Image 1: Foraging shorebirds at the study site, predominantly dunlin Calidris alpina (James Booty)
James Booty, University of East Anglia, reports on recently published research into top-down effects of shorebirds. Read the full paper here.
You might be familiar with the sight and sound of thousands of shorebirds on a mudflat, but have you considered what lies beneath their feet? In Essex, at Fingringhoe Wick on the Colne Estuary, researchers from the Universities of East Anglia and Essex have been studying how shorebirds affect ecosystem functioning with the finding that shorebirds do so, significantly, by changing the microalgal mass at the sediment surface.
Climate change, driven by increased atmospheric CO2, is one of the largest challenges faced by our planet, with huge consequences for our way of life as well as other species and the ecosystems which we all depend upon. Mudflats help mitigate atmospheric levels of CO2 by storing carbon and keeping it out of the atmosphere. Microalgae at the surface of mudflats aid transport of organic compounds, such as carbon and other nutrients, into the sediment, where they are broken down and recycled. Recycling of these nutrients is an ‘ecosystem function’, performed by microalgae, which provides an ‘ecosystem service’ (carbon storage) to humankind. We can measure these recycling rates by the ‘nutrient fluxes’ in and out of the mud, which tell us how much is taken up or released by the mud. Understanding the interactions between these microalgae and other organisms, and how these interactions affect ‘ecosystem functioning’, is very important if we are to mitigate climate change impacts. For example, on tidal salt marshes grazing cattle are called ‘ecosystem engineers’ because they increase soil compaction, changing the abiotic properties of the soil and enhancing carbon uptake and storage.
Intertidal biofilms are thin layers of photosynethetic algae, microbes, bacteria and very small invertebrates within the surface of the mudflat. They are bound together by glue-like secretions, and perform functions such as transporting nutrients in and out of the mud, physically stabilising the mudflat surface and providing food for invertebrates and birds. Despite being relatively unknown, they have long attracted attention from both scientists and artists. For some fantastic artwork and information on diatoms, see the Smithsonian magazine here.
Image 2: Diatoms (Prof. Graham Underwood)
Besides providing the base of the food chain, biofilms also perform other ecosystem functions including:
mediating ‘nutrient fluxes’
reducing the erodibility of the mudflat
stabilising the mudflat ‘buffer’ between land and sea, reducing wave energy before it reaches salt marsh or the sea wall.
The finding that some waders directly consume biofilm has widened the research field, with revised food webs and the potential for biofilm distribution to aid in shorebird distribution prediction. Here, we wanted to investigate 1) whether shorebirds affect biofilms, and if so, 2) how does the impact that shorebirds have on biofilms influence the ecosystem functions described above? In a wider context, the climate and biodiversity loss crises are receiving more global attention now than ever, as time is running out to reverse, or even mitigate, our impact on the planet. How do shorebirds fit into to this?
Our Experiment
Image 3: Dunlin (Calidris alpina) foraging around bird exclosures (James Booty)
To explore the effect of shorebirds on the properties and functioning of the mud, we needed to get up close and personal with this habitat. We undertook an exclusion experiment, which prevented shorebirds from accessing plots on the upper shore of Geedon mudflat. This enabled us to compare control areas where shorebirds walked and foraged as usual with exclusion plots where they did not.
Working on the mud is messy, frustrating and tiring. We used sledges to move equipment across the mud, pulling them behind us as we waded through the mud to reach the sampling plots. We repeated the same routes to minimise disturbance, but travelling the same route made subsequent trips more difficult due to increased churning of the mud.
Low points included:
sinking up to my waist in stinking mud while trying to work out why expensive gear had stopped working
abandoning sampling days by scrambling out of the mud in un-forecast heavy rain
trying to wash myself and gear down using the reserve hosepipe in temperatures barely above freezing.
High points included:
watching Peregrine Falcons, Marsh Harriers and Barn Owls hunting over the mudflat and saltmarsh while sampling
encountering foraging Dunlin only 20m away across the mud
working to the constant song of Skylarks above the saltmarsh
spotting Curlew Sandpipers during bird surveys
coffee and cake with invaluable voluntary field assistants in the visitor centre after wash down.
Biofilm mass and sediment erodibility were measured by taking samples from the surface of the mudflat. To measure nutrient fluxes, cores (see image below) were collected and submersed in carefully controlled tanks of seawater from the field site, maintaining the natural salinity, light levels, and temperature of the estuary. Water samples were taken from them at two-hourly intervals and their nutrient content analysed.
Our Results
Image 3: Flux cores in and out of the controlled mesocosms
We found that biofilm mass at the mudflat surface was significantly smaller in the areas where shorebirds were present compared to where the birds were absent. This change may have been due to direct consumption of the biofilm by the birds or as yet unknown mechanisms resulting from physical disturbance. The decrease in biofilm mass is likely led to ‘knock-on’ effects on ecosystem functioning. We found greater sediment erodibility and altered nutrient fluxes in the presence of shorebirds compared to absence.
The graphic below shows that dissolved organic carbon (DOC) is being taken into the mud where shorebirds are present and released into the atmosphere where they are absent.
Fig 1: Measured nutrient fluxes in and out of the sediment in shorebird presence and absence (DOC=Dissolved Organic Carbon)
Importance of Findings
This research has extended our knowledge of shorebird–biofilm interactions, suggesting that shorebird activity can significantly affect ecosystem functioning. This finding may mark the emergence of a research area where quantifiable ecosystem services are provided by shorebirds. With so many shorebirds on the brink of national or even global extinctions, identifying these benefits of their conservation has never been more important.
Our finding that the presence of shorebirds increases uptake of dissolved organic carbon (DOC) into coastal sediments gives clear additional human benefit to shorebird conservation. Tackling long-term climate change and resulting effects around the globe is an overwhelming challenge which is likely to continue for centuries to come. Our work here has identified that continued decline or extinction of overwintering shorebirds could increase CO2 emissions from mudflats, making the positive feedbacks of CO2 even more difficult to mitigate.
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