Abstract

The project Fondecyt 5960002-1996 "Primary production and its fate in the trophic web and ocean-atmosphere C02 exchange in the upwelling system off Antofagasta" focused on the study of carbon pathways from the primary production (PP) through the pelagic food web and deep-sea. Also the CO exchange between the ocean and the atmosphere was assessed in both coastal and oceanic zones off Antofagasta. The following carbon utilization/ transference pathways were studied: primary production (PP), community respiration (RC), bacteria] secondary production (PBS), zooplankton grazing (PZ) and particulate organic carbon sedimentation (SCOP) were estimated during pre-El Niño (January 1997) and El Niño (July 1997) conditions. In addition the following carbon pools: particulate (COP) and dissolved (COD) organic carbon together with oceanic and atmospheric pCO were measured. The major findings of the cruises were the following: (i) During January, cold filaments associated to upwelling events and propagating more than 100 nm from coastal toward oceanic areas were described. The water column structure under both pre-EI Niño and El Niño 1997-98 conditions showed a deepening of the thermocline from ca. 50 m during January (pr- El Niño), up to ca. 200 m depth in July 1997 (El Niño). (ii) The PP showed a large variability (range: 478 to 5 503 mg m-2 · d-1), and the smallest size-fractions (pico- y nano-phytoplankton, < 23 m) made an important contribution to total PP (>65%) and chlorophyll-a (>65%). (iii) The chitinous zooplankton was dominated by small-size calanoid copepods in abundance and for euphausiids in biomass. The predatory impact of these animals. estimated as percentage of total PP removed, was in the range of 6-14% (depending if a correction factor for taking into account possible pigment destruction to colorless compounds is used or not). On the other hand, salps aggregations of the species Salpa fusijormis were abundant during January 1997 and removed. on the average, slightly higher fractions of the total PP (ca. 16%) than the whole chitinous zooplankton combined. (iv) The BSP within the photic zone averaged 405 and 184 mgC · m · d during January and July 1997, respectively, which correspond to 33 and 14% of total PP during both periods. (v) The micro-zooplankton community respiration fluctuated in a broad range from 40 to 200% of total PP. The integrated respiration of the water column (0-50 m) showed a mean of 169.5 ± 68 and 58± 24 mmol O m d for January and July 1997. respectively. A positive correlation between microplanktonic respiration and temperature (r=0.76 in January and r=0.63 in July), dissolved oxygen (r=0.78 in January and r=0.25 in July) and fluorescence (r=0.72 in July) were found. The activity of the electron transport system (ASTE) of the total planktonic community (0.7 to 8000 mm) was significantly higher during the pre-ENSO. than ENSO periods. (vi) Sedimentation rates of POC fluctuated between 35 and 152 mgC · m-2 · d-1. Sedimented matter was dominated by faecal material, however. observations using microscopy methods indicated that, while copepod faecal pellets are efficiently recycled within surface layers of the ocean, krill faecal material might enhance vertical flux of POC to deep-sea. (vii) Overall pC02 values were higher in seawater than in the atmosphere, indicating that the study area behaved as a source of CO to the atmosphere during both January and July 1997. However, restricted coastal arcas characterized by high chl-a concentrations showed C02 values lower than those simultaneously recorded in the atmosphere. (viii) COD vertical profiles showed a wide range of concentrations (52-104 M) within the first 300 m water column and a narrow range(55-64 M) below 300 m depth. The "autotroph" versus "heterotroph" behaviour of the upwelling system off Antofagasta base on the balance between PP and CR was studied. On the average, the system showed an "autotroph" behaviour (PP > RC), however, a "heterotroph" behaviour (PP < RC) was described for the oceanic zone during January 1997. During this period, a COP and COD transport in cold- water-filaments generated from coastal-, and extending towards oceanic-areas, was suggested.