Contents lists available at ScienceDirect Ecological Indicators journal homepage: www.elsevier.com/locate/ecolind Original Articles Fluorescence indices of dissolved organic matter as early warning signals of fish farming impacts in a large tropical reservoir Ronaldo César Chaves a , Cleber Cunha Figueredo b, ⁎ , Iola Gonçalves Boëchat c , Juliana Trindade Marques de Oliveira d , Björn Gücker c, ⁎ a Graduate Program in Wildlife Ecology, Conservation and Management, Federal University of Minas Gerais, Belo Horizonte, Brazil b Department of Botany, Federal University of Minas Gerais, Belo Horizonte, Brazil c Department of Geosciences, Federal University of São João del-Rei, São João del-Rei, Brazil d Graduate Program in Geography, Federal University of São João del-Rei, São João del-Rei, Brazil ARTICLE INFO Keywords: Tropical aquaculture Fluorescence indices Net cage fish farming Nile tilapia Brazil ABSTRACT Dissolved organic matter (DOM) can be an important source of energy and nutrients in aquatic ecosystems and play important roles in carbon and nutrient dynamics of natural and impacted environments. In order to assess the effects of early stage fish farming on dissolved organic carbon (DOC) concentrations as well as DOM fluorescence indices, we took water samples along transects from six fish farms towards reference sites in a large tropical reservoir (Furnas Reservoir, Southeast Brazil) between April 2013 and December 2016. While DOC concentrations did no change along transects, small but significant changes in fluorescence indices were de- tectable in up to 100 m from fish farms. Higher fluorescence index (FI) values near fish farms pointed to small increases in microbial production due to fish farming. Only in the more pristine of the two main reservoir branches, small differences in the freshness index (β:α) and the peak T/peak C ratio along transects indicated that the relative contribution of recently produced DOM and the biochemical oxygen demand of DOM increased due to aquaculture. The humification index did not respond to fish farming. In summary, current early-stage fish farming did not appear to cause major impacts on DOM quantity and quality, but future increases in net cage and fish farm densities, as well as longer operation times may change this assessment. The fluorescence indices FI, β:α and peak T/peak C ratio may be useful early warning signals for monitoring fish farm impacts in tropical reservoirs. 1. Introduction The annual growth rate of global aquaculture has dropped from 10.8% to 5.8% from 1980 to 2016 (FAO, 2018). Nonetheless, aqua- culture continues to grow faster than other major sectors of animal protein production for human consumption. Among the major fish production zones, inland farming is the most important with 64.2% of the world’s production (FAO, 2018). Similar to other tropical devel- oping countries, freshwater aquaculture is expanding in Brazil, but basic principles of sustainability are often neglected, posing threats to the biodiversity, integrity and functioning of freshwater ecosystems (Lima Junior et al., 2018). Fish farming is often performed with high stocking densities and plentiful food addition, which may impact aquatic environments even when best management practices are followed (Cole et al., 2009). Such impacts include changes in water quality (Srithongouthai and Tada, 2017), alterations to the structure of the natural fish community (Macuiane et al., 2015) by the introduction of exotic species (Ortega et al., 2015), dissemination of diseases (Peeler et al., 2011), and pollution with toxic chemical elements, such as zinc, copper and cadmium (Dean et al., 2007). Regarding water quality, one of the main aquaculture effects is nutrient pollution from fish excreta (McGhie et al., 2000; Figueredo and Giani, 2005) and surplus fish feed (McGhie et al., 2000). Differences in the nutritional value and the quantity of supplied fish feed determine specific responses of water and sediment quality to fish farming (McGhie et al., 2000; Lachi and Sipaúba-Tavares, 2008; Alongi et al., 2009) and thus, the degree to which increases in the biomass of auto- trophs and primary production, i.e. eutrophication, occurs. https://doi.org/10.1016/j.ecolind.2020.106389 Received 29 September 2019; Received in revised form 27 March 2020; Accepted 5 April 2020 Abbreviations: DOC, dissolved organic carbon; DOM, dissolved organic matter; FI, fluorescence index; GLM, general linear model; GLMM, general linear mixed model; HIX, humification index; NH 4 -N, ammonium-nitrogen; NO 3 -N, nitrate-nitrogen; β:α, freshness index ⁎ Corresponding authors. E-mail addresses: cleberfigueredo@ufmg.br (C.C. Figueredo), guecker@ufsj.edu.br (B. Gücker). Ecological Indicators 115 (2020) 106389 1470-160X/ © 2020 Elsevier Ltd. All rights reserved. T