The Dawesville Channel now allows more than three times greater exchange of water between the estuary and the ocean than before, with 10% of the estuary's volume being flushed each day (Peel Inlet Management Authority, 1994). Prior to the opening of the Channel, a number of predictions were made on the effect this increased flushing would have on water quality in the estuary. The majority of these centred around significant improvements in water quality parameters.
It was predicted that the Dawesville Channel would have a two-fold effect on
salinity (Peel Inlet Management Authority, 1994):
1. an increase in salinity to approximately that of seawater
(35 ‰) for much of the year; and
2. a decrease in salinity range between wet and dry seasons.
Although salinity has become somewhat more stabilised, the change has not been as dramatic as predicted. Mean winter salinity levels in post Channel years (Figure 3) were still lower than those of the open ocean, with surface salinity of < 1‰ on a number of occasions at sites 4 and 31 adjacent to river inflows; and < 10 ‰ at sites adjacent to the Channel (Appendix 1).
During spring (previously a time of high Nodularia growth) salinity was significantly higher, with means > 20 ‰ at most sites. Prior to the opening of the Dawesville Channel, mean salinity, particularly in the Harvey Estuary was much lower, with levels remaining < 10 ‰ for much of this season (Appendix 1). Nodularia is a freshwater alga that requires near fresh conditions, particularly at the beginning of a bloom.
During summer, salinity was still above that of seawater and predominantly > 40 ‰. In this respect, there has been little change since the opening of the Dawesville Channel. Although the incidences of salinity > 50 ‰, which were recorded during the late 1970's, no longer occur, maximum values of 49 ‰ were obtained during post Channel years (Appendix 1).
4.2 Stratification and dissolved oxygen
The Peel-Harvey Estuary has a historical record of stratification with a layer of freshwater from runoff and river inflows overlying more dense, saline marine waters, resulting in deoxygenation of the bottom waters. In years preceding the construction of the Dawesville Channel, this phenomenon persisted for much of the year (e.g. 1981 and 1982, Hearn and Lukatelich, 1990).
It was predicted that the Dawesville Channel would result in decreased periods of stratification and increases in dissolved oxygen concentration (Peel Inlet Management Authority, 1994). Due to the variation in sampling frequency, it was difficult to determine the period of stratification. When samples were taken on a monthly basis, the result was only a snapshot of the system, and cannot be used to imply the conditions that persisted between sampling events.
However, the most significant change in stratification noted, since the opening of the Dawesville Channel, was a complete lack of deoxygenation of bottom waters during summer and a reduced number of incidences during spring (Figure 5). The greatest effect occurred at sites adjacent to the channel, where there were only two recorded dissolved oxygen stratifications at site 2 and none at site 58, after the opening of the Channel. At sites 4 and 31 (those closest to river influences) stratification still persisted for winter and much of spring.
Although there was no significant difference in dissolved oxygen concentrations during winter, between pre and post Channel years (Figure 4), the increased flushing has lead to significant increases in bottom water dissolved oxygen concentrations at sites closest to the Channel during summer. This has had a great influence on the concentrations of inorganic nutrients being released from the sediments (see section 4.4).
Surface dissolved oxygen concentrations have also changed since the opening
of the Dawesville Channel, particularly in the Harvey Estuary. Post Channel
concentrations were significantly lower in surface waters during spring. This
is most likely related to the absence of spring algal blooms (see
section 4.3). Surface waters of the Harvey Estuary, prior to the opening
of the Dawesville Channel were often super-saturated with dissolved oxygen as
photosynthesising phytoplankton released oxygen into the water column.
4.3 Chlorophyll aand water clarity
The predicted changes in chlorophyll a concentration and water clarity were as follows (Peel Inlet Management Authority, 1994):
1. loss of spring and summer Nodularia blooms;
2. loss of winter diatom blooms;
3. possibility of marine phytoplankton blooms; and
4. increases in water clarity.
There have been dramatic reductions of chlorophyll a concentrations during spring and summer since the opening of the Dawesville Channel (Figure 8). There have been no reported Nodularia blooms in the Harvey Estuary and reduced winter chlorophyll a concentrations, suggesting a reduction in winter diatom blooms. There was a large phytoplankton bloom centred at site 58 during July 1997, which extended into the middle of the estuary at site 1 (Appendix 1). This chlorophyll a concentration (245 m g L-1) was the highest recorded since the opening of the Dawesville Channel and was caused by a non toxic marine dinoflagellate (Heterocapsa triquetra, Wasele Hosja pers. comm.). This suggests that all the predictions about the effect of the Dawesville Channel in reducing chlorophyll a concentrations were correct.
The salinity of bottom waters during spring was predominantly > 20 ‰ which was higher than that required for the germination of Nodularia. According to Huber (1994) Nodularia akinetes (spores) reside in the sediment and germinate after several days of near fresh salinity. The increased flushing of the Dawesville Channel has caused an increase in salinity sufficient to prevent germination. However, there has still not been a season of high rainfall since the opening of the Channel. It is possible that the periods of low salinity recorded during winter could be extended if periods of high rainfall occurred during late winter or early spring. Post Channel salinity levels of < 10 ‰ have been recorded even at sites closest to the Channel. Therefore, it is possible for bottom salinity to reach the critical level required for germination of Nodularia. It should also be noted that there have been incidences of Nodularia blooms in the Serpentine River in 1997 and 1998 (D. A. Lord and Associates, 1998) which would be replenishing the akinete store in the sediments of the Harvey Estuary.
The reduction in chlorophyll a was reflected in light attenuation, with decreases in light attenuation coefficients, particularly in spring (Figure 7) after the opening of the Dawesville Channel. The effects of sediment-laden river water and possibly wind re-suspension of sediments, however, remains unchanged. Light attenuation has not changed significantly at sites close to river inflows, and actually slightly increased at site 4 in the Peel Inlet during winter.
The absence of algal blooms has affected the water quality in the Peel-Harvey Estuary in many ways. Post Channel pH levels have stabilised, and remain in the 7 to 8.5 range. Prior to the opening of the Channel, pH of surface waters > 10 were recorded during algal blooms, when available carbon dioxide was removed from the water column via photosynthesis; and bottom pH values were as low as 2, when bacterial breakdown of organic matter released large amounts of carbon dioxide into the system. This, together with deoxygenation of the bottom water lead to the release of inorganic nutrients from the sediment (McComb and Lukatelich, 1995). With a break in the cycle of winter diatom bloom and decay, followed by spring/summer Nodularia blooms and decomposition, sediment release of nutrients has been reduced (see section 4.5).
It was predicted that nutrients entering the system would no longer accumulate after the opening of the Dawesville Channel, but rather be flushed out to sea (Peel Inlet Management Authority, 1994).
There has been a dramatic decrease in the concentrations of particulate (organic) nitrogen and phosphorus since the opening of the Dawesville Channel, particularly during spring and summer (Figures 10 and 13). The majority of the organic nutrients in the water column, prior to the opening of the Dawesville Channel, could be attributed to spring and summer algal blooms (McComb and Lukatelich, 1995). With the decease in phytoplankton populations, since the Dawesville Channel, there has been a decrease in the biomass associated nitrogen and phosphorus. This has happened to a lesser extent with the decline in winter diatom blooms.
McComb and Lukatelich (1995) stated that the non-phytoplankton organic nitrogen and phosphorus concentrations were mostly due to wind related re-suspension of sediments. While this may still be a factor, the post Channel pattern of nitrogen and phosphorus concentrations, particularly during winter, indicate river water influences (Figures 10 and 13). Organic nitrogen and phosphorus concentrations were considerably higher at sites closest to river inflows (4 and 31) than sites adjacent to the Channel (2 and 58). This suggests that the rivers may be a significant source of particulate nutrients into the system, and that the flushing effect is only effective in the area adjacent to the Channel.
The concentration of particulate phosphorus at site 4 in the Peel Inlet was higher in post Channel years, than the five years prior. This suggests that the amount of phosphorus entering the system from the Serpentine and / or Murray Rivers has increased and this site is beyond the flushing abilities of the Channel.
There was no consistent pattern of mean inorganic nutrient reduction in post Channel years (Figures 9, 11 and 12). The distribution of filterable reactive phosphorus concentrations of high recordings closest to the river and low levels adjacent to the channel, indicates that the rivers are still a source of inorganic phosphorus, and that flushing is only effective in areas close to the Channel. Spring filterable reactive phosphorus concentrations at sites 4 and 31 were higher during post Channel years than the five years previous, indicating that there has been an increase in phosphorus loading from the catchment. The mean concentrations of filterable reactive phosphorus exceeded the ANZECC (1992) guidelines for the control of phytoplankton growth in estuarys of 5 to 15 m g L-1 at the majority of sites during winter and sites 4 and 31 during spring in post Channel years. This indicates that if all other conditions were conducive, phytoplankton blooms in the Peel Harvey Estuary would still be possible.
There have been dramatic reductions in ammonium concentrations since the opening of the Dawesville Channel, particularly in bottom waters. Prior to the opening of the Channel, elevated ammonia concentrations in the water column were linked in many cases to anaerobic conditions, and therefore indicative of sediment nutrient release. In anaerobic sediments, bacterial decomposition of organic matter can result in the release of ammonium and phosphate into the water column (Thornstenson and MacKenzie, 1974). As discussed in section 4.2, there has been an increase in bottom water dissolved oxygen concentrations and a stabilisation of pH levels. This coupled with the reduction in detrital biomass from phytoplankton has probably contributed to a decrease in inorganic nutrient release from sediments.
Ammonium concentrations at sites close to river influence were still high, and these were the sites that remained stratified for much of the winter period. Therefore, it is possible that inorganic nutrients in these areas may be from both river inflows and sediment release.
The biggest reductions in inorganic nitrogen between pre and post Channel years occurred during summer in the Harvey Estuary and site 2 of the Peel Inlet (Figures 11 and 12). It is likely that the high concentrations in years prior to the Channel opening were related to release from decaying phytoplankton cells after spring and early summer blooms. With the absence of large phytoplankton biomass there has been a corresponding decrease in inorganic nutrients at these sites.
The final significant change in inorganic nutrient concentrations was the dramatic increase in nitrate-nitrite concentrations at sites 4 and 7 in the Peel Inlet (Figure 11). The Murray River has, in the past, been cited as a source of nitrate-nitrite (McComb and Lukatelich, 1995). It is possible that this has not changed in over the last decade, and may have in fact increased. The flushing effects of the Channel do not appear to be sufficient to reach the areas located the most distance away, particularly during times of river flow.
There have been dramatic decreases in orthosilicate concentrations, especially during spring and summer, although winter inputs have remained high. Freshwater inflows bring silica from weathering of minerals by runoff into the estuary system. The concentrations of orthosilicate in river systems can be two orders of magnitude higher than those found in marine waters (Anderson, 1986). Therefore, it would appear that the winter concentrations of orthosilicate in the Peel-Harvey Estuary are primarily determined by river discharge.
Prior to the Channel opening, winter orthosilicates that entered the system were taken up by winter diatom blooms. The decline of these diatoms in the spring not only lead to a release of nitrogen and phosphorus, but may also have lead to a release of silicates into the water column. Although after death, diatom frustules dissolve slowly, this dissolution is enhanced by increasing salinity (Anderson, 1986). Anderson (1986) also noted that there was a release of orthosilica into the water column in an estuary after the decline of a diatom bloom. It is possible, therefore, that the decrease in winter growth of diatoms after the opening of the Dawesville Channel has stopped cycling of orthosilicate in this system.
The orthosilica that enters the system now, may simply be flushed out to sea
over the spring and summer months, or perhaps be taken up by marine diatoms,
which have become a more dominant part of the flora since the increases in salinity
(Wasele Hosja pers. comm.)