Present-day patterns in pelagic biodiversity are the result of the interaction of many factors acting at different scales. Developing an understanding of the processes that regulate the diversity of oceanic ecosystems is thus challenging. In this study, diversity of calanoid copepods was decomposed into species associations by means of the recent 'indicator value method' and multivariate analyses. For the first time, at an oceanic basin scale and with a spatial resolution approaching the mesoscale, species associations of calanoid copepods have been identified. Nine species associations were determined and have enabled us, (1) to improve the ecological partitioning of this region, and (2) to identify the main factors that regulate pelagic biodiversity in this area. It is shown that temperature, hydrodynamics, stratification and seasonal variability of the environment are likely to be the main factors contributing to the ecological regulation of diversity of calanoid copepods. The similar geographical pattern evident between currents/water masses and the species associations suggest that the species groups may be used as an environmental indicator to evaluate long-term changes in the marine environment related to climate change and other increasing human-induced influences. KEY WORDS: Pelagic diversity · Species associations · Calanoid copepods · Continuous Plankton Recorder (CPR) survey · Multivariate analyses Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 232: [179][180][181][182][183][184][185][186][187][188][189][190][191][192][193][194][195] 2002 the northern North Atlantic Ocean and adjacent seas with pronounced local variability. Multivariate analyses and a diversity index were used at both diel and seasonal scales to distinguish major biological regions with a relatively small spatial resolution (Beaugrand et al. 2001). Currents were clearly shown to play an important role in the regulation of pelagic biodiversity at both meso-and macroscales. The warm North Atlantic Current and the European shelf edge current and undercurrent have emerged as important parameters for the maintenance and regulation of biodiversity in the Bay of Biscay and west of the British Isles (Beaugrand et al. 2001).However, in these recent studies, information on taxa or assemblages that contributed to the diversity in a region of the survey area was not investigated. This represents a major gap in our understanding of the factors that maintain diversity at an ocean basin scale. Individual distributional ranges of many pelagic organisms has been determined, especially in the northern North Atlantic and the North Sea (Edinburgh Oceanographic Laboratory 1973). A number of aspects limit the application of this atlas. Firstly, it was only based on samples collected during the period from 1958 to 1968, with a restricted number of samples (about 45 000 in contrast to about 170 000 at present). Secondly, only the common species were divided into 3 categories of abundance; less co...

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