While detrimental to many marine species, ocean acidification (increased carbon dioxide, CO2) and nutrient loading will enhance the growth and spread of opportunistic macroalgae. This enhanced growth could be troubling for many ecosystems, but diverse communities may ultimately be resilient. As macroaglal consumers, grazers hold the key to controlling algal growth. For grazers to effectively control macroalgal abundance, the indirect effects of climate change (i.e. altered nutritional quality) will need to outweigh the direct effects. Here, we tested how macroalgal consumption rates and diet preferences of a common marine snail (Littorina littorea) were impacted by the interaction of increased CO2 and nutrients. Field-collected snails were placed in treatment mesocosms and were exposed to two pCO2 levels (400 and 1200 ppm) and two nutrient levels (10 and 200 μM TN), in a factorial design. Snails were given a choice of two common macroalgal species, Ulva lactuca and Fucus vesiculosus. Experiments ran for seven days while consumption rates and diet preferences were measured. Consumption experiments were also run using artificial, reconstituted algae to determine the role of nutritional quality and tissue toughness in snail diet preference. In addition, we investigated the direct effects of these environmental factors on snails by measuring respiration rates.
Results/Conclusions
High pCO2 resulted in a 50% reduction of snail consumption rates. Under ambient pCO2, snails fed on a mixed diet of both Ulva and Fucus, but under high pCO2 we observed a shift in diet where snails fed exclusively on Ulva. Despite increasing the quality of both Ulva and Fucus, increased nutrients did not affect the consumption rates or diet preference of snails. Consumption of artificial Ulva and Fucus was unaffected by environmental treatment, as snails consumed similar amounts of the reconstituted algal species, indicating that nutritional quality was not driving the diet shift observed. Snails, however, were directly impacted by acidification as respiration rates were significantly reduced under high pCO2. Here, the direct, physiological effects on snails had a greater effect than the indirect effects of increased food quality. As a result, L. littorea was unable to increase consumption to match expected macroalgal growth, ultimately enhancing algal growth and spread in this system. While L. littorea may not be able to control algal growth, grazer diversity will be important, as other species may be able to fill the void.