sinking cells of large phytoplankton
and of mesoplanktonic copepods; layers of high abundance of both occurred at 300–
500 m, at 600–650 m, and again at 800–900 m. In the last case, up to 100,000 cells m
−3
were associated with around 60 copepods m
−3 (Subba Rao and Sameoto, 1988).
To the west of the CRD, the same arrangement in the upper 200 m was found at the
BIOSTAT station, chosen to be representative of the unperturbed shallow-mixed layer of
the eastern tropical ocean. Here the standing stocks and rates of all biological variables
were lower, and features were deeper in the water column. The mixed layer was about
30 m deep, the DCM lay at about 40 m (<10 mg m
−3 ), the primary production maximum
was just slightly shoaler at 35 m (<20 mg C m
−2 hr
−1 ), and copepods occupied most of
the mixed layer above, though their depth centroid lay close to the depth of maximum
production.
Mesozooplankton at BIOSTAT could be partitioned among small herbivorous copepods whose highest abundance was just shoaler than the DCM, large Eucalanus spp.
that lay mostly below the DCM yet within the thermocline, and omnivorous copepods
and ostracods (Conchoecia spp.) that lay below the thermocline and well into the upper
365
enhancement from the coast out to 98
W. The SeaWiFS sequence of monthly chlorophyll
fields reveals the clear effects of the strong El Niño of 1997–98; surface chlorophyll
was significantly lower than in subsequent years, and seasonality was strongly depressed.
Recovery occurred at the end of 1998, and “normal” seasonality was re-established in
winter 1999–2000 and continued for at least the following two winters. These two events
remind us that investigations performed at sea in single seasons must be interpreted in
the light of the existing condition of the Southern Oscillation Index.
Seasonal images also show that to the east of the region investigated by Fiedler, the
NECC bloom exists in summer when it is absent farther to the west. In boreal summer
the NECC bloom is truncated at 120
W but extends westward to about 145
W.
During the period of greatest chlorophyll biomass there is a clear continuity between
enhancement in the PNEC and over the CRD, and there is clear separation between this
linear zone of higher chlorophyll than background, and the enhancement that occurs
within the PEQD province to the south. These observations are consistent with the
observations at sea of high chlorophyll both at the surface and in the DCM at 10–12
N
and 90–120
W during the EASTROPAC chlorophyll surveys in boreal winter. These
seasonal grids in the NECC also confirm a boreal winter primary production maximum
(300–400 mg C m
−2 d
−1 ) and a rather long period of high column-integrated chlorophyll
(February or March to June or July) reaching values of 25 mg chl m
−2 (Blackburn et al.,
1970). The divergent upwelling effect may be so strong as to remove the entire mixed
layer laterally and so expose the strong tropical thermocline (and its associated nutricline)
at the surface with significant biological consequences. The location of the CRD is shown
as values higher than background for nitrate, productivity, zooplankton volume in the
EASTROPAC Atlas, as it is in data for blue whale sightings maintained by the Southwest
Fishery Center, La Jolla, CA.
A typical expression of the ecology of the CRD was investigated in March and April
1981 (Herman, 1989; Longhurst, 1985a; Longhurst and Harrison, 1989), unfortunately
before the importance of autotrophic picoplankton was fully understood. The main
gradient of the thermocline mixed layer lay at 20–50 m and the DCM at 15–20 m with
a maximum concentration of 30 mg m
−3 , and primary production was maximal at
5–15 m with a maximal rate of about 80 mg C m
−3 hr
−1 . Copepods were the principal
mesozooplankton grazers, having their depth centroids closer to the depth of maximal
primary production than to the DCM. There was, however, a broad mesozooplankton
zone of high abundance from the surface down to about 50 m, below which layers of diel
migrants by day occurred at 200–250 m, well above the top of the deep oxygen minimum
zone. Repetitive profiles were much more variable within the CRD than beyond it. Below
250 m there was a remarkable association between sinking cells of large phytoplankton
and of mesoplanktonic copepods; layers of high abundance of both occurred at 300–
500 m, at 600–650 m, and again at 800–900 m. In the last case, up to 100,000 cells m
−3
were associated with around 60 copepods m
−3 (Subba Rao and Sameoto, 1988).
To the west of the CRD, the same arrangement in the upper 200 m was found at the
BIOSTAT station, chosen to be representative of the unperturbed shallow-mixed layer of
the eastern tropical ocean. Here the standing stocks and rates of all biological variables
were lower, and features were deeper in the water column. The mixed layer was about
30 m deep, the DCM lay at about 40 m (<10 mg m
−3 ), the primary production maximum
was just slightly shoaler at 35 m (<20 mg C m
−2 hr
−1 ), and copepods occupied most of
the mixed layer above, though their depth centroid lay close to the depth of maximum
production.
Mesozooplankton at BIOSTAT could be partitioned among small herbivorous copepods whose highest abundance was just shoaler than the DCM, large Eucalanus spp.
that lay mostly below the DCM yet within the thermocline, and omnivorous copepods
and ostracods (Conchoecia spp.) that lay below the thermocline and well into the upper