Spatial variation in zooplankton fatty acid composition in the northern Baltic Sea – a focus on offshore and coastal zone

Abstract

Zooplankton are essential in marine ecosystems, connecting primary producers to higher trophic levels and transferring essential nutrients such as polyunsaturated fatty acids (PUFA) to them. It is important to identify the factors influencing spatial differences in zooplankton fatty acid profiles to evaluate marine ecosystem health and resilience to ecological changes. This study investigated spatial variations in zooplankton fatty acid profiles between coastal and offshore areas in the Baltic Sea, examining dominant zooplankton communities and the effects of environmental factors on their distribution and fatty acid composition. Zooplankton samples were collected from offshore and coastal areas and their fatty acid profiles were analyzed using gas chromatography-mass spectrometry (GC-MS). Microscopic analysis of the zooplankton community was conducted. Results revealed surface temperatures were highest at Seili Archipelago Research Institute (18.24°C) and lowest at station BO3 (5.55°C), while salinity increased gradually with depth without sharp gradients. Coastal zones had lower surface dissolved oxygen (DO) levels (8–10 mg/L) than offshore areas (>12 mg/L), with DO decreasing with depth at all stations. Copepods dominate at all stations with coastal areas having moderate diversity, while offshore stations BO3 and SR5 had the lowest diversity. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were the dominant fatty acids at all stations with trace levels of arachidonic acid. Fluctuation of environmental variables influences phytoplankton composition and function causing changes in the quality and quantity of specific PUFAs. The considerable difference in fatty acid composition observed in offshore areas (average 32.15 %) compared to coastal regions (average 27.88%), is due to the higher abundance of copepods, their dependence on PUFA-rich phytoplankton, and the favourable environmental conditions in these offshore areas. The growth, function, and production of fatty acids in both phytoplankton and zooplankton are influenced by plankton taxa, seasonal changes, abiotic factors, nutrient availability and predation pressure.