Draft - WAMSI 2 - Dredging Node - 5.5 - Defining thresholds and indicators of primary producers - Seagrass pressure-response relationships - Experiment 2

Brief description

Objectives of Project 5.5 are to:
(1) determining the pressure:response relationships that relate the effects of dredging-generated sediments on tropical primary producers,
(2) examine the pathways, rates and timeframes of recovery from impacts and;
(3) identify and examine the effects of key environmental variables on the pressure:response relationships and recovery.

Specifically *Experiment 2* will be studying "Effect of different intensity & duration of sediment deposition on seagrasses (Laboratory and Field Experiment)"

Lineage

Statement: The effects of sediment burial on seagrasses will be determined in a series of laboratory and field-based
experiments. Laboratory experiments are desirable as they will remove a large number of potentially
confounding factors that have been known to influence earlier studies involving members of the study team (Ooi et al. 2011). Field studies will be used to provide field validation of the laboratory experiments. Unlike the light reduction experiments, sediment burial does not require the same degree of on-going maintenance, making it more feasible to conduct them in the field. The field experiments will be conducted in the NW, with sites in Exmouth Gulf the most likely location.

The laboratory experiments will be conducted as for experiment 1, but with treatments consisting of different depths of burial (3 levels; nominally 2, 4 and 10cm) and durations (nominally 2, 4 and 8 weeks plus a recovery period of 4 weeks). Variables to be measured are indicated in the thumbnail image.

An analogous field experiment will subject plots of seagrass meadow to different levels of sediment burial for different durations, and then track their recovery following removal of the burial treatment. The experiments will apply methods adapted from Ooi et al (2011) and Manzanera et al. (2011). Members of the study team were involved in the first of these studies and have experience in applying the techniques.

Cylinders of varying heights will be inserted into the meadow and filled with sediment to simulate burial. Based on the above literature, the treatments will be in the order of 0.2, 2, 4 and 10 cm and for durations of 1, 3 and 6 months after which recovery will be followed for up to a further 6 months (note, however, that these depths could change depending on the outcomes of Project 5.1 ). Rhizomes will be severed around the edge of the cylinders to prevent donal re-allocation of resources, which could support the plants; this more realistically represents the situation where a large-scale dredging programme produces an even burial effect over an entire meadow. It is possible that some of the sediments in the cylinders will be lost due to resuspension during the experiment. so the treatments will be 'topped-up' at each sampling interval (1 and 3 months). The levels of the depth and duration treatments may be adjusted following input from Theme 2/3. The variables to be measured are indicated in thumbnail.