Catherine Collier

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Responses of the seagrass Posidonia sinuosa to light availability: a mechanisitic approach

Seagrasses are both ecologically important and demonstrably sensitive to changes in their light environment. Therefore seagrasses are applied as key indicators of ecosystem health both locally and globally. With a large range of seagrass forms, both the responsive parameters and their sensitivity are highly variable.

This project was formulated to locally address the management application of seagrass indicators and contribute to the development of coastal monitoring programs in Cockburn Sound near Perth, Western Australia. Seagrass shoot density is the currently adopted Environmental Quality Criteria. Verification of the suitability of this monitoring tool and the identification of other potential tools will enable the development of a multi-parameter approach to monitoring.

Knowledge of the mechanisms by which seagrasses respond to and tolerate reduced light availability will build confidence in the cause-effect pathway of the indicators. As Posidonia species have slow growth rates and a poor capacity to recover following loss, the processes enabling their tolerance are crucial to their survival. Understanding these mechanisms will enable the development of management strategies to target survival and recovery mechanisms of this species and furthermore build confidence in the cause-effect pathway of the indicators.

The first research phase was a descriptive analysis of the morphological and physiological attributes of P.sinuosa down a depth gradient relating observed differences to the gradient of long-term light availability. Shoot density and biomass exhibited the greatest differences between the depths sampled and conformed to a priori expectations. Productivity and morphological parameters such as canopy height and leaf width demonstrated far more complex differences as the differences observed were dependant on sampling time. The physiological parameters examined showed little difference between depths.

This led on to an experimental test of the responses of P.sinuosa to imposed shading to validate these earlier findings and to determine the duration of reduced light that causes measurable seagrass responses. In situ shadings (3 different shading intensities; light (~19% reduction), moderate (~70% reduction) and heavy (~94% reduction)) were applied at 2 different depths in Cockburn Sound in early spring 2003. Repeated measurement of a range of parameters were taken throughout the 7 month shading period and the ongoing recovery period.

Within 106 days a significant difference in shoot density occurred between the control and the moderate and heavy treatment plots. Continued decline in these treatments resulted in shoot densities after 196 days that were 38% (moderate) and 6% (heavy) of the control plots. At the shoot level, the meadow-scale response (thinning) acted to offset changes in the light environment, resulting in comparable absolute light levels at the bottom of the canopy. Leaf production (mgdry.shoot-1.day-1) remained unaffected by light reduction throughout the shading period and may be supported by carbohydrate reserves in the persistent P.sinuosa rhizomes.

P.sinuosa in control plots in April 2004 after 196 days of treatment

P.sinuosa in the heavy treatment in April 2004 after 196 days of shading where only 6% of the shoots remain

In search of a rapid, non-destructive and early physiological response measure of P.sinuosa to light reduction, a number of measurements were made using the PAM fluorometer. After 2 weeks (and on a number of future sampling occasions) a typical “shade response” was observed in the heavy shade treatment only with lower relative maximum electron transport rate (rETRmax), higher alpha (α) and lower saturating irradiance (Ek). Overall, however, the response of the rapid light curve parameters to the shading treatment was inconsistent throughout the sampling period.

These data analysed to date, provide a preliminary indication that for P.sinuosa, shoot-scale responses (such as leaf width, height and photosynthetic characteristics) to light availability are less pronounced than meadow-scale responses (shoot density and biomass), a finding consistent with our earlier field observations for this species. A reduction of shoot density may not only reduce the respiratory load of the above ground material and open the canopy to greater light penetration, but may also allow resource allocation (e.g. fixed carbon compounds) to “prioritized” shoots.

Conceptual diagram summarizing results to date from the long-term shading of P.sinuosa

This study highlighted that shoot density is an appropriate monitoring parameter for P.sinuosa but that the application of the other measured parameters such as morphological characteristics and productivity may lead to confusing monitoring results. But watch this space, further analyses are ongoing and may reveal additional suitable monitoring parameters.

Previous studies have demonstrated the transfer of carbon between associated shoots over short time-scales which may be an important mechanism for survival during reduced light conditions. For the next phase of the research carbon translocation will be measured by supplying the stable carbon isotope (13C) in situ to seagrass shoots in control and shaded plots. The rate of 13C movement through the plant will be monitored by successive collections. The results of this research will contribute to our understanding of the mechanisms involved in seagrass responses to light availability and the role of seagrass clonal formation, that is, the scale at which a seagrass meadow responds to its light environment.

Membership of professional societies

Australian Marine Sciences Association

Previous research

Internal nitrogen cycling in three seagrasses: patterns and process

This research carried out for my Honours thesis examined the uptake, distribution and internal cycling of nitrogen in three seagrass species (Halodule uninervis, Zostera capricorni and Cymodocea serrulata) in Moreton Bay, Queensland, Australia, a potentially nitrogen limiting environment. 15N-NH4+ was added to whole seagrass shoots enclosed for 1 hour in plastic bags which were anchored into the sediment. The 15N content was analysed in plants collected over 25 days. Assimilation and redistribution of the 15N tracer occurred in all species. In addition, continued uptake of 15N from the sediment was inferred from continued 15N accumulation after the removal of the pulse. Remobilisation of N from older to younger tissues was detected in all species but the proportion of its contribution was variable. In Z.capricorni, rapid leaf uptake and re-distribution occurred, with ~65% of new leaf nitrogen derived from recycled N. C.serrulata also had rapid uptake and re-distribution with ~50% of new leaf N derived from recycled N. H.uninvervis had slow leaf uptake and re-distribution with only 12% of new leaf N derived from recycled N, instead N leaching from older leaves was inferred. A major portion of H.uninervis N appears instead to be sediment-derived. The studied seagrass species therefore appeared to have variable internal N cycling patterns and dependence on external N sources. A switch in the balance of N provision may therefore favour the growth of one species over another.

Publications

Collier, C.J. (2006) PhD thesis. Characterising responses of the seagrass Posidonia sinuosa to changes in light availability

Collier, C.J., Lavery, P., Masini, R. and Ralph, P.J. (2004) Responses of the seagrass Posidonia sinuosa to light availability and their application to environmental quality criteria. Strategic Research Fund for the Marine Environment 3rd Symposium. Floreat, Perth, Western Australia. 16th June 2003

Collier, C.J., Lavery, P., Masini, R. and Ralph, P.J. (2003) Characteristics of the seagrass Posidonia sinuosa down a depth gradient and their application to environmental quality criteria. Australian Marine Science Association Conference. Brisbane, Queensland. 9th-11th July 2003

Collier, C.J., Lavery, P., Masini, R. and Ralph, P.J. (2003) Responses of the seagrass Posidonia sinuosa to light availability and their application to environmental quality criteria. Strategic Research Fund for the Marine Environment 1st Symposium. Floreat, Perth, Western Australia. 18th June 2003

South East Queensland Regional Water Quality Management Strategy Team (2001) Discover the Waterways of South-East Queensland. South east Queensland Regional Water Quality Management Strategy, Brisbane

Dennison, W.C. and Abal, E.G. (Scientific Communicators - Rogers, J., Collier, C.J. and Gaus, C.) (1999) Moreton Bay Study: A scientific basis for the Healthy Waterways campaign. South East Queensland Regional Water Quality Management Strategy, Brisbane 246p.

Collier, C.J., Dennison, W.C., Schmidt, S. (1999) Internal nitrogen cycling in seagrasses. Australian Society of Plant Physiologists (Combio), Gold Coast, Queensland, Australia. 27th-31st September 1999

Udy, J.W., Collier, C.J., Longstaff, B., Prange, J. Dennison, W.C. (In prep.) Temporal and spatial differences in the response of the seagrass Zostera capricorni to nutrient enrichment.

Poster Presentations

Collier, C.J., Dennison, W.C., Grice, A. and Abal, E.G. (2001) Healthy Waterways: through effective scientific communication. Poster presentation at the Estuarine Research Federation, St Pete Beach, Florida, USA. 4th-8th November 2001.

Reports (non-reviewed)

Dennison, W.C., Prange, J., Collier, C., Rogers, J. (1999) Using biological assays and water column and sediment parameters to develop a monitoring program to assess the efficacy of stormwater treatment devices (wetlands/wetponds). Report prepared for Pine Rivers Shire Council.