Studying mudflats and intertidal biofilms using remote-sensing technologies

Tristan Douglas

Each fall and spring, millions of birds migrate to their breeding grounds via the Pacific Flyway — the westernmost of the four major North American bird migration routes stretching from Central and South America to Siberia and Alaska. Migration over thousands of kilometers is energetically demanding for birds, who stop to refuel at the estuarine ecosystems of major rivers. Abundant, predictable, and high-quality food resources at stopover estuaries are thus critical to the survival of many migratory bird species. Within estuaries, photosynthesizing benthic algae (mainly diatoms¹) and cyanobacteria (also known as “blue-green algae”), collectively form microphytobenthos, which can contribute up to 50% of the total primary production, representing a central component of the coastal food chain. Microphytobenthos secrete “sticky” extracellular polymeric substances that form aggregations with other microbes, mud-dwelling meiofauna,² and organic matter, which develop into “biofilms” on the surface of the seafloor.

These nutritious, labile biofilms are consumed by a range of taxa in estuarine food webs, from invertebrates to fish to birds. Indeed, biofilm is consumed directly or indirectly by at least 21 species of shorebirds that include sandpipers, shanks, and plovers, accounting for up to 68% of the daily energetic requirements of some migrating shorebird species. Specifically, it is the fatty acids produced by microphytobenthos within biofilms that are known to be a primary fuel for high endurance migration in birds. Birds assimilate fatty acids by grazing directly on surficial biofilm or by consuming meiofauna that have fed on biofilms or planktonic algae. In recent decades, a much greater understanding has developed about the importance of intertidal biofilms, especially as a high-energy food source for migratory birds. This has elicited calls for conservation strategies to preserve biofilms in estuaries and for further research on the specific processes underlying fatty acid production by the microphytobenthos.

Estuaries along the Pacific Flyway, including the Fraser River Estuary, are subjected to increasing pressures from human activity. Land transformations, urban development, eutrophication, pollution, disturbances associated with recreational and industrial marine traffic, and many other disturbances to ecologically important estuaries have contributed to a ~29% net loss of North American avifauna abundances—approximately 3 million birds—since the 1970s, along with steep declines in migratory shorebird species. The proliferation of artificial structures into intertidal and shallow subtidal areas often replaces natural habitats and can dramatically shift ecological structure, altering water turbidity, light availability, and sedimentation patterns in areas important for shorebird foraging. The impact of such habitat disturbances on food availability for biofilm-consuming shorebirds is significant but still understudied and likely site-specific. Within an estuary, biofilms are often patchily distributed, and their biomass and production of fatty acids are influenced by a complex interaction of environmental variables such as mudflat elevation and sunlight exposure times, salinity, proximity to shore and to freshwater discharge, and many other time-varying factors. In order to determine the viability of an estuary for shorebird foraging, microphytobenthos distributions need to be monitored frequently and accurately, taking into account changing mudflat morphology and many environmental variables.

In recent decades, remote-sensing methods that utilize satellite- or aircraft-acquired images have emerged as powerful tools for studying many facets of Earth's biosphere, and they have great potential for mapping, monitoring, and studying microphytobenthos within biofilm in estuaries. Estimations of chlorophyll-a content can be derived from multispectral and hyperspectral imagery containing wavelength ranges beyond the visible range of the electromagnetic spectrum. Since chlorophyll-a is used as a proxy for microphytobenthos biomass, it can be quantified and used to map microphytobenthos at a variety of spatial and temporal scales, from millimeters to kilometers. Many satellite missions collect multispectral and hyperspectral imagery with high spatial resolution and high acquisition frequency, much of which is free and open access. We can use these data to investigate decades of historical spring biofilm blooms and shorebird migration seasons in estuaries along the Pacific Flyway.

In addition, drones—formally referred to as “unoccupied aerial vehicles, or “remotely piloted aircraft systems”—can be equipped with similar multispectral cameras, able to detect chlorophyll content in surface sediments and used to detect extremely fine detail microphytobenthos patterning. Drone-acquired imagery can also be processed in such a way as to generate centimeter-scale 3D models of mudflat topography, offering great potential to investigate the interactions of biofilms and shorebirds with their physical environment.

Amidst surging human activity and steep declines in migratory shorebirds along the Pacific Flyway, new tools and strategies are needed to help monitor the health of intertidal ecosystems and survival success of the species that rely on intertidal biofilms. My research is aimed at resolving the nuanced, time-dependent, dynamic nature of intertidal biofilm distribution and abundance in order to make this available to policymakers; it will inform conservation strategies and reduce the current declines in migratory shorebirds and ecosystem integrity. Remote-sensing technologies offer great potential in this context, with an ability to help bridge gaps in our current understanding of the relationship between biofilms, shorebirds, estuaries, and humans.

¹ Diatoms are “algae that live in houses made of glass. They are the only organism on the planet with cell walls composed of transparent, opaline silica. Diatom cell walls are ornamented by intricate and striking patterns of silica”

² “Marine meiofauna are typically smaller than 1 millimeter (0.04 inches) and larger than 32 micrometers (32/1000 of a millimeter). These animals encompass a wonderfully diverse and very important, yet often overlooked part of marine ecosystems.” (https://oceanexplorer.noaa.gov/facts/marine-meiofauna.html).