KEY ECOSYSTEM CHALLENGES – LOSS OF SEAGRASSES
|The most significant impact on the ecology of Western Port to date has been the substantial losses of the seagrass communities. These losses have been recognized internationally as an ecological disaster, with a special case study segment of the United Nations Environment Program’s “World Atlas of Seagrasses” dedicated to this catastrophic loss. For Western Port, which is a recognized international Ramsar Wetland, and a declared United Nations Biosphere, these losses present a major body blow to the bay’s ecosystem.|
Significance of Seagrass Losses
Seagrasses are the foundation stone of Western Port’s ecology. The seagrass meadows represent the bay’s major solar collectors and chief converters of sunlight to energy. The biomass produced by the seagrasses plays a major role in fueling the entire food chain within the bay.
Seagrasses provide many ecosystem services, including;
History of Seagrass Losses
Western Port’s seagrasses were first mapped in detail as part of the Western Port Environmental Study in 1973/74. At that time concern was expressed about seagrass dying in the north and north-east corner of the bay, but it was considered to be localized and of minor significance.
Examination of aerial photographs from 1970 confirmed good seagrass cover in this area, but by 1975 similar photographs revealed a significant decline in seagrass cover. The next survey in 1983/84 calculated that 70% of the area covered with seagrass in 1973 was bare and the biomass had been reduced by 85%. Unfortunately, by the time these losses had been confirmed, too much time had elapsed to collect information that could pin point the major causal factors. To date, no scientific body has been prepared to state categorically the cause of this decline, but most believe there is a high probability of a multi-factorial effect.
Certainly seagrass losses were being observed elsewhere in Australia and around the world in this period and there was a growing body of evidence suggesting losses were more prevalent in coastal environments where water quality has been significantly influenced by catchment discharges. The Western Port catchment underwent major changes with the construction of the Koo Wee Rup swamp drainage system. This fundamentally altered the filtration mechanisms of the swamp and lead to ongoing discharges of sediments into the northern sector of the bay from erosion of the land and drains, especially during major flood events.
Increased use of fertilizer and biocides for agriculture and drain vegetation control added to the cocktail of pollutants entering the bay. The first big losses of seagrass in the Lang Lang delta region in 1965-1968 point strongly to catchment discharge effects, particularly smothering sediment loads. However strong and consistent correlations between catchment input episodes and recorded seagrass loss episodes across Western Port have not been proven.
There was for many years an active seagrass harvesting industry operating from French Island to supply the insulation market. This industry peaked at the time of the great decline and was based on collection of detritis washed ashore. This activity may have affected seagrass seed reserves but would not impact on seagrass beds. There is some reference to attempted mechanical harvesting, a much more impacting practice, but it is understood such efforts failed to be productive and in any event the natural seasonal leaf shedding process provided ample material for the market.
More recent analysis of temperature and low tide conditions by Dr Greg Parry (DPI) has suggested that desiccation of exposed seagrasses at low tide may also have been an important contributing factor. He suggests some seagrass loss episodes have coincided with the temperature increases during the 1982/83 and 1992/93 El Nino years across southern Australia. Linked to the desiccation issue is the height of seagrass beds and whether sediment build up has led to increased periods of low tide exposure and temperature impacts.
In 1998 EPA testing of seagrass meadows at various locations in Western Port showed seagrass health was directly linked to water quality, with turbidity and sedimentation being the key factors as these directly impacted on the capacity of these plants to photosynthesize. International studies have also shown that elevated nutrient levels in marine waters are linked with excessive algal growth on seagrass leaves (epiphytes) which significantly hinder photosythesis. So for seagrasses, sediments and nutrients are key pollutants. One thing is certain, as the seagrass meadows declined, increasingly large areas of bare mudflats were subjected to tidal erosion, with daily re-suspension of fine sediments, which in itself reduced water quality and the viability of remaining seagrass meadows. This problem still exists, and without a sediment budget (net loss or gain of sediments entering and leaving the bay), it is unclear if and when natural tidal processes will ameliorate the problem.
Map Showing Seagrass Distribution Changes
Factors Hampering Seagrass Recovery
Seagrasses depend on clean clear water to photosynthesize, grow and reproduce. Turbid and sediment laden water restricts light and smothers plants, whilst high levels of dissolved nutrients fosters algal and other epiphytic growth on seagrass leaves which in turn inhibits photosythesis. Occasional pollution episodes may also impact upon seagrasses.
Key sources are:
Changes to bank and channel morphology and water movement around the top of the bay may have affected the rate that these fine sediments can be flushed from the bay. In addition, changes in bank elevation may be affecting the extent and period that mudflats are exposed to extremes of temperature during low tides. These factors have not been adequately studied or assessed to date.
Unsuitable and Unstable Substrate
The very fine, slimy and mobile sediments in the northern sector of the bay are unsuitable for the establishment and anchoring of seagrass seeds and propagules. The erosion of the Koo Wee Rup swamp soils along the northern coastline is a major source of this problematic substrate. In addition, sand movements along the eastern sector have been shown to smother otherwise successfully growing seagrass transplants.
Seagrass Seed Stocks
The loss of so much seagrass and for such a prolonged period has reduced the stock of seagrass seeds and propagules needed for natural regeneration. This may have been exacerbated by commercial seagrass collection that peaked at the time of the worse losses.
There is clear evidence of physical damage to seagrass beds due to boat propellers and anchors. Damage due to seagrass harvesting and seine netting are no longer threats since being banned. Seagrass meadows on sandy substrates can withstand human activities like diving and fishing, but the mudflats and banks are very susceptible to human impact. Bait collecting and even scientific field studies and monitoring need to be undertaken with great care to avoid serious physical trauma to seagrasses in these soft substrates.
Strategic Management & Communication
Whilst there has been some good policy and strategy development covering Western Port and its catchment, including the State Environment Protection Policy Waters of Victoria (Schedule F8), and the Port Phillip and Western Port CMA Regional Catchment Strategy, it is very hard for community organizations like WPSP to determine the detailed status or the quality of decision making against critical actions, and how well these actions are being coordinated.
What Needs to be Done
Catchment sediment and dissolved nutrient loads entering Western Port need to be significantly reduced by:
Coastal erosion needs to be abated by:
An in-bay Sediment Management Strategy: This is required to ensure natural processes are assisted in bringing about long term improved substrate and water quality. It would include:
Strategic Management & Communication:
Coastal Erosion & Vegetation Losses
Significance of Mangrove and Saltmarsh Losses
Mangroves provide sustainable coastal stabilization, especially against wave action. They are a major source of nutrients in the overall bay nutrient pool and havens for young fish and other sea creatures. They provide nesting sites and food for migratory birds and numerous insects.
They also provide the buffer between the sea and land that creates the special niche required by saltmarsh species to survive. The saltmarshes are important feeding and sheltering zones for local and migratory bird species. Where mangroves have been removed, active erosion fronts are opened up, the coastline recedes and saltmarshes are eroded away.
History of Mangrove and Saltmarsh Losses
The earliest indication of mangrove distribution around Western Port are provided by early explorers and navigators sent to chart the region. The most detailed of these, conducted by George Smythe in 1842, included the location of mangroves and saltmarshes.
Interestingly, Smythe’s mapping shows a distribution not too dissimilar to the current distribution, with mangroves restricted to the northern regions of Western Port, away from the oceanic influences and high energy coastlines.
Mangroves were subsequently cleared to provide ship access to settlements and burnt to make barilla for soap, and even to get better views to the water.
The areas cleared in this fashion are not well documented. In some cases recovery has occurred naturally, as the conditions for re-establishment have remained favourable. Other areas have yet to stabilize and mangrove removal has resulted in ongoing coastal erosion, loss of saltmarshes and private and public land. Attempts to halt the erosion process have proven futile and extremely expensive.
Whilst mangroves are known to be shoreline colonizers, and in most scenarios seek seaward expansion, this is not the case at all locations around Western Port, as noted during the Shapiro report. It is quite natural for mangrove coastlines to wax and wane according to changed coastal dynamics and weather episodes, and as long as there is room to maneuver, the mangrove and saltmarsh communities will retreat and advance naturally.
If however the retreat pathway is blocked by artificial barriers or seawalls, the mangroves may displace saltmarshes and then finally be lost due to backwash. There are substantial areas along the north eastern coast where landholders have constructed earthern dykes to prevent peak tide inundation. In these zones, the mangroves and saltmarshes are providing vital protection, but ironically may in the long term be destroyed by the seawalls preventing natural peak tide energy alleviation. At one site where the artificial earth bund has been breached, this has resulted in a lower gradient shoreline and regeneration of saltmarsh. This may provide some insight into what a more sustainable land management regime should look like for this section of the coast.
Of course such a strategy would have consequences for current land holders and the Government would need to consider what needs to be done to assist landholders as part of the process of attaining a more sustainable coastline. There may have been some new opportunities for expansion of mangroves into drainage outlets built to empty the great Koo Wee Rup swamp, but this would have been counterbalanced by active drain clearance programs conducted by the drainage authorities.
In the areas to the north-east where mangroves were not recorded by Smythe, he observed the coastline to have many rills of water actively flowing into Western Port from the great swamp. This was an area with abundant off-shore seagrass meadows, but the coastal environment he describes would be quite unsuitable for saltmarsh and probably mangroves, because of the constant inputs of fresh water from both surface and subsurface flows.
Since the great swamps were drained and drainage systems deployed, the freshwater outputs have changed radically from ongoing widespread small inflow points to large single point inflows and significant flood event massive discharges of freshwater and sediments. The changes at the drain outlets would not favour mangrove colonization, but arguably that may have been beneficial where the rills ceased to flow. Still the north-east coast remains devoid of mangroves.
The situation would appear to be more complex and no doubt influenced by the massive losses of seagrasses along this section of the coast and active erosion processes that have set in. Costal erosion was recently measured at 1 metre / year, adding about 30,000 tonnes of sediment into the bay each year. It is unlikely that mangroves could of their own accord establish in this zone as the combination of tidal energy and slumping coastal mini-cliffs is quite inhospitable to seedling establishment. What is not yet clear is whether mangroves could be reestablished by plantings and whether they would go on to form a self sustaining population.
Original Mangrove Distribution
George Smythe’s survey of 1842, made before major European clearance activities, provides a very good indication of the original distribution of mangroves in Western Port.
The absence of mangroves and saltmarshes along the north east coast would seem to correlate with major freshwater drainage outlets from the large tea tree (Melaleuca ericifolia) swamps These swamps were later to be drained for agricultural pursuits.
Today these NE coastal areas are still devoid of mangroves and subject to very active coastal erosion. Whilst mangroves have yet to naturally colonize this NE Coastline, their capacity to form self sustaining colonies if assisted with seedling planting is yet to be ascertained. Current planting trials will provide the answer to this question.
Factors Hampering Mangrove and Saltmarsh Recovery
Historic evidence shows mangroves can be killed if inundated by coastal sand drift and deltic surges. These sand influxes cover mangrove pneumataphores and young seedlings. These episodes may occur naturally or result from coastal and channel engineering modifications. Silt from the Koo Wee Rup drainage area is known to be linked with smothering of both seagrasses and mangroves, especially near the outlet deltas.
Minor disturbances to mature mangrove and saltmarsh communities in protected inlets are generally self repairing, but major disturbances, especially along higher energy coastline can result in altered coastal dynamics and make recovery problematic. At this southern latitude mangroves grow slowly and even where they have the potential to recover from such removals, it happens slowly. In the interim the saltmarshes behind the mangroves become exposed to tidal energy and can be lost in a secondary coastal damage scenario.
These may form where mangroves have been removed. Eroding coastline and cliff faces present an inhospitable environment for mangroves, due to washback, soil slumps and eroding substrates.
Mangroves will not establish or survive along open high energy coastlines. Mature mangroves will however survive medium and infrequent tidal energy episodes, especially where retreat and advancement space is available. However, if such coastlines have been subjected to clearance, the tidal energy is often too severe for seedlings to establish naturally. Special replanting techniques are required, but even the use of mature seedlings requires persistence and replacement planting regimes until self sustaining plantations are achieved. There is no perfect formula for such efforts and much trial and error and case-by-case evaluation is required.
Where these are deployed to slow coastal erosion, they are usually very expensive and often counter productive. They inevitably collapse over time and ironically may prevent the establishment of natural self sustaining coastal line. In particular, “sea walls” built along the coast behind mangroves and saltmarshes prevent the natural gradual retreat and advancement process, resulting in saltmarshes being squeezed out and eventually mangroves being undermined by backwash energy. Mangroves and saltmarshes require gradated shorelines that can dissipate wave energy and allow periodic invasion of saltmashes by seawater. Mangrove and saltmarsh coastlines need sufficient buffer area to provide for retreat and advance phases that naturally occurs over time.
Stock and Human Access
Such access can severely impact on saltmarshes due to physical damage to these sensitive habitats which typically have slow recovery periods. Stock, horses, humans and trail bikes can cause significant physical trauma. Stock will also graze on mangrove and saltmarsh species, reducing their vitality and inhibiting their reproduction.
Chemical and freshwater pollutants are a concern. Saltmarshes in particular do not like constant freshwater inputs and the absence of mangroves at the old swamp discharge zone may suggest an intolerance to surface and subsurface freshwater inputs. Both mangroves and saltmarshes can be severely impacted upon by pollutants such as oil and biocides meaning that adjacent marine and farming operations need to be well managed. Altered water courses and drainage can also affect the salinity of these ecosystems, resulting in die off of saltmarshes.
Actions Needed to Protect Mangroves & Saltmarshes
As with seagrasses, halting coastal erosion is an important factor in protecting mangroves and saltmarshes.
The following additional steps need to be considered for these ecosytems: