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Overview
Mid-Summer Shelfwide Cruises Maps
Changes in Areal extent of Hypoxia 1985-2003 (Animation)
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Overview
The distribution of hypoxia on the Louisiana shelf has been mapped during mid-summer since 1985 during the expected maximal extent of hypoxia (usually mid July to early August). The sampling grid (approx. 60-80 stations) is similar from year to year and covered within a 5- to 6-d period. Abbreviated cruises were conducted in 1988 and 1989.
Each cruise starts at Southwest Pass of the Mississippi River to collect the ‘end member’ for river constituents. 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 prevent the complete mapping of the extent of hypoxia, either in shallow water, in the offshore direction or to the west. Thus, the areal extent of hypoxia generated from these surveys is a minimal estimate.
Histogram of estimated areal extent of bottom water hypoxia (< 2 mg l-1) for mid-summer cruises in 1985-2005 and Hypoxia Action Plan goal for reduced size. Long-term average and five-year running average as indicated.
Mid-summer hypoxic zones, between 1985 and 1992, generally formed in two distinct areas west of the Mississippi and Atchafalaya River deltas, averaging 8000 - 9000 km2. Since the Great Mississippi River Flood of 1993, mid-summer hypoxia on average has been twice the average size of areas in 1985-1992. The largest area mapped to date was 22,000 km2 in July 2002. In the 1988 drought year for the Mississippi River basin, hypoxia was confined to a single station off Terrebonne Bay when mapped in July. The relatively smaller extent in 2000 was proportional to the reduced Mississippi River discharge and nutrient flux. Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) to as deep as 60 m water depth but more typically between 5 and 30 m.
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| Yearly Mid-Summer Shelfwide Cruises |
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.
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