Nutrient availability and seawater temperature modulate algal nutrient quality and herbivore-algae interactions

Nutrient availability and temperature have a strong effect on the performance of organisms and ultimately determine the productivity of marine ecosystems. To evaluate how nutrient supply and temperature variability influence trophic dynamics, we simulated in the laboratory the main environmental conditions of upwelling and downwelling sites, contrasting high and low nutrient availability under current and end-of-the-century projected temperatures. We used a green alga ( Ulva sp.) and a prominent herbivore, the sea urchin Loxechinus albus , to test 2 hypotheses: (1) the positive effect of nutrient availability on algal biochemical composition, observed in cold upwelling zones, is attenuated or reversed under future warming scenarios due to the increased energy demand imposed by warmer temperatures; and (2) reduced nutrient availability in algal tissues reduces the physiological performance and thermal tolerance of herbivores compared to those consuming diets of higher nutritional content. Our results show that nutrient availability had a significant influence on the nutritional properties of the algae and indirectly on the food consumption and absorption efficiency rates of the sea urchins in addition to 2 thermal performance parameters. Temperature, and its interaction with nutrients, also had a significant effect on the herbivore’s growth rate and critical thermal maximum. Considering various climate model predictions that suggest warmer waters in or near upwelling systems, our results suggest negative effects for algal nutritional quality, and the feeding ecology of sea urchins and other consumers relying on these and possibly other macroalgae.