Understanding the nutritional status, diet, and demographic structure of Euphausia pacifica using organic markers

Abstract

Along the Oregon Coast, E. pacifica are the most abundant krill species and play a key link between primary producers and top predators. However, their ecology and vital rates are difficult to understand because this environment is strongly affected by physical forces that cause the spatial and temporal heterogeneity of environmental conditions. In this study, a suite of organic markers (including algal pigments and lipids) in animals and seston were analyzed to better understand the feeding ecology and nutritional status of E. pacifica by tracking diets and trophic transfer of energy. The results indicate that diatoms were the most abundant algal species in nearshore upwelling regions while greater diversity and smaller sizes of algal communities were found at offshore, displaying both temporal and spatial variations. Lipid levels and compositions in krill are closely linked to their life cycle (i.e. reproduction) and environmental condition (i.e. food availability). Furthermore, the lipid composition of E. pacifica collected across shelf transects in several areas reflect the varied food resources available in the water column, but also suggest selective feeding. Other lipid markers (fatty acids and sterols) are well correlated with algal pigment signatures including the distribution of diatom specific fatty acids (i.e.16:1(n-7) and 20:5(n-3)). Results of algal pigment and lipid analysis show substantial changes in many of these markers in animals and seston along physico-chemical features and coastal topography, most notably in upwelling regions where diatoms are important. An additional understanding of the precise demographic structure of E. pacifica is needed to assess growth and recruitment as well as to determine how changes in environmental conditions alter their condition and distribution. Since age determination of euphausiids cannot be accomplished using traditional approaches, we evaluated the potential of age pigments (collectively termed lipofusc