Appearing before Congress’s committee on energy and commerce, Samantha Rowe, professor of marine science at the University of Georgia, described the process by which the gulf oil spill will overwhelm the gulf’s natural ability to process and sublimate crude oil.
One likely consequence is apoxia, or a lack of oxygen in vast dead zones. Marine life will struggle to survive.
Rowe also blogs from a scientific perspective about the spill in the gulf.
Her testimony follows.
I am an Oceanographer in the Department of Marine Sciences at the University of Georgia (Athens, GA).
Over my career, a good deal of my research effort has focused on naturally occurring gas and oil seeps, commonly referred to as ‘cold seeps’, in the Gulf of Mexico.
My testimony will describe the role of hydrocarbons in Gulf of Mexico ecosystem, both in the natural context and with respect to the potential impacts of focused large inputs such as those resulting from the Deepwater Horizon spill.
A unique characteristic of the Gulf of Mexico is that its subseafloor sediments contain vast reserves of hydrocarbons. Some of this oil and gas (methane and higher alkanes) fluxes naturally from deep reservoirs through complex fault-networks to reach surficial sediments. In sediments, these reduced substrates fuel extremely high rates of microbial metabolism.
Some oil and gas escapes from the sediments and reaches the water column, where it is subject to additional oxidation.
Natural oil seepage from the seafloor creates slicks that can be quantified and mapped using satellite imagery. Naturally-occurring oil and gas seepage plays a key role in shaping the ecology, microbiology, and biogeochemistry of the Gulf of Mexico system, particularly its deep sediments and waters. Under most circumstances, natural seeps are the most important source of petroleum to the marine environment.
In the Gulf of Mexico, about 95% of offshore oil inputs are from natural seeps under normal conditions. Systems like the Gulf of Mexico are thus accustomed to slow, somewhat diffuse inputs of oil and gas, and the biological communities have adapted to endure and in some cases metabolize these materials such that negative impacts of such inputs are localized as opposed to widespread.
Seepage of oil and gas at the seafloor supports the establishment and proliferation of diverse chemosynthetic ecosystems that includes seep endemic sessile fauna (e.g. tubeworms and mussels), mobile fauna that tend to stay around seeps (e.g. clams, urchins, eels, fish, shrimps) as well as foraging species, such as demersal fish that likely migrate between seeps (e.g. six gill sharks).
Some endemic seep fauna harbor chemosynthetic symbionts (e.g. tubeworms, mussels, clams) while others are heterotrophic. While the macro-ecology of cold seeps in the Gulf of Mexico has been well described, the microbiology of these habitats is not, even though the microbial processes serve as the geobiological engine of cold seeps. Free-living microorganisms degrade oil and gas; under the anoxic conditions typical of seep sediments, oil and gas degradation are largely performed by sulfate reducing bacteria and the product of their metabolism (hydrogen sulfide) provides an inorganic energy source (hydrogen sulfide) to the chemosynthetic macrofauna.
The microbial degradation of oil and gas also generates carbonate ions, which subsequently drives precipitation of authigenic carbonates. These carbonate hardgrounds are colonized by deepwater corals (e.g. LopheJia), generating another unique seafloor ecosystem that is ultimately driven by natural seepage.
The impact(s) of natural oil and gas seepage on water column microbial communities has received little attention even though it is well known that both oil and gas are introduced into the water column at cold seeps in the Gulf of Mexico and elsewhere.
Microbial oxidation of oil is carried out by microorganisms like the gamma- proteobacterium Alcanivorax. Microbial oxidation of methane is carried out by a diverse assemblage of methane-eating, or methanotrophic, microorganisms.
Other low molecular weight alkane gases are similarly oxidized. Because the Gulf of Mexico experiences natural seepage, the natural microbial community here is poised to consume oil and gas. At least 1000 naturally occurring seeps along the Gulf of Mexico shelf and slope deliver from 1000-2000 barrels of oil per day into the Gulf’s waters.
The fact that this naturally derived oil does not accumulate on beaches underscores the ability of natural microbial and physical processes to consume it relatively quickly. However, as will become clear later in my testimony, the magnitude of this spill may saturate the microbial community’s ability to consume the introduced oil and gas.
In contrast to the naturally occurring hydrocarbon seepage, the BP blowout is injecting from 19,000 barrels (low-end estimate) to 70,000 barrels (high-end estimate) of oil per day into the water column via a focused, intense jet at a water depth of 5,OOO’.
The amount of gas being injected into the system has not been constrained, though BP has noted that the total flow could be as much as 40% gas. While natural seepage varies extensively in space and time, the BP blowout is an intense, localized input of labile organic matter to the deep ocean environment.
Thus, the BP blowout is an unprecedented perturbation to the Gulf of Mexico system that has no natural equivalent. It is virtually impossible to understand or quantify the ecological consequences of the BP blowout on the Gulf of Mexico ecosystem without knowing how much oil and gas has leaked from the wellhead. These numbers need to be estimated and corroborated independently based on available observational data.
Unfortunately, the leak rate was not quantified robustly during the first month of the spill (at least that information has not been made publically available). Unless we know how much oil is leaking from the wellhead, we cannot gauge the full extent of the ecological consequences in deepwater or surface water environments.
To put it bluntly, the scientific community is hamstrung until we know precisely how much oil and gas has leaked and is leaking from the wellhead. It is even more important to quantify the inputs from the wellhead since dispersants are being added to the fluid stream at the seafloor. The aim of deepwater dispersant addition is to break up oil and reduce formation of surface slicks.
The application of dispersants at the riser makes it impossible to estimate the size of the leak solely from surface observations (e.g. using satellite imagery). Given the importance of the estimating the magnitude of the spill, the challenge of monitoring hydrocarbons not only on the surface but also within mid-and deep waters, and of quantifying the hydrocarbon’s impact on ecosystem services in benthic, pelagic and littoral zones, it is critical that leak rates are quantified at least every other day by independent scientists until the well is capped and the leakage stopped.
There are many scientists who can make these measurements and I know they are willing and eager to help.
The Gulf of Mexico provides a number of ecosystem services to the public, including fisheries production, recreation and tourism, carbon sequestration and water purification in coastal marshes and mangroves, to name a few.
The potential coastal impacts of the BP blowout have received the most attention because this is where the direct human impacts are perceived to be the greatest. Certainly tourism, fisheries yield and production, and wetland and submerged aquatic vegetation (e.g. seagrass) habitats will be impacted.
But the food web of coastal and offshore habitats is likely to be impacted significantly. Everything from the base of the food web – microorganisms – to the higher order consumers – invertebrates, zooplankton, jellyfish, fish, birds, sea turtles, marine mammals – will suffer direct consequences of the BP blowout as long as there is oil in the system due the inherent toxicity of crude oil components.
This is why it is essential to recover as much of the spilled oil as possible and to remove it from the environment. While removing oil can be accomplished via skimming or burn offs on the surface ocean or clean up and removal from beaches and marshes, removing methane and other alkane gases is not possible. Other than evasion to the atmosphere, the fate of methane dissolved in water lies in the hands of microorganisms that can utilize methane as an energy source.
A secondary effect of the input of oil and gas on the oceanic system arises from the perturbation of the carbon and oxygen budgets in the system. Before the spill, oxygen concentrations in the water column reflected a “steady state” balance between sources (photosynthesis) and sinks (respiration). The direct injection of large quantities of oil and gas into the system has upset the delicate balance of oxygen in the offshore system.
Basically, the oxidation of the oil and gas has stimulated respiration such that oxygen is being consumed more rapidly than it is being supplied. We do not know what the end result of this infusion of oil and gas will be on the Gulf’s oxygen budget. But we can use well-studied coastal ecosystems to inform us of the possible consequences of extremely high organic matter loading.
In coastal ecosystems, excessive inputs of inorganic nutrients and hyper-production of labile organic carbon has driven increased respiration and heterotrophic oxygen consumption leading to the formation of coastal dead zones. Low oxygen (hypoxic) or zero oxygen (anoxic) waters have been documented in coastal systems across the globe in recent years. These dead zones are a direct result of perturbation of the carbon and oxygen budgets of these systems.
This imbalance between oxygen inputs and outputs, if sustained over an ample period of time, could lead to hypoxia or anoxia in the water column, which would have substantial and potentially widespread negative impacts on any oxygen-requiring animal populations and on the food web of the system.
Initial concerns regarding the BP blowout focused on coastal impacts and the need to keep oil from damaging critical coastal ecosystems and the coastal economy, which depends heavily on tourism and fisheries, in addition to the oil industry..
Certainly such concerns are valid and widespread efforts to protect the coastal zone from the oil are essential. It appears that the widespread use of dispersants in response to the BP blowout is due largely to the desire to keep the beaches clean and minimize the impact of the spill on coastal environments.
However, oil on the surface of the ocean and even on beaches can be cleaned up. Dispersed oil cannot be cleaned up; rather, it moves with the water and the oil and dispersants are likely to influence oceanic ecosystems for years to come. Because dispersed oil cannot be effectively recovered, its fate is largely tied to the activity of microorganisms that degrade it, assuming the dispersants have no negative impact on their metabolism.
The implication of this is that dispersed oil may stimulate the oxygen demand of the system and potentially promote subsurface hypoxia. Little attention has been given to the offshore oceanic impacts of the BP blowout and initial reports of subsurface oil were received with skepticism. The BP blowout is introducing both oil and methane gas into the deepwater.
The oil and gas mixture emitted from the pipe is derived from a very deep subsurface reservoir and the pressure/temperature field of the fluid is dramatically altered as it exists the riser pipe and enters the deep water. Previous studies of deepwater blowout events predicted and illustrated that a substantial fraction of the released oil and gas would become suspended in diffuse pelagic plumes.
Mid-water oil may derive from coagulation and settling of oil from surface waters or from slowly rising deepwater plumes. Satellite sea surface imaging has clearly illustrated how difficult it is to understand the movement of oil is in a two-dimensional setting.
Mapping and modeling movement of deep and mid-water plumes will be even more challenging. The fate of oil in the deepwater is likely to be very different from that of surface oil because some processes that occur on the • surface do not occur at depth. Most importantly, photooxidation and evaporative loss are important terms of oil breakdown (former) and removal (latter) in surface slicks. Photo oxidative processes transform crude oil into compounds that may or may not be susceptible to subsequent microbial oxidation.
Neither of these processes is important in deepwater, leaving microbially mediated oxidation and perhaps sedimentation along the seabed as the primary fate of the oil. For deepwater methane, the primary fate is likely microbial oxidation whereas both microbial oxidation and evasion to the atmosphere occur close to the surface.
Oxygen depletion in deepwater is a significant concern because deepwater oxygen is not replenished in situ by photosynthesis, as is the case for surface waters; rather, it is replenished by physical processes.
While surface water hypoxia/anoxia might be short-lived, deepwater hypoxia/anoxia could persist for years if decades. Hypoxia or anoxia would have multiple impacts on the deepwater system, including changes in microbial community composition and the associated processes they mediate, exclusion of oxygen-requiring fauna (e.g. zooplankton, gelatinous zooplankton, fish, squid, whales, etc.) and altered nutrient cycles.
For example, if the deepwater becomes anoxic, microbial respiration could switch to sulfate reduction, raising the possibility for generation of substantial volumes of anoxic, sulfidic water deep in the Gulf of Mexico.
Furthermore, if such anoxic waters were to intersect with sediments or be pushed into the coastal zone, the impacts could be severe and widespread. Coupled to the deepwater pelagic system is the benthic ecosystem. The seafloor in the vicinity of natural oil and gas seeps is home to diverse chemosynthetic ecosystems and colonies of cold water corals. Although these organisms can tolerate reduced oxygen concentrations and hydrocarbons, the impacts of the BP blowout will challenge the tolerance of sessile communities beyond any previous insult.
