CONDICIONES OCEANOGRÁFICAS FÍSICAS Y QUÍMICAS, ENTRE BOCA DEL GUAFO Y FIORDO AYSÉN (CRUCERO CIMAR 7 FIORDOS)*

NELSON SILVA ¹ DAFNE GUZMÁN ^{1, 2}

¹ Pontificia Universidad Católica de Valparaíso Av. Altamirano 1480, Valparaíso. E-mail: nsilva@ucv.cl ² Universidad del Mar. Carmen 446, Cerro Placeres, Valparaíso.

Recepción: 20 de junio de 2004 - Versión corregida aceptada: 19 de abril de 2006.

ABSTRACT

The CIMAR 7 Fiordos oceanographic cruise covered the area from Boca del Guafo (43^o 30' S) to Aysén Fjord (45^o 30' S) and consisted of two legs: the first between 7 and 21 July 2001 (46 stations) and the second between 11 and 27 November 2001 (47 stations). Temperature and salinity were taken during each leg, as were water samples for dissolved oxygen and nutrient analyses (phosphate, nitrate, and silicate). The oceanographic stations used for this research were distributed along the Boca del Guafo, Moraleda Channel, and Aysén Fjord. Vertical sections were prepared for each of the characteristics mentioned above, and seasonal and interannual variability were analyzed through comparisons with other cruises carried out in the area.

The vertical distribution confirms the existence of a vertical structure of two layers for most of the oceanographic variables analyzed. The sur face layer was characterized by greater variability whereas the deeper layer was more homogeneous. The surface layer consisted of warmer, oxygenated, less saline waters and lower in nutrient, than those of the deep layer. The only exception to this pattern was found at the head of Aysén Fjord; the wintertime temperature of the sur face layer was less than that of the deep layer and, in both seasons, silicate in the surface layer was greater than in the deep layer. No anoxic zones were detected during the cruise.

No seasonal differences were noticeable for salinity and dissolved oxygen between the two legs of the cruise. Temperatures were higher in spring than in winter and a weak seasonal thermocline was observed. Nutrients were distributed in two layers in spring, but in winter the vertical structure was more homogenous.

The analysis of the vertical distribution of physical and chemical variables in the water at the oceanographic stations and T-S diagrams allowed the determination of the presence of two water masses in the upper 200 m of the oceanic zone. These were the Subantarctic Sur face Waters (SAAW) and remnants of the Equatorial Subsurface Waters (AESS). In the channel and fjord area, the SAAW are mixed with freshwater (AD) from rivers, rain, and ice melting, which generates two bodies of water: Modified Subantarctic Waters (ASAAM) and Estuarine Waters (AE). As the oceanic waters enter through Boca del Guafo and move towards the interior channels and fjords, they are mixed with AD, gradually decreasing salinity and progressively generating variable amounts of ASAAM and AE. Depending on the proportion of fresh water/sea water, the AE is subdivided into: Salty Estuarine Waters (AE-Salada; 21 to 31 psu), Brackish Estuarine Waters (AE-Salobre; 11 to 21 psu), and Fresh Estuarine Waters (AE-dulce; >2 to 11 psu). Waters with salinities from 0 to ≤ 2 psu are considered to be AD.

Key words: Temperature, salinity, dissolved oxygen, nutrients, estuaries, channels, fjords, water masses, Chile.