Seagrasses are plants that are important cornerstones of our estuarine and inshore fisheries. Their leaves provide structure to otherwise featureless sandy bottoms, providing critical nursery habitat for a wide variety of fish and invertebrates through provision of food, shelter and protection from predators. Their root systems act to trap, stabilise and bind sediments, and they also oxygenate the water and the sediments they bind, reducing the buildup of toxic sulphides that otherwise form in anoxic sediments. A useful analogy is that they are the “lungs” of our estuaries and inshore environments.
It is common knowledge that seagrass habitats have undergone significant decline in the past 50-60 years. Unfortunately, this decline is predicted to continue throughout the 21st century. The first part of the decline begins with fragmentation of large continuous seagrass beds into smaller, more isolated patches, through processes such as storms or through human mediated mechanisms (e.g. boat propellers, nutrient loading).
Fragmentation does not affect fisheries as much as you’d think, because research shows that plankton and other fish foods (and therefore fish), concentrate mainly around the edges of seagrasses. Thus for a while fisheries productivity is not badly affected by early declines in seagrass, as fish remain abundant at the increased “edge areas”.
The next stage relates to catastrophic losses of seagrasses, often following flood events that bring sediments, nutrients and pollutants. Seagrasses can cope with a certain amount of sediment, and their growth can actually be increased by moderate nutrient loading. But in both cases too much of either results in mass seagrass diebacks. Like other plants, seagrasses need light for photosynthesis, but excess turbidity reduces light penetration, while nutrient loading encourages algal overgrowth that smothers seagrass. The most insidious pollutants though are the herbicides washed from agricultural and urban areas into the estuaries and bays during rainfall events. No one ever pulls weeds anymore, they spray, and there is now a significant body of scientific evidence showing levels of herbicides in inshore waters after rainfall events are high enough to cause seagrass dieback. Most herbicides work by blocking photosynthesis pathways – essentially they shade out the seagrass, resulting in its death.
The process of seagrass loss is sometimes reversed with seagrass regrowth during favourable periods, but mostly meadows never regain their original extent. Massive seagrass losses have been linked all over the world to loss of biodiversity, coastal erosion, collapse of fisheries, and loss of populations of dugongs and turtles (which eat seagrass).
Indeed, seagrasses are sensitive indicators of environmental quality, and because of this there are many seagrass monitoring and conservation programs in place around the world. Managing water quality (reducing nutrient and sediment inputs and herbicide inflows) and maintaining light availability through riparian and coastal buffer zones are critical approaches that support seagrass resilience.
In my local Moreton Bay Marine Park we have the Healthy Waterways ecosystem health monitoring program. This is a good start, but the program fails to relate its ecosystem quality ratings to something that key stakeholders such as fishers can understand – for example is the habitat with a “B” rating still suitable for seagrasses and recruitment of fish? Unfortunately, an assessment of their rating system against important yardsticks suggests the program is designed to produce overtly optimistic results. For example, high ranking estuaries such as the Noosa River (B+ rating) have issues with recruitment of sensitive native species such as Australian bass. The B rating is defined as “Good” – conditions meet all set ecosystem health values” . In reality, a B rating denotes a system under significant stress – seagrasses are stressed and fish recruitment begins to fail at this level. I know this via a recent trip to Moreton Island, where I witnessed a seagrass meadow severely clogged with algae 1 km north of the Tangalooma wrecks, in an area with a “B” rating. Their C rating is defined as “Fair” – Conditions meet some of the set ecosystem health values”. In reality, based on observations of loss of seagrasses, oysters and other bivalve molluscs (= fish food items) in Pumicestone Passage near where I live when it was rated “C”, it seems “C” indicates severe loss of ecosystem function. Pumicestone Passage just slipped down to a “D” rating this year. The D rating is classified as “Poor”, but in reality, D = Death of virtually any semblance of normal ecosystem function. This therefore suggests the Healthy Waterways F (= Fail) is completely redundant in that it is commonly associated with kills of adult fish in “F” areas such as the Albert-Logan river systems. Obviously the, sensitive seagrasses, larval fish and invertebrates are affected somewhere around “B” or “C”, well before death of adult fish, so do we have to wait for the “D” and “F” ratings before considering something is wrong with the system and it needs to be fixed ?
The Queensland Government recently alerted the public of the condition of the marine park with headlines such as “Marine Life Thriving in Moreton Bay Green Zones”. Of course, this is demonstrably false, as shown by the seagrasses and the Healthy Waterways results, which show that Moreton Bay is dying under the weight of relentless urban expansion. The worst part is the QLD government knows this, is letting it happen, and is even spinning the public message to try and somehow blame fishing for the decline.
A Halophila sp. seagrass bed at Tangalooma, Nov 2010, in an area with a “B” water quality rating. It is heavily choked with algae, a warning sign that does not bode well for the future of fisheries, dugong or turtles in Moreton Bay.
