Impacts of diverted freshwater on dissolved organic matter and microbial communities in Barataria Bay, Louisiana, U.S.A. Thomas S. Bianchi a, * , Robert L. Cook b, ** , E. Michael Perdue c , Paulina E. Kolic b , Nelson Green c , Yaoling Zhang c , Richard W. Smith a , Alexander S. Kolker d , Alex Ameen d , Gary King e , Loice M. Ojwang b , Caroline L. Schneider b , Anna E. Normand b , Robert Hetland a a Department of Oceanography, Texas A&M University, College Station, TX 77843-3146, USA b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803-7503, USA c School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA d Louisiana Universities Marine Consortium (LUMCON), Cocodrie, LA 70344, USA e Department of Biological Sciences Baton Rouge, LA 70803-7503, USA article info Article history: Received 25 May 2011 Received in revised form 14 September 2011 Accepted 15 September 2011 Keywords: Freshwater diversion Dissolved organic carbon Bacterioplankton Absorbance Fluorescence Gulf of Mexico Deepwater Horizon abstract Here we present results of an initial assessment of the impacts of a water diversion event on the concentrations and chemical composition of dissolved organic matter (DOM) and bacterioplankton community composition in Barataria Bay, Louisiana U.S.A, an important estuary within the Mississippi River Delta complex. Concentrations and spectral properties of DOM, as reflected by UV/visible absor- bance and fluorescence, were strikingly similar at 26 sites sampled along transects near two western and two eastern areas of Barataria Bay in July and September 2010. In September 2010, dissolved organic carbon (DOC) was significantly higher (568.1e1043 mM C, x ¼ 755.6 þ/ 117.7 mM C, n ¼ 14) than in July 2010 (249.1e577.1 mM C, x ¼ 383.7 þ/ 98.31 mM C, n ¼ 14); conversely, Abs 254 was consistently higher at every site in July (0.105e0.314) than in September (0.080e0.221), averaging 0.24  0.06 in July and 0.15  0.04 in September. Fluorescence data via the fluorescence index (FI 450/500 ) revealed that only 30% (8 of 26) of the July samples had an FI 450/500 above 1.36, compared to 96% (25 of 26) for the September samples. This indicates a more terrestrial origin for the July DOM. Bacterioplankton from eastern sites differed in composition from bacterioplankon in western sites in July. These differences appeared to result from reduced salinities caused by the freshwater diversion. Bacterioplankton communities in September differed from those in July, but no spatial structure was observed. Thus, the trends in bac- terioplankton and DOM were likely due to changes in water masses (e.g., input of Mississippi River water in July and a return to estuarine waters in September). Discharge of water from the Davis Pond Fresh- water Diversion (DPFD) through Barataria Bay may have partially mitigated some adverse effects of the oil spill, inasmuch as DOM is concerned. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The Deepwater Horizon (DWH) MC252 oil spill released 4.4 million barrels (6.4  10 8 L  20%) of light south Louisiana crude oil (Crone and Tolstoy, 2010) into the northern Gulf of Mexico (GOM). A massive diversion of Mississippi River water was initiated on 30 April, 2010 through the Davis Pond Freshwater Diversion (DPFD) in an effort to limit influxes of oil into Barataria Bay, a major estuary within the Mississippi River (MR) Delta complex. This paper examines the impact of inflow from the DPFD on the dissolved organic matter (DOM) and microbial communities in Barataria Bay, as revealed through two sampling trips in July and September, 2010. The MR Delta is one of the most modified aquatic coastal ecosystems in the world, and experiences as much as 80% of the wetland loss nationwide (peak rates of 60e90 km 2 y 1 ; Gagliano et al., 1981). These wetland losses in the deltaic plain exceeded 690,000 acres between the 1930s and 1990 (Britsch and Kemp, 1990; Penland et al., 2000). The preponderance of natural wetland loss has been attributed to the effects of compaction- induced sediment subsidence exacerbated by a starvation of new sediment to wetland surfaces that resulted from levee construction * Corresponding author. ** Corresponding author. E-mail addresses: tbianchi@tamu.edu (T.S. Bianchi), rlcook@lsu.edu (R.L. Cook). Contents lists available at SciVerse ScienceDirect Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev 0141-1136/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marenvres.2011.09.007 Marine Environmental Research 72 (2011) 248e257