322
S.E. Ibarra-Obando eta!.
nant role in determining the composition of species assemblages or in
modulating the differences in abundance of the species ( Chelton et al.
1982).
21.4 Lagoonal Ecosystems
The better known coastal lagoons along the Pacific coast of Baja California
are La Salina, Estero de Punta Banda, Bahia San Quintin, the lagoonal complex of Guerrero Negro-Ojo de Liebre, Laguna San Ignacio, and Bahia
Magdalena (Fig. 21.1). Although the lagoons differ in geological origin, the
sediments are mostly sandy and alluvial. Since land-to-ocean stream flux is
limited, the advected nutrient-rich waters of the ocean directly (i.e. Bahia
San Quintin; Milhin-Nufiez et al. 1982) or indirectly (Estero de Punta
Banda and Laguna Ojo de Liebre; Amador-Buenrostro et al. 1995) affect
most biogeochemical processes in the lagoons (Fig. 21.1). Owing to a
dominance of evaporation over precipitation and lack of inflowing surface
or groundwater, moderately hypersaline conditions are common and salinity tends to increase from the mouth (approx. 33.5) towards the inner part
(approx. 40). However, average salinity and residence times vary between
35 and 14 days (Bahia San Quintin; Camacho-Ibar et al. 1997), 36 and
16 days (Laguna Guerrero Negro), and 45 and 100 days (Laguna Ojo de
Liebre; Postma 1965), respectively. Pacific tides are predominantly semidiurnal with Lower Low Water (LLW) following Higher High Water
(HHW). These conditions cause high velocities during ebb tide which flush
suspended sediments and inhibit or retard the sedimentary filling processes typical in these environments.
Most of the coastal lagoons are pristine or are barely modified and they
represent highly productive and diverse environments. Only some seasonally ephemeral lagoons (La Salina, 32°N, and Laguna Figueroa, 30°N) have
low diversity because few species can tolerate the prolonged dry periods
and extreme salinity variations. Owing to their high productivity (i.e.
Laguna Ojo de Liebre; Phleger and Ewing 1962; Bahia Magdalena; MillanNufiez et al. 1987) and protection, the lagoons and estuaries are feeding
and resting sites and millions of migrant shorebirds and waterfowl overwinter in these areas. The tidal vegetation, sea grasses and algae between
California (34°N) and the northern half of Baja California are similar and
thus seem to belong to the same biogeographic province. However,
between 27-24°N (Bahia Magdalena) mangroves (Rhizophora mangle,
Laguncularia racemosa) replace salt marshes, and tropical and subtropical
red algae ( Corallina vancouverensis, Gelidium rob us tum, Gracilaria rob us-
S.E. Ibarra-Obando eta!.
nant role in determining the composition of species assemblages or in
modulating the differences in abundance of the species ( Chelton et al.
1982).
21.4 Lagoonal Ecosystems
The better known coastal lagoons along the Pacific coast of Baja California
are La Salina, Estero de Punta Banda, Bahia San Quintin, the lagoonal complex of Guerrero Negro-Ojo de Liebre, Laguna San Ignacio, and Bahia
Magdalena (Fig. 21.1). Although the lagoons differ in geological origin, the
sediments are mostly sandy and alluvial. Since land-to-ocean stream flux is
limited, the advected nutrient-rich waters of the ocean directly (i.e. Bahia
San Quintin; Milhin-Nufiez et al. 1982) or indirectly (Estero de Punta
Banda and Laguna Ojo de Liebre; Amador-Buenrostro et al. 1995) affect
most biogeochemical processes in the lagoons (Fig. 21.1). Owing to a
dominance of evaporation over precipitation and lack of inflowing surface
or groundwater, moderately hypersaline conditions are common and salinity tends to increase from the mouth (approx. 33.5) towards the inner part
(approx. 40). However, average salinity and residence times vary between
35 and 14 days (Bahia San Quintin; Camacho-Ibar et al. 1997), 36 and
16 days (Laguna Guerrero Negro), and 45 and 100 days (Laguna Ojo de
Liebre; Postma 1965), respectively. Pacific tides are predominantly semidiurnal with Lower Low Water (LLW) following Higher High Water
(HHW). These conditions cause high velocities during ebb tide which flush
suspended sediments and inhibit or retard the sedimentary filling processes typical in these environments.
Most of the coastal lagoons are pristine or are barely modified and they
represent highly productive and diverse environments. Only some seasonally ephemeral lagoons (La Salina, 32°N, and Laguna Figueroa, 30°N) have
low diversity because few species can tolerate the prolonged dry periods
and extreme salinity variations. Owing to their high productivity (i.e.
Laguna Ojo de Liebre; Phleger and Ewing 1962; Bahia Magdalena; MillanNufiez et al. 1987) and protection, the lagoons and estuaries are feeding
and resting sites and millions of migrant shorebirds and waterfowl overwinter in these areas. The tidal vegetation, sea grasses and algae between
California (34°N) and the northern half of Baja California are similar and
thus seem to belong to the same biogeographic province. However,
between 27-24°N (Bahia Magdalena) mangroves (Rhizophora mangle,
Laguncularia racemosa) replace salt marshes, and tropical and subtropical
red algae ( Corallina vancouverensis, Gelidium rob us tum, Gracilaria rob us-