Abstract
AbstractOne of the largest human‐caused areas of bottom‐water oxygen deficiency in the coastal ocean is on the northern Gulf of Mexico continental shelf adjacent to the Mississippi River, which discharges nitrogen and phosphorus loads into its surface waters. The beginnings of seasonal hypoxia (≤2 mg l−1 dissolved oxygen) in this area was in the 1950s with an acceleration in the worsening of severity during the 1970s. Currently, the bottom area of hypoxic areas can approach 23,000 km2, and the volume, 140 km3. Ecosystems, people, and economies are now at risk within the Mississippi River watershed and in the northern Gulf of Mexico. Strengthened nitrogen and phosphorus mitigation, altered agriculture practices, and reduction in carbon and nutrient footprints are key to the recovery of these systems. In this article, we review the past, present, and possible future conditions of the northern Gulf of Mexico and provide insight into possible management actions.
Highlights
We review the past, present, and possible future conditions of the northern Gulf of Mexico and provide insight into possible management actions
There is no doubt that many areas of the coastal ocean are receiving enriched loads of nitrogen (N) and phosphorus (P) that are implicated in noxious, and often toxic, harmful algal blooms, and in decreasing dissolved oxygen (DO) concentrations in the bottom waters to ecologically depressed levels (Breitburg et al 2018)
One well-documented area for these ecosystem changes is in the northern Gulf of Mexico adjacent to the discharge of the Mississippi River where it discharges its constituents to affect nearshore and mid-depth areas of the adjacent Louisiana continental shelf and lead to bottom-water low oxygen conditions (Rabalais et al 2007a; Rabalais et al 2010; Rabalais et al 2014)
Summary
There is no doubt that many areas of the coastal ocean are receiving enriched loads of nitrogen (N) and phosphorus (P) that are implicated in noxious, and often toxic, harmful algal blooms, and in decreasing dissolved oxygen (DO) concentrations in the bottom waters to ecologically depressed levels (Breitburg et al 2018). These changes began primarily in the mid-1950s when North American watersheds underwent landscape use change or even earlier in European watersheds during the Industrial Revolution and as human population expanded and fossil fuel use increased. Our research findings originate from 30+ years of summertime research cruises across the Louisiana shelf, cruises on two transects 100 and 250 km west of the Mississippi River, continuous bottom-water oxygen measurements on moorings, and analyses of Mississippi River landscape and nutrient load changes revealed on the continental shelf by palaeoindicators in dated sediments
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