Impacts of shrimp farm effluent on water quality, benthic metabolism and N-dynamics in a mangrove forest (New Caledonia) Nathalie Molnar a, c, * , David T. Welsh b , Cyril Marchand c , Jonathan Deborde c , Tarik Meziane a a UMR CNRS BOREA 7208, Muséum National Histoire Naturelle, CP 53, 61 rue Buffon, 75231 Paris cedex 05, France b Environmental Futures Centre and School of Environment, Griffith University, Queensland, Australia c IRD, UR 206, UMR 7590 e IMPMC, F-98848 New Caledonia, France article info Article history: Received 20 July 2011 Accepted 24 July 2012 Available online 21 August 2012 Keywords: mangrove shrimp farm nutrient fluxes denitrification DNRA New Caledonia abstract Water quality parameters, sediment oxygen demand (SOD), dissolved organic and inorganic nutrient fluxes, and N-cycle processes (nitrification; denitrification; dissimilatory nitrate reduction to ammonium (DNRA)) were determined in a New Caledonian mangrove receiving shrimp farm effluent and a natural mangrove. Effluent was enriched in nutrients and organic matter, and significantly stimulated SOD and nutrient regeneration rates in the receiving sediments. All N-cycling processes were stimulated between w2 and 12-fold in the sediments receiving effluents compared to the natural mangrove. However, due to the preferential enhancement of DNRA compared to denitrification, there was no significant increase in net nitrogen elimination compared to the significant increase in sediment nutrient regeneration rates. These results indicate that the mangroves are only a partial filter for the shrimp farm effluent, as confirmed by the elevated nutrient concentrations measured in an external, marine creek of the effluent receiving mangrove. Ó 2012 Published by Elsevier Ltd. 1. Introduction Mangroves are dynamic ecosystems, which develop at the interface between terrestrial and marine environments along tropical and subtropical coastlines (Hogarth, 1999). Mangroves are important as they stabilise sediments, provide physical protection to coastlines and are nursery environments for many fish and crustaceans, including commercially important species (Nerot et al., 2009). They also play a fundamental role in the trans- formation, turnover and export of particulate and dissolved organic and inorganic nutrients of terrestrial origin (Marchand et al., 2006; Kristensen et al., 2008; Nagelkerken et al., 2008). One of the largest documented threats, to mangrove ecosys- tems, is the development of aquaculture ponds, especially shrimp farming (Alongi, 2002; Duke et al., 2007). Worldwide, shrimp farming has increased almost exponentially since the mid-1970’s due to short production cycles and high product values, reaching a total annual production of over 2.3 billion tonnes in 2008 (Bostock et al., 2010). In tropical and sub-tropical zones, particularly in South America, Indonesia and Thailand, shrimp farms have been devel- oped at the expense of mangrove forests, which are cleared for the establishment of the rearing ponds (Menasveta, 1997). In addition to the direct loss of mangroves during construction, shrimp farms also impact the adjacent mangroves through the release of large quantities of effluents, rich in particulate and dissolved organic and inorganic nutrients (Paez-Osuna, 2001; Jackson et al., 2003; Thomas et al., 2010). During a production cycle, it has been esti- mated that only 29 and 16%, respectively, of total nitrogen (N) and phosphorus (P) added to the ponds as food and fertilizers inputs, is actually assimilated by the shrimps (Avnimelech and Ritvo, 2003). As a consequence, most of the added N and P is released to the environment in effluents rich in particulates (e.g. uneaten feeds, faeces and phytoplankton) and dissolved organic and inorganic species (Jackson et al., 2003; Thomas et al., 2010). Several studies have investigated the impacts of shrimp farm effluents on mangrove ecosystems, especially on water column processes in mangrove creeks (McKinnon et al., 2002; Burford et al., 2003; Thomas et al., 2010). Effluents significantly increase water column chlorophyll a, dissolved inorganic nitrogen (NH 4 þ and NO x (NO 3  þ NO 2  )) and total nitrogen and phosphorus concentrations (Costanzo et al., 2004; Thomas et al., 2010). In addition, effluents enhance primary and bacterial production (McKinnon et al., 2002; Burford et al., 2003) and induce significant increases in biological oxygen demand, promoting water column hypoxia or anoxia * Corresponding author. E-mail addresses: molnar@mnhn.fr (N. Molnar), D.Welsh@griffith.edu.au (D.T. Welsh), Cyril.Marchand@ird.fr (C. Marchand), jonathan.deborde@ird.fr (J. Deborde), meziane@mnhn.fr (T. Meziane). Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ecss.2012.07.012 Estuarine, Coastal and Shelf Science 117 (2013) 12e21