Environmental variables driving spatial variation in fatty acid composition of brackish-water zooplankton communities

Eutrophication, warming, and salinity fluctuations are altering the fatty acid composition of marine plankton communities and the quality of available food to consumers. The planktonic content of essential fatty acids 22:6n-3 and 20:5n-3 is prone to changes due to the warming of seas, with cascading effects on the food webs, and zooplankton transfers these fatty acids from phytoplankton to fish. We studied the effect of temperature, salinity and eutrophication on zooplankton community samples collected during two weeks in August on R/V Aranda. The study area has high spatial representativeness and covers six basins of the Baltic Sea. The zooplankton lipids were converted to fatty acid methyl esters, which were identified by GC-MSD and analysed for quantitative composition by GC-FID. The fatty acid compositions of the zooplankton community differed between the sea areas. Temperature and eutrophication together were responsible for higher share of the fatty acid markers of heterotrophic bacteria in the zooplankton community, whereas salinity correlated with low EPA/DHA and high n-3/n-6 polyunsaturated fatty acid ratios. Salinity likely affected the fatty acid compositions indirectly by shaping the species composition: the low-salinity areas were dominated by cladocerans typically rich in EPA. This study showed that the Baltic zooplankton communities have significant spatial variation in terms of fatty acid nutritional quality, and this variation is driven by multiple environmental variables reflecting climate change. Our results help to understand how phytoplankton fatty acids mediate the effects of climate change to organisms at higher trophic levels by altering their supply of essential fatty acids.