Primary production and its fate in the pelagic food web and deep-sea, and ocean-atmosphere CO2 exchange in the northern Humboldt Current (23 degrees S): Possible effects of the 1997-98 El Nino in Chile

The project Fondecyt 5960002-1996 "Primary production and its fate in the t rophic web and ocean-atmosphere CO2 exchange in the upwelling system off An tofagasta" focused on the study of carbon pathways from the primary product ion (PP) through the pelagic Food web and deep-sea. Also the CO2 exchange b etween the ocean and the atmosphere was assessed in both coastal and oceani c zones off Antofagasta. The following carbon utilization/transference path ways were studied: primary production (PP), community respiration (RC), bac terial secondary production (PBS), zooplankton grazing (PZ) and particulate organic carbon sedimentation (SCOP) were estimated during pre-EI Nino (Jan uary 1997) and El Nino (July 1997) conditions. In addition the following ca rbon pools: particulate (COP) and disserved (COD) organic carbon together w ith oceanic and atmospheric pCO(2) 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 oc eanic areas were described. The water column structure under both pre-El Ni no and El Nino 1997-98 conditions showed a deepening of the thermocline fro m ca. 50 m during January (pre-El Nino), up to ca. 200 m depth in July 1997 (El Nino). (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 mu m) made an important contribution to total PP (>65%) and chloroph yll-a (>65%). (iii) The chitinous zooplankton was dominated by small-size c alanoid 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 acc ount possible pigment destruction to colorless compounds is used or not). O n the other hand, salps aggregations of the species Salpa fusiformis were a bundant during January 1997 and removed, on the average, slightly higher fr actions of the total PP (ca. 16%) than the whole chitinous zooplankton comb ined. (iv) The BSP within the photic zone averaged 405 and 184 mgC . m(-2) . d(-1) during January and July 1997, respectively, which correspond to 33 and 14% of total PP during both periods. (v) The micro-zooplankton communit y 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-2 m(-2) . d(-1) for January and July 1997, resp ectively. A positive correlation between microplanktonic respiration and te mperature (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 plankto nic 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 mate rial, however, observations using microscopy methods indicated that, while copepod faecal pellets are efficiently recycled within surface layers of th e ocean, krill faecal material might enhance vertical flux of POC to deep-s ea. (vii) Overall pCO(2) values were higher in seawater than in the atmosph ere, indicating that the study area behaved as a source of CO2 to the atmos phere during both January and July 1997. However, restricted coastal areas characterized by high ch1-a concentration s showed CO2 values lower than those simultaneously recorded in the atmosph ere. (viii) COD vertical profiles showed a wide range of concentrations (52 -104 mu M) within the first 300 m water column and a narrow range(55-64 mu 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 oce anic zone during January 1997. During this period, a COP and COD transport in coldwater-filaments generated from coastal-, and extending towards ocean ic-areas, was suggested.