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 LossesSeagrasses 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;
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History of Seagrass LossesWestern 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. Lang Lang River / drain – source of sediment delta that smothered seagrass and mangrove communities. Photo: T. Ealey WPSP. 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. Left: Sunburnt leaves of intertidal Zostera muelleri, but no conclusive evidence of mortality. Photo: T. Ealey WPSP. 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 RecoveryWater QualitySeagrasses 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:
GeomorphologyChanges 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 SubstrateThe 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 StocksThe 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. Physical DisturbancesThere 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 & CommunicationWhilst 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 DoneCatchment 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:
Seagrass Restoration:
Strategic Management & Communication:
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Factors Hampering Mangrove and Saltmarsh RecoverySand DriftsHistoric 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. ClearancesMinor 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. Erosion HeadsThese 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. Coastal EnergyMangroves 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. Artificial BarriersWhere 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 AccessSuch 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. PollutionChemical 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 & SaltmarshesAs 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:
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