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

The chain of islands including Desertores, Apiao and Quemchi located in the Inner Sea of Chiloé, in northern Chilean Patagonia (41°-44°S), acts as a natural barrier for the flux of water in the north-south direction, influencing exchange and circulation between the northern and southern micro basins. The northern micro basin is mainly characterized by the dominance of surface estuarine waters whereas the southern micro basin is affected by the inflow of Subantarctic Surface and Equatorial Subsurface Waters from the adjacent coastal ocean. This setting determines key differences in the spatial distribution, abundance and size composition of phytoplankton biomass (measured as chlorophyll-a). These chlorophyll-a qualitative and quantitative differences were evaluated in contrasting seasons twice during 2004 and 2005 surveys (Cruceros de Investigación Marina, CIMAR 10 and 11 Fjords). In both micro basins, physical, chemical and bio-optical data from upper mixed layer water samples were analysed along with 8-days composite satellite images of chlorophyll-a (MODIS-AQUA). The largest difference in environmental heterogeneity between zones was in the concentration of nitrate and silicic acid. In spring the abundance of chlorophyll-a showed high variability in both sub-basins and predominance of micro-phytoplankton. In austral winter the micro-phytoplankton fraction was also dominant in a wide range of environmental with conditions, but closely linked to regions with high silicic acid concentrations. The smaller fractions, however, dominated in a more homogeneous environment characterized by higher nitrate concentration. Although nutrients are not depleted in the study area, the nitrate/silicic acid ratio clearly influenced the autotrophic community abundance, size composition and spatial structure under lower light availability during winter and more complex oceanographic processes during spring.