Changes in growth, internode-distance and nutrient concentrations of the seagrass Halophila ovalis with exposure to sediment sulphide

Brief description

Between November 2005 and December the short-term effects of sulphide on the growth, nutrition and morphology of the seagrass Halophila ovalis were investigated in situ in the Swan River Estuary. Sediments within a H. ovalis meadow were enriched with Na2S equivalent to 0, 1.1 and 4.2 g m-2. The hypothesis tested was that sediment sulphide enrichment would have a negative effect on growth of H. ovalis. Biomass, internode-distance and growth rate (mg apex-1 day-1) were determined, and plant material analysed for soluble carbohydrate, carbon, nitrogen and phosphorus.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Twenty four plots were randomly assigned in a 5 x 5 m grid, at a spacing of 1 metre in a Halophila ovalis meadow in shallow water (< 0.5 m deep at low tide) in the Swan River Estuary. Plastic pots with the base removed (165 mm diameter with 22 two cm diameter holes in the sides to allow sideways porewater movement) defined each plot. Treatments were assigned randomly and were control (3 diffusion tubes containing distilled water), low sulphide (3 diffusion tubes delivering a total of 0.023 g of Na2S per plot - dissolved in distilled water) and high sulphide (3 diffusion tubes delivering a total of 0.09 g of Na2S per plot - dissolved in distilled water). Each diffusion tube held 0.75 mL of either DI water, or sodium sulphide solution, and had 2 mm diameter holes drilled such as to release the treatment solution via diffusion at a depth of 5-8 cm into the sediment. PVC tape, temporarily sealing the holes, was removed concurrently with insertion of diffusion tubes into the sediment. Sulphide treatments were estimated to enrich sediment porewater by 1 - 4 mM (based on 20 % porosity of sediment and the arbitrary assumption of a diffusion zone of 5 cm depth within the pot). Although these solutions of sodium sulphide were basic (pH 13 - 14) the treatments were not expected to produce a substantial rise in porewater pH, given that mixing of porewater over a 5 cm diffusion sphere would only produce a rise in pH in the order of 0.4 pH units without the additional effect of pH buffering of the sediment porewater. Pots were left in the field for approximately 3 weeks, and monitored by photoquadrats prior to harvest. Approximately 100 underwater photographs of the plots were taken approximately every four days, using an Olympus digital camera in underwater housing. The clearest of these for each of the plots was then colour and contrast adjusted in Corel Photo-paint 11 to enhance the image for counting the leaves. Estimates of leaf growth were made by counting leaves in ~ 150 digitally enhanced photographs as a relative percentage increase from the initial number of leaves present on day 0. Porewater was extracted in-situ with porewater sippers at an integrated depth of 5-8 cm and analysed for sulphide (TPS ion selective electrode - Ag pellet) at the end of the experiment prior to harvesting of plant biomass. Whole plants (root, rhizome and leaves) were harvested with plant material placed on dry ice (to halt respiration) at the site, and then at -20° C until analysis. For analysis, plant material was dissected into above-ground and below-ground biomass. Internode-distance (distance between the 2nd and 3rd leaf pairs) and leaf and apex number per pot were determined. Since initial biomass within each plot could not be determined prior to experimental manipulation, leaf number counted by photographs at the outset of experiment was used to estimate growth rate. Leaf growth, as mg apex-1 day-1, was calculated by difference between initial and final leaf number multiplied by average leaf mass, then divided by the number of apices present at the end of the experiment and the length of the experiment. Leaf growth rate was converted proportionally via relative biomass allocation of above and below-ground parts to give total growth rate expressed as mg apex-1 day-1. Epiphytes were removed from leaves by wiping with a tissue, and scraping with a razor blade when necessary. Plant material was then re-frozen and freeze dried. Samples were finely ground with a Retsch Eppendorf grinder, then analysed for % C and % N with LECO CHN analyser using a sample size of ~100 mg. Plant tissue was digested and analysed for phosphorus colourimetrically by a method modified from Solorzano and Sharp (1980) by Fourqurean and Zieman (1992). Soluble carbohydrates were extracted from ~50 mg of finely ground tissue (above and below-ground) for 20 minutes with 80 % ethanol at 75 °C and concentration determined absorbance at 620 nm (Shimadzu 1240 UV-Vis spectrophotometer) by the anthrone colorimetric method (Yemm & Willis 1954) with glucose standards.

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)