What is alkalinity?
Alkalinity is a measure of the buffering capacity of water, or the capacity of the water to neutralise acids and resist pH change. It is expressed in milligrams per litre as calcium carbonate (mg/L as CaCO3) and is present in the water predominantly due to the weathering of carbonate minerals in the catchment.
Alkalinity tends to increase in the main Lower Lakes water body when inflows to the River Murray are low. Alkalinity decreases in the Lower Lakes during flood inflows from the River Murray and tributaries which are dominated by rainwater with low alkalinity levels. Alkalinity can be reduced or completely consumed as acid is released from acid sulfate soils which have become exposed and then been rewet. When water acidification occurred in some areas of the Lower Lakes during the 2007-2009 hydrological drought, limestone was added to restore the alkalinity in Boggy Lake.
Past alkalinity levels
Historically, alkalinity levels within this region have been between 80 and 250 mg/L as CaCO3. During the 2007-2009 hydrological drought, alkalinity was high in the main lake water body but was very low or completely disappeared at some locations on the lake margins.
What is pH?
pH is an indicator of acidity or alkalinity. pH is a logarithmic scale and an increase or decrease of one pH unit is a 10 fold change. Neutral water has a pH of 7, acidic solutions have values between 0-6 and alkaline solutions have values between 8-14.
pH can increase when there are low inflows in the River Murray, and consequently minimal flushing of the Lower Lakes. pH can decrease during River Murray and tributary flood inflows as rainwater has a lower pH. pH can also decrease if exposed acid sulfate soils become rewet by rainfall or rising lake levels.
Past pH levels
The pH in the Lower Lakes is typically between 8 and 9. During the 2007-2009 hydrological drought, the pH remained relatively stable within the main body of Lake Alexandrina. However, acidic water (pH 2 - 4) was identified in several fringing areaas (e.g. Boggy Lake, Loveday Bay) following the exposure and rewetting of acid sulfate soils.
What is salinity?
Salinity is a measure of the amount of dissolved salts in the water. Saline water conducts electricity more readily than freshwater, so electrical conductivity (EC), measured in micro siemens per centimeters (µS/cm) is routinely used to measure salinity.
As salinity increases, it may become harmful to native freshwater organisms. Salinity increases occur as water evaporates from the Lower Lakes (highest in summer) and also during times when inflows to the River Murray are too low to flush the salt out over the barrages and through the Murray Mouth. Salinity decreases occur when rain falls directly onto the Lower Lakes, and during times of high inflows in the River Murray and other tributaries which flush salt from the lakes.
Past salinity levels
Prior to the 2007-2009 hydrological drought, salinity was on average less than 700 EC in Lake Alexandrina (at Milang). During the 2007-2009 drought, levels reached as high as 9000 EC in Lake Alexandrina. Seawater has a salinity of approximately 55,000 EC.
What is total phosphorus?
Total phosphorus (TP) is the total amount of phosphorus present in the water body. Phosphorus is a nutrient, and can be present in different forms (eg organic phosphorus, phosphate). High concentrations of nutrients typically indicate excessive growth of aquatic plants such as cyanobacteria, phytoplankton, macrophytes and filamentous algae.
Total phosphorus fluctuations
TP levels tend to increase during low flow periods as algae and other organic material accumulate when lake flushing rates are low. Higher flows tend to reduce TP levels as nutrients are flushed out over the barrages and through the Murray Mouth. The Lower Lakes have high TP levels but generally there are very low soluble nutrients. This indicates that algae rapidly take up any soluble nutrients.
Past total phosphorus levels
TP is on average about 0.1 mg/L in Lake Alexandrina. Much higher total phosphorus levels (>0.2 mg/L TP) were recorded during the 2007-2009 hydrological drought due to the lack of flushing of the Lower Lakes.
What is turbidity?
Turbidity is a measure of the cloudiness or murkiness in water caused by suspended solids (eg sediment, algae). Turbidity is expressed in Nephelometric Turbidity Units (NTU) and is measured using a relationship of light reflected from a given sample. Turbidity can also prevent sunlight from penetrating the water column, leading to reduced plant growth. It can also potentially affect the gills of aquatic organisms.
Turbidity is very variable in the Lower Lakes and can change on short time scales due to wind events. High wind conditions and/or low water levels increase turbidity in the Lower Lakes as fine sediments on the lake bed are disturbed and resuspended. Turbidity tends to decrease when River Murray inflows to South Australia are dominated by River Murray water opposed to Darling River water which is generally more turbid. High salinities and low flow conditions also encourage the settling of suspended sediment to the lake bed, reducing the turbidity of the water.
Past turbidity levels
Turbidity levels, on average, are about 50-80 NTU in the Lower Lakes; however, this value was often greater than 100 NTU during 2007-2009 due to the low water levels.