Seagrass forms an incredibly important ecosystem worldwide, providing a wide range of functions. However, as with many other lifeforms, their health has been threatened by human advancements.

Significant declines of seagrass meadows extent (up to a third of meadows along the Adelaide Metropolitan Coastline) leading up to the early 2000s, has been due to poor waste- and storm-water quality. Significant work has been done to improve water quality being released into the coast, showing significant regrowth over the past 20 years. Assessing the effectiveness of seagrass regeneration requires fast and accurate feedback data. This has been significantly enhanced with the use of up-and-coming monitoring techniques.

Seagrass Restoration Information Video (SA Department of Environment and Water)

Seagrass and Wrack – Why Is It Important and What Is Affecting It?

Seagrass meadows provide many ecological benefits. Locally, their roots form a network within the sediment to stabilise the ocean floor, and extensive meadows filled with food and shelter form a nursery habitat for many species. Globally, seagrass can draw carbon out of the atmosphere and store it in their leaves (similar to trees storing carbon).

Natural shedding of dead seagrass leaves and roots produces a substance called wrack (the combination of dead seagrass leaves, and other dead matter from the coast (Figure 1)). This forms in seasonal cycles, from annual changes in temperature and light.

Significant decline in seagrass meadows and changes to wrack cycles have been observed over the past 50+ years, which is likely due to human induced factors. Rising ocean temperatures affect seagrass’ ability to photosynthesise, which reduces growth rates in seagrass meadows. Poor wastewater quality causes small algae living on seagrass leaves to thrive, which prevents seagrass leaves from getting sunlight. These human induced factors likely enhance wrack production. Currently, there is no way to measure or track how much wrack is present, meaning there is not much known about the natural or enhanced wrack production.

Where improvements in water quality have occurred, substantial seagrass regrowth has been noticed, however, monitoring wrack dynamics in these studies would allow a refined understanding in these dynamics.  

How Can Wrack Be Monitored?

There are currently many techniques being used for monitoring seagrass in Adelaide, however these techniques cannot assess wrack at the scale needed. Satellite imagery cannot detect fine scale changes in meadows and manual dives and underwater videos don’t provide enough detail to make generalised descriptions of the whole study site. Wrack dynamics needs to be measured over a relatively large study site to determine changes occurring, but still require fine scale detail as seagrass changes occur slowly. Seagrass meadows are a highly dynamic system, therefore frequently collected data is also required to monitor these dynamic changes throughout changing seasons. Fortunately, there are emerging techniques to assess wrack when monitoring seagrass meadows. Drones can be used with high quality cameras to image large areas with very high detail (Figure 2). Collecting images to monitor an area gives us more information than what we could collect on the ground and takes a fraction of the time. Another novel technique uses a range of sensors on drones to build high detail imagery. This technique digitally reduces the effects of underwater features that interfere with seeing the seafloor. Capturing images through water can often result in darkened and murky images. The technique enhances the seafloor features that are of interest in the study and mitigates the darkness and murkiness.

  • Figure 2. Imagery capture using high powered drone

Different colours in the image are used to identify what the cover type is, whether it is living seagrass, patches of seagrass wrack or just sand (Figure 3). Having improved the quality of the imagery, separating the different cover types based on colour can be much easier. As images are collected throughout a year, changes to where wrack coloured cover is present can give a representation of where wrack is moving in its natural cycle.

  • Figure 3. Drone imagery showing different cover types

Why Do We Need to Monitor Wrack?

There is currently very little known about seagrass wrack cycles and its movement patterns, as currently techniques don’t measure sites with enough detail or frequently. By developing a new technique, questions about wrack production and dynamic patterns can be answered. Monitoring wrack frequently over seasonal periods can improve understanding about the natural cycle of wrack production. Vast changes from the natural cycle can be investigated as being caused by human induced factors, providing an early indication of poor health in a system. Management efforts to improving water quality will also benefit from wrack monitoring as it provides a more timely form of information to be used as feedback for management teams to make decisions. Monitoring is a key step in the process to re-establish these highly beneficial ecosystems back to a naturally healthy state.