The Many Pathways (and Impacts) of the Oil - from Surface to Seafloor


 Historically, oil spill response efforts (and research) have focused on impacts to wildlife and coastal ecosystems.  This focus is understandable because the surface destruction is visible, and our reaction visceral. 

 Also, we’ve learned from previous oil spills that impacts to our shoreline can be devastating to a diversity of animals, for long periods of time.  This is especially so for salt marshes, which can act like sponges, soaking up the oil and sequestering it in its toxic form for decades.  Long-term contamination of marshes on the Gulf Coast - which comprise 60% of the coastal marshes in the United States - could cause substantial ecological and economic harm, as these wetlands serve as nurseries for commercially important fisheries such as shrimp, oysters, and crab.

 Protecting the coast is the rationale behind the heavy use of dispersants in this disaster.  Currently, the reigning response strategy is to do everything possible to keep the oil off of coastlines.  

 But, it’s important to keep in mind that the oil that hits the shoreline is what well-respected marine ecologist Jeremy Jackson calls, the ‘ring around the bathtub.’ This is the oil that we can see.  And, in the case of Gulf oil disaster, it is only a fraction of the more than 18 million gallons of oil that have flooded into the Gulf of Mexico.  The majority of it remains dissolved or dispersed throughout the Gulf’s waters – largely out of our sight.

May 19 - Thick oil pools in the waters near Pass a Loutre, on Louisiana’s coast. Credit: Office of Gov. Bobby Jindal

 So what are the impacts of this offshore contamination?  Scientists know surprisingly little about these impacts because they are grossly under-studied. 

 But we do know that oil is toxic and that it will harm just about everything it encounters before it degrades (via chemical oxidation or microbial degradation).  Ultimately, the damage done will depend on how large an area it contaminates, how dense it is, and where it goes.

 There are numerous scenarios for each of the oil’s various physical states and pathways:


Floating Oil

 Oil slicks at sea cause serious harm to animals that need to spend a portion of their time at the surface to breath, feed or rest. This includes animals like birds, turtles, and mammals that will not, or cannot, avoid oiled water.

 When air-breathing species such as whales, turtles, and dolphins come to the surface, they may inhale toxic volatile chemicals, leading to respiratory irritation and absorption of the toxins which can cause organ damage, reproductive failure, or death.  If these animals die, they will likely sink offshore and many casualties will remain uncounted.


May 6 - Pod of bottlenose dolphins - Chaneleur Sound, LA. Photo credit: Alex Brandon, AP


Dissolved oil and chemically dispersed oil in the upper water column

 In the Gulf spill, natural mixing, and the extensive use of dispersants, caused much of the floating oil to dissolve or be dispersed into the upper layer of water below the surface, where life is concentrated in the ocean. 

 This is where sunlight penetrates and plants grow.  Invertebrates like copepods graze on the plants and small forage fish eat the invertebrates.  This chain of life could prove highly susceptible to the chemically dispersed oil.  What is not wiped out by acute short-term exposure will be contaminated by toxic hydrocarbons.  These toxins will then be transferred up the food chain, with cascading impacts to top predators, such as dolphins, seabirds, and large fish.  Past research shows that exposure to hydrocarbons through the food chain can result in reduced survivorship, reproductive failure, and reduced reproductive capacity.

 Scientists are especially worried about the eggs and larvae that have just arrived with spring and that float in this layer.  Many have no protective covering and are essentially permeable, making them especially sensitive to toxins.  Fisheries experts predict significant death rates for these juveniles, attrition that could translate to future reduced catch levels of the adults into which they grow.




Splash of seawater.  David Liittschwager / National Geographic Stock

sword fish.JPG

Larval swordfish.  David Liittschwager / National Geographic Stock

 Of particular concern is the impact to the beleaguered western Atlantic blue fin tuna, which uses the northern Gulf of Mexico as their only breeding grounds.


Dissolved and dispersed oil in deep water

 Being a deep water spill, the Gulf disaster is exceptional.  For example, the bulk of the oil remains below the surface.  The documented miles-long plumes of sub-surface oil could contaminate any organisms that live and travel through the deep and mid-water.  This includes hundreds of fish species, invertebrates such as squid, and marine mammals such as the Gulf’s resident sperm whales.



Whale shark up close.  Photo credit: shutterfly.


Dispersed oil and oil-sedimentation on the bottom

 If these plumes of oil, which are said to be as deep as 3,000 ft, contact the bottom, or collide with the continental shelf, they could smother and contaminate the rich bottom habitat of the Gulf, including fields of sponges, sea fans, and coral reefs.  The Gulf of Mexico is home to prolific deep water coral reefs.  And, corals are particularly sensitive to chemically dispersed oil. 

 Oil can also be adsorbed by sediment in the water column causing it to sink and accumulate on the sea floor.  Such contamination could cause additional problems for commercial fishing because many fish, such as snapper and grouper, reside in coral reefs on the floor of the Gulf.  Even closer to shore is essential habitat for the important recreational fish like red drum, which develop in seagrass beds near the coast for three years before emigrating offshore as adults.


Coral and brittle stars on the floor of the Gulf credit: G.P. Schmahl, Flower Garden Banks National Marine Sanctuary


When will we know the scope of the harm?

 Currently, we have been forced to speculate about impacts to the Gulf’s diverse ecosystems.  This will need to change.  Outstanding questions – such as the toxicity of the chemically dispersed oil – need to be answered. 

 According to the Oil Pollution Act of 1990, the federal government is responsible for overseeing a natural resource damage assessment (NRDA) of this disaster.  To properly assess the damage, the oil needs to be quantified, tracked, and its impacts evaluated.  Some of these factors are ephemeral and require immediate observation. 


Essential Fish Habitat for Red Drum in the Gulf of Mexico (boundary delineatoins from National Marine Fisheries Service).  Matthew McKinzie, NRDC

 Given that this disaster has been unfolding for over a month, the necessary data to begin understanding the types and scope of the damage should be available.  The lack of available open ocean data is disconcerting.  The federal government should provide the Natural Resource Damage Assessment plan for public comments (as it has with BP) and post incoming data on a central website for the public to see – so we know which paths the oil is taking, which wildlife its harming, and the full scope of its impact.