This dataset summarises daily integrals of photosynthetically active radiation (PAR) at six sites in Cleveland Bay from March 2017 to October 2018. PAR was measured every 15 minutes (as µmol m-2 s-1), and converted to daily light integrals (mol m-2 d-1).

Seagrasses are dependent on light for photosynthetic carbon fixation, growth and biomass production. Within the Great Barrier Reef, seagrasses occupy sediment-rich banks in relatively shallow water in order access light and where nutrients are abundant to support growth to depths of ~60m. These habitats are at risk from terrestrial discharge and from coastal development. Benthic light is affected by water clarity, water depth and incoming solar radiation. Benthic light levels within seagrass habitat of the GBR follow clear seasonal, but with interannual variability particularly associated with periods of high river discharge.

There has been considerable research into seagrass light requirements, with a focus on short-term (weeks-months) light requirements that resemble typical durations of dredging within ports for channel maintenance and periods of elevated suspended sediment/low light from wet season run-off. Light requirements for the most common species occurring in areas of high risk were used to model sediment load reduction targets for the Reef 2050 WQIP (Brodie et al. 2017). In NESP TWQ project 3.2.1, sediment loads from rivers influencing Cleveland Bay were connected to seagrass condition indicators. Benthic light levels were measured at six sites as a component of project 3.2.1 linking sediment loads to seagrass condition, and to validate spatially explicit benthic light predictions. Light loggers were deployed at six sites within the Bay.

In making this data publicly available for management, the authors from the TropWATER Seagrass Group request being contacted before using this data to ensure it is fully understood.

Methods:

The sampling methods used to measure photosynthetically active radiation (PAR), are described in a number of reports (e.g. Chartrand et al. 2012, Bryant and Tasker 2014, Chartrand et al. 2016). Submersible OdysseyTM photosynthetic irradiance autonomous loggers (light loggers) were deployed with automatic wiper units which clean the optical surface of the sensor every 15 minutes to prevent marine organisms fouling the sensors. At each location, two independent light loggers and wiper units were deployed on the seabed and affixed to a metal stake or a frame. The presence of dual loggers reduces the risk of data loss through unit malfunction, fouling or disturbance during the three months between equipment exchange and download. Odyssey PAR loggers log a cumulative reading at 15-minute intervals, which is calibrated and summed to gain total daily PAR (mol m-2 day-1). The raw data captured by the loggers requires calibrating to a known light value. Sensors were calibrated against a certified reference irradiance sensor (LI-CORTM LI-192SB Underwater Quantum Sensor) in controlled laboratory conditions. A custom-built calibration device held the light loggers a consistent distance from the source light and excluded all external light. Because the light loggers are calibrated in air, a multiplication factor of 1.33 was applied to data to allow for the differences in light absorption properties between air (calibration medium) and water (deployment media). These adjustments were also applied to data during submersion periods based on the tide record (supplied by MSQ) and the depth of each logger. If both loggers were collected free of fouling, then the average value of both loggers was taken as the true value.

Limitations of the data:

Due to tampering with deployed equipment, not all data sets are continuous.

Format:

This dataset consists of one csv file including daily PAR values as mol photons m-2 d-1 for each day and site that data were collected.

References:

Lambert, V. M., K. O’Brien, C. J. Collier, J. Brodie, M. P. Adams, M. Baird, Z. T. Bainbridge, A. Carter, S. Lewis, M. Rasheed and M. Saunders (2020). Connecting Sediment Load Targets to Ecological Outcomes for Seagrass. Report to the National Environmental Science Program. Cairns, Reef and Rainforest Research Centre Limited: 153pp.

Data Location:

This dataset is filed in the eAtlas enduring data repository at: data\NESP3\3.2.1-Eco-load-targests-seagrass