The effect of sea water salinity on the morphology of Emiliania huxleyi in plankton and sediment samples
Introduction
Variations in salinity and temperature result in density differences of ocean water which is the single most important parameter that defines water masses, ocean stratification and deep water thermohaline circulation. All of these parameters are major factors affecting the climate system and therefore, the precise knowledge of past ocean salinity and temperature is of utmost importance to identify mechanisms that drive the climate system. In contrast to temperature, salinity cannot be easily reconstructed from geological archives with the same accuracy and reliability as temperature because several assumptions need to be made that lead to a significant error of the salinity estimates (Rohling and Bigg, 1998, Rohling, 2000). This explains why no widely accepted paleo-salinity proxy is currently available (Henderson, 2002, Bollmann and Herrle, 2007).
Recent approaches to develop a reliable salinity proxy utilize single celled marine algae, coccolithophores. Culture experiments indicate a significant relationship between the hydrogen isotopic composition of long chain alkenones synthesized by the species Gephyrocapsa oceanica and Emiliania huxleyi and salinity (Schouten et al., 2006). Although the growth rate of G. oceanica and E. huxleyi also affects the fractionation factor, the hydrogen isotopic composition of alkenones appears to have great potential for the reconstruction of past salinities, especially if it is used in a multi proxy approach (van der Meer et al., 2007, van der Meer et al., 2008).
Also the morphological variation of the world's most abundant coccolithophore species, E. huxleyi, in core-top samples is related to sea surface salinity (Bollmann and Herrle, 2007). Bollmann and Herrle (2007) established a salinity transfer function based on the morphological variation of E. huxleyi and used it to reconstruct the ocean salinity during the LGM. Although the results are in good agreement with published values for the LGM, this promising technique needs to be further constrained by plankton analysis and controlled laboratory culture experiments. Towards this goal, we have analysed the morphology of E. huxleyi from globally distributed plankton samples.
Section snippets
Materials and methods
A total of 28 plankton samples were collected from the Atlantic, Pacific and Southern oceans (Fig. 1, Table 1) covering an in-situ salinity gradient from 32.6 to 38.8 and an in-situ temperature gradient from 1.93° to 28.31 °C.
Results
The morphology of around 1400 placoliths of the coccolithophore species E. huxleyi was analyzed from 28 globally distributed plankton samples that cover an in-situ salinity gradient from 32.6 to 38.8 and an in-situ temperature gradient from 1.93° to 28.31 °C. From the morphometric measurements several parameters such as the length and width of the distal shield (DL, DW) and the area of the central area (CAA) were calculated (Fig. 2, Fig. 3). Multiple linear regression analysis was applied to
Discussion
Our results indicate that the morphological variability of E. huxleyi placoliths is significantly related to salinity variability and support the results of Bollmann and Herrle (2007). Their regression model showed a significant relationship between morphology of E. huxleyi placoliths and sea surface salinity based on Holocene sediment core-top samples. However, their stepwise regression analysis of the sediment data set revealed that width and not length (as was found in this study) of the
Conclusions
We have demonstrated that the morphology of E. huxleyi, the world's most abundant coccolithophore species, varies systematically in plankton samples over a large salinity gradient. Our analysis confirms that the morphology of E. huxleyi responds in plankton samples in a similar way to that reported from culture experiments (Green et al., 1998) and surface sediments (Bollmann and Herrle, 2007). However, the derived multiple linear regression model for in-situ salinities is different from that
Acknowledgements
We thank Claire Findlay, Patrick Holligan, Ralf Schiebel, Toby Tyrrell and Cornelius Veltkamp. The manuscript benefited from the comments of five anonymous reviewers. Financial support came from JOH and JB's Natural Sciences and Engineering Research Council Canada Discovery Grant and JOH's support from Canada Research Chairs.
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