Salinity stratification refers to the situation where there are layers of water of different salinity sitting on top of one another. Salinity stratification occurs because the density of water in the Inlet depends on its salinity. When a layer of fresher water overlies a layer of saltier water a situation that resists mixing exists. The stratification resists mixing because the saltier water is heavier than the fresh and therefore it takes a lot more energy to mix the two layers together than it would if they were the same density. Winds over the Inlet provide the major source of energy for mixing, and when salinity stratification occurs it can take days or even weeks to mix the water column if winds are light.

Figures 25 (top) and 26 (bottom). These transects of salinity (Figure 25) and dissolved oxygen (Figure 26) through the Inlet in September 1996 illustrate the effects of stratification. Marine water is entering the Inlet at the left of each figure and freshwater from the right. A lens of saltier, marine derived, severely deoxygenated water lies in the deeper parts of the Inlet. Although in this case the depth of deoxygenated water is no more than 0.5 m it is commonly as much as 1 m.

Dissolved oxygen in the Inlet can come from two sources. It dissolves into the Inlet from the atmosphere, and plants and algae may photosynthesize and produce oxygen themselves; but only during the day when there is sunlight present. As a result there is usually plenty of oxygen in the water at the surface of the Inlet and around the shore of the Inlet (except where there are accumulations of rotting biomass around the shoreline). However the oxygen can easily be used up at the bottom of the Inlet, especially in the deeper areas.

The major cause of deoxygenation in Wilson Inlet is reduced mixing during periods of salinity stratification and light winds. The most prolonged periods of light winds during the year usually occur in October and April (see Figure 14, previous page) while the period most prone to stratification is the first few months following bar opening; roughly August to November. Dissolved oxygen loggers placed on the floor of the Inlet demonstrate that deoxygenation occurs within one to four days of stratification being achieved (Figure 27). This data also demonstrates that decreases and increases in dissolved oxygen concentrations at the sediment water interface may occur on timescales shorter than our weekly to monthly water profile sampling. However it shows that the weekly sampling runs have roughly approximated the major features of the pattern of deoxygenation, and that deoxygenation events occur when our weekly data shows salinity stratification to be occuring.

Figure 27. Bottom water deoxygenation for spring 1996 at site WI6. The plot compares a time series trace of dissolved oxygen (mg/L) recorded at 15 minute intervals (blue line) from a logger placed on the floor of the Inlet, against the bottom water dissolved oxygen as measured by weekly profiles (red line) and the relative strength of stratification as measured by weekly profiles (green line).

When deoxygenation occurs, the animals and bacteria that require oxygen are unable to survive in the deoxygenated waters. Many of these animals and bacteria regulate the nutrient cycling at the sediment water interface. Without them, and with a low dissolved oxygen concentration, nutrient cycling at the sediment water interface changes. Nutrients that were previously trapped by chemical processes in the sediments or were recycled in forms that plants and algae were unable to use are released into the water column in forms that plants and algae can use.

The key processes controlling nitrogen and phosphorus availability at the sediment water interface are denitrification and the trapping of orthophosphate by iron-oxyhydroxide compounds respectively. Denitrification is a two stage process that results in the conversion of ammonium (readily available to plants and algae) into nitrogen gas (not readily available to plants and algae). The first stage of this process requires oxygen, therefore without oxygen ammonium is not denitrified and instead becomes available to plants and algae. In terms of phosphorus, iron-oxyhydroxide compounds become unstable in the absence of oxygen and therefore release their trapped orthophosphate back into the water, which is then available to plants and algae.

Sediment bio-geochemistry work undertaken by the Australian Geological Survey Organisation (AGSO), to be published in Wilson Inlet report to the community number eight, demonstrated the importance of these dissolved oxygen regulated nutrient cycling processes at the sediment water interface. Under the present incidence of bottom water deoxygenation, it was estimated that about 500 tonnes of nitrogen and 20 tonnes of phosphorus were delivered to the Inlet from the sediments lining the estuary (all of it available to plants and algae). Furthermore, it was also found that a further 500 tonnes of nitrogen per year and an unknown amount of phosphorus could also be delivered to the Inlet from the sediments lining the estuary in the event of further deoxygenation of bottom waters in the Inlet.

As the nutrient cycle diagram in the previous boxed aside illustrated, nutrients are delivered to the Inlet from two main sources; the catchment and the sediments lining the floor of the estuary. The two main sinks for nutrients on the other hand are the ocean and the sediment processes (with subsequent loss to the atmosphere or burial). The estimated loads from these sources and exports to these sinks are tabulated below in Figure 28. This data demonstrates the relative importance of sediment processes compared to catchment inputs and losses to the ocean. Given the importance of sediment processes in nutrient cycles, and the reliance of these processes on the availability of oxygen, clearly the maintenance of dissolved oxygen concentrations at the sediment water interface must be a prime management concern.

Figure 28. Loads of nutrients, internal recycling, denitrification and exports for Wilson Inlet (average of 1995-1997 and AGSO data). This work will be expanded on in greater detail in Wilson Inlet report to the community number eight. The biologically available fraction is highlighted.

Figure 29 (left) and 30 (right): Two of the consequences of eutrophication in Wilson Inlet. Figure 29 shows short stands of Ruppia on the floor of Wilson Inlet, smothered with the macroalgae Spyridia and Polysiphonia (photo B. Dudley) while Figure 30 shows macroalgae (Cladophora in this case) growing on Ruppia in the shallows of the Inlet (photo W. Hosja).