Wilson Inlet 5 - water quality cycle

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

The annual water quality cycle

Each year there is a regular, relatively predictable, annual sequence of events in the water quality of Wilson Inlet. This sequence reflects the annual cycles in the major drivers of the water quality. The external forces that drive the annual water quality cycle in Wilson Inlet and their effects on the Inlet are described below. Data for some of the drivers of the Inlet's water quality and the water quality data itself for the 1996/1997 period are presented so as to illustrate major elements of the annual water quality cycle in the Inlet (the data are for site WI6 in the centre of the Inlet).

Rainfall and river flow

In concert with landuse practice in the catchment, the rainfall and runoff determine the timing and load of nutrients delivered from the catchment to the Inlet, the salinity of the Inlet, the water colour and the water level in the Inlet. The availability of nutrients in turn contributes to plant and algal growth, whilst the salinity of the Inlet determines the species of plants, algae and animals present, and plays a role in determining the occurrence of salinity stratification, finally the water level determines the bar opening date. As can be seen in Figures 9 and 11 both rainfall and runoff (and hence nutrient delivery) have a clear seasonal pattern with some 80% of flow and 60% to 70% of rainfall occurring between June and October. The river flow and nutrient delivery characteristics of the catchment are described in detail in Wilson Inlet report to the community number three.


Figure 9: Daily rainfall at Denmark (mm)		Figure 11: Daily runoff (GL/day) to the Inlet

Ocean water levels (including astronomic tides and barometric tides)

When the bar is open the ocean water levels (along with the bar channel dimensions) are the major determinant of the volume of marine exchange between the ocean and the Inlet. The subsequent marine exchange plays a role in determining the Inlet salinity, water colour, salinity stratification, and the export of nutrients to the ocean. The most important seasonal difference in the ocean water levels is that there are larger, more frequent, shifts from high to low water levels, due to the passage of low pressure systems, (and hence the potential for more pumping of water into and out of the Inlet) during the winter and spring periods than the summer. The ocean water levels and the marine exchange are plotted in Figures 12 and 15. For comparison the Inlet water level is also plotted in Figure 16.


Figure 12. Marine exchange (GL/day) to the Inlet		Figure 15: Daily max and min sea level (cm)

Figure 16: Inlet water level (cm above mean sea level)

Winds (speed, direction, and duration)

Winds govern the vertical mixing in the Inlet (and hence play a role in determining stratification) and also affect the rate of evaporation and hence water level. Salinity stratification in turn effects bottom water oxygen status and nutrient recycling which has an impact on the availability of nutrients for plants and algae. The major seasonal variation in the winds is the shift in the predominant wind direction. As shown in Figure 13, usually in about April the predominant wind direction shifts from south easterly to north westerly and then in October shifts back again. These shifts in direction are periods when the average wind speeds, and hence mixing activity in the Inlet, are at a minimum for the year as shown in Figure 14.


Figure 13: Average daily wind direction in degrees		Figure 14: Average daily wind speed in knots

Ocean wave conditions

Ocean wave conditions largely affect the sand transport on Ocean Beach, this in turn affects the infill of the delta and the date of bar closure, which also have implications for the dimensions of future bar channels and marine exchange. The ocean wave climate is more intense over the winter months.

Solar radiation (day length, angle of the sun, and cloudiness)

Solar radiation plays a major role in determining the water temperature which in turn effects growth rates of plants, algae and bacteria and hence the rates of nutrient uptake and recycling. Solar radiation also provides the energy for plants and algae to grow in the first place and therefore is of critical importance to nutrient cycles. Solar radiation has a strong seasonal response with a summer peak and a winter minimum. The number of daylight hours are presented in Figure 10 which gives some idea of the seasonality of solar radiation.

Figure 10: Daily sunshine (hours)

Air temperature, pressure, and humidity

These atmospheric parameters largely affect evaporation, water temperature, and in the case of the air pressure the ocean water level. No data for these are plotted here, in summary though air temperature is at a maximum in summer and minimum in winter whilst the reverse is the case for humidity. The air pressure has already been mentioned in the context of ocean water levels.

Water temperature

The water temperature in the Inlet, plotted in Figure 18, reaches a maximum of about 24°C in January and a minimum of about 12°C in July. The shallows may get slightly warmer and cooler than this in summer and winter respectively. The surface to bottom temperature difference is usually no more than a couple of degrees, and often the difference is negligible.

Figure 18: Temperature (°C), red line is surface and blue bottom water

Water colour and clarity

The water colour, Figure 17, which reflects the varying contributions of marine and catchment water, only deviates significantly above its baseline during the period of river flow from the catchment when the Inlet is flooded with brown tannin stained waters. The clarity drops at this time in response to the particulate material carried into the Inlet by flows and by subsequent blooms of phytoplankton algae clouding the water.

Figure 17: Relative water colour (red line) and clarity (blue line)

Salinity

At the start of the year the Inlet is fully mixed with a salinity about two thirds that of seawater (Figure 19). Over late summer and autumn the Inlet remains mixed but the salinity increases, due to evaporation, reaching a peak in May. Over the same period the Inlet water level may drop between 10 cm and 80 cm due to evaporation. From early June until bar opening the water level rises and the salinity falls as freshwater flows into the Inlet from the catchment. In a typical year, by the time of bar opening the salinity has fallen to less than half that of seawater. Shortly after bar opening marine water intrudes into the Inlet causing a sharp rise in bottom water salinities, however surface water salinities continue to fall until about October.

Figure 19: Salinity (ppt), red line is surface and blue bottom water

This phenomenon whereby the bottom waters may be significantly saltier than surface waters is called salinity stratification (see boxed aside). Periods of salinity stratification may last from days to weeks depending on wind mixing. When mixing does occur stratification is usually re-established shortly afterwards. Episodes of salinity stratification persist until about December, during which time the marine intrusions are mixed into the surface and the overall salinity of the Inlet again reaches about two thirds that of seawater.

Dissolved oxygen

At the start of the year the dissolved oxygen concentration in surface waters is saturated at about 8 mg/L (Figure 20). From late summer until mid winter the concentration increases because colder, fresher water holds more oxygen than warmer, saltier water. By mid-winter the surface waters may hold something like 10 mg/L of oxygen. The spring period typically sees dissolved oxygen concentrations in surface waters rise above 10 mg/L due to the activity of phytoplankton. By the end of the year surface dissolved oxygen concentrations have fallen back to about 8 mg/L.

Figure 20: Dissolved oxygen (mg/L), red is surface and blue bottom

Bottom waters in the Inlet display very different seasonal behaviour. Bottom water concentrations may be lower than surface waters because there is more organic material and bacterial activity at the bottom of the Inlet and it is more difficult to mix oxygen into deeper bottom waters than it is into shallow surface waters. Bottom water concentrations in January are usually about 8 mg/L, increasing through winter to about 10 mg/L as the water cools. Following bar opening, and the establishment of stratification, deoxygenation of bottom waters may occur with dissolved oxygen concentrations falling below 2 mg/L (see stratification section). Episodes of deoxygenation in bottom waters, lasting days to weeks, persist through spring and summer while stratification is present. Concentrations in bottom waters rise to about 8 mg/L once mixing is again effective.

Nutrients

Nutrient concentrations in the Inlet are generally low through most of the year. Orthophosphate, ammonium and nitrate concentrations are often very close to or below their respective analytical detection limits of 0.003 mg/L, 0.005 mg/L and 0.005 mg/L (Figures 21 to 23). These low concentrations are attributed to the rapid biological uptake of the nutrients in the Inlet by plants and algae. In fact the only occasions when dissolved nutrients are detectable in the water is at times when their rate of delivery to the Inlet outstrips their rate of biological uptake. The two annual events that lead to measurable nutrients in the water column are the winter runoff from the catchment and the recycling of nutrients from the sediment under low dissolved oxygen conditions.


Figure 21: Ammonium (mg/L), red is surface and blue bottom water		Figure 22: Nitrate (mg/L), red line is surface and blue bottom water

Figure 23: Orthophosphate (mg/L), red is surface and blue bottom

² The process of converting biologically available nitrogen in the water into biologically unavailable nitrogen gas.

While river flow is an obvious source of nutrients, the nutrients that are actually biologically available to plants and algae only make up a part of the total catchment nutrient load. About half of the phosphorus and more than 80% of the nitrogen from the catchment are bound up in nutrient forms that plants and algae are unable to directly utilise, these accumulate in the water over winter and most are lost to sea with marine exchange. The majority of catchment derived nutrients that are biologically available to plants and algae are trapped by the Ruppia, its epiphytes and macroalgae and are only briefly detected in the water.

Nitrate is usually only detectable in the Inlet after significant river flow, around about July. Nitrate is primarily detected in surface waters. After the flow subsides, around about October, nitrate is no longer detected in the Inlet; it has either been taken up by biota or denitrified². Both ammonium and orthophosphate are usually only detectable in the Inlet during periods of bottom water stratification as they are mainly regenerated from the sediments (see stratification section). They are usually detected in bottom waters between October and January before they are taken up by plants and algae. Ammonium and orthophosphate concentrations may also increase following the breakdown of algal bloom material.

Phytoplankton algae

³ The annual 'die back' at the end of the peak growth season.

The annual cycle of phytoplankton events in Wilson Inlet is usually dominated by the spring bloom (see Figure 24). It has been demonstrated that all of the major spring phytoplankton blooms have been related to a chain of events beginning with bar opening and marine exchange leading to stratification and nutrient release. As discussed in the boxed aside, stratification precipitates the deoxygenation of bottom waters and the subsequent increase in the recycling of nutrients into the water column from the sediment. With warming water temperatures and increasing day lengths these pulses of sediment derived nutrients lead to blooms of diatoms and dinoflagellates. Late summer bloom events have occurred in some years also, these also appear to be a result of nutrient recycling, in this case related to the senescence³ of Ruppia, its epiphytes and macroalgae. On occasion summer blooms have been related to wind mixing stirring up bottom sediments.

Figure 24: Chlorophyll a (mg/L), red is surface and blue bottom