Climate variability and planktonic communities: The effect of an extreme event (severe drought) in a southern European estuary So ´nia Cotrim Marques * , Ulisses Miranda Azeiteiro, Filipe Martinho, Miguel A ˆ ngelo Pardal Department of Zoology, F.C.T., IMARdInstitute of Marine Research, University of Coimbra, 3004-517 Coimbra, Portugal Received 4 December 2006; accepted 13 March 2007 Available online 25 April 2007 Abstract As a consequence of climate change, flood and drought events are increasing in frequency throughout the world. Nevertheless, knowledge of the effects on zooplankton estuarine communities is still scarce. The present study aimed to examine zooplankton ecology over two contrasting environmental conditions: regular years and extreme dry years, in a shallow temperate southern European estuary, the Mondego Estuary (Portugal). Monthly samples were carried out during three consecutive years: 2003 characterized as a regular temperate year concerning precipitation and river flow, and an extremely dry period during 2004e2005. The spatial and temporal structure of the biological data was evaluated by a three-mode principal component analysis (PCA), which allowed us to distinguish three distinct ecological areas based on their biological composition and their relationship with hydrologic parameters. The severe drought in 2004e2005 was responsible for spatial shifts in the estuary regarding zooplankton community and interannual variability, with an increase in abundance and diversity during the period of low freshwater flow. This freshwater flow regime influenced the composition of the zooplankton community at the most upstream section of Mondego estuary (zone 3), with a replacement of the freshwater community by one predominantly dominated by estuarine organisms. The occurrence of such estuarine community contributed to the increase in zooplankton abundance which is ascribed to the estuarine species Acartia tonsa. The comparison with previous data obtained for this estuarine ecosystem, demonstrated the occurrence of a different scenario at times of high freshwater flow, being defined the existence of two sub-estuarine systems, the north and south arm, presenting the south one the highest values of abundance. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: mesozooplankton community; interannual variability; extreme events; three-mode principal component analysis (PCA); estuary 1. Introduction Understanding how climate change will affect the planet is a key issue worldwide (Hays et al., 2005). Estuaries are rela- tively shallow coastal ecosystems thus sensitive to environ- mental parameters and frequently under stress because of human population growth and coastal socio-economic devel- opment and can be particularly vulnerable to climate changes. Significant environmental factors that affect the structure (e.g. faunal composition) and function (e.g. secondary produc- tion and nutrient cycling) of estuarine systems are expected to be sensitive parameters of global climate change: temperature (e.g. heat waves), freshwater availability, precipitation (e.g. extreme floods and droughts) and runoff (Kennedy et al., 2002). Increases or decreases in precipitation and runoff may create extreme events, floods or droughts respectively, and are increasing in frequency worldwide (Gleick, 2003; Mirza, 2003). The influence of freshwater flowing into estuaries on biological processes and ecosystems development has long been recognized (e.g. Attrill et al., 1999; Kimmerer, 2002). Understanding mechanisms by which estuarine ecosystems * Corresponding author. E-mail address: scotrim@ci.uc.pt (S.C. Marques). 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.03.010 Estuarine, Coastal and Shelf Science 73 (2007) 725e734 www.elsevier.com/locate/ecss