Pacific Coastal Biome
407
with high chlorophyll at that season along the eastern coast of the Gulf of California.
I have not seen a description of this effect but I suggest that the observation is
consistent with the presumed slight upwelling tendency that may be expected along
this coast at this season. It appears to be separable as a phenomenon from the Gulf
of Tehuantapec enhancement immediately to the south.
Gulfs of Tehuantapec, Papagaya, and Panama: The Gulf of Tehuantapec is important
oceanographically because it lies west of a major gap in the Central American sierra
chain, the Chivela Pass, through which the transmontane winds from the Gulf of
Mexico (only 150 km to the east) have easy passage (Blackburn, 1962). These winds,
mostly from the northeast, force southward surface flow in the gulf. Interaction
between this and the northward flow along the Mexican coast in the Costa Rica
Coastal Current generates a meridional thermocline (pycnocline and nutricline)
ridge. In boreal winter, the mixed layer above this feature may be completely eroded
by wind stress, and surface temperature, salinity, and nutrient levels resemble values
in the pycnocline. After extreme transmontane wind events, a plume having such
characteristics may stretch several hundred kilometers to the southwest from the
gulf. Surface productivity is high (<150 mg C m
−3 day
−1 ) and the plume, then lying
above the Tehuantapec Ridge, becomes a feature of the surface chlorophyll field
(see Color plate 19). Gonzales-Silvera et al. (2004) analyzed SeaWiFS and AVHRR
data and found that surface chlorophyll ranged over two order of magnitude and
SST by almost 10
C between upwelling events and stasis. They found that the
Gulf of Tehuantapec produces a larger number of eddies, each being associated
with several smaller cyclonic eddies; these rarely survive long, whereas the major
anticyclonic eddies mentioned above were followed for up to 3 months and over
trajectories of <700 km.
Upwelling also occurs, and for the same reason, in the Gulfs of Papagaya and Panama
and at approximately the same season, though the occurrence of upwelling in one gulf
does not necessarily denote upwelling in the others. In some years this process can
produce a major feature in the sea-surface thermal field of the Gulf of Panama, which
reaches seaward into the region of the Galapagos Islands. At such times, the northern
thermal front off Panama carries 500-km waves that pass westward along the front
at about 30 km day
−1 (Legeckis, 1988). Transmontane winds into the Gulf of Chiriqui
appear not to cause upwelling.
Beyond these exceptional events the ecology of the coastal regions of Central America
can best be regarded as comprising two systems: (i) those off arid coasts with little freshwater input (such as much of the Mexican coast and particularly the Gulf of California)
where water clarity is high, and (ii) those within the area of tropical rainfall where
coasts are invested with mangrove, there is significant river effluent, and water clarity is
seasonally very low.
The inshore Gulf of Panama will serve as a model for the latter type of pelagic
ecosystem: here the ecological balance and biota appear to closely resemble those of
the mangrove coasts of the Gulf of Guinea (see GUIN). Here, there is an algal bloom,
apparently dominated as in the Gulf of Guinea by large diatoms and at the same season
(January–March); of course, wind-forced upwelling at the mouth of the Gulf of Panama
is also implicated and perhaps the relaxation of river flow and consequent reduction
in water clarity is less important here than in Gulf of Guinea. Less than 10% of daily
production by phytoplankton can apparently be consumed by the standing stock of
mesozooplankton; since these are nitrate and diatom-dominated blooms, it seems likely
that modern investigations of the role of picoplankton and their protist grazers will
modify the models based on older techniques less than they will in the open ocean.
Another case of apparent surplus production, requiring an export process or a sink for
organic matter, is the Gulf of Guayaquil, to the south of the Gulf of Panama.
407
with high chlorophyll at that season along the eastern coast of the Gulf of California.
I have not seen a description of this effect but I suggest that the observation is
consistent with the presumed slight upwelling tendency that may be expected along
this coast at this season. It appears to be separable as a phenomenon from the Gulf
of Tehuantapec enhancement immediately to the south.
Gulfs of Tehuantapec, Papagaya, and Panama: The Gulf of Tehuantapec is important
oceanographically because it lies west of a major gap in the Central American sierra
chain, the Chivela Pass, through which the transmontane winds from the Gulf of
Mexico (only 150 km to the east) have easy passage (Blackburn, 1962). These winds,
mostly from the northeast, force southward surface flow in the gulf. Interaction
between this and the northward flow along the Mexican coast in the Costa Rica
Coastal Current generates a meridional thermocline (pycnocline and nutricline)
ridge. In boreal winter, the mixed layer above this feature may be completely eroded
by wind stress, and surface temperature, salinity, and nutrient levels resemble values
in the pycnocline. After extreme transmontane wind events, a plume having such
characteristics may stretch several hundred kilometers to the southwest from the
gulf. Surface productivity is high (<150 mg C m
−3 day
−1 ) and the plume, then lying
above the Tehuantapec Ridge, becomes a feature of the surface chlorophyll field
(see Color plate 19). Gonzales-Silvera et al. (2004) analyzed SeaWiFS and AVHRR
data and found that surface chlorophyll ranged over two order of magnitude and
SST by almost 10
C between upwelling events and stasis. They found that the
Gulf of Tehuantapec produces a larger number of eddies, each being associated
with several smaller cyclonic eddies; these rarely survive long, whereas the major
anticyclonic eddies mentioned above were followed for up to 3 months and over
trajectories of <700 km.
Upwelling also occurs, and for the same reason, in the Gulfs of Papagaya and Panama
and at approximately the same season, though the occurrence of upwelling in one gulf
does not necessarily denote upwelling in the others. In some years this process can
produce a major feature in the sea-surface thermal field of the Gulf of Panama, which
reaches seaward into the region of the Galapagos Islands. At such times, the northern
thermal front off Panama carries 500-km waves that pass westward along the front
at about 30 km day
−1 (Legeckis, 1988). Transmontane winds into the Gulf of Chiriqui
appear not to cause upwelling.
Beyond these exceptional events the ecology of the coastal regions of Central America
can best be regarded as comprising two systems: (i) those off arid coasts with little freshwater input (such as much of the Mexican coast and particularly the Gulf of California)
where water clarity is high, and (ii) those within the area of tropical rainfall where
coasts are invested with mangrove, there is significant river effluent, and water clarity is
seasonally very low.
The inshore Gulf of Panama will serve as a model for the latter type of pelagic
ecosystem: here the ecological balance and biota appear to closely resemble those of
the mangrove coasts of the Gulf of Guinea (see GUIN). Here, there is an algal bloom,
apparently dominated as in the Gulf of Guinea by large diatoms and at the same season
(January–March); of course, wind-forced upwelling at the mouth of the Gulf of Panama
is also implicated and perhaps the relaxation of river flow and consequent reduction
in water clarity is less important here than in Gulf of Guinea. Less than 10% of daily
production by phytoplankton can apparently be consumed by the standing stock of
mesozooplankton; since these are nitrate and diatom-dominated blooms, it seems likely
that modern investigations of the role of picoplankton and their protist grazers will
modify the models based on older techniques less than they will in the open ocean.
Another case of apparent surplus production, requiring an export process or a sink for
organic matter, is the Gulf of Guayaquil, to the south of the Gulf of Panama.