Coccolithophore community response to ocean acidification and warming in the Eastern Mediterranean Sea: results from a mesocosm experiment

Abstract

Mesocosm experiments have been fundamental to investigate the effects of elevated CO 2 and ocean acidification (OA) on planktic communities. However, few of these experiments have been conducted using naturally nutrient-limited waters and/or considering the combined effects of OA and ocean warming (OW). Coccolithophores are a group of calcifying phytoplankton that can reach high abundances in the Mediterranean Sea, and whose responses to oA are modulated by temperature and nutrients. We present the results of the first land-based mesocosm experiment testing the effects of combined OA and OW on an oligotrophic Eastern Mediterranean coccolithophore community. Coccolithophore cell abundance drastically decreased under OW and combined OA and OW (greenhouse, GH) conditions. Emiliania huxleyi calcite mass decreased consistently only in the GH treatment; moreover, anomalous calcifications (i.e. coccolith malformations) were particularly common in the perturbed treatments, especially under OA. Overall, these data suggest that the projected increase in sea surface temperatures, including marine heatwaves, will cause rapid changes in Eastern Mediterranean coccolithophore communities, and that these effects will be exacerbated by OA. CO 2 anthropogenic emissions into the atmosphere have been increasing since the industrial revolution, especially in the last decades. This process alters the climate system and the ocean uptake of anthropogenic CO 2 , causing shifts in marine carbonate chemistry (i.e. ocean acidification, OA) 1. Moreover, the extra heat trapped in the atmosphere by greenhouse gases is largely transferred to the ocean, causing ocean warming (OW), enhancing water column stratification (i.e. a process that hampers the supply of nutrients to the upper ocean layers) 2,3 , and increasing the frequency, intensity, extent and duration of marine heatwaves (i.e. periods of few days to few months characterized by extremely high surface ocean temperatures) 4,5. The Mediterranean region is considered particularly vulnerable to climate change 6-9. Anthropogenic CO 2 has already invaded the whole Mediterranean basin 10 and a pH lowering of 0.245-0.457 units has been estimated for its surface waters by year 2,100 based on two IPCC atmospheric CO 2 scenarios 11,12. Meanwhile, atmospheric warming is expected to proceed in the Mediterranean area 20% faster than the global average 13. The results of a linear black box model suggest that this warming might induce a 5.8 °C increase in sea surface temperatures (SST) by the end of this century (compared to the average SST for the period 1986-2015) 14. According to the IPCC RCP8.5 (a scenario consistent with the worst-case emissions 15), the Mediterranean Sea will be subjected to long-lasting marine heatwaves, occurring at least once per year, by the end of the twenty-first century 4. Interestingly, the Mediterranean Sea is already seasonally subject to vertical stratification, especially in its oligotrophic

Particulate Scattering and Backscattering in Relation to the Nature of Particles in the Red Sea

“…However, the higher chlorophyll-specific particulate backscattering associated with the nano/micro-phytoplankton population than for the picoplankton, at green and red wavelengths, is not consistent with previous studies (Morel, 1987;Kostadinov et al, 2009). It is possible that some nano-sized phytoplankton groups that have high backscattering efficiencies like coccolithophores, that have been found in the nearby Mediterranean Sea (D'Amario et al, 2020), are present in Red Sea waters. It is also possible that some of the optical environments where larger cells are present in the Red Sea (i.e., near Coral Reefs) include high concentrations of other covarying constituents, such as detritus, bacteria and viruses, that backscatter light with high efficiency (Ahn et al, 1992).…”

Microbial Ecology of Oxygen Minimum Zones Amidst Ocean Deoxygenation

“…The direct effect of increased temperature and lowered pH on OMZ microeukaryotes is less clear. Acidification will likely decrease the abundance of any calcium test-bearing microeukaryotes in OMZs like it does for coccolithophores (D'Amario et al, 2020). Further, rising temperatures have the potential to increase grazing rates of bacteria by microeukaryotes in oxic waters (Gu et al, 2020;Cabrerizo and Marañón, 2021), but it is unclear whether this can overcome the observed negative effect low oxygen concentrations have on grazing.…”

Combined effects of ocean warming and acidification on the larval stages of the European abalone Haliotis tuberculata

“…The combined impacts of OA and OW have been addressed with contrasting responses, such that the effects of OA and OW are either exacerbated (e.g., Di Santo 2015;D'Amario et al, 2020;Zittier et al 2018;Rodolfo-Metalpa et al, 2011) or ameliorated (e.g., Kroeker et al 2014;Knights et al, 2020;García et al, 2015;Jiang et al, 2018) in the presence of the other stressor (). Meta-analyses have suggested that the combined impacts of OA and OW on species physiology are especially devastating for the larval stages of many species (Przeslawski et al, 2015;Kroeker et al, 2013), presenting a major bottleneck for population persistence under changing oceanic conditions (Przeslawski et al, 2015).…”