Nelson Silva

Chlorophyll-a distribution and associated oceanographic conditions in the upwelling region off northern Chile during the winter and spring 1993

The mesoscale distributions of chlorophyll-a (Chl-a) and associated physical and chemical variables off the northern coast of Chile are described using quasi-synoptic data obtained during the austral winter (August–September) and spring (November–December) of 1993. Surface temperature, salinity and chlorophyll-a data were collected in the region bounded by 18–24°S and 70–72°W. An area of persistent coastal upwelling (19–22°S, out to 74 km from the coast) was selected to examine the vertical distribution (0–100 m) of these variables together with dissolved oxygen (O2) and nutrient concentrations (phosphate, nitrate and nitrite). Surface temperature indicated that the occurrence of coastal upwelling was not well defined during the winter compared to the spring, but the vertical distribution of temperature, density, dissolved Oz and nutrients within the selected area confirmed that there was activity during both seasons. Low dissolved O2 ( 2 μM within the upper 100 m, together with the vertical salinity distribution, suggested that the main subsurface component of the upwelling water within the selected area was water of equatorial subsurface origin. Surface Chl-a distribution in the region was mostly heterogeneous during the winter, with higher concentrations ( > 1 mg m−3) both inshore and offshore, while during the spring these values were confined to within 37 km of the coast, in association with the upwelling band. Surface concentrations, however, were not significantly different between the two seasons over the whole of the region, and maximum values (> 10 mg m−3) were found in the same zone, inshore off Iquique (20°S), on both occasions. Within the selected upwelling area, most of the Chl-a was present in the upper 25 m layer during both seasons, and surface values were correlated with Chl-a integrated over this layer. Water column Chl-a concentrations in this area were significantly higher during the spring, and, during both periods, the highest Chl-a concentrations ( > 2 mg m−3) were located inshore (<37 km from the coast), with maximum values off Iquique. The predominance of net-phytoplankton was mostly associated with the highest total Chl-a concentration, and it increased towards the coast except inshore off Iquique and offshore off Arica-Iquique. These results are discussed in terms of their implications for the overall production of the pelagic system and the important fisheries that it sustains.

Chlorophyll- a distribution and mesoscale physical processes in upwelling and adjacent oceanic zones off northern Chile (summer–autumn 1994)

(HumboldtCurrent System), during the austral summer (February-March) and autumn (May) of 1994, were studied in the region bounded by 18-24`S and 70-72°W (out to 200 km from the coast ; 0-100 in depth) ; within this region, nutrients were measured in an area of persistent coastal upwelling (^-19-22`S, out to 80 km from the coast) . Temperature and salinity distributions, as well as nutrient concentrations, indicated the occurrence of active upwelling during both cruises . Also, and together with maps of geopotential anomaly (0/200 dbar) and depth of the thermocline (15°C isotherm), their distribution suggested the presence of a mainly equatorward flow, anticyclonic eddy-like structures, and intrusions of warm (>19 -C).high salinity (> 35 .0 psu), subtropical water towards the coast . A tongue of cooler and lower salinity water, and of lower flow fields, extended from the coast towards the offshore zone during both sampling periods, in association with higher chl-a concentrations (> 1 mg m -3, > 20 mg m -2 between 0 and 25 in depth) and predominance of netphytoplankton (> 20µm) . The comparison of these results with those for the winter and spring of 1993 in the same area suggest a relatively weak seasonal signal in chl-a concentration during the 1993-1994 period, with higher water column concentrations during the summer and spring periods in the selected upwelling area, though surface chl-a concentrations for the whole of the area did not vary significantly .

Variability of environmental heterogeneity in northern Patagonia, Chile: effects on the spatial distribution, size structure and abundance of chlorophyll-a

La cadena de islas incluyendo Desertores-Apiao y Quemchi situada en el Mar Interior de Chiloe, en la Patagonia chilena norte (41-44°S), actua como una barrera natural para el flujo de agua en direccion norte-sur que influye en el intercambio y circulacion entre las micro cuencas norte y sur. La micro cuenca norte se caracteriza por el dominio de las aguas estuarinas superficiales mientras que la micro cuenca sur se encuentra afectada por aguas Subantarticas y Ecuatoriales Subsuperficiales desde el oceano costero adyacente. Esta situacion determina diferencias claves en la distribucion espacial, abundancia y composicion por tamano de la biomasa autotrofica (medida como clorofila-a). Estas diferencias cuali y cuantitativas de clorofila-a se evaluaron en dos epocas contrastantes durante expediciones en 2004 y 2005 (Cruceros de Investigacion Marina, CIMAR 10 y 11 Fiordos). En ambas micro cuencas, se analizaron parametros fisicos, quimicos y bio-opticos de la capa de mezcla superficial junto con imagenes satelitales compuestas de 8 dias de clorofila-a (MODIS-AQUA). La principal diferencia de la heterogeneidad ambiental entre zonas fue determinada por la concentracion de nitratos y acido silicico. En primavera la abundancia de la clorofila-a mostro alta variabilidad en ambas micro cuencas y con predominancia del micro-fitoplancton. En el invierno austral el micro-fitoplancton tambien fue dominante en una amplia gama de condiciones ambientales, pero estrechamente vinculada a zonas con alta concentracion de acido silicico. Las fracciones mas pequenas, sin embargo, dominaron en un entorno mas homogeneo caracterizado por una mayor concentracion de nitratos. Aunque los nutrientes no se agotan en el area de estudio, la razon nitrato/acido silicico influencia claramente la abundancia, la composicion por tamano y la estructura espacial de la comunidad autotrofica durante invierno bajo menor disponibilidad de luz y procesos oceanograficos mas complejos durante la primavera.