Ecological investigations on the zooplankton community of Balsfjorden, northern Norway: Seasonal changes in body weight and the main biochemical composition of thysanoessa inermis (krøyer), T. Raschii (M. Sars), and Meganyctiphanes norvegica (M. Sars) in relation to environmental factors

Abstract

An eco-biochemical study of the euphausiids Thysanoessa inermis (Krøyer), T. raschii (M. Sars), and Meganyctiphanes norvegica (M. Sars) has been carried out in Balsfjorden, northern Norway, (70°N). The seasonal variation in wet and dry weights, and protein, lipid, and ash content of 0-, I- and II-groups have been related to their population dynamics and to environmental data. The weight increases and changes in biochemical content were closely related to the primary production cycle. 0-group Thysanoessa showed an increase in weight from July to September, while there was “stagnation” during the winter. In I-group of the two Thysanoessa species a great increase in weight could be seen from March–April to August–September; wet and dry weights, and protein and lipid contents increased by > 350, 600, 500, and 1200%, respectively. During winter the weight decrease in I-group Thysanoessa could partly be explained by a decrease in the mean carapace length of the population. The proportion of protein and ash was higher and lipid lower in the 0-group than in I- and II-groups for the three investigated species. The decrease in the relative amount of lipid in the 0-group Thysanoessa during winter can be related to both low phytoplankton production and decreasing food quality. There was a decrease in lipid for I-group T. inermis and Meganyctiphanes norvegica during the winter and this can be associated with the use of energy for over-wintering and growth of gonads. The difference in the protein/lipid content between the three species indicate different metabolic responses towards overwintering. The uses of biochemical variation in examination of ecological niches are discussed and it is suggested that the high lipid level (48%) in I-group Thysanoessa inermis indicates that this species is a more “obligate” herbivore than I-group T. raschii and Meganyctiphanes norvegica with lower lipid levels ( ≈ 37%). The biochemical composition for the three species from different geographical areas have been compared. The lipid contents were considerably higher in euphausiids from the subarctic Balsfjorden (70 °N) than those levels published for temperate-boreal waters (50–60 °N). This supports the supposition that lipid is especially important in the ecology of Zooplankton from higher latitudes. It is concluded that differences in lipid levels between Zooplankton from different latitudes reflect differences in light climate and its influence on primary production.