Piers Chapman

Hypoxia in the Northern Gulf of Mexico: Does the Science Support the Plan to Reduce, Mitigate, and Control Hypoxia?

We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.

The science of hypoxia in the Northern Gulf of Mexico: a review.

The Mississippi River is one of the worlds 10 largest rivers, with average freshwater discharge into the northern Gulf of Mexico (GOM) of 380km(3) 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.

Short-term variability during an anchor station study in the southern Benguela upwelling system: Chemical and physical oceanography

Abstract During March and April 1987, an anchor station experiment was conducted in 45m water depth in St Helena Bay (32°30′S) on the west coast of South Africa, “downstream” of a major upwelling centre at Cape Columbine. Temperature, salinity, oxygen, chlorophyll and nutrient concentrations were measured at seven depths down to 43m at four-hourly intervals over a 30 day period. These measurements were accompanied by current metering and weather station observations. Additional biological measurements of primary and secondary productivity were also carried out (see other papers in this suite). The period of the anchor station covered one major upwelling/decay cycle during the first fortnight of the experiment, followed by a second upwelling-favourable period. Throughout the experiment, stratification was observed in all parameters. The physical oceanography data suggested that the gyral circulation in the area tends to trap organic matter on the shelf as part of a two-layer system. Coastally-trapped internal waves and tidal periodicity control the relative importance of surface and bottom layers, which show considerable shear across the interface, because of the difference in cross-shelf circulation in the two layers. Despite sudden changes in bottom current velocities, little change was found in the distribution of chemical parameters below the thermocline, suggesting that relatively little net advection occurs into the bay, in contrast to the steeper slope regime. Chemical parameters showed consistent variability over the inertial period (22.5h). The inverse relationship between nitrate, phosphate or silicate and dissolved oxygen confirmed the importance of nutrient regeneration in the bottom mixed layer (BML), although considerable differences existed between nitrate and the other two nutrients. Rates of uptake during phytoplankton growth and replenishment during upwelling suggests that minimum upwelling rates in the Cape Columbine area were of the order of 0.5–1.0md−1 during the anchor station experiment. A considerable flux of ammonia from the sediment to the BML was also recorded. Data obtained during this study are compared to those obtained from similar anchor stations off Walvis Bay (22°S). Considerable differences in chemical parameters are related to changes in the oceanographic regimes at the two sites, together with changes in source water masses.

Satellite observations of upwelling on the continental shelf south of Madagascar

We report on upwelling seen in satellite AVHRR sea surface temperature imagery over the continental slope and shelf of southern Madagascar during February and March 2000. The upwelling is concurrent with anomalously high pseudo wind-stress over the region during this period. However, the western boundary East Madagascar Current, which is seen over the continental slope region, may contribute to the upwelling effect. The upwelling covers an area of 2° longitude by 1° latitude and at its peak is about 3–5°C cooler than the local ambient sea surface temperature. The paucity of in situ wind and current data in the region, however, prohibit a quantitative assessment of the relative forcing.

The western Agulhas Bank: circulation, stratification and ecology

Studies of the physical oceanography of the western Agulhas Bank are reviewed, pointing out the unique position of this shelf region between the eastern boundary Benguela system and the western boundary Agulhas system. New observations from moorings off Quoin Point during summer 1986/87 permit a fuller description of the dynamics of circulation and stratification and the identification of three subregions. The inner shelf is dominated by wind-forcing, as in eastern boundary upwelling systems, whereas the outer shelf is dominated by oceanic forcing, as over western boundary shelves. The mid-shelf is characterized by strong stratification separating near-surface oceanic water from near-bottom upwelling Central Water. A lack of correlation between currents and temperatures in these three subregions of the western Agulhas Bank suggests that they function independently. This synthesis of the physical oceanography, which includes discussion of the hydrodynamic coupling of the western Agulhas Bank with shelf reg...

Flow at intermediate depths around Madagascar based on ALACE float trajectories

During 1994–1996, 215 Autonomous Lagrangian Circulation Explorers (ALACE floats) were released at a nominal 900 m depth in the Indian Ocean as part of the World Ocean Circulation Experiment. Of these, 66 entered the region around Madagascar (2–30°S, 35–55°E), generally at a depth of 800–900 m. Floats approaching the island from the east were deflected either northward or southward depending on latitude, with the bifurcation point being near 20°S. Mean southward velocities in the western boundary current were 8.5 cm s-1 during each 25-day observation period, with mean northward velocities 7.7 cm s-1. Speeds past Cape Amber were about 11 cm s-1. These figures are comparable to those obtained from in situ current meter measurements. Floats rounding the island to the north frequently drifted north of the Comores for many months with no discernable pattern, before exiting the region either to the north in the East African Coastal Current and the equatorial current system or to the south via the Mozambique Channel. Flow rates in this region were highly skewed towards low (less than 5 cm s-1) velocities. Floats passing south of Madagascar showed little northward movement into the Mozambique Channel, but tended to move steadily westwards towards the African coast, becoming entrained in the Agulhas Current and its recirculation gyres near 28°S. Similarly, floats released within the southern portion of the Channel all tended to move to the south and west despite occasional entanglement with eddies. Mean flow in the Agulhas region was about 18 cm s-1, with maximum velocities over 25 days of up to about 35 cm s. All flows were extremely variable because of the ubiquity of eddies. In essence, the flow observed near 800 m resembled closely that seen at the surface from TOPEX/POSEIDON altimetry and at 845 m in the OCCAM global model. Temperature data collected by the floats were used to determine when a given float was being acted on by an anticyclonic eddy. Deviations, which were typically 0.5°C or more from the background temperature field, were found only in eddies south of 12°S and occurred in about 9% of float records within the Mozambique Channel. The results support the idea of a net southward flow of water through the Channel from the tropics to the Agulhas Current, with the eddies playing a major role in the transport in the southern part of the region. In the northern and eastern parts of the region, however, eddies were far less prevalent, and topographic steering by the Mascarene Ridge became important.

Characteristics of the South Atlantic subtropical frontal zone between 15°W and 5°E

Abstract In this paper we present data from a number of crossings of the boundary between subtropical and subpolar water in the 15°W–5°E region of the South Atlantic and discuss the implications. The previous paucity of synoptic data sets near 40°S between 25°W and the Greenwich Meridian meant that up to now it has not been possible to fix the position of the boundary in the mid-South Atlantic or to deduce the effects of the midocean ridge on the strength of the South Atlantic Current (SAC). Using hydrographic and chemical tracer data we confirm that the transition from subtropical to subpolar waters occurs within a Subtropical Frontal Zone (STFZ), which constrains the South Atlantic Current flow and is bounded on each side by a distinct front. The northern one, the Northern Subtropical Front (NSTF), varies by only 1.5° of latitude, whereas the southern one, the South Subtropical Front (SSTF), the traditional Subtropical Convergence (STC, defined by Deacon, 1937), migrates over 2.5° of latitude and remains south of the island of Tristan da Cunha. This finding goes some way in resolving the disparity in the literature to the position and seasonal migration of the STC. The data confirm the existence of a subsurface salinity maximum lobe, which appears to be formed at the NSTF rather than at the SSTF. By closely investigating the mesoscale structure and comparing it with historical data from a number of meridional cruises, we have shown strong seasonality in the frontal structure between 30 and 45°S in the South Atlantic. Having resolved the fine structure, we have made an estimate of the geostropic flow of the SAC and suggest that there are east to west differences, which may be related to recirculation in the Argentine Basin rather than to a slowing down by the midocean ridge.

Controlling Hypoxia on the U.S. Louisiana Shelf: Beyond the Nutrient‐Centric View

As the Earths population continues to increase, the projected effects of contaminant loading and human encroachment on biodiversity remain unclear. One area of intense interest is coastal eutrophication and associated hypoxia events (with hypoxia defined as oxygen <2 milligrams per liter = 1.4 milliliters per liter = ˜63 micromoles per cubic decimeter). On average, the Mississippi River discharges to the northern Gulf of Mexico (GOM) 550 cubic kilometers of freshwater and approximately 60–70×109 moles of dissolved inorganic nitrogen per year [Dagg et al., 2004], which contains anthropogenic nitrogen derived primarily from agricultural fertilizer as well as effluent from animal feedlots.

Changes in iodine speciation in the Benguela current upwelling system

Iodine speciation in the oxygen-depleted water mass along the west coast of South Africa was investigated in February 1979. The low oxygen water was related to the presence of a stable density layer with σt = 26.8, which persists over long time periods. The remineralisation of the area is discussed in terms of the nutrient and iodine profiles and contrasted with deep-sea iodine regeneration. The presence of total iodine concentrations (dissolved and possibly particulate) of up to 6.6 μM is probably accounted for by sediment release. Calculations of possible breakdown rates of phytoplankton suggest a continuous reduction process as the water body travels south, rather than a non-local source of the reduced waters.

Nutrients chlorophyll and oxygen relationships in the surface layers at the agulhas retroflection

Abstract During the Agulhas Retroflection cruise (ARC) in November–December 1983, data were obtained on the distribution of nutrients, oxygen and chlorophyll a in the area between the south of Africa and the Subtropical Convergence. Several frontal systems were observed, which were distinguished by enhanced sea surface chlorophyll a and fluorescence values. Subsurface waters could be differentiated by means of nutrient and chlorophyll concentrations; in particular, the lowest concentrations of all parameters were found in the zone between the Agulhas and Agulhas Return currents, while the Subtropical Convergence zone exhibited the highest nutrient levels. Agulhas Current water showed a substantial oxygen minimum layer, derived from the tropical surface water of the Indian Ocean, at a depth of 50–200 m. This water penetrated under certain conditions as a lobe almost unchanged as far west as 16°E during the cruise period; it can be found at least as far west as 13°E at 34°S. Detached Agulhas rings showed this feature, which may be a useful index of the penetration of Indian Ocean thermocline water into the Atlantic. Nutrient concentrations in the rings varied depending on the degree of mixing between water of Indian and Atlantic Ocean origin.