Trends and spatial and temporal variability

Some mean nutrient concentrations in the Clyde and Shoalhaven Rivers differ from those reported for other temperate estuaries. A comparison of published information on water quality in temperate and sub-tropical estuaries in NSW, Australia, and of other temperate estuaries in North America, Asia and Europe was undertaken (Scribner et al., 1985; Eyre and Twigg, 1997; Eyre and Balls, 1999; Fisher et al., 1999; Rask et al., 1999; Songsangjinda et al., 2000; Wyong Shire Council, 2000; Dell'Anno et al., 2002; Moore, 2003; Paterson et al., 2003a; Piola, 2003; Soletchnik et al., 2005; Drewry et al., 2006 in press).

The parameter that differs most in the Clyde R compared to publish values was FRP-P. Concentrations of FRP-P were extremely low, being 3 to 10 times lower, on average, than in other estuaries on the south coast of NSW. The maximum concentrations of NH4+ and DSi in

the Clyde R. were equal to the average concentrations in other temperate estuaries. Mean concentrations of TSM, POM, POC and the mean value of C:N in the Clyde R. were similar to mean values for other temperate estuaries (Scribner et al., 1985; Paterson et al., 2003a; Piola, 2003; Drewry et al., 2006 in press).Ammonia concentrations in the Shoalhaven River were lower than average values in temperate estuaries.

Other nutrient concentrations in the Shoalhaven and Clyde Rivers were higher than average values in other temperate NSW estuaries. Concentrations of DIN (2 to 10 times higher), PN and PC:Chl were markedly higher compared to temperate estuaries in mid and south coast NSW (Scribner et al., 1985; Paterson et al., 2003a; Piola, 2003). Some parameters such as Chl-a vary across temperate estuaries according to: a) geographic location along the NSW coast due to different water temperatures, with higher values of Chl-a in the north and; b) major catchment development with increased nutrient delivery. Chl-a in the Clyde R. corresponded to typical concentrations for a pristine estuary on the south coast of NSW. These levels are significantly lower than those found in estuaries with a higher level of development within the catchment (Paterson et al., 2003a; Piola, 2003). Chl-a levels across the Clyde R. locations varied between 0.2 and 3µg/l except during rain events when values reached 5ug/l. Chl-a level in the Shoalhaven sampling sites were similar to the Clyde, although some of the oyster growing grounds had consistently higher chl-a concentrations (Shoal-5) than any of the grounds in the Clyde River.

The low nutrient concentrations, especially DIN, TN, FRP-P, TP, TDN, TOC, DOC and POC, increased in flood conditions. The decrease in levels of dissolved nitrate and phosphorus following a major rain event might be a result of phytoplankton depleting these resources. During these ‘wet’ periods, the concentration of phosphorus in the estuary seems to be limited

as illustrated by the values of DIN:DIP ratios (more information is discussed in following section 5.4.5.on nutrient limitation).

Physico-chemical parameters did not vary as much between locations as they did between seasons. Of all the sampled variables and seston quality ratios, FRP-P, TSM, DSi, Chl-a, TOC, DOC and the PC:TSM ratio showed strong gradients along the estuaries at a highly significant level (P<0.001). In the Clyde River FRP-P and TSM levels were higher at the oceanic sites while the other parameters were higher at the top mid-stream or upstream sites (see Table 5-13). This may be due to resuspension of benthic deposits at the Clyde River mouth. In the Shoalhaven most of the mean parameters decreased down the estuary approaching the ocean (Table 5-12). This is consistent with the level of development in the Shoalhaven mid-catchment compared with that in the Clyde.

Water quality parameters generally showed more variability between seasons than between sampling sites. A summary of the seasonal variations found with a significance level of P<0.001 is given in Table 5-13. Shoalhaven water quality data was only collected for 1 year and 3 months and consequently, no replications for seasons could be analysed and any statements regarding seasonality in this estuary need to be treated with care.

In each season the relative importance of the various nutrient forms is different. For instance, on average winter in the Clyde River was characterized by high levels of NOx and low levels of NH4+ whilst autumn was the reverse. For the particulate nitrogen, maximum levels

occurred in summer with low levels in spring. In addition, TP and TDP were significantly high during winter, while the dissolved inorganic fraction was high in autumn and the particulate fraction dominated in summer.

During dry periods in winter in the Clyde River, TSM and POM concentrations were extremely low, enhancing water clarity. Despite the low turbidity, Chl-a and Phaeo concentations were also low, presumably due to nitrogen limitation during dry winters and as a result of low water temperatures. However, the analyses indicate that POC, TP and TDP are available in the water column during this season. In contrast, in the Shoalhaven most of the nutrients reached high levels during winter, except for Chl-a, POM, TP and PP. During summer in the Clyde River, the particulate fractions of N and P, as well as the organic component of TSM, are higher. This, in combination with the significantly high levels of DSi, contributed towards the high concentrations of Chl-a during summer in the Clyde.

Table 5-12: Schematic diagram showing the spatial variability of the water quality parameters that had a strong gradient along the Clyde River and the Shoalhaven River

Clyde

Shoalhaven

Ocean Midstream Upstream Ocean Midstream Upstream

DSi DSi DSi DSi

FRP-P FRP-P FRP-P FRP-P

Chl-a Chl-a Chl-a Chl-a

TSM TSM TSM TSM

TOC TOC TOC TOC

DOC DOC DOC DOC

Table 5-13: Seasonal variability in water quality parameters for Clyde (C ) and Shoalhaven (S) Rivers. Highest and lowest significant levels are shown with triangles (at the significance level P<0.001)

WINTER SPRING SUMMER AUTUMN NOx NH4 PN FRP-P TP TDP PP DiSi Chl-a Phaeo TSM POM POM:TSM Chl:TSM PC:Chl PN:TSM C:N

Triangular shape pointing down represents the lowest significant value found in the season where the icon appears for the Clyde (C ) and Shoalhaven (S) respectively

Triangular shape pointing up represents the highest significant value found in the season where the icon appears for the Clyde (C ) and Shoalhaven (S) respectively