Water Quality

• Turbidity
• Photic Depth
• Benthic Metabolism

Turbidity

Turbidity within the Murray-Darling province is generally high, but can be low at upland or headwater sites. At lowland sites, turbidity remains high even during the sometimes long periods between flows (Bailey, 2001; Bunn et al., 2003). High turbidity has always been a natural feature of the river water in the Murray-Darling Basin (Crabb, 1997).

Light penetration is generally limited to < 16 cm.

Information about primary productivity and light climate

Information about methods

Photic Depth

Light penetration of the water column has been investigated at 15 sites on the Warrego River in the Murray-Darling province. Photic zone depth is shallow, reflecting the high turbidity of the system (Christy Fellows, unpublished data). Light is one factor governing rates of instream primary production because the growth of most aquatic plants is limited by light availability.

Benthic Metabolism

Benthic metabolism has been measured twice at 30 sites throughout the Condamine Balonne system (NR&M unpublished data), four times from four sites and twice from a further 11 sites in the Warrego catchment (Christy Fellows, unpublished data) and once from 11 sites in the Border Rivers (Barwon, Weir and Macintyre catchments) (Christy Fellows, unpublished data). Recorded GPP is on average low in the province, but is variable. Benthic respiration rates are on average higher than benthic GPP, meaning benthic communities in the province are typically net consumers of carbon. In some cases however, this situation is reversed, and they can be net producers of carbon. In contrast to rivers in the Lake Eyre and Bulloo province, rates of benthic metabolism in the Murray-Darling province are relatively low, despite similarities in climate, turbidity and nutrient status (Bunn et al., 2006; Christy Fellows, unpublished data; NR&M unpublished data). The underlying reasons for this are not known but may relate to differences in waterhole geomorphology, disturbance by introduced Carp (which are absent in the Lake Eyre and Bullo province) or hydrology with more frequent flows in the Murray-Darling province (Bunn et al., 2006).

Phytoplankton production can also occasionally be high in the surface waters of these turbid systems in Australia during periods of no-flow, as indicated by significant daily variations in dissolved oxygen (Bunn et al., 2003). Actual rates of water column production in waterholes have been estimated using measurements of dissolved oxygen change in sealed light and dark bottle chambers. These rates range from 1.5 mg C L^-1 day^-1 to 500 mg C L^-1 day^-1, during extended periods of no-flow (Bunn et al., 2006).

Primary production and food webs in isolated waterholes have been studied in turbid floodplain rivers, typical of much of the Murray-Darling Province (Bunn et al. 2003).

In upland segments of some rivers within the Murray-Darling province, where turbidity is lower and flow less intermittent, light penetration is not as limited so benthic production is unlikely to be limited to littoral zones. In such situations submerged macrophytes also contribute to primary production.