336
S. Alvarez-Borrego
tral regions of the Upper Gulf (Alvarez-Borrego et al. 1973). Surface
nutrient concentrations tend to increase from the central and northern
Gulf of California (south of31 oN; 0.7-1.0 11M P0 4 ,0-4.0 11M N0 3 ,6.1-1811M
Si0 2 ; Alvarez-Borrego et al. 1978) towards the Upper Gulf adjacent to Montague Island (0.3-3.111M P0 4 , 3.3-18.311M N0 3 ; Hernandez-Ay6n et al.
1993). Throughout the year, nutrient concentrations are highest in the
estuary (2-1511M N0 2 , 40-5311M N0 3 , 5-11.511M P0 4 , 60-9211M Si0 2 ) but
without any clear seasonal pattern. High concentrations in the estuary are
possibly due to application of fertilizers in upstream agricultural areas,
groundwater input, and sediment resuspension mainly during spring tides
(Hernandez-Ay6n et al. 1993).
22.4 Biological Communities
Despite the ecological impact caused by the construction of dams, life in
the estuary is abundant, even during the long periods without surface
freshwater input. At most locations of the Gulf photosynthetic pigment
concentrations vary seasonally, with maxima during strong upwelling
events in "winter" (November to May) and minima under "summer" conditions between June and October. Owing to strong tidal mixing, seasonal
variations of pigment concentrations in the Upper Gulf are less evident
and concentrations are generally higher than 4 mg m - 3 (SantamariaDel-Angel et al. 1994). Despite the patchy distribution of phytoplankton,
chlorophyll-a concentrations may reach values between 5 and 20 mg m- 3 in
turbid surface waters off El Golfo de Santa Clara and off San Felipe. Zooplankton biomass in the Upper Gulf of California generally increases
west or northwestward and varies between 25-150mg m- 3 in August and
1-4 mg m- 3 in October, though biomass values in the channels around
Montague Island are consistently higher (up to 154mg m- 3 ). In general,
calanoid copepods are the most abundant taxonomic group, both numerically and in terms of biomass (Farfan and Alvarez-Borrego 1992 ).
Of the 73 fish species collected from soft bottoms (Guevara-Escamilla et
al. 1973), 29 species are panamic, 31 occur in California waters and 13 species might be endemic to the area. The juveniles of most fish species occur
in areas of high turbidity north of a line between El Golfo de Santa Clara
and San Felipe, possibly owing to protection and food. Some species, like
the endemic totoaba ( Totoaba macdonaldi), supported an important
fishery in the Upper Gulf since 1929; however, the drastic decline of stocks
at the end of the 1960s placed the species on the endangered species list
(Guevara-Escamilla et al. 1973). Despite a general scarcity of ecological
S. Alvarez-Borrego
tral regions of the Upper Gulf (Alvarez-Borrego et al. 1973). Surface
nutrient concentrations tend to increase from the central and northern
Gulf of California (south of31 oN; 0.7-1.0 11M P0 4 ,0-4.0 11M N0 3 ,6.1-1811M
Si0 2 ; Alvarez-Borrego et al. 1978) towards the Upper Gulf adjacent to Montague Island (0.3-3.111M P0 4 , 3.3-18.311M N0 3 ; Hernandez-Ay6n et al.
1993). Throughout the year, nutrient concentrations are highest in the
estuary (2-1511M N0 2 , 40-5311M N0 3 , 5-11.511M P0 4 , 60-9211M Si0 2 ) but
without any clear seasonal pattern. High concentrations in the estuary are
possibly due to application of fertilizers in upstream agricultural areas,
groundwater input, and sediment resuspension mainly during spring tides
(Hernandez-Ay6n et al. 1993).
22.4 Biological Communities
Despite the ecological impact caused by the construction of dams, life in
the estuary is abundant, even during the long periods without surface
freshwater input. At most locations of the Gulf photosynthetic pigment
concentrations vary seasonally, with maxima during strong upwelling
events in "winter" (November to May) and minima under "summer" conditions between June and October. Owing to strong tidal mixing, seasonal
variations of pigment concentrations in the Upper Gulf are less evident
and concentrations are generally higher than 4 mg m - 3 (SantamariaDel-Angel et al. 1994). Despite the patchy distribution of phytoplankton,
chlorophyll-a concentrations may reach values between 5 and 20 mg m- 3 in
turbid surface waters off El Golfo de Santa Clara and off San Felipe. Zooplankton biomass in the Upper Gulf of California generally increases
west or northwestward and varies between 25-150mg m- 3 in August and
1-4 mg m- 3 in October, though biomass values in the channels around
Montague Island are consistently higher (up to 154mg m- 3 ). In general,
calanoid copepods are the most abundant taxonomic group, both numerically and in terms of biomass (Farfan and Alvarez-Borrego 1992 ).
Of the 73 fish species collected from soft bottoms (Guevara-Escamilla et
al. 1973), 29 species are panamic, 31 occur in California waters and 13 species might be endemic to the area. The juveniles of most fish species occur
in areas of high turbidity north of a line between El Golfo de Santa Clara
and San Felipe, possibly owing to protection and food. Some species, like
the endemic totoaba ( Totoaba macdonaldi), supported an important
fishery in the Upper Gulf since 1929; however, the drastic decline of stocks
at the end of the 1960s placed the species on the endangered species list
(Guevara-Escamilla et al. 1973). Despite a general scarcity of ecological