Spatial patterns of diatom and dinoflagellate seasonal cycles in the NE Atlantic Ocean

Abstract

Within the phytoplankton community, diatoms and dinoflagellates have diverse roles, different spatial patterns and contrasting trophic value: diatoms are the foundation of the copepod- fish food web while dinoflagellates appear less valuable. Changes in relative abundance of these two phytoplankton groups have been linked to pressures such as climate change and eutrophication. Spatially comprehensive data on the seasonal distribution of diatoms and dinoflagellates in non- coastal waters is limited; thus, information concerning their distribution in the open ocean is particu- larly useful. Here we present spatial and temporal patterns of diatom and dinoflagellate seasonal cycles in the coastal and open NE Atlantic Ocean based on >100 000 Continuous Plankton Recorder (CPR) samples. This analysis is presented in the form of monthly composite maps of the spatial distri- bution of diatoms and dinoflagellates and their relative abundances from 1958 to 2003, whose monthly time scale allows the biogeographical exploration of seasonal cycle patterns for each group. The diatom bloom peaks first during May, with a smaller peak in late summer, while dinoflagellate abundance reaches its peak in August. Spatially, the blooms of both groups begin in the North Sea and spread outward across the NE Atlantic region. Throughout the year, dinoflagellates and diatoms are generally most abundant in the central and southern North Sea, while the minimum abundances of both groups occur to the south of Iceland. These spatially detailed seasonal data are not yet avail- able from remote sensing sources and may be used for the validation of current models and research as well as coastal and resource management.