Elevating Dissolved Oxygen—Reflections on Developing and Using Long-Term Data

This prospectus took me about as long to generate as my 36—year record of working on the issue of northern Gulf of Mexico (nGOM) oxygen deficiency, or so I felt. There was so much to cover, but I focused on the issue of hypoxia on the Louisiana continental shelf from the early 1980s to present and my participation in the research and outreach. Not that I was ignoring other aspects of my academic research career (e.g., stone crab populations and their differences in physiology and larval development along the nGOM coast; settlement of crab megalopae, especially blue crabs, on artificial substrates and their timing with tidal events; oil and gas pollutant discharges in coastal waters of Louisiana, and as Director of the Coastal Waters Research Consortium (CWC) of the Gulf of Mexico Research Initiative (GoMRI), and marsh infaunal researcher. I must say, however, that the journey through the documentation of low dissolved oxygen on the Louisiana continental shelf, and its linkage to the changes in the Mississippi River nutrient loads to the coastal waters of the nGOM, marked a dominant part of my career. This prospectus follows my research and outreach career from my first journey offshore in an outboard to set stations for the transect off Terrebonne Bay in early summer of 1985 to now

Global Change And Eutrophication Of Coastal Waters

The cumulative effects of global change, including climate change, increased population, and more intense industrialization and agri-business, will likely continue and intensify the course of eutrophication in estuarine and coastal waters. As a result, the symptoms of eutrophication, such as noxious and harmful algal blooms, reduced water quality, loss of habitat and natural resources, and severity of hypoxia (oxygen depletion) and its extent in estuaries and coastal waters will increase. Global climate changes will likely result in higher water temperatures, stronger stratification, and increased inflows of freshwater and nutrients to coastal waters in many areas of the globe. Both past experience and model forecasts suggest that these changes will result in enhanced primary production, higher phytoplankton and macroalgal standing stocks, and more frequent or severe hypoxia. The negative consequences of increased nutrient loading and stratification may be partly, but only temporarily, compensated by stronger or more frequent tropical storm activity in low and mid-latitudes. In anticipation of the negative effects of global change, nutrient loadings to coastal waters need to be reduced now, so that further water quality degradation is prevented

Nitrogen And Phosphorus Phytoplankton Growth Limitation In The Northern Gulf Of Mexico

We conducted 158 bioassays to determine phytoplankton growth limitation in the northern Gulf of Mexico and made the following observations. Light limitation occurred where salinity was \u3c 20; at higher salinities, phytoplankton biomass yield became mostly limited by N or by a co-limitation of N plus P (NP). The number of N-limited bioassays was 5 times greater than the P-limited bioassays. NP synergism occurred where salinity was \u3e 20, and represented 59% of all samples that were not light-limited. The interaction of N and P co-limitation was frequently synergistically additive, i.e. the combined effects of N and P limitation together created a greater response than the sum of either separately. The dissolved inorganic nitrogen: phosphate ratio (DIN:Pi) and various concentrations of DIN and Pi did not offer reliable chemical boundaries describing likely areas of exclusive N or P limitation in these bioassays. We conclude that reducing N loading to the shelf is a prudent management action that will partially remediate eutrophic conditions, including those that lead to hypoxia, but the omission of a concurrent reduction in P loading would be shortsighted

Effects Of The Deepwater Horizon Oil Spill On Coastal Marshes And Associated Organisms

Oil gushed from the Macondo Mississippi Canyon 252 well into the Gulf of Mexico for 87 days after the Deepwater Horizon drilling rig exploded and sank. A concern, after widespread dispersant use offshore on surface waters and at the wellhead, was that the oil/dispersant mixture would reach valuable, and vulnerable, coastal ecosystems. Standardized oil spill response methodology identified 1,773 km of the 7,058 km of surveyed shoreline as oiled, with 1,075 km oiled in Louisiana. This paper synthesizes key results of published research on the oiling effects on coastal habitats and their inhabitants from microbes to vertebrates. There were immediate negative impacts in the moderately to heavily oiled marshes, and on the resident fish and invertebrates. Recovery occurred in many areas within the two years following the oiling and continues, but permanent damage from heavily oiled marshes resulted in eroded shorelines. Organisms, including microbial communities, invertebrates, and vertebrates, were diminished by acute and chronic hydrocarbon exposure. However, the inherent variability in populations and levels of exposure, compounded with multiple stressors, often masked what were expected, predictable impacts. The effects are expected to continue to some degree with legacy hydrocarbons, or the marsh ecosystem will reach a new baseline condition in heavily damaged areas

Dynamics And Distribution Of Natural And Human-Caused Hypoxia

Water masses can become undersaturated with oxygen when natural processes alone or in combination with anthropogenic processes produce enough organic carbon that is aerobically decomposed faster than the rate of oxygen re-aeration. The dominant natural processes usually involved are photosynthetic carbon production and microbial respiration. The re-supply rate is indirectly related to its isolation from the surface layer. Hypoxic water masses (\u3c 2 mg L-1, or approximately 30% saturation) can form, therefore, under \u27natural\u27 conditions, and are more likely to occur in marine systems when the water residence time is extended, water exchange and ventilation are minimal, stratification occurs, and where carbon production and export to the bottom layer are relatively high. Hypoxia has occurred through geological time and naturally occurs in oxygen minimum zones, deep basins, eastern boundary upwelling systems, and fjords. Hypoxia development and continuation in many areas of the world\u27s coastal ocean is accelerated by human activities, especially where nutrient loading increased in the Anthropocene. This higher loading set in motion a cascading set of events related to eutrophication. The formation of hypoxic areas has been exacerbated by any combination of interactions that increase primary production and accumulation of organic carbon leading to increased respiratory demand for oxygen below a seasonal or permanent pycnocline. Nutrient loading is likely to increase further as population growth and resource intensification rises, especially with increased dependency on crops using fertilizers, burning of fossil fuels, urbanization, and waste water generation. It is likely that the occurrence and persistence of hypoxia will be even more widespread and have more impacts than presently observed. Global climate change will further complicate the causative factors in both natural and human-caused hypoxia. The likelihood of strengthened stratification alone, from increased surface water temperature as the global climate warms, is sufficient to worsen hypoxia where it currently exists and facilitate its formation in additional waters. Increased precipitation that increases freshwater discharge and flux of nutrients will result in increased primary production in the receiving waters up to a point. The interplay of increased nutrients and stratification where they occur will aggravate and accelerate hypoxia. Changes in wind fields may expand oxygen minimum zones onto more continental shelf areas. On the other hand, not all regions will experience increased precipitation, some oceanic water temperatures may decrease as currents shift, and frequency and severity of tropical storms may increase and temporarily disrupt hypoxia more often. The consequences of global warming and climate change are effectively uncontrollable at least in the near term. On the other hand, the consequences of eutrophication-induced hypoxia can be reversed if long-term, broad-scale, and persistent efforts to reduce substantial nutrient loads are developed and implemented. In the face of globally expanding hypoxia, there is a need for water and resource managers to act now to reduce nutrient loads to maintain, at least, the current status

Linking Landscape And Water Quality In The Mississippi River Basin For 200 Years

Two centuries of land use in the Mississippi River watershed are reflected in the water quality of its streams and in the continental shelf ecosystem receiving its discharge. The most recent influence on nutrient loading-intense and widespread farming and especially fertilizer use-has had a more significant effect on water quality than has land drainage or the conversion of native vegetation to cropland and grazing pastures. The 200-year record of nutrient loading to offshore water is reflected in the paleoreconstructed record of plankton in dated sediments. This record illustrates that the development of fair, sustained management of inland ecosystems is linked to the management of offshore systems. Land use in this fully occupied watershed is under the strong influence of national policies affecting all aspects of the human ecosphere. These policies can be modified for better or worse, but water quality will probably change only gradually because of the strong buffering capacity of the soil ecosystem

Suspended Sediment, C, N, P, and Si Yields from the Mississippi River Basin

The annual loads of C,N,P, silicate, total suspended sediment (mass) and their yields (mass area−1) were estimated for six watersheds of the Mississippi River Basin (MRB) using water quality and water discharge records for 1973 to 1994. The highest load of suspended sediments is from the Missouri watershed (58 mt km2 yr−1), which is also the largest among the six major sub-basins. The Ohio watershed delivers the largest load of water (38%). The Upper Mississippi has the largest total nitrogen load (32%) and yield (1120 kg TN km2 yr−1). The loading of organic carbon, total phosphorus and silicate from the Upper Mississippi and Ohio watersheds are similar and relatively high (range 2.1–2.5, 0.068–0.076, and 0.8–1.1 mt km2 yr−1, respectively). The yields of suspended sediments, total phosphorus, total nitrogen, and silicate from the Lower Mississippi watershed are disproportionately the highest for its area, which is the smallest of all the watersheds and has the weakest monitoring network. The loading from the Red and Arkansas watersheds are of lesser importance than the others for most parameters investigated. The total nitrogen loading to coastal waters increased an additional 150% since the early 1900s, and is now dominated by loads from the Upper Mississippi watershed, rather than the previously dominant Ohio watershed. An analysis of trends for 1973–1994 suggests variability among years, rather than uni-directional change for most variables among 11 key stations. Explanatory relationships were established or confirmed to describe TN and TP loadings in terms of the now largely human-created landscape arising mostly over the last 150 years