Tracing the effects of oil in the open ocean

Oil spills not only cause problems for the shores they wash up on but also impact open ocean ecosystems. Amy Hirons invites us into the planktonic world and tells us about her...
08 July 2010

Interview with 

Amy Hirons, NOVA Southeastern University

Sarah -   Oil spills don't just affect the shorelines that they wash up on.  It can also cause important changes out at sea.  I spoke with Amy Hirons from Nova Southeastern University in Florida to find out about the effects of oil in the pelagic realm.

Amy -   Well I'm a Biological Oceanographer and my interests really stem with looking at how ocean production or food is transferred from the smallest organisms, the phytoplanktons, the plants, and the zooplankton, the animals, and how that's passed through the food web in the ocean all the way to apex predators - things like sharks and whales and dolphins.  The method that I used is utilizing stable isotope ratios.  A lot of people understand, or at least have heard of, radio isotopes which naturally decay over time, such as carbon-14 which can be used for dating.  Diatoms - a key Phytoplankton groupA stable isotope is somewhat similar except that it doesn't decay and so it's used as a natural tracer.  There are stable isotopes of many elements such as carbon and nitrogen.  So I can actually take a piece of tissue or bone or sediment and combust the sample.  I use a mass spectrometer then to look at the ratio of heavier and lighter isotopes such as carbon or nitrogen.  That allows me to look at how carbon and nitrogen, which are composed in our food, is then passed through a food web.  That provides this information on many different levels.  One, basically, [with nitrogen] who's eating whom?  Carbon can actually give us information about geographic location.  If an organism is found in the near shore in the estuarine environment versus offshore in the pelagic, or whether it's pelagic in the open water versus benthic on the ocean floor.

Sarah -   So, by tracing the carbon isotopes, you're effectively tracing the impacts of oil spills through the food chain.

Amy -   Yes and by working with environmental chemists who can look at various fractions of the petroleum product itself and then comparing that to the same kind of components of cells of fish and invertebrates and plankton, we can determine if certain components of petroleum are truly being incorporated physiologically in an organism and then being passed on through the food web.  Then we can follow any kind of detrimental impact, certainly looking at an organism directly and at cellular structures, but also by looking at population numbers and community structures.  Previous work that I did after the Exxon Valdez oil spill in Prince William Sound, Alaska, indicated that, for many different organisms, there were different impacts.  Certainly, there was the external: the direct impacts of reduced primary production because the phytoplankton wasn't able to stay in the water column and photosynthesise.  But then there were fish that developed ulcers and tumours.  There were decreased numbers of populations of various organisms such as steller sea lions and harbour seals that we began to be concerned that this carbon and other elements or other components of petroleum products were actually being incorporated and damaging the cells of these organisms.  Our major concern right now is that we don't have a lot of information about some of the detrimental impacts, particularly at the base of the food web, in the phytoplankton and the zooplankton.

Sarah -   And of course, it's not just the oil that's having effects.  It's how we're dealing with the oil as well.  Is that right?

Amy -   That's correct. Many oil companies use a chemical dispersant to help break apart the oil, much like a dish detergent is added to a sink full of dirty dishes that may have oil deposits to help break apart the oil or the little globs.  So BP has been using their own chemical dispersant called Corexit and they've been releasing it at the source of the well from the Deepwater Horizon.  What most people don't know, as far as I am aware, is what impact that dispersant is actually going to have on the cellular level of different organisms.  So where we've been talking about the potential impact of petroleum and how it's traced through the food web, we don't know about the chemical dispersant and that also could be having a detrimental effect to a degree that we just don't know.  One thing we're trying to do right now is - there are many different researchers out in different platforms out in the Gulf of Mexico, all the way down into the Florida straits and in the areas of the Florida Keys, collecting water samples, collecting microscopic organisms, looking and doing counts of the larger fish and invertebrates, trying to determine: have their population numbers changed?  So, is there a detrimental impact that could be killing the organisms or damaging the organisms outright, or is it something that can also be perpetuated through the food web, through bioaccumulation in the tissues?  We don't know that and that has to be studied.

Larval fishSarah -   So what do you think, with the spill in the Gulf of Mexico, do you think we're going to be seeing the impacts in the accumulation of the oil from that in seals, sea birds, that sort of thing in years to come?

Amy -   I absolutely believe that to be the case based on what we've seen in other locations like Prince William Sound.  Another serious impact that the oil can have is not just on the phytoplankton and zooplankton, but juveniles - those organisms that have recently been hatched or spawned. The time of the oil spill from the Deepwater Horizon was also similar timing to when many organisms actually spawn.  The water column is now full of developing ichthyoplankton or baby fish that may be impacted.  We may lose year classes of fish or invertebrates.  We just don't know at this point.  It's just too early to tell.

Sarah -   I suppose you see on the news the impacts with sea birds and the large sized animals, but people don't necessarily think about the impact on the microscopic life out in the open ocean.

Amy -   Oftentimes, my concern is that people have this out-of-sight, out-of-mind feeling about oil or an oil spill.  People tend to not think about what they can't see.  That's human nature I believe.  "Oh, we've cleaned it up.  Oh, we don't see tar balls anymore.  We don't see evidence of an oil slick."  But the fact is, just because you can't see it with the naked eye doesn't mean it's not still in the environment, either the deep ocean, in the benthos, being incorporated and continually being reutilised.  And again, I've repeated it several times, they're at the base of the food web.  Without the phytoplankton and the zooplankton, you don't have anything else in the ocean and that's why it's critical for us to find out what the impact of petroleum products are having.

Find out more

Amy Hirons. 
Research Professor, Oceanographic Centre, NOVA Southeastern University  


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