ORIGINAL ARTICLE Temporal change in UK marine communities: trends or regime shifts? M. Spencer 1 *, S. N. R. Birchenough 2 *, N. Mieszkowska 3 *, L. A. Robinson 1 *, S. D. Simpson 4 *, M. T. Burrows 5 , E. Capasso 3,6 , P. Cleall-Harding 6 , J. Crummy 7 , C. Duck 8 , D. Eloire 9,10 , M. Frost 3 , A. J. Hall 8 , S. J. Hawkins 3,6 , D. G. Johns 11 , D. W. Sims 3,12 , T. J. Smyth 9 & C. L. J. Frid 1 * 1 School of Environmental Sciences, University of Liverpool, Liverpool, UK 2 Cefas Laboratory, Pakefield Road, Lowestoft, Suffolk, UK 3 Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, UK 4 School of Biological Sciences, University of Bristol, Bristol, UK 5 The Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, UK 6 School of Ocean Sciences, Bangor University, Menai Bridge, Ynys Mon, UK 7 British Geological Survey, Murchison House, Edinburgh, UK 8 Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews, St Andrews, UK 9 Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, Devon, UK 10 Laboratoire Ecosyste ` me Lagunaire, UMR 5119, CNRS – Universite ´ Montpellier II – IRD – IFREMER, CC093, Montpellier, France 11 Sir Alister Hardy Foundation for Ocean Science, Plymouth, UK 12 Marine Biology and Ecology Research Centre, Marine Institute, School of Marine Sciences and Engineering, University of Plymouth, Drake Circus, Plymouth, UK Introduction A growing number of studies report major changes in biological systems (Reid et al. 2001; Rudnick & Davis 2003; Lees et al. 2006; Beaugrand et al. 2008; Carpenter & Lathrop 2008; Greene et al. 2008; Hagerthey et al. 2008; Heath & Beare 2008; Hemery et al. 2008; Petersen et al. 2008). These, often high profile, observations have con- tributed to a move towards more holistic and integrated ‘ecosystem-based’ environmental management (United Keywords Abundance; population trends; principal components; regime shift detection; regime shifts; time series; UK marine ecosystems. Correspondence Matthew Spencer, School of Environmental Sciences, University of Liverpool, Liverpool L69 7ZB, UK. E-mail: m.spencer@liverpool.ac.uk *These authors wish to be considered as joint first authors. Accepted: 16 November 2010 doi:10.1111/j.1439-0485.2010.00422.x Abstract A regime shift is a large, sudden, and long-lasting change in the dynamics of an ecosystem, affecting multiple trophic levels. There are a growing number of papers that report regime shifts in marine ecosystems. However, the evidence for regime shifts is equivocal, because the methods used to detect them are not yet well developed. We have collated over 300 biological time series from seven marine regions around the UK, covering the ecosystem from phytoplankton to marine mammals. Each time series consists of annual measures of abundance for a single group of organisms over several decades. We summarised the data for each region using the first principal component, weighting either each time series or each biological component (e.g. plankton, fish, benthos) equally. We then searched for regime shifts using Rodionov’s regime shift detection (RSD) method, which found regime shifts in the first principal component for all seven marine regions. However, there are consistent temporal trends in the data for six of the seven regions. Such trends violate the assumptions of RSD. Thus, the regime shifts detected by RSD in six of the seven regions are likely to be artefacts caused by temporal trends. We are therefore developing more appropriate time series models for both single populations and whole commu- nities that will explicitly model temporal trends and should increase our ability to detect true regime shift events. Marine Ecology. ISSN 0173-9565 10 Marine Ecology 32 (Suppl. 1) (2011) 10–24 ª 2010 Blackwell Verlag GmbH