Draft - WAMSI 2 - Dredging Node - 5.3 - Defining thresholds and indicators of primary producers - Seagrass pressure-response relationships - Experiment 1

Brief description

Project 5.3 will address three of the tasks required in the Dredging Science Node Science Plan (tasks 5.3, 5.4 and 5.5).

Objectives of Project 5.3 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 1* will be studying "Effect of different intensity, duration & timing of light reductions on seagrasses (Laboratory)"

Lineage

Statement: This experiment is one of two core experiments for the project (along with the sediment deposition experiment - #3). This experiment will provide the critical information on the pressure:response relationships of the two seagrass species to light reductions, as well as their capacity for recovery. Seagrasses will be collected in the NW of Western Australia and transferred to Perth, as has been done previously by members of the research team. The plants will be established in large 'tanks' (approximately 0.75 x 5 m) with flowing seawater over a 1-2 months period.

Seagrasses will be grown in tanks for 1 month prior to each experiment. Multiple shoots or small cores will be planted into the corners of trays (0.25 x 0.6 m). The trays will be constructed from plastic mesh seedling trays with a cotton liner to retain a homogenised sediment. Up to 25 trays will be placed into a single large seawater tank using a randomized block or split plot design. At each sampling time, trays will be removed and sacrificed for measurements.

Plants will be subjected to one of three intensities of light reduction (control, moderate and high level of
reduction) for one of three durations (in the order of 1-6 months) in order to determine the effect of intensity and duration of light reductions. After the final duration, all shading will be removed and the plants will be monitored for a further 3 months to measure their capacity to recover from the previously imposed stress. Four replicate plots of each treatment combination will be established for each species.

Ideally, this experiment would have a fully orthogonal design. However, to do this would require an extraordinarily large number of mesocosms (in order of 1). Logistically this presents unsurmountable problems in terms of the space available, the costs of the units and the number of staff required to maintain so many units.

The only way to run an orthogonal design would require running the experiment sequentially for each species but this would result in the laboratory being unavailable for the subsequent experiments (2-5) and the project could not be completed within a three year period. Instead, we will employ a repeated measures design, whereby each treatment plot will be sampled repeatedly at each duration to determine the effects of the treatments.

Light will be supplied through banks of halogen and LED lamps set at a constant intensity and with shading screens (neutral density) to reduce the intensity at the plant canopy to the desired levels. The plants will be maintained at one of two temperatures, the median temperatures for the 'dry' and 'wet' seasons in NW Australia. Temperature will be maintained through a combination of ambient air temperature control in the laboratory and heating of the flowing seawater prior to its entry into the tanks.

At the beginning of the experiment and at each duration, a range of plant and 'meadow' variables will be measured in order to characterise the plants' responses to the pressures (see thumbnail picture). Plants will be sampled, processed and analysed using the methods described by Lavery et al. (2009), modified for the laboratory situation.