Upwelling and Lagoonal Ecosystems of the Dry Pacific Coast of Baja California
317
peninsula has a semi-warm regime (18-22 °C) which becomes a warm
regime (22-26 °C} at the southern tip of the peninsula. In general, the northern half of Baja California receives most of the annual rainfall (mean
225 mm) from November to March. In contrast, most of the annual precipitation (mean 160 mm) at the southern peninsula occurs during the summer months. However, both precipitation regimes experience extreme
inter-annual variability (i.e. 32.5°N, range 63-433 mm and 24.4°N, range
46-608mm).
21.3 The Upwelling Ecosystem of Baja California
Although most intense in spring and early summer (March to June), upwelling off western Baja California persists during the entire year (Bakun
and Nelson 1977), and advected upwelled waters bathe the entire coastline
of the peninsula. However, the influence of upwelling is characteristically
stronger between Punta Baja (30°N) and Punta Abreojos (26°N), owing to
negative wind stress curl (maximum Ekman convergence) which favors
the formation of fronts and convergent patches of upwelled water. North
and south of this region divergent Ekman conditions prevail and upwelling
(although reduced) is stronger offshore than inshore. The coastal ecosystem off Baja California (-250 km) is the southern extension of the cold
California Current ecosystem. However, owing to latitudinal temperature
gradients between regions off Northern California (mean annual SST
12 oc, seasonal SST 3.5 oq and Southern Baja California (mean annual SST
23 °C, seasonal SST > 10 °C}, the abundance of several species changes.
Furthermore, limited communication between semi-permanent cyclonic
eddies (Fig. 21.1) north and south of Punta Eugenia (28°N) appears to
induce discontinuities among a wide variety of taxa. The distribution of
oceanic euphausiids and copepods, which are indicators of subarctic
equatorial and central waters, terminates at Punta Eugenia. Several coastal
pelagic fish species, like the northern anchovy (Engraulis mordax), the
Pacific sardine (Sardinops caeruleus) and the Pacific hake (Merluccius productus), display distinct discontinuities (i.e. morphometric and meristic
measurements), suggesting a restricted flow of genes between populations
north and south of 28°N. A provincial boundary in the vicinity of Punta
Eugenia appears to separate northern warm-temperate faunal elements
from biota of uncertain origin with reduced tropical elements, high
endemism, and insular characteristics of a province tentatively called
Panamic between Punta Eugenia and Cabo St. Lucas (Hewitt 1981).
317
peninsula has a semi-warm regime (18-22 °C) which becomes a warm
regime (22-26 °C} at the southern tip of the peninsula. In general, the northern half of Baja California receives most of the annual rainfall (mean
225 mm) from November to March. In contrast, most of the annual precipitation (mean 160 mm) at the southern peninsula occurs during the summer months. However, both precipitation regimes experience extreme
inter-annual variability (i.e. 32.5°N, range 63-433 mm and 24.4°N, range
46-608mm).
21.3 The Upwelling Ecosystem of Baja California
Although most intense in spring and early summer (March to June), upwelling off western Baja California persists during the entire year (Bakun
and Nelson 1977), and advected upwelled waters bathe the entire coastline
of the peninsula. However, the influence of upwelling is characteristically
stronger between Punta Baja (30°N) and Punta Abreojos (26°N), owing to
negative wind stress curl (maximum Ekman convergence) which favors
the formation of fronts and convergent patches of upwelled water. North
and south of this region divergent Ekman conditions prevail and upwelling
(although reduced) is stronger offshore than inshore. The coastal ecosystem off Baja California (-250 km) is the southern extension of the cold
California Current ecosystem. However, owing to latitudinal temperature
gradients between regions off Northern California (mean annual SST
12 oc, seasonal SST 3.5 oq and Southern Baja California (mean annual SST
23 °C, seasonal SST > 10 °C}, the abundance of several species changes.
Furthermore, limited communication between semi-permanent cyclonic
eddies (Fig. 21.1) north and south of Punta Eugenia (28°N) appears to
induce discontinuities among a wide variety of taxa. The distribution of
oceanic euphausiids and copepods, which are indicators of subarctic
equatorial and central waters, terminates at Punta Eugenia. Several coastal
pelagic fish species, like the northern anchovy (Engraulis mordax), the
Pacific sardine (Sardinops caeruleus) and the Pacific hake (Merluccius productus), display distinct discontinuities (i.e. morphometric and meristic
measurements), suggesting a restricted flow of genes between populations
north and south of 28°N. A provincial boundary in the vicinity of Punta
Eugenia appears to separate northern warm-temperate faunal elements
from biota of uncertain origin with reduced tropical elements, high
endemism, and insular characteristics of a province tentatively called
Panamic between Punta Eugenia and Cabo St. Lucas (Hewitt 1981).