The science of hypoxia in the Northern Gulf of Mexico: A review

Abstract

The Mississippi River is one of the world's 10 largest rivers, with average freshwater discharge into the northern Gulf of Mexico (GOM) of 380 km³ year^− 1. In the northern GOM, anthropogenic nitrogen is primarily derived from agricultural fertilizer and delivered via the Mississippi River. The general consensus is that hypoxia in the northern Gulf of Mexico is caused primarily by algal production stimulated by excess nitrogen delivered from the Mississippi–Atchafalaya River Basin and seasonal vertical stratification of incoming stream flow and Gulf waters, which restricts replenishment of oxygen from the atmosphere.

In this paper, we review the controversial aspects of the largely nutrient-centric view of the hypoxic region, and introduce the role of non-riverine organic matter inputs as other oxygen-consuming mechanisms. Similarly, we discuss non-nutrient physically-controlled impacts of freshwater stratification as an alternative mechanism for controlling in part, the seasonality of hypoxia. We then explore why hypoxia in this dynamic river-dominated margin (RiOMar) is not comparable to many of the other traditional estuarine systems (e.g., Chesapeake Bay, Baltic Sea, and Long Island Sound). The presence of mobile muds and the proximity of the Mississippi Canyon are discussed as possible reasons for the amelioration of hypoxia (e.g., healthy fisheries) in this region. The most recent prediction of hypoxia area for 2009, using the current nutrient-centric models, failed due to the limited scope of these simple models and the complexity of this system. Predictive models should not be the main driver for management decisions. We postulate that a better management plan for this region can only be reached through a more comprehensive understanding of this RiOMar system—not just more information on river fluxes (e.g., nutrients) and coastal hypoxia monitoring programs.

Introduction

Increases in the occurrence of hypoxia, when dissolved oxygen (DO) is < 2 mg L^− 1 (1.4 mL L^− 1 = ∼ 63 μmol O₂ dm^− 3) in coastal waters have made this environmental problem a global issue (Diaz and Rosenberg, 2008). The expansion of hypoxic zones around the world is largely associated with eutrophication (Conley et al., 2009), where nutrient inputs to coastal waters result in excess primary production, followed by depletion of oxygen in waters, due to the high rates of microbial respiration (Bianchi, 2007, and references therein). In particular, there has been a significant debate in the literature on the processes that control spatial and temporal variability of hypoxia in bottom waters of the northern Gulf of Mexico (GOM), purported to be the largest in the western hemisphere (Rabalais et al., 2002a, Rabalais et al., 2002b, Rabalais et al., 2007). Despite 25 years of consistent monitoring there remains considerable uncertainty in the processes that control hypoxia here and their relative importance (Krug, 2007, Krug and Merrifield, 2007, Dagg et al., 2007, Bianchi et al., 2008, Hetland and DiMarco, 2008, Boesch et al., 2009). In this review, we will compare and contrast existing data on this hypoxic zone in terms of: 1) the spatial and temporal effects of a two-plume system (e.g., Atchafalaya River Plume [ARP] and Mississippi River Plume [MRP]) on the hypoxic region; and 2) the processes that are reinforcing and weakening hypoxia.