1 3 Oecologia DOI 10.1007/s00442-015-3242-5 PHYSIOLOGICAL ECOLOGY - ORIGINAL RESEARCH CO 2 and inorganic nutrient enrichment affect the performance of a calcifying green alga and its noncalcifying epiphyte Laurie C. Hofmann · Kai Bischof · Cecilia Baggini · Andrew Johnson · Ketil Koop-Jakobsen · Mirta Teichberg Received: 17 July 2013 / Accepted: 16 January 2015 © Springer-Verlag Berlin Heidelberg 2015 (L.) J.V. Lamouroux) and its common noncalcifying epi- phyte (Dictyota sp.) in a 4-week fully crossed multifacto- rial experiment. Inorganic nutrient enrichment (+NP) had a strong influence on all responses measured with the excep- tion of net calcification. Elevated CO 2 alone significantly decreased electron transport rates of the photosynthetic apparatus and resulted in phosphorus limitation in both spe- cies, but had no effect on oxygen production or respiration. The combination of CO 2 and +NP significantly increased electron transport rates in both species. While +NP alone stimulated H. opuntia growth rates, Dictyota growth was significantly stimulated by nutrient enrichment only at elevated CO 2 , which led to the highest biomass ratios of Dictyota to Halimeda. Our results suggest that inorganic nutrient enrichment alone stimulates several aspects of H. opuntia physiology, but nutrient enrichment at a CO 2 con- centration predicted for the end of the century benefits Dic- tyota sp. and hinders its calcifying basibiont H. opuntia. Keywords Halimeda opuntia · Dictyota · Calcification · Ocean acidification · Eutrophication Introduction In recent decades, the biological and ecological effects of increasing CO 2 concentrations in surface ocean waters (ocean acidification) as a result of anthropogenic CO 2 input into the atmosphere has been widely studied. Although calcifying marine organisms show a variety of responses to increasing CO 2 concentrations (Langer et al. 2006; Ries et al. 2009; Fabricius et al. 2011; Hurd et al. 2011), a general trend is emerging that benthic marine commu- nities with a mixture of calcifying and noncalcifying spe- cies may become dominated by the latter under future CO 2 Abstract Ocean acidification studies in the past decade have greatly improved our knowledge of how calcifying organisms respond to increased surface ocean CO 2 levels. It has become evident that, for many organisms, nutri- ent availability is an important factor that influences their physiological responses and competitive interactions with other species. Therefore, we tested how simulated ocean acidification and eutrophication (nitrate and phosphate enrichment) interact to affect the physiology and ecology of a calcifying chlorophyte macroalga (Halimeda opuntia Communicated by Elena Litchman. L. C. Hofmann (*) · K. Bischof Marine Botany, Bremen Marine Ecology Centre for Research and Education, University of Bremen, Leobener Str. NW2, 28359 Bremen, Germany e-mail: laurie.c.hofmann@gmail.com C. Baggini Marine Biology and Ecology Research Center, Plymouth University, Plymouth, Devon PL4 8AA, UK A. Johnson Department of Biological Sciences, Virginia Institute of Marine Science, College of William and Mary, 1375 Greate Road, Gloucester Point, VA 23062, USA K. Koop-Jakobsen HGF-MPG Group for Deep-Sea Ecology and Technology, Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany K. Koop-Jakobsen Center for Marine Environmental Sciences, Leobener Str., 28359 Bremen, Germany M. Teichberg Leibniz Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany