Figure 20: Manning River chlorophyll-a for the period 2015-2018

MANNING RIVER - CHLOROPHYLL A

Chlorophyll-a exceeded the ANZECC trigger on several occasions but generally results are below 3 ug/L. Chlorophyll-a is a marker for algae levels, which are driven by nutrients. The data set suggests higher chlorophyll-a levels generally occurred during low flow events. This may align with variables such as higher water temperature. Low flow conditions may also present less of a physical threat to algal growth.

Figure 20: Manning River chlorophyll-a for the period 2015-2018

Algae caused by excess nutrients and high temperatures in the stagnant water of Dewitt Creek during the 2019 drought

In summary, the data suggests nutrient loading is the biggest issue, along with algae blooms which are caused by high nutrients. However, nutrients are mobilised by sediment inputs. The data correlates with the Rapid Site Assessment conducted in 2019, which found most of the 175 sites assessed in the freshwater catchments were rated as having poor to fair instream condition. Instream condition scores in both fresh and estuarine subcatchments showed indicators of elevated nutrients from fertilised pastures and crops, manure, and stock defecation (Swanson 2020).