A deep dive on seagrass monitoring in the Cockburn Sound

If you head south from the mouth of Perth’s Swan River towards Rockingham, you’ll be traversing the waters of the Cockburn Sound.

Flanked by the mainland to the east and Garden Island to the west along much of its length, the Sound is home to extensive seagrass meadows—an ecological community that our Marine Science team is working hard to understand, monitor and protect.

Here, our Marine team shares their insights from work in the underwater gardens of southern Western Australia (WA).

Tell us a little bit about seagrasses!

How do they differ from seaweeds, and what environmental conditions do they need to grow?

Seagrasses are marine flowering plants that have roots, shoots and leaves – similar to the grass in your backyard. They reproduce via pollination – tiny flowers and fruits are transported by ocean currents to sand or mud where they can grow. As a plant that photosynthesises, seagrasses require light to grow and so are generally found in shallow marine waters.

Seaweeds on the other hand are actually algae and include things like giant kelp forests and ‘sea lettuce’ that can be anchored to the sea floor, or float through the ocean in a raft.

Seagrass

Why are seagrasses so important ecologically?

What types of marine diversity do they support and what role do they play in our fight against climate change?

Seagrasses form extensive and highly productive meadows in coastal marine ecosystems. Seagrass meadows act as ecological engineers, providing essential services by stabilising sediment, nutrient cycling and primary production, and they provide food and habitat for other organisms.

Seagrasses have been recorded to capture carbon up to 35 times faster than tropical rainforests and currently absorb 10 percent of carbon in the world’s oceans – this is what we call ‘blue carbon’.

This means that preserving and restoring seagrass meadows is essential in the fight against climate change. We need to ensure that conditions (water temperatures, clarity and acidification, for example) are optimal for seagrasses to thrive, and that removal or damage to seagrass meadows via anchoring and development is reduced.

Tell us about the Long-Term Monitoring of Seagrass Communities initiative

RPS has been working with the Cockburn Sound Management Council on the Long-Term Monitoring of Seagrass Communities initiative. When did this program start and why is it important?

Monitoring of seagrass meadows has been an important part of the Cockburn Sound management strategy since 1990s, aiming to identify long-term trends in seagrass cover. This was sparked by disturbances leading to 77% of seagrass habitat being lost between 1967 and 1999.

Despite improvements in water quality within Cockburn Sound there has been no signs of large-scale recovery of seagrasses. Understanding trends in the seagrass cover is important as it will allow for informed decisions on the environmental management of Cockburn Sound.

RPS marine scientist aboard a boat with clip board and hi-vis workwear while completing seagrass monitoring in the Cockburn Sound, between Perth and Rockingham in Western Australia.

What methods do you use when completing field monitoring surveys?

How do you complete the measurements, and what are you looking for?

The seagrass measurements required for the Cockburn Sound survey include shoot density counts, shoot height measurements, estimates of percentage cover and lower depth limit. These data give us an indication of how much of each species of seagrass is present and how healthy it is, which can be compared to historical data to see trends over time.

At each survey location a team of divers would locate pickets that indicate the start of fixed transects (areas that are surveyed each time so we can monitor changes). The divers would swim along each transect and count the number of individual shoots within several 20x20 cm quadrats.

Seagrass shoot height is measured as both maximum and mean shoot height. Maximum shoot height is the height of the tallest shoot within the quadrat while mean shoot height is the height at which 80% of shoots reach.

Percentage cover is a visual estimate of how much space within a quadrat the seagrass occupies. If seagrass fills approximately half of the quadrat, the percentage cover would be 50%.

Lowest depth limit is the maximum depth where light intensity allows for photosynthesis to exceed respiration. This is measured by divers swimming along a transect down a depth gradient observing 1 m each side of the transect. The depth at which there is no longer any seagrass 1 m either side of the transect was the lowest depth limit, at this point divers use their dive computer to record the depth.  

Ever come across anything surprising when working in the field?

Garnet Hooper

"A WWII mine caught in a trawl survey. The Navy divers and bomb disposal team had to come and do a controlled detonation on it."

Garnet Hooper, Principal Marine Consultant

Garnet Hooper

Rebecca Watson

"Our team was lucky enough to have an encounter with three curious humpback whales on a recent survey. They played around the boat in the glassed-off waters for two hours."

Rebecca Watson, Senior Marine Environmental Consultant

Rebecca Watson, Senior Marine Environmental Consultant, Perth Western Australia

Mike Mackie

"I once had a heart attack when a large grey shape loomed out of the murky water of Mangrove Bay, Ningaloo, when I was head down counting fish. Shark? No! Curious dugong..."

Mike Mackie, Principal Scientist

Mike Mackie, Principal Scientist, Perth Western Australia

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