There are serious concerns that ocean acidification will combine with the effects of global warming to cause major shifts in marine ecosystems, but there is a lack of field data on the combined ecological effects of these changes due to the difficulty of creating large-scale, long-term exposures to elevated CO 2 and temperature. Here we report the first coastal transplant experiment designed to investigate the effects of naturally acidified seawater on the rates of net calcification and dissolution of the branched calcitic bryozoan Myriapora truncata (Pallas, 1766). Colonies were transplanted to normal (pH 8.1), high (mean pH 7.66, minimum value 7.33) and extremely high CO 2 conditions (mean pH 7.43, minimum value 6.83) at gas vents off Ischia Island (Tyrrhenian Sea, Italy). The net calcification rates of live colonies and the dissolution rates of dead colonies were estimated by weighing after 45 days (May-June 2008) and after 128 days (July-October) to examine the hypothesis that high CO 2 levels affect bryozoan growth and survival differently during moderate and warm water conditions. In the first observation period, seawater temperatures ranged from 19 to 24°C; dead M. truncata colonies dissolved at high CO 2 levels (pH 7.66), whereas live specimens maintained the same net calcification rate as those growing at normal pH. In extremely high CO 2 conditions (mean pH 7.43), the live bryozoans calcified significantly less than those at normal pH. Therefore, established colonies of M. truncata seem well able to withstand the levels of ocean acidification predicted in the next 200 years, possibly because the soft tissues protect the skeleton from an external decrease in pH. However, during the second period of observation a prolonged period of high seawater temperatures (25-28°C) halted calcification both in controls and at high CO 2 , and all transplants died when high temperatures were combined with extremely high CO 2 levels. Clearly, attempts to predict the future response of organisms to ocean acidification need to consider the effects of concurrent changes such as the Mediterranean trend for increased summer temperatures in surface waters. Although M. truncata was resilient to short-term exposure to high levels of ocean acidification at normal temperatures, our field transplants showed that its ability to calcify at higher temperatures was compromised, adding it to the growing list of species now potentially threatened by global warming. Marine Ecology. ISSN 0173-9565 Marine Ecology 31 (2010) 447-456 ª

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Effects of ocean acidification (OA on the colonization/settlement pattern of the epibiont community of the leaves and rhizomes of the Mediterranean seagrass, Posidonia oceanica, have been studied at volcanic CO 2 vents off Ischia (Italy), using "mimics" as artificial substrates. The experiments were conducted in shallow Posidonia stands (2-3 m depth), in three stations on the north and three on the south sides of the study area, distributed along a pH gradient. At each station, 4 rhizome mimics and 6 artificial leaves were collected every three months (Sept 2009-Sept 2010. The epibionts on both leaf and rhizome mimics showed clear changes along the pH gradient; coralline algae and calcareous invertebrates (bryozoans, serpulid polychaetes and barnacles) were dominant at control stations but progressively disappeared at the most acidified stations. In these extremely low pH sites the assemblage was dominated by filamentous algae and non calcareous taxa such as hydroids and tunicates. Settlement pattern on the artificial leaves and rhizome mimics over time showed a consistent distribution pattern along the pH gradient and highlighted the peak of recruitment of the various organisms in different periods according to their life history. Posidonia mimics at the acidified station showed a poor and very simplified assemblage where calcifying epibionts seemed less competitive for space. This profound difference in epiphyte communities in low pH conditions suggests cascading effects on the food web of the meadow and, consequently, on the functioning of the system.