Morphological Characteristics of the Seagrass Posidonia sinuosa along a Depth-Related Gradient of Light Availability

Brief description

Morphological and growth characteristics of the meadow-forming seagrass Posidonia sinuosa, were measured along a depth-related gradient of light to infer its response to long-term differences in light availability. Morphometric measurements were carried out at 6 depths between 1.6 and 9.0 m in summer and winter in 2002 and 2003 at Cockburn Sound and summer only at Warnbro Sound in south-western Australia.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Study site- The study was conducted at Cockburn Sound (CS) and Warnbro Sound (WS) near Fremantle, Western Australia. Two locations were used to verify whether the trends in the measured Posidonia sinuosa characteristics across depths were consistent between different locations. At these locations, mostly monospecific stands of P. sinuosa grow on steep sub-tidal depth gradients ranging from 1 to 9 m depth. The CS sites were located north-east of Garden Island (see thumbnail). Sampling was carried out at 6 depths; 1.6, 4.0, 5.7, 6.5, 8.3 and 9.0 m (lowest astronomical tide), which will be referred to as Sites CS1, CS2, CS3, CS4, CS5 and CS6, respectively. Sampling effort was concentrated nearer to the depth limit where the greatest differences between depths were expected as light reaches levels that are limiting to the long-term maintenance of the meadow. The 5 deepest sites were located within close proximity to each other on a steep slope leading to a basin (32° 09 37.0S, 115° 40 47.3 E), while the shallowest site was located closer to the island shore approximately 800 m away (32° 09 35.6 S, 115° 40 16.4 E). WS was sampled in the north-east of the Sound (see thumbnail) at the same 6 depths as CS, and will be referred to as sites WS1, WS2, WS3, WS4, WS5 and WS6. Again, the 5 deepest sites were located within close proximity (32° 18 57.4 S, 115° 42 51.5 E), while the shallowest site was located approximately 150 m away, closer to the mainland shore (32° 18 53. 7 S, 115° 42 50.3 E).

Statement: - Sampling - At CS, sampling occurred in winter (June) 2002 and in summer (January - February) 2003 with follow-up sampling on both occasions. WS was sampled in summer (January - February) 2003 only. The sites were sampled for environmental and morphological parameters and leaf growth. Environmental parameters: Photosynthetic photon flux density (PPFD) was recorded every 15 min using submersible 2* loggers (Submersible Odyssey Photosynthetic Irradiance Recording System, Dataflow Systems) deployed just above canopy height at 4.0 m and 9.0 m depth from September 2002 to September 2003. Surface PPFD was also recorded at the Point Peron education facility, near Rockingham (see thumbnail). The loggers were calibrated using a LI-192SA underwater quantum sensor (LI-COR) and then corrected for immersion effect using a factor of 1.33 (Kirk 1994). Automated sensor cleaners wiped the sensor free from fouling materials every 30 min (Carruthers et al. 2001). Continued technical difficulties prevented the collection of complete annual data at WS, so extinction coefficients for a part of the summer (December 2002) and winter periods (July 2003) are given. At CS, small gaps in data were interpolated by taking the means of data blocks from a 2 wk period on either side of the missing data. For further methodology see section 3.2.2.1 of thesis. Biomass and morphological parameters: Biomass sampling was carried out using up to 12 replicate quadrats (25 × 25 cm) in a stratified design. Percent cover of seagrass was estimated visually within a 10 m2 area at each depth. Where percent cover was less than 100%, the number of replicates was reduced to the corresponding proportion of 12 samples, with the remaining samples given a zero value. All leaf and sheath material was collected from within each replicate quadrat and placed directly into a plastic bag. These were later rinsed and sorted to retain only the above-ground material. Leaves were scraped free of epiphytes and, together with the leaf sheaths, were dried at 60°C for 48 h and weighed. The number of shoots in the sample was counted to determine shoot density. Below-ground biomass was collected in summer only to a depth of 30 cm using a stainless steel corer with a diameter of 10 cm. The samples were placed immediately into mesh bags (1.5 mm mesh size) and transferred to plastic bags at the surface. Below ground components were separated into dead material and roots + rhizomes, then dried at 60°C for 48 h prior to weighing. As shoot and root/rhizome biomass were collected independently, estimates of above-/below ground biomass ratios were carried out on mean values of these parameters and, as such, no statistical analysis is available. Morphological measurements were made on the above-ground biomass samples. As the 3 deepest sites had some replicate samples with zero biomass, additional samples were collected at these depths for analysis of these morphological parameters. From each replicate sample, 15 shoots were randomly selected and the number of leaves per shoot recorded. As Posidonia sinuosa shoots usually have only 1 fully mature leaf with or without 1 emergent leaf, all parameters were measured on the mature leaf, including leaf length (from bottom of sheath to leaf tip), leaf width, leaf thickness (using Mitutoyo dial calipers, 505-633-50) and epiphyte biomass (dry weighted), quantified by scraping the leaf free of epiphytes using a razor blade and drying at 60° for 48 h. Leaf area index (LAI) was calculated for the mature leaf only by multiplying the leaf length (for the leaf above the sheath only) by the width to obtain area per shoot and then multiplied by shoot density. Leaf growth: Leaf growth was measured using the leaf hole punch technique (Kirkman & Reid 1979). At each site, 6 replicate groups of 15 to 20 shoots were hole-punched using a leather punch. On average, 10 to 15 shoots were recovered after 2 to 3 wk of growth. New growth was removed, the length measured, dried at 60°C for 48 h and weighed. Shoot turnover time was estimated from the product of the time taken to produce one leaf and the number of leaves per shoot.

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)

Purpose
To identify characteristics responsive to light in order to form indicators of long-term light reduction.