Circulation and the nutrient budget in Myall Lakes

Abstract

A two-dimensional model is used to study transport of material within Myall Lakes and is coupled to the ocean using a one-dimensional model of Lower Myall River. Runoff from the catchment is calculated from water levels and compares favourably with rainfall assuming an average runoff yield of 28% and a runoff coefficient of 47% for events above the 90th percentile. Most of the runoff enters Bombah Broadwater which is also the only basin connected to the ocean. Intermittent runoff events rapidly displace water from Bombah Broadwater into Boolambayte Lake and from Boolambayte Lake into Myall Lake. Displaced water is mixed within basins by winddriven circulation within the time scale that levels fall as water drains from the lakes to the ocean. Only 7% of the nutrient load entering Bombah Broadwater becomes resident Myall Lake. About 40% of the nutrient load entering Boolambayte Lake becomes resident in Myall Lake. Median time scales for loss of conservative material entering each basin with the runoff are: 140 days for Bombah Broadwater, 118 days for Boolambayte Lake, and 535 days for Myall Lake. Salinity fluctuates greatly in Bombah Broadwater but is stable in Myall Lake. Material loss from Bombah Broadwater is characterized by many time scales associated with runoff from the catchment and low-frequency changes in ocean water level. Comparison of observed distributions of total nitrogen with simulations indicates that there are sources of total nitrogen in Boolambayte Lake and Myall Lake and sinks of total nitrogen in Bombah Broadwater. There appear to be sinks of total phosphorus throughout the Myall Lakes.

Mechanisms affecting biomass and distribution of charophytes and Najas marina in Myall Lake, New South Wales, Australia

“…Salinity is relatively low (2-3 ppt) and stable in Myall Lake because an interplay of physical mechanisms buffers the impact of drought and flood (Sanderson, 2008). Najas marina grows optimally at alinities in the range 2.7-4.2 ppt, with germination only mildly suppressed when salinity was increased to 5.6 ppt .…”

“…2 and 11). Wind waves can be expected to resuspend material and the wind-driven currents (Sanderson, 2008) would transport material offshore. Indeed, these strong winds drive a gyre in the eastern basin of Myall Lake (Sanderson, 2008) that would transport suspended material to an area near site 7 where there is a mound of soft sediment (Fig.…”

“…Soft bottom substrate is found in the deeper sections of Two Mile Lake, so bottom substrate is probably not a constraint. Salinity fluctuations are higher near Bombah Broadwater (Sanderson, 2008) so this may sometimes be a constraint based on species-salinity diagrams for charophytes in Australia (García & Chivas, 2006). The distributions of charophytes and Najas marina are associated with the down-lake gradient in water transparency.…”

“…Myall Lake was found to be different from most other coastal lakes in New South Wales because it has abundant charophytes and Najas marinarather than seagrass. The uncommon assemblage corresponds with Myall Lake being a poorly flushed backwater (Sanderson, 2008).…”

Nutrient and phytoplankton responses to a flood event in a series of interconnected coastal lakes: Myall Lakes Australia

“…This is because elevated nutrient concentrations are rapidly depleted over hours or days due to dilution and mixing, nutrient cycling, and uptake by phytoplankton and macroalgae (Ferguson et al, 2004;Moore et al, 2006). However, as up to 70% of the total runoff can be delivered during runoff events (Sanderson, 2008), these events are significant to the hydrology, ecology, and nutrient status of coastal lakes.…”