Wilson Inlet 5
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Water Quality in Wilson Inlet from 1995 to 2002
Water quality for 1995 to 2002
The water quality that occurs in any year is a usually a rough approximation of the annual cycle described above. However interannual variations in the drivers of the system and the subsequent variations in the conditions at the start of a year (due to the summed effects of past interannual variations) result in some variability in the water quality between years.Bar opening
In each of the years between 1995 and 2001 the sand bar was artificially breached at its western end once water levels within the Inlet reached approximately 1 m above mean sea level. The average period of bar opening in the past 50 years has been about 190 days. Therefore the duration of opening in 1995 and 1996 were average, 1998 and 1999 were above average, and 1997, 2000 and 2001 were below average. The duration of opening ranged from about 50 to 289 days. The sixth Wilson Inlet report to the community describes the sand bar in more detail. However the inter annual differences are largely attributed to variations in ocean water levels, ocean wave conditions, and rainfall and runoff.Rainfall and runoff
Considerable variation in the runoff from year to year was a major factor driving interannual variations in the system. The highest volumes of river flow in the monitoring period occurred in 1996 and 1998, with the lowest volume, at less than half the highest, occurring in 2000. Notably, none of the years in the monitoring period had river flow to the Inlet above the long term average, which is estimated to be approximately 200 GL. Similarly the rainfall in each of the past six years at Denmark was below the long term average of 1120 mm.Marine exchange
Despite similar opening regimes (western opening when the Inlet level reached approximately 1m above mean sea level) there were marked differences from year to year in the volume of marine exchange. The major drivers of this variation were differences in the rainfall and runoff between years, differences in ocean wave conditions, differences in bar channel dimensions and perhaps most importantly differences in the ocean water levels, especially the timing of the passage of low pressure systems. (For example during the 1997/1998 season within the first four months after bar opening there had been only 50GL of marine exchange, whilst in 1998/1999 after four months there had been 140GL of exchange. The difference being due largely to variations in the ocean water levels driven by the passage of low pressure systems.)Water colour and clarity
While it was not possible to determine any significant difference in water clarity between years, there was a clear difference in water colour between years. The difference in water colour was primarily a result of the different volumes of river flow (rather than marine exchange). In high runoff years the Inlet water was twice as dark as in low runoff years.Salinity and stratification
Whilst the annual salinity cycles in the Inlet largely conformed to the typical pattern described above there were some notable exceptions (Figure 32). In particular salinities following the 1998 bar opening were much higher than previous years. Conversely the maximum salinities following the 2000 opening were low and, unusually, some salinity stratification persisted through the autumn of 2000.
Figure 32: Surface and bottom salinities in ppt in Wilson Inlet. The red line is the average of surface data and blue line is the average of bottom data for sites: WI6, WI35, WI12, WI14 and WI9. Where the lines diverge stratification is occurring.
As expected the rate of salinity decrease in any winter/ spring was clearly correlated with the volume of inflow and rainfall in that period. Whilst the rate of salinity increase in any spring/summer was strongly influenced by the volume of marine exchange. There was a reasonable relationship between the final salinity in any year and the volumes of rainfall, inflow, and marine exchange, although it was complicated by the apparent variations in mixing between years. On the whole there was a reasonable correlation between the intensity of stratification and the marine exchange; the more marine exchange, the more strong stratification.
Overall the interannual variation in salinity (and stratification) in the Inlet was primarily due to variations in rainfall and runoff, the sea level (particularly barometric conditions), wind energised mixing, ocean wave conditions (effecting the bar closure and subsequent bar channel dimensions) and possibly the timing of bar opening.
| Year | Breached | Shoaled | Duration open | Rainfall | River flow | Exchange |
|---|---|---|---|---|---|---|
| 1995 | 19 August 95 | 26 February 96 | 191 days | 950 mm | 105 GL | 105 GL |
| 1996 | 5 August 96 | 23 February 97 | 202 days | 970 mm | 185 GL | 165 GL |
| 1997 | 18 August 97 | 19 January 98 | 154 days | 830 mm | 115 GL | 60 GL |
| 1998 | 7 August 98 | 23 May 99 | 289 days | 1050 mm | 185 GL | 250 GL |
| 1999 | 30 June 99 | 6 March 00 | 250 days | 1000 mm | 135 GL | 110 GL to 25 October |
| 2000 | 21 July 00 | 30 November 00 | 132 days | 1030 mm | 90 GL | Not available |
| 2001 | 03 October 01 | 15 February 02 | 135 days | 930 mm | Not available | Not available |
Figure 31: Dates of bar opening and closing, duration of opening, the rainfall for the Denmark town site, the gauged flows from the catchment to the Inlet, and the volume of marine exchange (estimated using hydraulic modelling) for the 1995-2001 period (where available).
Dissolved oxygen
The annual dissolved oxygen cycles in the Inlet largely conformed to the typical pattern described above (Figure 33). Periods of super-saturation (eg October 1995 or August 1999) due to high phytoplankton productivity were often recorded post bar opening. Deoxygenation of bottom waters clearly tracked the stratification (there was a reasonable correlation between deoxygenation and stratification in the water profile data; a correlation coefficient of 0.7 when filtered for stratification due to marine intrusions). While complete anoxia was rarely recorded in water profiles, it was commonly recorded in bottom logger data (as in Figure 27, previous page) indicating perhaps a shortcoming of weekly to monthly profiling.
Figure 33: Dissolved oxygen in Wilson Inlet in mg/L. The red line is the surface average, the dark blue line the bottom average and the light blue line the bottom minimum from sites: WI6, WI35, WI12, WI14 and WI9. From 2000 onwards the frequency of sampling was reduced and therefore fewer low dissolved oxygen events may have been recorded. Where the surface and bottom concentrations diverge, in most cases deoxygenation is occurring.
The number of bottom water samples that were severely deoxygenated (below 10%) ranged from about 10% of bottom water samples in 1996 to 1% in 1999. Like the stratification, there was no single clear relationship between the frequency of deoxygenation in any year and the volume of river flow, marine exchange or wind strengths; all three factors play an interacting role in the occurrence of deoxygenation. The apparent lack of very low dissolved oxygen events from 2000 onwards is probably an artefact of the sampling frequency being reduced from weekly to monthly, and possibly also reflects weaker stratification due to poor bar openings and poor river flow. An apparent downward trend in dissolved oxygen concentrations over this period may be an artefact of a change of monitoring instrument on 25 February 2000 or may be a real, but as yet unexplained feature of the water quality data.
Nutrients
The annual nutrient cycles in the Inlet largely conformed to the typical patterns described above (Figures 34 to 38). The peaks in ammonium concentrations (Figure 34) strongly tracked the occurrence of stratification and deoxygenation (as discussed in the boxed aside this is due to the lack of dissolved oxygen preventing denitrification). Elevated ammonium concentrations began to be detected once dissolved oxygen fell below about 4 mg/L; in other words measurements of anoxia in the profile data were not a good gauge of the extent of sediment nutrient cycling. There was a 7-fold variation in maximum and a 5-fold variation in average ammonium concentrations between years. Higher ammonium concentrations were generally measured in years where more deoxygenation was also measured.
Figure 34: Ammonium concentrations in Wilson Inlet in mg/L. The dark blue line is the average and the light blue line the maximum for bottom waters and the red line the average for surface waters of sites: WI6, WI35, WI12, WI14 and WI9.
The measured nitrate concentrations (Figure 35) reflected the strength of river flow. Nitrate concentrations were generally lower than ammonium. There was a 30-fold variation in maximum and a 3-fold variation in average nitrate concentrations between years.
Figure 35: Nitrate concentrations in Wilson Inlet in mg/L. The red line is the average and the light orange line the maximum for surface waters and the blue line the average for bottom waters of sites: WI6, WI35, WI12, WI14 and WI9.
The peaks of orthophosphate (Figure 36) tracked stratification and deoxygenation (as discussed in the boxed aside, this is due to changes in the binding of phosphorus to sediment in the absence of oxygen) and the recycling of algal bloom material. The relatively high concentrations (compared to the normal background) through 1999 and the first half of 2000 appears uncharacteristic. The phenomenon has not been adequately explained as yet but may be related to the high salinities following the bar opening of 1998/1999. There was a 5-fold variation in maximum and a 2-fold variation in average orthophosphate concentrations between years.
Figure 36: Orthophosphate concentrations in Wilson Inlet in mg/L. The dark blue line is the average and the light blue line the maximum for bottom waters and the red line the average for surface waters of sites: WI6, WI35, WI12, WI14 and WI9.
In Figures 37 and 38 the total nutrient concentrations are compared to the ANZECC guidelines for water quality in south west Australian estuaries. The total nitrogen and phosphorus concentrations have largely (85% and 70% of sample period respectively) been below the guideline trigger values of concern for eutrophication impacts in slightly disturbed estuaries.
Figure 37: Surface and bottom water total phosphorus concentrations in Wilson Inlet in mg/L. The blue line is the average of bottom and the red line the average of surface concentrations for sites: WI6, WI35, WI12, WI14 and WI9. The green line is the ANZECC guideline for total phosphorus.
Figure 38: Surface and bottom water total nitrogen concentrations in Wilson Inlet in mg/L. The blue line is the average of bottom and the red line the average of surface concentrations for sites: WI6, WI35, WI12, WI14 and WI9. The green line is the ANZECC guideline for total nitrogen.
Phytoplankton algae
Spring blooms are evident in the first four years of the data set and in 2001, as are weaker late summer blooms in most years. Spring blooms were also present, but much less intense in 1999 and 2000. All of the spring phytoplankton blooms were caused by the same regular chain of events, described above, that follows bar opening; stratification, deoxygenation, nutrient recycling increase and subsequent bloom. There was a 3-fold variation in maximum chlorophyll a concentrations between years and a 20-fold variation in maximum phytoplankton cell numbers between years. The phytoplankton algal flora was dominated by small diatoms of the genus Chaetoceros with important contributions, especially during blooms, from the dinoflagellate Prorocentrum minimum. Over the seven years of data there is a decreasing proportion of flagellates to diatoms. The intensity of algal blooms between years was clearly affected by many changing interannual factors such as winds, the availability of nutrients, timing of bar opening etc. A detailed analysis has not been completed for all factors, however, summer bloom events appeared to be at a maximum in a period when above average orthophosphate concentrations were measured. Spring bloom intensity on the other hand correlated well with the availability of ammonium following deoxygenation and stratification.
Figure 39: Surface water chlorophyll a concentrations in Wilson Inlet in mg/L. The red line is the average and the light orange line the maximum for surface waters and the blue line the average for bottom waters of sites: WI6, WI35, WI12, WI14 and WI9.
