Shade-induced Response and Recovery of the Seagrass Posidonia sinuosa

Brief description

A number of physiological, morphological and growth characteristics (photosynthetic photon flux density, sediment redox potential, sediment nutrients, shoot density and biomass, morphometrics, growth, carbohydrates, chlorophyll, photosynthetic characteristics, crude wall fibre, nutrients and 13C) previously known to respond to changes in light availability were measured in P. sinuosa that were exposed to a range of shade intensities. This experiment occurred from 1st October 2003 to mid-May 2005 in Cockburn Sound, north-east of Garden Island.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Experimental design - Shade screens were set-up in Cockburn Sound at north-eastern Garden Island in depths of 3 - 4 m (32º 09' 42.7" S, 115º 40' 38.5" E) and 7 - 8 m (32 09' 39.0" S, 115º 40' 45.7" E). Here the seagrass is mostly a monospecific P. sinuosa meadow. The site is partially protected from the dominant south-westerly wind and swell by Garden Island. Shade screens consisting of ½ inch monofilament net (light shade; LS), 50% (moderate shade; MS) and 80% (heavy shade; HS) reducing shade cloth were established in situ over the meadow. HS was only established at the shallow site. The screens consisted of 2.4 m metal star pickets hammered into each corner with an additional picket in the centre of each side. A stainless steel wire was run around the pickets, suspended from stainless steel shackles. The shade cloth was fastened to the wire using clips (Coolaroo shade cloth fixing butterfly clips) and cable ties. The final dimension of the screens was 4.5 m x 3 m. Control plots were established by marking out an equivalent area with a permanent metal picket placed in each corner. P. sinuosa responses to light availability at the deeper site, four replicates control plots as well as LS and MS were arranged in a line along the depth contour to avoid the array traversing a depth change. At the shallower site, four replicate plots each of control, LS, MS and HS were arranged in a random order within three parallel rows. Because epiphyte accumulation on the screens was rapid, they were cleaned every 2 - 3 weeks for the first two months. As this was relatively ineffective at controlling fouling, the screens were subsequently removed and replaced with clean shade cloth every 3 - 4 weeks. The experiment was initiated on 1st October 2003 and the screens were left in place until mid-April 2004, a total shading duration of 206 days. Sampling occurred immediately prior to attachment of screens and after 21, 105, 158 and 206 days. Sampling was then conducted during the recovery period at 15, 115, 203 and 384 days after removal of shading. The day 21 (shading) and day 15 (recovery) sampling was for physiological parameters only, all other sampling events were for physiological and morphological variables throughout the shading (206 days) and recovery (384 days) period. In order to quantify the penetration of light under the shade screens and to determine the appropriate sampling area within each plot, the profile of light reduction under the shade screens was recorded at noon in January, when peak irradiance is expected, and at 16:30 h in April, when the angle of solar irradiance is greater. Using an underwater quantum sensor (LiCor LI-192SA, Nebraska USA) a single reading of Photosynthetic Photon Flux Density (PPFD) was recorded every 20 cm along two transects running along the central axes of the shaded area. Light attenuation was consistent from about 75 cm inside the edge of the shaded screens, so the final sampling area excluded the outer 75 cm band, leaving a 1.5 m × 3 m area of fairly uniform PPFD.

Statement: - Environmental sampling - See section 5.2.2.1 for Light See section 5.2.2.2 for Sediment redox potential See section 5.2.2.3 for Sediment nutrients - Shoot density and biomass - Shoot density was repeatedly measured in a 20 cm × 20 cm quadrat located in the centre of each experimental unit and permanently marked by stainless steel pegs. Measurements were made on each of the morphological + physiological sampling occasions and conducted by the same person to minimize the risk of observer-induced variation. An additional, more detailed analysis of shoot recovery was carried out in two of the most heavily impacted HS plots at the shallow site. Four transects, 50 cm apart were established along the length of the sampling area. Shoot density was counted in a 20 cm × 20 cm quadrat at 50 cm intervals along the transects. This was carried out after 203 and 384 days of recovery. Biomass was collected on the same occasions as shoot density. A 20 × 20 cm quadrat was randomly placed within the sampling area and all of the shoots were removed. A peg with floating marker was placed into the center of the harvested area to avoid re-sampling. On the final sampling occasion within the shaded period (206 days), both above-ground and below-ground biomasses were sampled from within the same quadrat. An 11 cm i.d. stainless steel corer was placed randomly within the quadrat and forced into the sediment to a depth of 30 cm. The sample was extracted and placed into a 1 mm mesh bag whilst underwater, then later sorted into shoots and below-ground material. The remainder of the above-ground material (shoots) within the quadrat was harvested and later combined with the above-ground material from the core. All harvested material was placed temporarily on ice prior to storage at -18°C. - Morphometrics - In the laboratory, 15 shoots were randomly selected from each biomass sample for morphometric analysis. On each shoot the following measurements were carried out on the youngest mature leaf (leaf one or two): the number of leaves and their length (including the sheath); leaf width (using Mitutoyo dial calipers, Japan); leaf thickness (using a Mitutoyo digital micrometer Japan). Epiphytes were then scraped from the leaf surface using a razor blade and retained for measurement of biomass. These shoots were then combined with the remaining shoots (also scraped free of epiphytes) with only the inner-most sheath retained on each shoot. The below-ground biomass sample was sorted into roots, rhizome, sheath and dead material. The biomass samples were rinsed in fresh water, then dried at 60°C and weighed. - Growth - On each morphological sampling occasion, leaf extension and net weight gain were determined according to the methods of Kirkman and Reid (1979). Twenty shoots in each plot were marked with a leather punch at the base of the ligula. Following two to three weeks of growth 15 ? 20 of the shoots were recovered and stored at -18°C prior to analysis. Nearer to the end of the shading period, after considerable shoot loss had occurred in the MS and HS treatments, the number of marked and recovered shoots in each plot was reduced to 5 ? 10. The harvested area was again marked to avoid later resampling. For each recovered shoot, the material produced in the growth period was removed, its length measured, dried at 60°C for 48 h and weighed. Mean length and weight of growth was determined for each replicate plot. Areal leaf growth was determined from this mean and multiplied by the shoot density for that plot. - Physiology - See section 5.2.5.1 for Carbohydrates See section 5.2.5.2 for Chlorophyll See section 5.2.5.3 for Photosynthetic characteristics See section 5.2.5.4 for Crude wall fibre, nutrients and 13C

Notes

Credit
Strategic Research Fund for the Marine Environment (SFRME)