Gulf of Mexico Hypoxia, A.K.A. “The Dead Zone”

Abstract

The second largest zone of coastal hypoxia (oxygen-depleted waters) in the world is found on the northern Gulf of Mexico continental shelf adjacent to the outflows of the Mississippi and Atchafalaya Rivers. The combination of high freshwater discharge, wind mixing, regional circulation, and summer warming controls the strength of stratification that goes through a well-defined seasonal cycle. The physical structure of the water column and high nutrient loads that enhance primary production lead to an annual formation of the hypoxic water mass that is dominant from spring through late summer. Paleoindicators in dated sediment cores indicate that hypoxic conditions likely began to appear around the turn of the last century and became more severe since the 1950s as the nitrate flux from the Mississippi River to the Gulf of Mexico tripled. Whereas increased nutrients enhance the production of some organisms, others are eliminated from water masses (they either emigrate from the area or die) where the oxygen level falls below 2 mg l−1 or lower for a prolonged period. A hypoxia-stressed benthos is typified by short-lived, smaller surface deposit-feeding polychaetes and the absence of marine invertebrates such as pericaridean crustaceans, bivalves, gastropods, and ophiuroids. The changes in benthic communities, along with the low dissolved oxygen, result in altered sediment structure and sediment biogeochemical cycles. Important fisheries are variably affected by increased or decreased food supplies, mortality, forced migration, reduction in suitable habitat, increased susceptibility to predation, and disruption of life cycles.

Nitrous oxide fluxes and dissolved N gases (N2 and N2O) within riparian zones along the agriculturally impacted San Joaquin River

“…of this primary limiting nutrient to freshwater and marine ecosystems (Vitousek et al 1997;Caraco and Cole 2001;Howarth and Marino 2006;Hakanson et al 2007). In particular, N fertilization has led to degradation of water quality, depletion of oxygen and decreases in species abundance and richness (Rabalais et al 2002;Morrisey et al 2003;USEPA 2013). Vegetated riparian buffer zones, located at the interface between terrestrial and aquatic ecosystems, are one management strategy that can play a key role in mitigating nitrogen inputs into waterways (Jordan et al 1993;Cey et al 1999;Pinay et al 2007).…”

Modeling eutrophication and oligotrophication of shallow-water marine systems: the importance of sediments under stratified and well-mixed conditions

“…The intensity of these processes was considered to differ under wellmixed and stratified conditions. When lasting for long enough periods, anoxia may completely eradicate the larger benthic animals (e.g., Rabalais et al, 2002), and it may take years before significant populations of macrofauna return (Josefson & Widbom, 1988). In contrast, smaller organisms are less affected and may recover more rapidly upon restoration of oxic conditions (Josefson & Widbom, 1988).…”

An integrated ocean circulation, wave, atmosphere, and marine ecosystem prediction system for the South Atlantic Bight and Gulf of Mexico

“…Notable results of these impacts are coastal eutrophication, recurring hypoxia [aka the 'Dead Zone' (Rabalais et al 2002)], and coastal ocean acidification (Cai et al 2011) on the Louisiana-Texas shelf. The coastal regions of the South Atlantic Bight and Gulf of Mexico (SABGOM) are also periodically threatened by surges induced by storms, which have exhibited a statistically significant trend of increasing in their frequency over the past 90 years (Grinsted et al 2013).…”