Cruise Definition

Hypoxia on the Louisiana shelf has been measured and mapped since 1985, during the expected maximal extent that usually occurs between mid-July and early August. The sampling grid, comprised of approximately 60-80 stations, is similar from year to year and visited over a period of 5-7 days. Abbreviated cruises were conducted in 1988 and 1989.

Each cruise begins at the Southwest Pass of the Mississippi River to collect the end member constituents of this important water mass. Then, stations along transects perpendicular to shore are sampled from east to west for a series of physical, chemical, and biological characteristics. Time or other logistical constraints often prevents the complete mapping of the extent of hypoxia, either closest to shore, far offshore, or at the western limit. Therefore, the areal extent of hypoxia generated from these surveys should be considered a conservative estimate.

Past Hypoxia

Mid-summer hypoxic zones generally form in two distinct areas west of the Mississippi and Atchafalaya River deltas (see Frequency of Hypoxia). Since the major Mississippi River flood of 1993, the area of mid-summer hypoxia has approximately doubled on average. The largest area mapped to date was 22,000 km2 in July 2002. In 1988, a drought year for the Mississippi River basin, hypoxia was confined to a single station off Terrebonne Bay in July. The relatively small extent area of hypoxia in 2000 was proportional to reduced Mississippi River discharge and nutrient flux. Hypoxic water depths are typically distributed from shallow near shore (4 to 5 m) to as deep as 60m offshore, but typically range between 5 and 30 m shelfwide.

Long-Term Monitoring

Routine, continuing and consistent data collections since 1985 have provided the necessary long-term data with which we can:

Document the temporal and spatial extent of hypoxia

Identify the hydrographic, chemical, and biological data related to the development and maintenance of hypoxia

Develop predictive models of relationships between riverine nutrient loading and the size of the hypoxic zone and its severity.

Understanding the physics is relevant to the study of biological processes, nutrient cycling, and dissolved oxygen dynamics. Likewise, physical measurements alone do not provide sufficient information to understand the development and maintenance of hypoxia. Documenting the extent of hypoxia over large areas and through time is essential in understanding its formation through an annual cycle. These observations provide a synthetic, interdisciplinary dataset that directly supports improved modeling, better understanding, and important strategies related to nutrient management scenarios within the Mississippi River basin and resource management in the Gulf of Mexico.