Hierarchical effects of biogeography and upwelling shape the dietary signatures of benthic filter feeders

Benthic filter feeders have key functional roles in coastal ecosystems as an inter mediate trophic level in food webs and as bioengineers. The food available to such species depends on the local composition of the water column, reflecting factors operating across multiple spatial scales. Here, we examined how upwelling and biogeography influence their dietary signatures. The diet regimes of 2 mussels and 1 barnacle were investigated using fatty acid (FA) and stable isotope (SI) analyses across 13 sites spanning 3000 km across 3 biogeographic regions corresponding to the west, south and east coasts of South Africa. SI and FA signatures showed similar patterns for all taxa. delta N-15 signatures of filter feeders increased from the oligotrophic east to the eutrophic west coast, with no difference between upwelling and non-upwelling sites, while delta C-13 signatures significantly decreased at upwelling sites. The lower delta C-13 signature associated with upwelling indicates different food sources from non-upwelling sites. FA signatures changed among coasts, with west coast samples differing from the south and east coasts. Upwelling affected the FA signature of filter feeders only on the west coast, where upwelling events are stronger. These specimens were enriched in polyunsaturated FA, a marker of high food quality. Hence, a powerful effect of upwelling was detectable in filter feeder diets, but depended on upwelling intensity and frequency, and was nested within the overriding effects of biogeography.