ALBERT

Description: Limacina helicina is a key component of matter and energy fluxes in Arctic marine ecosystems. This study aimed to investigate unresolved aspects of its life history, feeding behaviour, and grazing impact in autumn. L. helicina and particular organic matter were sampled weekly from end of August until early October in Adventfjorden, Svalbard, for fatty acid and carbon and nitrogen stable isotope analysis. Moreover, population structure and body composition of L. helicina were determined. Adults disappeared from surface waters end of August, but persistent spawning indicated the presence of at least some adults until October. Veligers and juveniles were actively feeding and grew 0.0044-0.0072 mm/d. The fatty acid composition of particular organic matter was largely mirrored in the fatty acid composition of L. helicina, revealing phytoplankton and detritus being its major food sources in autumn. This was supported by only slightly enriched d15N of L. helicina compared to particular organic matter (‰〈4). Elevated amounts of calanoid copepod markers in L. helicina suggest feeding on copepod debris to a small extend. Increasing levels of(n-3) polyunsaturated fatty acids in growing L. helicina indicate that these compounds were decisive for growth. Differing trends in particular organic matter suggest that L. helicina regulated its food quality to a small degree which is supported by considerable enrichment or depletion of some fatty acids. Chlorophyll-a clearance rates of L. helicina veligers/juveniles were measured to estimate their grazing rates. Veligers/juveniles did not top-down control primary producers, but at high abundances and times of low primary production they may have a considerable impact. This study documented the growth and omnivorous feeding of L. helicina veligers/juveniles in autumn in the Arctic for the first time. To fully understand L. helicina's life history, further research on its population structure and feeding strategy during the poorly studied winter is recommended.