Benthic light as an ecologically-validated water quality indicator

Reef and Rainforest Research Centre

Dr Barbara Robson from the Australian Institute of Marine Science presents the webinar Benthic light as an ecologicaly-valicated water quality indicator, with the Office of the Great Barrier Reef. This webinar was created for the NESP TWQ Round 5 - Project 5.3 – Benthic light as ecologically-validated GBR-wide indicator for water quality: Drivers, thresholds and cumulative risks.

Barbara talks about the development of a light based water quality indicator that can be used for understanding the amount of light stress that seagrasses and coral reefs experience due to reduced light as a result of poor water quality from flood plumes or resuspension, algal blooms. This indicator was designed to assist in reporting on the status of the Great Barrier Reef. Barbara outlines the details of the new indicator and shows how it varies over time and with location. She also outlines some of the latest research into how much light corals and seagrass need to grow.

The new light metric assesses how much light each location gets in a given day compared to the near highest light level (95th percentile) that each location has received over the long term. In this way, the indicator is scaled by the amount of light a given location would expect to receive in near ideal water quality conditions. Areas that are deep or normally turbid (which restricts light) are not penalised compared to shallow reef areas that receive lots of light. The reason that this approach is important is that organisms tend to settle in locations that they are adapted to. In deeper locations, where the light is lower due to the depth, organisms, such as seagrass and coral, that settle there are adapted to the low light conditions. They only experience light stress when the light drops below normal levels. As light levels increase both coral and seagrass grow faster, but only up to a point. Above a certain threshold there is more light than can be used for growth. The new indicator metric stops accumulating once the light level is above this maximum as any additional light in that day would not help with growth.

The indicator algorithm uses MODIS satellite imagery to estimate the water quality characterists at each pixel in the imagery. The amount of light reaching the sea floor is estimated from knowing the depth and the water quality. Barbara also presents a comparison between the indicator results estimated from the MODIS remote sensing and that estimated from the eReefs BioGeoChemical (BGC) model.