Water Quality

Primary productivity and light climate

Light availability

Light is important in streams because it is necessary for plants (algae and macrophytes) in order for photosynthesis to occur. During photosynthesis, inorganic carbon (CO²) is transformed into carbohydrates. Light is one factor governing rates of instream primary production because the growth of most aquatic plants is limited by light availability.

Ambient sunlight available to aquatic plants is limited by two principal factors:

• shading by riparian vegetation; and
• turbidity of the water itself.

Riparian vegetation shades streams, decreasing the amount of sunlight reaching the water surface and reducing daily and seasonal extremes of water temperature. Water temperature influences pH and dissolved oxygen concentration, which affects the species composition and abundance of invertebrates and fish (Rutherford et al., 2004). Shading controls primary productivity within the stream by reducing light availability. The degree of shade created by riparian vegetation is influenced by several factors, including canopy height, foliage density, channel width and orientation, valley topography, latitude and season. The effect of shading on the structure and function of stream ecosystems is greatest in small streams (Bunn et al., 2002).

Suspended particles are the dominant influence on light penetration in most natural waters (Davies-Colley and Smith, 2001), with the exception of highly coloured waters where absorption can be more significant. Turbidity is a measurement of the degree to which light traveling through the water column is scattered by suspended organic and inorganic particles. The scattering of light and thus turbidity increases with a greater suspended load. Turbidity therefore reduces the depth to which sunlight penetrates into water and the higher the turbidity, the greater this effect. The depth to which light penetrates is the photic depth of the waterbody, which is the depth below which there is insufficient light for plants to photosynthesise. This depth can be shallow in highly turbid rivers with instream primary production limited to floating or emergent plants and benthic, planktonic or submerged plants only within the shallow photic zone. Rates of primary production in deeper mid-channel areas can be two orders of magnitude lower than in shallow littoral areas (Bunn and Davies, 1999). Primary production is then limited to the uppermost layers of the water. Where turbidity is high and light penetration subsequently low, phytoplankton assemblages are usually dominated by either flagellated eukaryotic algae or vacuolate cyanobacteria which are selectively favoured due to their ability to actively position themselves within a suitable light environment.

Turbidity is strongly influenced by river flows and run-off. An example is given in Shafron et al. (1990) for changes in turbidity in the Darling River. Turbidity recorded during a drought was approximately 25 NTU, but increased twenty-fold during a flood event.

Productivity

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). It was expected that, given the high turbidity and thus limited light penetration, aquatic primary production would be light-limited and the riverine food web would be dependent on terrestrial carbon. However the band of algae along the shallow littoral zone of waterholes, where light can penetrate to the streambed, was found to be highly productive (about two orders of magnitude greater than that measured in the main channel). Stable carbon isotope analysis showed that terrestrial carbon was not an important source of energy in the aquatic food web, despite its presence in large quantities. However, the littoral band of algae was the major source of energy for aquatic consumers, ultimately supporting large populations of invertebrates and fish. Much of the observed spatial variation in benthic primary production in turbid river waterholes can be explained by variations in turbidity and this in turn may be influenced by waterhole morphology (Bunn et al., 2003).

In-channel flows in these turbid rivers are likely to have a significant influence on aquatic primary production. Depth increases of only 20 cm are likely to curtail benthic primary production as algae become submerged below the photic depth. Flow events that last for days to weeks may have significant consequences for consumers dependent on algal food resources (Bunn et al., 2006). In these turbid systems, the contribution to productivity by the phytoplankton is low. Generally the phytoplankton is dominated by flagellated cryptophytes, eugleonphytes and chlorophytes which are able to actively maintain their position in the water column, and in relation to the very shallow photic depth (McGregor et al., 2006). The other major algal community is restricted to the benthos. Many of these species form littoral bands along the shoreline of streams and waterholes and are motile, thus potentially capable of tracking fluctuations in water depth (McGregor et al., 2006). Isotopic signatures further indicated that zooplankton (presumably feeding on phytoplankton) was the other likely major carbon source (Bunn et al., 2003).

The infrequently inundated floodplains typical of much of the Murray-Darling province are another important location for aquatic primary production. Primary production on similar floodplains in the Lake Eyre and Bulloo province has been studied by Bunn et al. (2006). Shortly after floodplain inundation rates of benthic and pelagic gross primary production were low, although there was a high rate of benthic respiration. After one month of inundation, high rates of benthic metabolism were recorded. Although rates were not as high as those observed in waterholes during prolonged dry periods, Bunn et al., (2006) estimated that, because of the large floodplain areas inundated, the amount of algal carbon produced on the floodplain during a single day of inundation at this time was equivalent to over 80 years of aquatic production in the permanent waterholes during the dry.

Jeffries and Mills (1990) state that there is a particular pH range in which the photosynthetic activity of dense plant growth or algal blooms can cause a rise in the pH of the water during the daytime that is lethal to fish.