Nitrogen enrichment offsets direct negative effects of ocean acidification on a reef-building crustose coralline alga.

Ocean acidification (OA) and nutrient enrichment threaten the persistence of near shore ecosystems, yet little is known about their combined effects on marine organisms. Here, we show that a threefold increase in nitrogen concentrations, simulating enrichment due to coastal eutrophication or consumer excretions, offset the direct negative effects of near-future OA on calcification and photophysiology of the reef-building crustose coralline alga, Projected near-future pCO levels (approx. 850 µatm) decreased calcification by 30% relative to ambient conditions. Conversely, nitrogen enrichment (nitrate + nitrite and ammonium) increased calcification by 90-130% in ambient and high pCO treatments, respectively. pCO and nitrogen enrichment interactively affected instantaneous photophysiology, with highest relative electron transport rates under high pCO and high nitrogen. Nitrogen enrichment alone increased concentrations of the photosynthetic pigments chlorophyll , phycocyanin and phycoerythrin by approximately 80-450%, regardless of pCO These results demonstrate that nutrient enrichment can mediate direct organismal responses to OA. In natural systems, however, such direct benefits may be counteracted by simultaneous increases in negative indirect effects, such as heightened competition. Experiments exploring the effects of multiple stressors are increasingly becoming important for improving our ability to understand the ramifications of local and global change stressors in near shore ecosystems.