Myall Lakes (Australia)
The Myall Lakes comprise four connected basins: Bombah Broadwater, Two Mile Lake, Boolambayte Lake, and Myall Lake, and is the biggest lagoon system in New South Wales, Australia. Bombah Broadwater receives most of the catchment discharge and is weakly connected to the ocean via Myall River. Myall Lake is the ultimate backwater, receiving very little catchment discharge and having a constricted, shallow connection to Boolambayte Lake which, in turn, connects to Bombah Broadwater via the narrow Two Mile Lake. Infrequent flood events and droughts cause large salinity changes in Bombah Broadwater but Myall Lake remains stable. Plankton assemblages vary in Bombah Broadwater in ways that are related to salinity. In Myall Lake chlorophyll is stable and low. Meadows of charophytes and Najas marina are widespread in Myall Lake and these macrophytes are associated with high water clarity and a thick layer of ammonium-rich gyttja. Rising sea level, associated with climate change, has the potential to disrupt the hydrologic stability of Myall Lake and induce an ecological change of state.
KeywordsLagoon Hydrological stability Salinity Hypsometry Water clarity Charophytes Gyttja
The Worimi people were the aboriginal inhabitants in the vicinity of the Myall Lakes. European settlement first occurred during 1800–1830 to harvest timber and develop agriculture. The town of Bulahdelah (present population about 1,500) was established on the upper Myall River in 1840. The timber industry declined as the resource was depleted. Beef and dairy farming became established in the early 1900s and continue to this day. In 1968, Bulahdelah established a sewage treatment plant (STP) that discharged into a tributary of upper Myall River. The STP was upgraded to tertiary treatment in 1996 (Drew et al. 2008). Much of the catchment is now a designated National Park or managed by State Forests. In 1999, the Myall Lakes, a nearly pristine ecosystem, was listed as a Ramsar Site (OEH 2012).
The relatively large uppermost lake, Myall Lake, is the ultimate backwater, being weakly connected to both the ocean and the catchment and with little human activity. As there are no rivers that directly enter Myall Lake, the waters are both low in salinity (usually 2–3 ppt) and distinctively clear. Myall Lake supports extensive growth and high biomass of charophytes, Chara fibrosa and Nitella hyalina, which are limited to waters with low salinity (Garcia and Chivas 2006). The combination of clear water and abundant charophytes surrounded by an undeveloped catchment marks Myall Lake as a place with rare beauty and natural serenity. This remarkable ecosystem is on the one hand accessible and on the other hand made remote from human disturbance due to an unusual degree of hydrodynamic isolation.
Given the limited catchment inputs and low levels of human use of the system, a cyanobacterial bloom observed in the Myall Lakes during April 1999 was unexpected. Prior to the bloom there had been one ecological study (Atkinson et al. 1981) and no long-term water quality monitoring in the Myall Lakes. A flurry of scientific investigations was undertaken following the bloom (Wilson 2008).
The upper Myall River and Nerong Creek have catchment areas 240 km2 and 88 km2, respectively, and they both drain into the northwestern end of Bombah Broadwater. Bombah Broadwater is the southernmost basin and receives most (about 65%) of the total catchment discharge into Myall Lakes. The southern side of Bombah Broadwater is connected to the outer harbor of Port Stephens by the 26 km long lower Myall River (inset in Fig. 1). Morphology of the outer harbor of Port Stephens is rapidly changing (Vila-Concejo et al. 2007), and although the morphology of the entrance to lower Myall River has also changed, its connection to the ocean remains open. Nevertheless the connection to the ocean is strongly choked, with the 1.5 m tidal range in Port Stephens being reduced to 1–2 cm in Bombah Broadwater. The oscillating tide loses amplitude as it travels up lower Myall River, and its momentum is converted into a stress which raises the mean water level, a phenomenon known as tidal setup. Tidal setup is about 0.2 m and oceanic processes with sub-tidal frequencies can sometimes raise the water level of the lakes by a further 0.2 m.
Being broad and shallow (average depth 2.4 m), with a weak connection to the ocean, Bombah Broadwater is usually well mixed. At Bombah Point, a constricted channel connects Bombah Broadwater to the southern end of Two Mile Lake. Two Mile Lake is deep (average 2.7 m) and narrow with thalweg aligned north–south. Roughly 25% of the catchment (100 km2) discharges from Boolambayte Creek into the northern end of Two Mile Lake. Under drought conditions, saltwater intrudes from Bombah Broadwater, and the water column can stratify in Two Mile Lake, as well as in the deeper channels of Boolambayte Lake (Fig. 2).
The northern end of Two Mile Lake opens onto Boolambayte Lake which is shallow (average depth 2 m) and has no significant rivers. Myall Lake connects to Boolambayte Lake via a deep, narrow passage at Violet Hill. Shallow water at the southwestern corner of Myall Lake (Fig. 1) restricts intrusion of deep, saline water. These shallows are critical for maintenance of low salinity in Myall Lake. Myall Lake has a small catchment (68 km2) with no river discharge.
Myall Lake is remotely connected to catchment discharge and even more remotely connected to the ocean. Remarkably, salinity is sometimes observed to increase in Myall Lake after a catchment discharge event. This happens because salinity can slowly build in the three lower lakes (Bombah, Two Mile, and Boolambayte) during a long dry period. A large discharge event then drives freshwater into Bombah Broadwater causing water level to rise due to constricted drainage to the ocean. As water level rises, the more salty water of Boolambayte Lake can only be driven into Myall Lake. Given its large volume and weak coupling to the ocean, the salinity in Myall Lake (typically 2–3 ppt, measured at 6 ppt during a severe drought) is very stable compared to the other basins and compared to other coastal lakes and lagoons in NSW.
With an average depth of 2.9 m, Myall Lake is the deepest of the four interconnected basins and has the largest surface area, 65 km2, greater than the other basins combined. Displacing the combined volume of the other three lakes into Myall Lake would raise water level in Myall Lake by 1.5 m. Catchment discharge into Bombah Broadwater infrequently raises water level to that extent (Sanderson 2008).
Myall Lake is hydrodynamically isolated from the modified Bombah Broadwater catchment, but up to 40% of discharge from the Boolambayte catchment could be washed into Myall Lake (Sanderson 2008). The Boolambayte catchment is in a relatively natural state, so Myall Lake can be considered to be a large, pristine ecosystem with only weak coupling to ecosystems that have been modified by human activity.
Macrophytes and Gyttja
In the Myall Lakes region, land has been rising relative to sea level for the last 6,000 years (Lambeck 1996). Skilbeck et al. (2005) examined cores which showed that Myall Lake made the transition from marine estuary to charophyte lake about 1,100 years BP. A thick layer of hydrous organic sediments known as “gyttja” has accumulated since then. Samples obtained from shallow sites in Myall Lake show a continuous transition from growth at the charophyte apex to a decaying base and then to gyttja (Sanderson et al. 2008).
The hypsometry of Myall Lake is deep and flat bottomed compared to the other basins. Shallow flats are found in sheltered bays, mostly along the north–west shoreline which is in the lee of the strongest winds. These shallow flats are formed by accumulations of thick layers of gyttja. The southeastern shorelines are sandy, consistent with mechanical stress by wind waves and currents driven by the strongest winds.
The top few cm of the gyttja layer is soupy, floc-like, and easy to disturb when without overlying macrophytes. Sometimes flocculent gyttja is swept onto sandy areas, for a time, before being washed away by waves and currents. Beneath this surface layer the gyttja sediment is more gelatinous and more coherent. From 5 to 30 cm beneath the sediment surface, the density of gyttja is not particularly high (1,028 ± 7 kg/m3), but its static strength (11 ± 1 N/m2) is sufficient to resist erosion by the typically small wind-driven currents in Myall Lake. When disturbed, it settles quickly.
Conditions are calm on shallow flats that have charophytes growing on a gyttja substrate, even when onshore winds have raised a surface chop offshore. In part the dense vegetation might damp wind-driven waves as they propagate into shallow water (Granata et al. 2001). Additionally, gyttja may deform with the pressure of passing waves in which case it might be approximated as a viscous fluid which damps wave energy (Dalrymple and Liu 1978). Measurements suggest that dynamic viscosity is 1 ± 0.6 kg/m/s. Given this value for viscosity, the available theory is broadly consistent with qualitatively observed wave damping.
Physical stability, low salinity, decoupling from catchment nutrient discharge, and relatively deep and flat-bottomed hypsometry of Myall Lake are all factors that favor charophytes over angiosperms (Andrews et al. 1984). In Myall Lake, Chara fibrosa dominates biomass and is abundant over the depth range 0.5–4 m. Ultraviolet radiation may restrict charophytes from more shallow waters (Asaeda et al. 2007). The other commonly found charophyte, Nitella hyalina, is found over the same depth range as Chara fibrosa but is patchy in distribution and less abundant where depth is greater than 2 m. Both species show little seasonal variability, consistent with the small annual range of water temperature (13–28 °C).
The dominant angiosperm, Najas marina , has great seasonal variability and can achieve very high biomass in waters 1.5–2.7 m deep. Water temperature and light can support two growing seasons for Najas marina. This species requires soft sediment in order to establish roots (Handley and Davy 2002); mechanical disturbance by wind-driven waves and currents can be high in the spring, so the greatest biomass is often observed in the fall. The distribution of Najas marina is very patchy. Shading by dense stands of Najas marina is associated with the patchy distribution and reduced biomass of the charophyte Nitella hyalina in deeper water (Sanderson et al. 2008).
Charophyte meadows increase water clarity by stabilizing the sediment and trapping particulate material (Blindow et al. 2002). The coefficient of light attenuation is 0.5 m−1 in charophyte-dominated Myall Lake. Light attenuation increases and charophyte abundance diminishes progressing through Boolambayte Lake and into Two Mile Lake. There are no charophytes in Bombah Broadwater where the coefficient of light attenuation is typically 2 m−1 and can be much higher near upper Myall River and Nerong Creek.
While hydrology makes Myall Lake favorable for charophytes, it is the charophytes that do much to determine the physical and chemical characteristics of Myall Lake.
Nutrients and Plankton
Calculation of nutrient budgets from catchment loads (Sanderson 2008) demonstrated that Myall Lake should have a nitrogen source and a phosphorus sink. Chemical analyses of gyttja cores have shown ammonium (NH4+) concentrations of 10 mg/L in lower gelatinous layer, reducing to 1–4 mg/L near the top flocculent layer (Sampaklis 2003). The large accumulation of NH4+ in gyttja suggests that Myall Lake could be an overall nitrogen sink while still having the capacity to be a nitrogen source when disrupted or when ecological state changes. Sampaklis demonstrated that physical disturbance of gyttja released high concentrations of NH4+ and silicate into the water column. Siong and Asaeda (2008) find that incomplete decomposition of charophytes results in burial of phosphorus in the gyttja.
Little is known about the microphytobenthos community in Myall Lakes. Biomarkers indicate that diatoms and dinoflagellates contribute to the organic matter in the sediments of Bombah Broadwater, and Microcystis filaments have been reported to emerge from surface sediments (Heggie et al. 2008). The gyttja in Myall Lake provides substrate for large benthic diatoms and abundant Cyanophyceae (mostly Chroococcales and Nostocales). Potentially toxic cyanobacteria, Microcystis flos-aquae, is also found in the gyttja.
Bombah Broadwater is strongly and variably influenced by both catchment discharge and marine input. Chlorophyll-a measurements in 2003 varied with respect to both time and location (Cohen 2004). Average chlorophyll-a was 4.1 μg/L (±3.4 μg/L) and range 1.6 to 15.6 μg/L. Anabaena blooms were observed during freshwater periods in 1999 and early 2000 (Ryan et al. 2008). Redden and Rukminasari (2008) demonstrated that small increases in salinity (1.5–5.5 ppt) negatively affected growth of Anabaena circinalis collected from Bombah Broadwater.
The composition of the plankton assemblage in Bombah Broadwater might change for many reasons. Salinity is one factor, and injection of either marine species from lower Myall River or freshwater species from upper Myall River is another. Bombah Broadwater was dominated by marine dinoflagellates in the 2003 autumn when salinity was 8–9 ppt. By winter the salinity reduced to 2–3 ppt and Chlorophyceae dominated. Redden and Rukminasari (2008) incubated natural phytoplankton assemblages and found that elevating salinity levels from 4 ppt to 8 ppt caused chlorophytes, particularly Scenedesmus sp., to decline. Salinity of Bombah Broadwater rose to 5 ppt in spring and 6 ppt in summer, and this heralded a switch to dominance by Cyanophyceae (Cohen 2004).
The stable physical and chemical environment of Myall Lake is reflected by the stability of its phytoplankton community. Chlorophyll-a is low and stable in the water column of Myall Lake. Measurements spanning 2003 at multiple sites (Cohen 2004) showed the average value for Chlorophyll-a was 2.3 μg/L (±0.7 μg/L) and range 1.4 to 3.8 μg/L. Cohen (2004) found that the Cyanophyceae consistently dominated throughout 2003 in Myall Lake. Similarly, Ryan et al. (2008) report that Chroococcus, Merismopedia, and chlorophyte taxa were most common from 2000 through 2002. Two Mile Lake and Boolambayte Lake have phytoplankton assemblages that might be characterized as a mix of those found in Bombah Broadwater and Myall Lake.
Copepods dominate mesozooplankton assemblages in both Bombah Broadwater and Myall Lake, but counts are both higher and more variable with respect to time and site within Bombah Broadwater than within Myall Lake (Cohen 2004). Large numbers of very small rotifers are also found in both lakes. Nevertheless, the zooplankton community of stable Myall Lake has clear differences from more variable Bombah Lake. Myall Lake is distinguished by large populations of the rotifer Brachionus baylyi (Timms 1976a). Cohen (2004) observed that Bombah Broadwater was set apart by abundant cyclopoids in the fall of 2003. In 2001, Muschal (2003) found that the two lakes differed due to high densities of polychaete larvae in Bombah Broadwater. Vertical migration causes zooplankton to become more abundant at nighttime in Myall Lake but not so in Bombah Broadwater (Timms 1976b).
Microzooplankton dilution studies (Cohen 2004) shed light upon the growth and daytime grazing dynamics of the plankton communities in the water columns of Myall Lake and Bombah Broadwater. In Myall Lake both zooplankton grazing and phytoplankton growth rate were lower and much less variable than in Bombah Broadwater. Phytoplankton growth rate of samples from Myall Lake was not stimulated by the addition of phosphorus and/or nitrogen, whereas samples from Bombah Broadwater were often found to be nitrogen limited and sometimes phosphorus limited. Mechanisms that limit phytoplankton growth in Myall Lake remain unknown.
Threats and Future Challenges
Myall Lake and Boolambayte Lake have expansive charophyte meadows in shallow, calm water. Both the charophytes and gyttja are easily disturbed by motorized craft. Gyttja entombs high concentrations of NH4+ which are released into the water column when sediment is disturbed. A recent map (RMS 2013) designates some bays as being restricted to paddle craft and others to idle speeds. Interestingly, some of the restricted areas are sandy beaches, and many charophyte meadows (including shallow and easily disturbed Bibby Harbour) are unprotected.
The modern-day ecology of Myall Lakes is believed to have begun about 1,000 years ago as a consequence of 6,000 years of land rising relative to the level of the ocean. Now the long-term trend has turned for water level. Sea level is rising, at an accelerating rate, as anthropogenic emissions warm the planet. The entrance of lower Myall River is protected by Yacaaba barrier where sand presently accretes, although erosion is severe at neighboring locations (Vila-Concejo et al. 2007). Should further sea level rise shift the pattern of erosion, the entrance might become exposed and more susceptible to closure. Alternatively, perhaps inevitably, continued rising sea levels and increased storminess might breach sand barriers, opening direct communication of Bombah Broadwater with the nearby ocean (inset of Fig. 1).
The apparent stability of vast charophyte meadows and the lack of response of phytoplankton to nutrient enrichment experiments should not be construed as indicating that Myall Lake will be robust to anthropogenic disturbance. Eutrophication and/or changed hydrology can be expected to be followed by invasion of other species of phytoplankton which might grow and fundamentally change the ecosystem. It has happened before.
The ecological stability of Myall Lake is predicated upon physical stability and isolation from nutrient loading by the catchment. Shallow water in the southwestern corner of Myall Lake plays a critical role, preventing rapid intrusion of saline water during drought. The dominant submerged vegetation – charophytes and Najas marina – seems to grow best when salinity is low (Garcia and Chivas 2006). To maintain this unique, pristine ecosystem, it is critical to preserve the shallow barrier. Presently this area is mapped as a “channel,” and there are no restrictions placed upon the draft of motorized vessels passing through it (RMS 2013).
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