We used carbon and nitrogen stable isotope (δ13C and δ15N) analyses to investigate the trophic network of the CWC reef habitat off Angola (SE Atlantic Ocean). Samples were collected in January 2016 during the M122 (“ANNA”) expedition on board R/V Meteor. In total, 18 reef sites, including seven CWC mound settings over a bathymetric range of 250 to 530 m water depth, were sampled for stable isotope analyses. Samples of organisms belonging to the taxa Porifera, Cnidaria, Arthropoda, Annelida, Echinodermata and Chordata were collected by means of a box corer, a Van-Veen grab sampler and the Remotely Operated Vehicle (ROV) SQUID (MARUM, Bremen, Germany). To investigate potential food sources of the benthic megafauna, three types of Particulate Organic Matter (POM) were collected: Suspended Particulate Organic Matter (SPOM) was collected with a McLane phytoplankton pump; settling SPOM was collected with a sediment trap (SPOM trap) and sediment samples were collected with a box corer and a grab sampler. Analyses of benthic megafauna were performed using a Elementar IsoPrime 100 isotope ratio–mass spectrometry (IR–MS) (IsoPrime Ltd.) coupled to a CNS elemental analyzer (Elementar Vario Pyro Cube EA CNS; Elementar Analysensysteme GmbH), while POM samples were analyzed by a Delta V Advantage IR–MS coupled online to an elemental analyzer (Flash 2000 EA-IRMS) by a ConFlo IV (Thermo Fisher Scientific Inc.). Vienna Pee Dee belemnite (V.P.D.B.) for carbon, and atmospheric N2 (Air) for nitrogen, were used as reference materials, and stable isotope values are reported in respect to that.

Coccolithophores are a calcifying unicellular phytoplankton group that are at the base of the marine food web, and their lipid content provides a source of energy to consumers. Coccolithophores are vulnerable to ocean acidification and warming, therefore it is critical to establish the effects of climate change on these significant marine primary producers, and determine potential consequences that these changes can have on their consumers. Here, we quantified the impact of changes in pH and temperature on the nutritional condition (lipid content, particulate organic carbon/nitrogen), growth rate, and morphology of the most abundant living coccolithophore species, Emiliania huxleyi. We used a regression type approach with nine pH levels (ranging from 7.66 to 8.44) and two temperatures (15°C and 20°C). Lipid production was greater under reduced pH, and growth rates were distinctly lower at 15°C than at 20°C. The production potential of lipids, which estimates the availability of lipids to consumers, increased under 20°C, but decreased under low pH. The results indicate that, while consumers will benefit energetically under ocean warming, this benefit will be mitigated by ocean acidification. The carbon to nitrogen ratio was higher at 20°C and low pH, indicating that the nutritional quality of coccolithophores for consumers will decline under climate change. The impact of low pH on the structural integrity of the coccosphere may also mean that coccolithophores are easier to digest for consumers. Many responses suggest cellular stress, indicating that increases in temperature and reductions in pH may have a negative impact on the ecophysiology of coccolithophores.

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