Coccolithophore community response along a natural CO₂ gradient off Methana (SW Saronikos Gulf, Greece, NE Mediterranean)

<div><p>A natural pH gradient caused by marine CO₂ seeps off the Methana peninsula (Saronikos Gulf, eastern Peloponnese peninsula) was used as a natural laboratory to assess potential effects of ocean acidification on coccolithophores. Coccolithophore communities were therefore investigated in plankton samples collected during September 2011, September 2016 and March 2017. The recorded cell concentrations were up to ~50 x10³ cells/l, with a high Shannon index of up to 2.8, along a pH gradient from 7.61 to 8.18, with values being occasionally <7. Numerous holococcolithophore species represented 60–90% of the surface water assemblages in most samples during September samplings. <i>Emiliania huxleyi</i> was present only in low relative abundances in September samples, but it dominated in March assemblages. Neither malformed nor corroded coccolithophores were documented. Changes in the community structure can possibly be related to increased temperatures, while the overall trend associates low pH values with high cell densities. Our preliminary results indicate that in long-termed acidified, warm and stratified conditions, the study of the total coccolithophore assemblage may prove useful to recognize the intercommunity variability, which favors the increment of lightly calcified species such as holococcolithophores.</p></div

Coccolithophore corroded specimens in Methana acidified environments (Ω<1).

<p>1. <i>Rhabdosphaera clavigera</i>, P2-2 m, September 2016. 2. <i>Syracosphaera pulchra</i>, P2-8 m, September 2011. 3. <i>Syracosphaera pulchra</i>, P2-20 m, September 2011. 4. <i>Syracosphaera mediterranea</i> HOL <i>(hellenica)</i>, P1-20 m, September 2011. 5. <i>Emiliania huxleyi</i>, P1-2 m, September 2016. 6. <i>Homozygosphaera arethusae</i>, P1-2 m, September 2016.</p

Sample information.

<p>Stations’ location, sampling dates, physicochemical parameters, total coccolithophore density (cells l^-1) resulting from both inverted microscopy and Scanning Electron Microscopy techniques and coccolithophore diversity (<i>H’</i>) based on SEM countings.</p

The structure of plankton community.

<p>Abundance (cells l^-1) of the major plankton groups Dinophyceae, Bacillariophyceae and the Coccolithophores component, during the two sampling periods.</p

Seawater carbonate chemistry, average nutrient concentrations and total chl-a values in the sampling sites.

<p>Seawater carbonate chemistry, average nutrient concentrations and total chl-a values in the sampling sites.</p

Coccolithophores of Methana acidified environments.

<p>1. <i>E</i>. <i>huxleyi</i>, P1-5 m, September 2016 (Ωmin<1). 2. <i>E</i>. <i>huxleyi</i>, P2-5 m, March 2017 (pH<8). 3. <i>Pontosphaera syracusana</i>, P2-2 m, March 2017. 4. <i>Syracosphaera halldalii</i>, P1-2 m, September 2016 (Ωmin<1). 5. <i>Syracosphaera ossa</i>, P1-2 m, September 2016. 6. <i>Algyrosphaera robusta</i> HOL, P1-2 m, September 2016 (Ωmin<1). 7. <i>Syracosphaera mediterranea</i>, P1-2 m, September 2016 Ωmin<1). 8. <i>Rhabdosphaera clavigera</i>, P1-2 m, September 2016 (Ωmin<1). 9. <i>Algyrosphaera robusta</i>, P1-2 m, September 2016 (Ωmin<1). 10. <i>Syracolithus ponticuliferus</i>, P1-2 m, September 2016 (Ωmin<1). 11. <i>Algyrosphaera robusta</i> HOL, P1-2 m, September 2016 (Ωmin<1). 12. <i>Syracosphaera mediterranea</i> HOL <i>wettsteinii</i> type, P1-2 m, September 2016 (Ωmin<1).</p

Corellation of various coccolithophore groups and coccolithophore diversity with <i>in situ</i> pH data.

<p>Holococcolithophores and particularly <i>A</i>. <i>robusta</i> HOL showed a clear increasing trend with lower pH during the warm period (September 2016), forcing diversity (<i>H’</i>) to display an opposite pattern. (obtained p values below 0.05 indicate statistically significant correlation at the 95% confidence level).</p

Study area.

<p>A. Map of the central Aegean Sea (NE Mediterranean) with sampling sites visited in the present study; Methana represents the western end of the Aegean Volcanic Arc. Image resource: NASA Worldview. The inlet map presents the dominant tectonic structure of the Aegean Sea domain [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0200012#pone.0200012.ref048" target="_blank">48</a>]. B. Bathymetry and hydrography of Saronikos Gulf. Bathymetry data are provided by HCMR (Hellenic Centre for Marine Research). The map was designed with ArcGiS software (ESRI) v.10.4. Hydrographic data are redrawn from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0200012#pone.0200012.ref046" target="_blank">46</a>]. C. Sample location around Methana peninsula. Image resource: NASA Worldview.</p