232
J. Tarazona and W. Arntz
16.2.3 Oceanographic Features
Interannual sea levels off Peru fluctuate by more than 40 em (seasonal differences <15 em), with even higher amplitudes during El Nino events. Tidal
amplitude along the Peruvian coast does not exceed 1.2 m and semidiurnal
tides predominate north of 5°S while diurnal tides occur towards the
south. Sea surface temperatures in Peru and Chile tend to increase towards
the west and north, thus creating both a zonal and latitudinal gradient, respectively, the latter being mainly a consequence of coastal upwelling. The
average seasonal variation of surface water temperatures off Peru (winter
13-17 °C,summer 17-27°C) and Chile (winter 11-17°C,summer 13-24 °C)
is 5-7 oc and 2-8 oc, respectively. Temperatures increase in El Nino years
(1982-1983, 1986-1987, 1991-1992, 1997-1998), with an increment of
almost 10 oc above the long-term mean during the 1983 event. Salinity ranges from 34.8 to 35.1 in nearshore upwelling zones off Peru and between
33.8 and 35.2 north of 34°S off Chile. During El Nino events, equatorial
waters with lower salinity are displaced towards the south and oceanic
water (salinity >35.1) approaches the coast (Tarazona et al. 1985).
Dissolved superficial oxygen concentration off the Peruvian and Chilean coast vary between 2-7 and 4-8 ml I- 1 , respectively. Off the central
Peruvian coast, oxygen minima ( <0.5 ml I- 1 ) and frequent anoxia occur
mostly between 50 and 700 m depth, owing to high organic matter accumulation and microbial activity (Rowe 1985). However, low oxygen values
may also be associated with shallow depths (<20m) in protected bays
(Rosenberg et al. 1983; Tarazona et al. 1991). Although oxygen concentrations at the sea floor tend to diminish from north to south, below the
minimum oxygen zone they rapidly increase with depth. Owing to an
influx of equatorial water, dissolved oxygen values increase strongly
during El Nino events (Arntz et al. 1991) and, on the whole, dissolved
oxygen concentrations seem to be the principal selective factor for the
benthic, demersal, and pelagic invertebrate and fish fauna on the continental shelf off Peru.
Coastal waters of the upwelling system off Peru are characterized by
high phosphate (maximum >2.5 flg 1- 1 ) and silicate (up to 20 flg 1- 1 ) concentrations, while nitrite (0.1-1.6 flg I- 1 ) and nitrate (0.5-20 flg I- 1 ) concentrations tend to decrease towards offshore areas (approx. 90km). High
nitrate concentration corresponds to upwelling centers and denitrification
is an important process because it is restricted to oxygen-deficient situations that characterize the coastal upwelling ecosystem. High rates of organic matter deposition are typical of sediments in upwelling areas. The
magnitude of organic matter loss is controlled by the rate of heterotrophic
activity and euphotic production. The remineralized nitrogen can be
J. Tarazona and W. Arntz
16.2.3 Oceanographic Features
Interannual sea levels off Peru fluctuate by more than 40 em (seasonal differences <15 em), with even higher amplitudes during El Nino events. Tidal
amplitude along the Peruvian coast does not exceed 1.2 m and semidiurnal
tides predominate north of 5°S while diurnal tides occur towards the
south. Sea surface temperatures in Peru and Chile tend to increase towards
the west and north, thus creating both a zonal and latitudinal gradient, respectively, the latter being mainly a consequence of coastal upwelling. The
average seasonal variation of surface water temperatures off Peru (winter
13-17 °C,summer 17-27°C) and Chile (winter 11-17°C,summer 13-24 °C)
is 5-7 oc and 2-8 oc, respectively. Temperatures increase in El Nino years
(1982-1983, 1986-1987, 1991-1992, 1997-1998), with an increment of
almost 10 oc above the long-term mean during the 1983 event. Salinity ranges from 34.8 to 35.1 in nearshore upwelling zones off Peru and between
33.8 and 35.2 north of 34°S off Chile. During El Nino events, equatorial
waters with lower salinity are displaced towards the south and oceanic
water (salinity >35.1) approaches the coast (Tarazona et al. 1985).
Dissolved superficial oxygen concentration off the Peruvian and Chilean coast vary between 2-7 and 4-8 ml I- 1 , respectively. Off the central
Peruvian coast, oxygen minima ( <0.5 ml I- 1 ) and frequent anoxia occur
mostly between 50 and 700 m depth, owing to high organic matter accumulation and microbial activity (Rowe 1985). However, low oxygen values
may also be associated with shallow depths (<20m) in protected bays
(Rosenberg et al. 1983; Tarazona et al. 1991). Although oxygen concentrations at the sea floor tend to diminish from north to south, below the
minimum oxygen zone they rapidly increase with depth. Owing to an
influx of equatorial water, dissolved oxygen values increase strongly
during El Nino events (Arntz et al. 1991) and, on the whole, dissolved
oxygen concentrations seem to be the principal selective factor for the
benthic, demersal, and pelagic invertebrate and fish fauna on the continental shelf off Peru.
Coastal waters of the upwelling system off Peru are characterized by
high phosphate (maximum >2.5 flg 1- 1 ) and silicate (up to 20 flg 1- 1 ) concentrations, while nitrite (0.1-1.6 flg I- 1 ) and nitrate (0.5-20 flg I- 1 ) concentrations tend to decrease towards offshore areas (approx. 90km). High
nitrate concentration corresponds to upwelling centers and denitrification
is an important process because it is restricted to oxygen-deficient situations that characterize the coastal upwelling ecosystem. High rates of organic matter deposition are typical of sediments in upwelling areas. The
magnitude of organic matter loss is controlled by the rate of heterotrophic
activity and euphotic production. The remineralized nitrogen can be