A pathway for contamination

Common terns are obligate piscivores, meaning they feed almost exclusively on fish. What this also means is that they are near the top of the food chain and at high risk to contaminants that biomagnify  from one trophic level to the next. Polychlorinated Biphenyls (PCBs) are one of these biomagnifying contaminants, and to date no study has elucidated the biological pathway of PCBs from their discharge into the environment to ultimate accumulation in terns.

A recent study by M. Ward from the University of Illinois has attempted to describe this biological pathway using a population of common terns breeding within a naval station harbour on Lake Michigan, Illinois. Terns are endangered in Illinois and because PCBs can impact fitness, reduce reproductive success, and in some cases lead to mortality, it is useful to understand how they accumulate in terns.



Ward and collegues had two hypotheses: 1) Resuspension of PCB contaminated sediments or atmospheric deposition into the water column leads to assimilation by seston and detritus and transfer to small pelagic fish and then onto terns, or 2) Invasive and exotic zebra mussels filter PCBs from the water column and biodeposit a large portion into excreted feces that are later ingested by fish and in turn are preyed upon by  piscivorous birds (i.e. terns). To test these hypotheses, Ward set up two experiments.

The first consisted of eight PVC tubes suspended in the water column inside the harbour. Four tubes contained zebra mussels (treatment) and the other four contained gravel of similar size to the mussels (control). Bags were attached at the bottom of each tube and emptied every two weeks. If zebra mussels were biodepositing PCBs via feces, making them more bioavailable, the bags attached to the treatment tubes would have significantly more PCBs then control bags.

The second experiment consisted of six small fish enclosures containing small pelagic fish collected from the tern's foraging habitats and known to be consumed by the tthem. Three enclosures contained zebra mussels at the top of the cage (treatment) and the other three contained gravel (control).  A sub-sample of the fish were tested for PCB concentrations prior to the experiment. Again if zebra mussels were facilitating PCB exposure to fish, then those fish within the treatment cages would exhibit  significantly higher PCB concentrations then fish in control cages.

The results of their first experiment showed that the bags attached to control PVC tubes had more PCBs then bags from treatment tubes.  What this means is that mussel biodeposits are probably not a signficant source of bioavailable PCBs, becase the mussels appear to accumulate the contaminants
in their tissue and shells. The second experiment also showed no significant difference in PCB concentrations in fish from the treatment cages vs. the control cages, indicating again that zebra mussels do not facilitate PCB exposure to fish consumed by terns.

In addition to the above experiments, Ward and colleagues evaluated stable carbon and nitrogen isotopes in terns, forage fish, amphipods, and algae to help elucidate the trophic pathway of nutrients, which is essentially the pathway of contamination. Results from their mixing model indicated that alewives are the dominant forage fish among terns.

From the two experiments and isotope analysis, the authors suggest that the dominant pathway for PCB accumulation in terns is via seston, amphipods, and alewives. This is  a similar contaminant pathway described for many pelagic sport fish such in the Great Lakes  (i.e. lake trout and chinook salmon). This is useful information because it helps managers gauge the impact of zebra mussels on another component of the ecosystem: bioavailability of contaminants to piscivorous birds.


While I agree with their interpretation of the result, I question the necessity of both  experiments. They both proved the same thing: zebra mussel biodeposits are not a significant source of bioavailable PCBs within the higher trophic guilds. I think they could have just easily proven their point using the mixing model and either experiment.

Ward et al. also evaluated PCB content for several tern eggs. they found PCB concentrations equivalent to nearly three times what is known to cause hatching failure in eggs. They also observed chicks with lesions and chicks that were unable to develop flight feathers. And 95% of the PCB load was attributed to a single congener, Aroclor 1254- a PCB used in electrical devices, but was banned in 1950. This shows just how persistant PCBs are in the environment.

This is the last known tern colony breeding in Illinois and it would have been useful if they said something about the population dynamics so that the reader could better appreciate the effects of PCBs at the population scale. Presumably this data is available and could have easily been written into the discussion. Thankfully we can say that the arrival of the zebra mussel does not appear to be increasing contamination in higher trophic organisms.