Wilson Inlet 5 - introduction

The Wilson Inlet Catchment Committee Inc (WICC)

Wilson Inlet 5

Cover page
Introduction
Nutrients
Water Quality Cycle
Stratification
Water Quality
Comparative data
Summary of Findings

References

Water Quality in Wilson Inlet from 1995 to 2002

Introduction

Wilson Inlet is a seasonally closed estuary on the south coast of Western Australia. The Inlet has a surface area of 48 km², is 14 km long from east to west, and is about 4 km wide. A sand bar isolates the estuary from the Southern Ocean for about half of the year (usually about February to July). The bar is artificially breached each year to prevent flooding of land next to the estuary. The Inlet has an average depth of 1.8 m below mean sea level, and a maximum depth of about 4 m below mean sea level. Note that when the sand bar across the Inlet mouth is breached the water level is usually about 1 m above mean sea level.

The Inlet's partially cleared catchment covers some 2300 km². The average rainfall at the coast is approximately 1100 mm and at the inland boundary of the catchment is about 600 mm. About 10% of the rain that falls on the catchment reaches the Inlet itself, delivered largely by the Inlet's five major tributaries: the Hay, Denmark, Sleeman, Little and Cuppup.

¹ Eutrophication means nutrient enriched.

There is ongoing concern that as a consequence of land use changes in its catchment since the 1950s Wilson Inlet is becoming increasingly eutrophied¹. An increase in benthic vegetation, mainly the seagrass Ruppia megacarpa, its epiphytes and various macroalgae, has to date been the major symptom of eutrophication in the Inlet (see Figure 2). There is also evidence suggesting that there have been concurrent increases in estuarine fish numbers over this period. The community is concerned that without adequate management the eutrophication of the Inlet will increase and eventually the runaway growth of plants and algae and the internal recycling of nutrients will lead to the collapse of the present ecosystem, similar to the case of the Peel- Harvey estuary in the 1980s.

Figure 2. Series of aerial photos of the Wilson Inlet delta, dated March 1946, January 1971 and December 1992 respectively. The series illustrates the increase in benthic vegetation in the Inlet over time. The clean sands present in the 1946 photo in the back of the delta (centre of photos), next to Poddyshot Point (top right of photos) and on the southern shore of the Inlet (bottom right of photos) have become increasingly vegetated in the 1971 and 1992 photos compared to 1946.

To understand the nutrient status of the Inlet and the processes controlling it the Water and Rivers Commission has regularly monitored the water quality in Wilson Inlet at a number of sites since 1995. The locations of the monitoring sites are shown in Figure 3. At these sites measurements of the water temperature, salinity and dissolved oxygen concentration, are made from the surface down to the bottom using Hydrolab datasonde instruments. Water samples are collected from the surface and the bottom at a number of these sites and are analysed for nutrients (nitrogen, phosphorus, and silica) and the composition and concentration of phytoplankton.

Figure 3. Map of Wilson Inlet showing water depths below mean sea level and the location of the eight commonly used water quality monitoring sites: WI2, WI30, WI6, WI7, WI9, WI35, WI12 and WI14.

Depending on the season and the year monitoring has taken place from weekly to monthly. During periods of stratification loggers have been placed on the bottom of the Inlet recording oxygen, temperature and salinity at 15 minute intervals.

Water quality monitoring has been a part of a broader investigation into the nutrient status of Wilson Inlet. The Water and Rivers Commission has also undertaken extensive catchment nutrient monitoring (discussed in the third Wilson Inlet report to the community). While the National Eutrophication Management Program (NEMP) and the Water and Rivers Commission have jointly funded investigations into the seagrass dynamics, sediment geochemistry, and phytoplankton of the Inlet. The Commission has also funded studies into coastal processes and the bar, and the fisheries of the Inlet. The NEMP projects are summarised in the fourth Wilson Inlet report to the community. Future community reports (numbers six to ten) will discuss the findings of each of these projects individually.