Under a powerful microscope, the edges of a Western Sandpiper’s tongue look more like an old shag carpet, or the head of a toothbrush that’s been used far too long. Their tongues are covered in tiny keratinous bristles that they use to slurp up biofilm – a superfood slime on the surface of tidal mudflats. Biofilm is made up of bacteria and single-celled plants called diatoms that – in the right conditions –photosynthesize to produce carbohydrates and omega-3 fatty acids. As the tide recedes, it reveals a shimmering slimy layer of biofilm – a substance scientists have only begun to study in the past 25 years.
Detail of edge and tip of Western Sandpiper tongue.
Elner, R.W., P.G. Beninger, D.L. Jackson and T.M. Potter. 2004. Evidence of a new feeding mode in western sandpiper (Calidris mauri) and dunlin (Calidris alpina) based on bill and tongue morphology and ultrastructure. Marine Biology 146: 1223 – 1234.
Upon first realization that migratory shorebirds might be eating biofilm, it was assumed that they consumed it as a byproduct of probing for their preferred diet of marine invertebrates. We now know that not only are they eating it on purpose, but different species – even different subsets of one population – might have different physical adaptations to access the best biofilm at their specific stopover sites.
When Western Sandpipers (Calidris mauri) make it to the mudflats of Roberts Bank each spring, it’s one of their last stops before returning to their breeding grounds in western Alaska. Roberts Bank is in the center of the Fraser River Estuary, a WHSRN Site of Hemispheric Importance in Vancouver, British Columbia. It is an important stopover for many species of migratory shorebirds, but for Western Sandpipers, it’s crucial. More than 95% of all Western Sandpipers stop at Roberts Bank to rest and refuel before the last leg of their long journey — a non-stop 1,000km flight. In all likelihood, every member of the entire species will use the Fraser River Estuary during some phase of their life.
Dr. Bob Elner has been studying Western Sandpipers and biofilm at Roberts Bank for 25 years. By examining sandpiper tongues and bill structures under electron microscopes, cataloging the contents of their stomachs, and recording extremely slow-motion footage of foraging behavior, Elner and his colleagues have put together the puzzle pieces that have begun to unveil these shorebirds’ reliance on biofilm.
His decades of observational data also uncover an important understanding: in the 8,000 hectares of mudflats at Roberts Bank, Western Sandpipers flock primarily to a condensed area in the upper intertidal zone, just 300 meters from the shore. This is where the biofilm is thickest, most nutritious, and exposed for longest, and it’s right at the mouth of the Fraser River, where fresh water mixes with sea water to create just the right level of salinity for fatty-acid-producing diatoms.
“The birds are very selective of how they’re feeding on biofilm,” said Elner. “Not all mudflats are equal.”
Western Sandpipers. Photo: Kim Stark.
This delicate balance is at risk of being disrupted at Roberts Bank. Not only is the site adjacent to the huge urban center of Vancouver, but it’s at the heart of a major port. The existing port was originally built in 1968 — before an environmental review was required – and has been added to several times since. But the Vancouver Fraser Port Authority is now proposing its biggest expansion yet.
The Roberts Bank Terminal 2 project, or RBT2, is currently engulfed in Canada’s environmental assessment process. The 7,000 page Environmental Impact Statement (EIS) that the Port Authority submitted in 2015 did make minor mention of the importance of biofilm, but without distinguishing the difference between biofilm produced by fresh water diatoms versus marine diatoms. The expansion will double the size of the existing artificial island and causeway, which already holds Canada’s largest shipping container facility. Because the proposed terminal would alter the flow of salt and freshwater across the mud flats the project could actually make more biofilm, by creating a new freshwater tidal basin.
But will Western Sandpipers eat biofilm produced by fresher water? Will they be able to access it further out on the bank? Will it contain the same essential nutrients as the biofilm they’re eating now?
Elner doesn’t think so, and cautions that we don’t know enough yet to predict how the port expansion will change this ecosystem.
Artist rendering of the expanded Roberts Bank Terminal. The island on the left is the proposed Terminal 2 project. The island in the center is the existing facility, that already includes a coal terminal and a container terminal. Elner has found that the Western Sandpipers flock to a condensed area where the biofilm is thickest, most nutritious, and exposed for longest. This area sits between the proposed new terminal and the shore. Image courtesy of Government of Canada, Asia-Pacific Gateway and Corridor Initiative, via http://www.robertsbankterminal2.com/news-information/video-images/
Other scientists studying biofilm at key WHSRN stopover sites echo this precautionary principle. Dr. Diana Hamilton studies sandpiper diet and feeding behavior in the Bay of Fundy, focused on a close relative of Western Sandpipers, the Semipalmated Sandpiper (Calidris pusilla). Not only do Semipalmated Sandpipers also have hairy tongues to help hoover film from the flats, but Hamilton’s team has conducted isotope analyses that consistently show biofilm as a significant part of these sandpipers’ diet. It’s the long-chained fatty acids they’re after – the critical component that, in Elner’s words, are “undoubtedly required as a biochemical signaling agent that leads to the physiological changes needed for long-distance migration.”
Hamilton adds that “sandpipers can’t synthesize these fatty acids themselves – they must get them through diet.” Marine diatoms are the ultimate source of these fatty acids, whether consumed directly or after incorporation into other foods. Past studies have documented a Semipalmated Sandpiper diet dominated by Corophium volutator, an amphipod rich in polyunsaturated fatty acids. As Corophium populations have declined, the birds have filled the void by increasing their consumption of polychaete worms. Hamilton and her team, though, have shown that the fatty acid makeup of biofilm is similar to Corophium and polychaete worms. If consumed in large volume, biofilm could provide sufficient quantities of these essential fatty acids to prepare Semipalmated Sandpipers for migration. But – even though the Bay of Fundy is blanketed in biofilm, Hamilton has found that the sandpipers consistently eat more of it in one arm of the Bay than the other. In Shepody Bay, the biofilm contains a higher proportion of polyunsaturated fatty acids, and makes up a more significant part of the sandpipers’ diet. In Minas Basin, the biofilm contains more of other kinds of less valuable fatty acids, and the sandpipers are eating less biofilm and more polychaete worms. Is this a choice? Hamilton’s research seeks to find the answer. “Can birds distinguish the difference [between different types of biofilm]?”
Biofilm on San Francisco Bay, another crucial stop on the Pacific Flyway for Western Sandpipers. Photo: Monica Iglecia.
Back in Vancouver, Western Sandpipers will soon be migrating north, flocking once again to the mudflats of Roberts Bank. Half a million Western Sandpipers have been observed at Roberts Bank in a single day; their global population is estimated at 3.5 million, almost all of which stop here at some point along their annual journey.
An independent review panel acting on behalf of the Canadian Environmental Assessment Agency (CEAA) is currently collecting evidence on the potential impacts of the proposed Roberts Bank Terminal 2 project. Once the panel has decided it has enough evidence to determine the potential impacts, the review panel will invite participation in public hearings. Just this February, Environment and Climate Change Canada (ECCC) submitted a response to the panel “that there is insufficient, science-based information to support the Proponent’s finding that the Project would not adversely impact intertidal biofilm and consequently, migratory shorebirds in general, and the Western Sandpiper species in particular.” ECCC further deemed that the impact of the expansion on Western Sandpipers is “potentially high in magnitude, permanent, irreversible, and continuous,” but the fate of the RBT2 proposal is still up in the air – as is the future for Western Sandpipers.
We are only beginning to learn the complexities of biofilm, and the intricacies of its importance to migratory shorebirds. But the port decision at Roberts Bank could have species-level consequences for the Western Sandpiper. When it comes to an ecosystem rich in the right kind of biofilm, we simply don’t yet know enough to make an irreversible change.
For more information, please contact Dr. Bob Elner at rwelner@sfu.ca or David Bradley, Bird Studies Canada, at dbradley@bsc-eoc.org
UPDATE, 27 September 2018: The environmental review process for the Robert Banks Terminal 2 project is currently open to public comment. Decision-makers must decide if the potential impact is acceptable. Comments can be submitted to Panel.RBT2@ceaa.gc.ca before next Friday October 5. Click here to learn more.
