The chlorophyll a concentrations of post Channel years were markedly lower than those recorded prior to the opening of the Dawesville Channel (Figure 8). This was significant at all sites in both surface and bottom waters during spring and summer; and at sites 2 and 1 at the surface and sites 2, 31, 1 and 58 in the bottom water during winter. Mean chlorophyll a concentrations during post Channel years were all < 10 µg L-1, while in pre Channel years, mean concentrations were mostly >10 µg L-1 and at site 1 in the Harvey >100 µg L-1. Mean chlorophyll a concentrations were higher in Harvey Estuary sites than Peel Inlet sites for both pre and post Channel years.
The variability of chlorophyll a concentrations was lower in post Channel years, with pre Channel concentrations ranging from 0.1 µg L-1 on a number of occasions, to 2302 µg L-1 at site 58 (29th October, 1985). Chlorophyll a concentrations often exceeded 100 µg L-1, particularly at sites in the Harvey Estuary prior to the opening of the Channel (Appendix 1). During post Channel years, chlorophyll a values ranged from 0.1 µg L-1 on a number of occasions to 253 µg L-1 at site 58 (15th July, 1997; this was the only occasion that a chlorophyll a value > 100 µg L-1 was recorded during post Channel years).
3.7.1 Filterable reactive phosphorus
There was no clear change in mean filterable reactive phosphorus concentration between pre and post Channel years (Figure 9). Surface filterable reactive phosphorus concentrations during pre Channel years were significantly lower than post Channel years at site 2 during summer. Bottom reactive phosphorus concentrations were significantly lower during post Channel years site 58 during winter, spring and summer; and site 2 during spring and summer (sites adjacent to the Channel). This was also the case for site 7 in the Peel Inlet during spring.
Mean inorganic phosphorus concentrations were higher at sites 4 and 31 (adjacent to river inflows) than other sites during both pre and post Channel years. Highest inorganic phosphorus values during pre and post Channel years were recorded at site 4 in the Peel Inlet. The maximum reactive phosphorus concentration recorded in pre Channel years was 325 µg L-1 (21st June, 1988) and during post Channel years was higher at 391 µg L-1 (12th July, 1994). Inorganic phosphorus concentrations exceeded 100 µg L-1 on a number of occasions in both sample groups (Appendix 1).
Mean organic (particulate) phosphorus concentrations were predominantly lower during post Channel years than those recorded prior to the opening of the Dawesville Channel (Figure 10). This was statistically significant in surface and bottom water at sites 2 and 58 during winter and all sites, with the exception of site 4 during spring and summer. However, winter organic phosphorus concentrations at site 4 in the Peel Inlet were higher during post Channel years than prior to the opening of the Dawesville Channel. This was statistically significant in the bottom water.
The seasonal variation in particulate phosphorus concentrations has also changed
since the opening of the Dawesville Channel. During pre Channel years, higher
concentrations were recorded in the Harvey Estuary during summer; with the maximum
organic phosphorus concentration of 1460 µg L-1 recorded in
the surface water of site 1 (3rd December, 1985). Organic phosphorus
concentrations were more evenly distributed throughout the year after the opening
of the Dawesville Channel, with maxima recorded during winter at sites 4 and
31 closest to river inflows (292 µg L-1 in the surface water
12th July, 1994; and 334 µg L-1 in the bottom water
6th August, 1996; respectively).
Mean nitrate-nitrite concentrations were predominantly lower during post Channel years, with the exception of winter in the Peel Inlet (Figure 11). During winter, nitrate-nitrite concentrations were significantly higher in the surface water at site 4 and in the bottom water at sites 4 and 7 during post Channel years. Nitrate-nitrite concentrations in the bottom water at the remainder of the sites (2, 31, 1 and 58) were significantly lower during winter in post Channel years. Bottom concentrations of nitrate-nitrite were also significantly lower in post Channel years at sites 2, 31 and 58 during spring and 7, 2, 31, 1 and 58 during summer. Surface nitrate-nitrite concentrations were significantly lower after the opening of the Dawesville Channel at sites 2, 31, 1 and 58 during summer.
Winter concentrations of nitrate-nitrite were generally higher during winter at all sites, with means > 100 µg L-1 in both pre and post Channel years. The maximum post Channel value recorded (2113 µg L-1) was in the surface water at site 4 on the 2nd of August, 1996. However, the maximum nitrate-nitrite concentration during pre Channel years (2880 µg L-1) was recorded during summer (24th February, 1987) at site 31 in the Harvey Estuary (Appendix 1).
Mean ammonium concentrations in the Peel-Harvey Estuary were predominantly lower during post Channel years (Figure 12). Ammonium concentrations in surface and bottom waters of the Harvey Estuary were significantly lower during post Channel years during winter, spring and summer at all sites with the exception of site 31 (bottom water spring and surface and bottom water summer). The ammonium concentrations recorded at site 2 in the Peel Inlet were also significantly lower during post Channel years during all three seasons.
Ammonium concentrations were generally higher during winter than other times of the year in both pre and post Channel years. With Harvey Estuary sites (31, 1 and 58) generally higher than those in the Peel Inlet pre Channel, and the sites closest to the river inflows (4 and 31) higher than those adjacent to the Channel in post years. Maximum ammonium concentrations were recorded in bottom waters of Harvey Estuary sites during winter in both pre and post Channel years. The pre Channel maximum was 1584 µg L-1 recorded at site 1 (9th July, 1985) and greatest ammonium concentration recorded after the opening of the Dawesville Channel was 712 µg L-1 at site 31 (2nd August, 1994).
Organic (particulate) nitrogen concentrations were significantly lower during post Channel years in the surface and bottom water during winter, spring and summer at all six sites (Figure 13).
Organic nitrogen concentrations prior to the opening of the Dawesville Channel were highest during spring and summer and higher in the Harvey Estuary than the Peel Inlet. There was a more even distribution of particulate nitrogen concentrations during post Channel years, with higher concentrations recorded at sites 4 and 31 (adjacent to river inflows) than sites 2 and 58 (close to the Dawesville Channel).
The maximum organic nitrogen concentration recorded during pre Channel years was 26,126 µg L-1 in the surface water of site 58 in the Harvey Estuary (5th November, 1985). In contrast, the maximum organic nitrogen concentration recorded during post Channel years was 2035 µg L-1 in the bottom water of site 4 in the Peel Inlet (an order of magnitude lower).
Mean orthosilicate concentrations were predominantly lower post Channel than those of pre Channel years (Figure 14). This was statistically significant in the bottom water during winter, spring and summer at all sites except site 4 in the Peel Inlet. Surface water orthosilicate concentrations were significantly lower during post Channel spring and summer at all sites in the Harvey Estuary, as well as site 2 in the Peel Inlet. Winter orthosilicate concentrations of surface water were statistically similar between pre and post Channel years at all sites.
Orthosilicate concentrations were highest in winter both before and after the
opening of the Dawesville Channel. Summer concentrations in the Harvey Estuary
were also high during pre Channel years, but at a minimum after the opening
of the Channel. Pre Channel orthosilicate concentrations ranged from 15 µg
L-1 in the surface water of site 7 (28th October, 1986)
to 8300 µg L-1 in the surface water of site 2 (3rd
December, 1985). Post Channel orthosilicate concentrations ranged from 13 µg
L-1 in the bottom water of site 31 (4th February, 1998)
to 4100 µg L-1 in the bottom water of site 4 (23rd
July, 1996).