Comparison of coccolith and dinocyst assemblages in the northern North Atlantic: How well do they relate with surface hydrography?

Coccolith and dinoflagellate cyst (or dinocyst) population counts were compiled from existing surface sediment databases as well as new counts in order to establish an 87-sample database for which assemblages of both microfossil groups are known. This database allowed a direct comparison of the distribution of coccolith and dinocyst assemblages in the subtropical to subpolar North Atlantic. In addition, the relationship between these assemblages and sea surface environmental parameters was addressed, in order to identify possible differences in the ecology of the two plankton groups. The comparison highlights an excellent correspondence between dinocyst assemblages, coccolith assemblages and the distribution of the surface water masses represented in our database, notably in the subtropical and temperate domains. In the subpolar domain, coccolith assemblages are much less diversified than dinocyst assemblages in terms of species. As a result, the discrimination between the subpolar water masses based on coccolith assemblages is not as clear as in the subtropical/temperate regions, whereas dinocyst assemblages show a distribution pattern closely related with surface hydrography. Canonical correspondence analyses performed on coccolith and dinocyst assemblages show that sea surface temperature is the primary environmental parameter influencing the distribution of both groups. Dinocyst assemblages also seem to respond to the distance to the coast, and may therefore bring additional information compared with coccolith assemblages. Other significant environmental factors include sea surface salinity and productivity, but their relative importance changes depending on the inclusion of samples from extreme environmental settings in the database. Results suggest that the complexity of the interrelationships between the various environmental parameters makes it difficult to adequately bring to light all the different environments and their associated coccolith/dinocyst assemblages in multivariate analyses. However, each surface water mass represented in our database is characterized by a unique combination of environmental parameters as well as by distinct associations of coccolith and dinocyst assemblages, thus showing that these microfossil groups closely relate to sea surface conditions, including temperature, salinity and productivity.