Is coccolithophore distribution in the Mediterranean Sea related to seawater carbonate chemistry?

A Oviedo,P Ziveri,M Álvarez,T Tanhua

doi:10.5194/os-11-13-2015

Abstract

Abstract. The Mediterranean Sea is considered a "hot spot" for climate change, being characterized by oligotrophic to ultra-oligotrophic waters and rapidly increasing seasurface temperature and changing carbonate chemistry. Coccolithophores are considered a dominant phytoplankton group in these waters. As marine calcifying organisms they are expected to respond to the ongoing changes in seawater carbonate chemistry. We provide here a description of the springtime coccolithophore distribution in the Mediterranean Sea and relate this to a broad set of in situ-measured environmental variables. Samples were taken during the R/V Meteor (M84/3) oceanographic cruise in April 2011, between 0 and 100 m water depth from 28 stations. Total diatom and silicoflagellate cell concentrations are also presented. Our results highlight the importance of seawater carbonate chemistry, especially [CO32−] but also [PO43−] in unraveling the distribution of heterococcolithophores, the most abundant coccolithophore life phase. Holo- and heterococcolithophores respond differently to environmental factors. For instance, changes in heterococcolithophore assemblages were best linked to the combination of [CO32−], pH, and salinity (ρ = 0.57), although salinity might be not functionally related to coccolithophore assemblage distribution. Holococcolithophores, on the other hand, showed higher abundances and species diversity in oligotrophic areas (best fit, ρ = 0.32 for nutrients), thriving in nutrient-depleted waters. Clustering of heterococcolithophores revealed three groups of species sharing more than 65% similarities. These clusters could be assigned to the eastern and western basins and deeper layers (below 50 m), respectively. In addition, the species Gephyrocapsa oceanica, G. muellerae, and Emiliania huxleyi morphotype B/C are spatially distributed together and trace the influx of Atlantic waters into the Mediterranean Sea. The results of the present work emphasize the importance of considering holo- and heterococcolithophores separately when analyzing changes in species assemblages and diversity. Our findings suggest that coccolithophores are a main phytoplankton group in the entire Mediterranean Sea and can dominate over siliceous phytoplankton. They have life stages that are expected to respond differently to the variability in seawater carbonate chemistry and nutrient concentrations.

Highlights

Marine phytoplankton constitutes about 1–2 % of the global biomass among primary producers (Falkowski, 1994); it contributes to ∼ 46 % of the primary production in a global scale (Field et al, 1998)
Our results highlight the importance of seawater carbonate chemistry, especially [CO23−] and [PO34−] in unraveling the distribution of heterococcolithophores, the most abundant coccolithophore life phase
Close to the 4 Results M84/3 trajectory, wind-induced upwelling and downwelling zones are found in different seasons: for instance, (1) downwelling along the southern coastal boundary during winter, reversing to coastal upwelling off Libya in spring and off Algeria in summer and Overall, phytoplankton cell density was highest in the western Mediterranean Sea, in the Strait of Gibraltar and in the fall; (2) strong upwelling in the eastern Aegean Sea throughout the year, becoming remarkably intense in the summer and Alboran Sea

Summary

Introduction

Marine phytoplankton constitutes about 1–2 % of the global biomass among primary producers (Falkowski, 1994); it contributes to ∼ 46 % of the primary production in a global scale (Field et al, 1998). Coccolithophores represent ∼ 10 % of global phytoplankton biomass (Tyrrell and Young, 2009). They play an important role in biogeochemical cy-. A. Oviedo et al.: Coccolithophore distribution along carbonate chemistry gradients cles, contributing to both the organic and inorganic carbon pumps through photosynthesis and calcification. Oviedo et al.: Coccolithophore distribution along carbonate chemistry gradients cles, contributing to both the organic and inorganic carbon pumps through photosynthesis and calcification The latter is the main process controlling PIC : POC (rain ratio). In the eastern Mediterranean Sea, they are the main contributor to the inorganic carbon pump (CaCO3 production and flux) throughout the year (Knappertsbusch, 1993; Ziveri et al, 2000; Malinverno et al, 2003)

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Discussion

Conclusion