Molecular and carbon isotopic variations in lipids in rapidly settling particles during a spring phytoplankton bloom

The lipid class, fatty acid molecular assemblage and the compound-specific δ13C composition of individual fatty acids were examined as part of a study to elucidate the transfer of primary photosynthate from the euphotic zone to benthic/hyperbenthic environments in Conception Bay (Newfoundland, Canada). Total lipid and lipid classes from rapidly settling particles showed a decrease in fluxes between the sediment traps collecting at 80 and 220 m depths. The flux difference is most pronounced in the wax and steryl esters fraction where hardly any discernible amounts are recorded in the 220 m traps. The difference in total lipid fluxes with depth likely reflects active heterotrophic utilization at the intervening depths, but with the wax and steryl esters fraction also influenced by zooplankton activity at the trap deployment depths. The latter is most clearly apparent when poisoned and non-poisoned trap fluxes of wax and steryl esters are compared indicating significant contribution from zooplankton (swimmers) in the poisoned traps. Temporal variations in fatty acid molecular signatures agreed with previously recognized shifts in predominant phytoplankton taxonomy during spring blooms in cold ocean environments; i.e. the diatom contribution maximized at the very early stages of the spring bloom and dinoflagellate input was highest at the latest stages of the bloom. Compound-specific fatty acid δ13C values are highest at the peak of the bloom, and become 13C-depleted towards the end of the bloom. The δ13C temporal shift is consistent with the severe limitation in the availability of dissolved inorganic carbon substrate in the euphotic zone at the height of the bloom. However, the specific effects of growth rate and geometric parameters in fatty acid isotopic composition are also likely reflected in the observed δ13C temporal trend.