Nekton response to freshwater inputs in a temperate European Estuary with regulated riverine inflow E. González-Ortegón a, b , M.D. Subida a , A.M. Arias a , F. Baldó c , J.A. Cuesta a , C. Fernández-Delgado d , C. Vilas e , P. Drake a, ⁎ a Instituto de Ciencias Marinas de Andalucía (CSIC), Avda. República Saharaui 2, 11519 Puerto Real, Cádiz, Spain b School of Ocean Sciences, Bangor University, Menai Bridge LL59 5EG, United Kingdom c Instituto Español de Oceanografía, Apartado 2609, 11006 Cádiz, Spain d Departamento Biología Animal, Edificio C-1, Campus Universitario de Rabanales, Universidad de Córdoba, 14071 Córdoba, Spain e IFAPA Centro El Toruño, Camino Tiro de Pichón s/n, 11500 El Puerto de Santa María, Cádiz, Spain abstract article info Article history: Received 17 April 2012 Received in revised form 15 June 2012 Accepted 15 June 2012 Available online xxxx Keywords: Estuaries Nekton Environmental conditions Freshwater inputs Resilience Guadalquivir estuary The aim of this 12-year study was to assess the nekton (fish, decapod crustaceans) response to freshwater inputs (rainfall, dam discharges) in a temperate estuary with regulated riverine inflow. Although interannual variability in river discharges to the Guadalquivir estuary has been extremely high since the construction of a dam in 1930, a significant decreasing trend in the dam's discharges has been observed in the last 80 years. During this study, an alternation of wet, standard and dry years occurred in the estuarine area but no signif- icant long-term trend was observed. River discharge, in turn, showed a considerable interannual variability and a significantly decreasing long-term trend. Freshwater inputs had an immediate effect on estuarine salin- ity and turbidity, and consequently on prey availability (mysids). Although 124 nektonic species were col- lected, only 47 of them (adding up to 99.7% of total abundance) were regularly present in the estuary: 32 marine migrants, 13 estuarine species and 2 diadromous species. Well-defined temporal changes in species composition and abundance yielded clear seasonal patterns in the estuarine nektonic community. Consider- able intermonth and interannual changes were occasionally observed relating to freshwater inputs, mainly in winter/autumn of wet years. Thus, within each two-month period, some significant interannual differences in the nektonic community were also observed, with marine migrants tending to be more abundant in dry years. However, changes in the studied nektonic community did not show long-term trends. In conclusion, natural and human-controlled freshwater inputs currently play a significant role in determining the physico- chemical conditions and the biota of the Guadalquivir estuary. However, although freshwater input seemed to transitorily affect the estuarine nekton, either directly (flushing out) or indirectly (through changes in sa- linity, turbidity and prey availability), a quick reestablishment of the estuarine nekton (strong resilience) was observed following freshwater inputs together with the recovery of environmental conditions within the estuary. © 2012 Elsevier B.V. All rights reserved. 1. Introduction An adequate freshwater supply to estuaries is essential to ensure estuarine productivity (Abril et al., 2002) and to establish the charac- teristic physicochemical and biological structure of these systems (Bate et al., 2002; Whitfield, 2005). However, river discharges are reg- ulated mainly for economic purposes, such as hydroelectric power generation, irrigation and/or water supply to urban settlements, (Kennish, 2002; Vasconcelos et al., 2007). Such water management usually causes a decrease in the freshwater input to estuaries and an increase in the residence time of suspended matter and organisms within estuaries (Uncles et al., 2003; González-Ortegón et al., 2010b), altering estuarine natural hydrological regime (Fernández- Delgado et al., 2007; Snoussi et al., 2007). In the same way, a high pulse of freshwater also influences estuarine biota by eliminating salinity gradients or by flushing suspended particulates and organisms (mainly small-size individuals/species) out of the estuary during flood events (Abril et al., 1999; González-Ortegón and Drake, 2012). Thus, in addition to the traditional anthropogenic activities, which have a high impact on estuaries (habitat loss and alteration, exces- sive nutrient and sewage inputs, chemical contaminations and overfishing), freshwater diversions are turning into a problem (Kennish, 2002), especially in the most arid regions, where extraor- dinary dry water years periodically occur (Baptista et al., 2010; Cyrus et al., 2011). Estuaries have large mesozooplanktonic populations of detri- tivorous consumers whose densities are usually controlled by both Science of the Total Environment xxx (2012) xxx–xxx ⁎ Corresponding author. Tel.: +34 956832612; fax: +34 956 83 47 01. E-mail address: pilar.drake@icman.csic.es (P. Drake). STOTEN-13696; No of Pages 11 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2012.06.061 Contents lists available at SciVerse ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Please cite this article as: González-Ortegón E, et al, Nekton response to freshwater inputs in a temperate European Estuary with regulated riverine inflow, Sci Total Environ (2012), doi:10.1016/j.scitotenv.2012.06.061