icolls - Sustainable Tourism CRC

ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS
Title of paper: Predicting responses of New South Wales Coastal Lakes to changing nutrient loads
Author: Barbara Robson
Author's email: barbara.robson@csiro.au
Abstract: The coast of New South Wales is dotted with more than 100 shallow lagoons that receive fresh water
from their catchments and have intermittent or permanent connections to the ocean. As the human population
continues to expand along the coast, coastal catchments are subject to increasing development pressure, which
leads to changes in the quantity and quality of fresh water reaching coastal lagoons. To facilitate management of
these ecosystems, there is a need for better understanding of their resilience and likely response to increased
nutrient loads. In this study, an ecological box model with input from hydrodynamic and catchment models has
been applied to eight New South Wales coastal lakes (Cudgen Lake, Coila Lake, Merimbula Lake, Back Lagoon,
Burrill Lake, Narrawallee Inlet, the Myall Lake system and Lake Wollumboola). By applying the model with a
range of nitrogen load scenarios, response curves have been generated for each system in terms of macroalgae,
seagrass, microphytobenthos and phytoplankton biomass and seasonality. The results highlight the importance of
factors such as lagoon area and morphology, residence time and denitrification efficiency in determining the
sensitivity of coastal lagoons to changes in nitrogen loads. The study also indicates where more work is needed
to fill gaps in essential data for these systems.
Title of paper: A comparison of the Hydrodynamics of an Opening Event in two Coastal Lagoons
Author: Emma Gale (UWA), Charitha Pattiaratchi (UWA), Roshanka Ranasinghe (DIPNR)
Author's email: gale@cwr.uwa.edu.au
Abstract: Intermittently Closing and Opening Lakes and Lagoons (ICOLL’s) are an important sub category of
lagoons, and are characterised by their intermittent opening events to the ocean. The opening events, usually
made artificially, are related to variable rainfall events that, over time, produce super-elevated lake levels that
breach the beach berm and release the lake water to the ocean. An opening event introduces appreciable water
level fluctuations (1-3m’s) accompanied by large salinity variability’s (7 – 30ppt) on a sub daily time scale and
the duration and frequency of the opening may also vary between each ICOLL, from weeks to months, and inter
and intra annually, respectively. The opening event for two ICOLLs, located in eastern Australia, Smiths Lake
and Wamberal Lagoon, have been examined as a comparative study due to their similar bathymetry’s yet
significantly different opening regimes. It has been found that the dominant physical forcing during these events
appear to be a combination of low frequency sea level forcing, tidal forcing and an eroding-accreting inlet. The
varying nature of these forcings and their interactions generate changes to the dominant processes controlling
internal mixing and circulation, allowing the setup and breakdown of stratification, and the potential depletion of
dissolved oxygen at depth. This talk shows that defining the hydrodynamics throughout the different stages of
the opening event is important in understanding the water quality.
Title of paper: Physical Processes of Intermittently Closed and Open Estuaries in South Africa
Author: Piet Huizinga, Lara van Niekerk
Author's email: p.huizinga@adept.co.za
Abstract: The physical processes operating in the temporarily open and closed estuaries, including semi-closed
systems will be discussed in the paper, by using examples from several studies of estuaries in South Africa.
Approximately 70 per cent of the more than 250 estuaries in South Africa are intermittently closed and open.
This aspect has a major influence on the ecological processes of these estuaries. Increases in closed mouth
conditions can prevent fish and invertebrate recruitment from the sea into the estuary. Many of the small and
very small estuaries in South Africa (often one to five km long) are temporary open and closed, but also some of
the largest systems such as St Lucia, Bot and Swartvlei are in this category. The major force keeping the mouth
of such an estuary open is the river flow, but large differences in river flow required to keep an inlet open occurs
between different estuaries. For two small estuaries of similar size the differences in flow required to keep the
inlet open can be twenty times or more. The river flow needed to maintain an open inlet is a major component of
the freshwater requirements of estuaries and it is therefore most important that for each estuary this flow is
determined. An interesting feature is that sometimes in small estuaries a semi-closed state of the inlet between
open and closed can be distinguished. The inlet is then in a perched position on the sand berm and continuous
over flow occurs from the estuary to the sea. However, this position can be so high that even at high tide influx
of seawater into the estuary does not occur. This results in changes in the physico-chemical characteristics and
biota of the estuary.
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