Flood Plumes: What do they mean for water quality?

The recent flooding across our region shows just how variable our weather can be. After two years of below average rainfall, the more active monsoonal weather patterns have returned to the Wet Tropics bringing significant rainfall.

These flooding events will have an effect on the water quality in our waterways by causing nutrients, sediments and pollutants to flush off the land.

The most visible evidence of this is the plumes of brown water extending from the mouths of rivers. These are mainly caused by high concentrations of total suspended solids, in other words, particles such as soil, clay and organic matter suspended in the water.

Eventually, these visible brown primary plumes subside fairly close to shore as the solid particles meet the ocean and begin to settle.

However, the less visible dissolved components of the plume continue to extend considerably further from the coast. These secondary plumes include nutrients such as dissolved inorganic nitrogen (DIN), which is released from some fertilisers and is one of the main impacts affecting water quality in the Wet Tropics.

Microscopic algae, known as phytoplankton, tend to be a characteristic of secondary plumes where they show increased growth. While these microscopic plants are a natural feature of marine waters, the combination of increased nutrients and the greater light penetration in the clearer water stimulates extra growth.

Plumes continue to extend even further to the outer reefs and, while they become more dilute the further they travel, evidence of their higher nutrient concentrations is still found in offshore waters.

Flood plumes from rivers that flow into the GBR are monitored by the Marine Monitoring Program (MMP). They use ocean colour to map and track the extent and duration of flood plumes during the high flow periods and conduct water sampling transects to measure concentrations of sediment, nutrients and pollutants in the river and marine environments.

flood plume diagram

 Figure source: Marine Monitoring Program Wet Season Summary Report

The results of this monitoring is published annually by Australian Institute of Marine Science (AIMS) and James Cook University (JCU)  and the water quality data is used for reporting the health of the inshore marine zones for the Wet Tropics Healthy Waterways Report Card.

**Top image thanks to Oz Cyclone Chasers

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