Calcification, photophysiology and treatment parameters for laboratory experiment

Ocean acidification and coastal nutrient enrichment threaten the persistence of nearshore ecosystems, yet little is known about their combined effects on marine organisms. Here, we show that elevated nitrogen concentrations, in the forms of nitrite + nitrate and ammonium, offset the negative effects of near-future OA on calcification of the reef-building crustose coralline alga, Porolithon onkodes. Projected near-future pCO2 levels (~850 µatm) decreased P. onkodes calcification by 30% relative to ambient conditions. Conversely, nitrogen enrichment increased P. onkodes calcification by 90% and 130% in ambient and high pCO2 treatments, respectively, relative to ambient controls. pCO2 and nitrogen enrichment interactively affected instantaneous photophysiology. Relative electron transport rates (rETR) were highest in high pCO2 and high nitrogen conditions. Nitrogen enrichment alone increased concentrations of the photosynthetic pigments chlorophyll a, phycocyanin and phycoerythrin by ~80-450%, regardless of pCO2. These results demonstrate that nutrient enrichment can mediate organismal responses to OA, which has far-reaching implications for nearshore coral reefs that experience persistent or episodic nutrient enrichment via eutrophication or consumer excretions. Multi-stressor OA experiments increasingly are becoming important for improving our ability to predict the response of marine organisms and coral reefs to simultaneously occurring local and global change stressors.