Cross-examining the influence of upwelling and seaweed quality on herbivores' feeding behavior and growth

At the regional scale, upwelling conditions are known to influence ecosystems and communities and their primary and secondary productivity. However, the influence of upwelling on local herbivore-algae interactions is less well understood. We address this question by cross-examining herbivores and seaweeds from sites associated with upwelling and downwelling conditions along the Humboldt Current System. Specifically, we quantified the feeding and benefits attained by the black sea urchin (Tetrapygus niger) and the black sea snail (Tegula atra) while consuming a widespread kelp species (Lessonia spicata). We hypothesized that food quality drives herbivores' preference, consumption, and growth rates, regardless of the origin or "prior" conditions of the consumers. Laboratory trials measured algal consumption rates with (preference) and without a choice, and consumer's growth rates, to assess the influence of food quality (algae from upwelling vs downwelling sites) and the site of origin of the consumers. Our results showed that algal quality was a prevailing factor for both herbivores: they chose, consumed more, and grew faster on high quality (upwelling) algae. By comparison, the origin of the consumer was only significant for sea snails: those coming from an upwelling site, consumed significantly more and grew faster than those from downwelling. The bulk of our results provided strong support to our hypothesis and suggest that the high nutritional quality of algae associated with upwelling centers has a strong influence on consumers' preferences, consumption, and performance (growth). The fact that origin was found to be relevant for one of the herbivores suggests that the conditions in which species grow may dictate some of their efficiency as consumers.