Just Cerfing Vol. 7, Issue 8, August 2016 Volume 5, Issue 3, March, 2014 | Page 14

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Spatial and Temporal Variations in Nutrients and Water-Quality Parameters
in the Mississippi River-Influenced Breton Sound Estuary
Christopher J. Lundberg, Robert R. Lane, and John W. Day, Jr.
Department of Oceanography and Coastal Sciences
Louisiana State University
Baton Rouge, LA 70803, U.S.A.
ABSTRACT

INTRODUCTION

The purpose of this study is to investigate the long-term temporal and
spatial nutrient patterns in the Breton Sound estuary, an estuarine wetland
complex in coastal Louisiana that is highly influenced by the Caernarvon
river diversion. A water-quality data set spanning 8 years of monthly sampling was analyzed. Analysis of salinity mixing diagrams indicates the estuary to be a source of ammonium and chlorophyll a, and a sink for nitrate,
total nitrogen, and total suspended sediments. The estuary served as either
a source or sink for phosphate, total phosphorus, and silicate depending on
season. The NOx loading rate ranged from 1.1 g N m−2 y−1 during fall to
4.9 g N m−2 y−1 during spring, with an overall mean of 3.5 g N m−2 y−1. Nitrate removal efficiency varied seasonally, with highest efficiency during the
fall (98%), summer (92%), and spring (87%) and lowest during the winter
(74%). There was an inverse relationship between nutrient removal efficiency
and nutrient loading rate. The results of this study indicate that the estuary is
effective in water-quality amelioration through nitrate removal.

The Mississippi River drains approximately 40% of the contiguous United States and annually exports an average of nearly 1x106 tons of nitrate (NO3-)
to the Gulf of Mexico, and concentrations have more than doubled since
1950 (Goolsby and Battaglin, 2001). Reasons for this increased nitrogen
export include increased fertilizer application, more efficient drainage of
agricultural fields, and expansion of corn and soybean production (Donner, Kucharik and Foley, 2004; Howarth et al., 2002; Zhang and Schilling,
2006). Additionally, extensive construction of flood control levees during
the 20th century along the lower Mississippi Valley has prevented seasonal
flood waters from reaching alluvial and deltaic wetlands, which serve as natural sinks for nutrients and sediments (Day et al., 2000; Day et al., 2003a;
Mitsch et al., 2001; Mitsch et al., 2005). Nitrogen export is partially responsible for the large seasonal hypoxic zone in the Gulf of Mexico (Bianchi
et al., 20 10; Justic, Rabalais and Turner, 1995; Lohrenz et al., 2008; Rabalais et al., 1996; Rabalais, Turner and Scavia, 2002).
Mississippi River deltaic wetlands were historically stabilized and enhanced
by freshwater, sediment, and nutrient input from seasonal river flooding, as
well as from hurricanes and and other large storm events and other large
storm events (Day et al., 2000). However, during the 20th century, there
was a loss of about 25%, 4500 km2, of coastal wetlands in the Mississippi
delta (Barras et al., 2003). One reason for this was decreased sediment de-

ADDITIONAL INDEX WORDS: River diversion, removal efficiency, nitrate loading, water quality amelioration.

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