The effect of light reduction on the nutrient dynamics and growth of Halophila ovalis, in culture.

Brief description

The effect of light reduction on the nutrient (nitrogen and phosphorus) dynamics of the seagrass Halophila ovalis was investigated in a short-term (<50 days) laboratory experiment under controlled growth conditions in Autumn 2004. Leaf growth rate was measured and converted to growth rate while nitrogen, phosphorus, ammonia and soluble reactive phosphate were also measured for different light treatments.

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Statement: The effect of light reduction on nutrient dynamics of Halophila ovalis was investigated in a laboratory experiment in Autumn (March - May) 2004. 54 Cores (90 mm diameter PVC pipe) of H. ovalis with ~ 9 cm depth of associated sediment and 18 cores of unvegetated sediment were collected from shallow water (~1 m deep) at Freshwater Bay in the Swan River Estuary (32°00 S, 115°46 E).

Cores were retained in the PVC tube throughout the experiment. Initial leaf density was 26 ± 1.4 (S.E.) leaves per core. Initial biomass allocation was assumed to be distributed similarly across all cores (in terms of above and below-ground ratios). This assumption was supported by analysis of extra cores collected at the outset of the experiment, harvested immediately, where leaves accounted for 37 ± 1 % of total biomass. Cores were placed into individually aerated aquaria (dimensions 10 cm x 10 cm x 25 cm) filled with river water and were allowed to stabilize for two days at full light. Cores were capped at the base which contained the sediment on transfer into aquaria, however these were not water-tight so diffusion of water through the cores could still occur to some degree. Throughout the experiment, water was refreshed twice weekly with site water (volume exchanged ~0.75 L). Light reduction treatments were imposed on seagrass-vegetated and unvegetated cores by covering aquaria with shade cloth (Hessian). Light reduction corresponded to 100 and 40 umoles of photons m-2s-1 PAR, with the controls receiving saturating irradiance at 300 umoles of photons m-2s-1 PAR. Cores containing seagrass were harvested at 17 days, 30 days and 50 days, and
unvegetated cores at 50 days. There were six replicates for each treatment at each incubation time. Plant material, frozen immediately on harvest, was divided into roots, rhizomes and leaves. Since biomass within each core could not be determined at the start of the experiment, leaf number was counted at the outset of the experiment and used to estimate growth rate. Leaf growth, as mg apex-1day-1, was calculated as the difference between initial and final leaf number multiplied by the average leaf mass, then divided by the number of apices present at the end of the experiment and the duration in days 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 DW apex-1day-1. Epiphytes, where present, were removed from leaves by wiping with a tissue. Plant tissue was dried to constant weight at 60 °C and finely ground in an Eppendorf ball mill grinder. Plant tissue was digested by sulphuric acid with copper sulphate as a catalyst at 365 °C then analysed for nitrogen (Total Kjeldahl Nitrogen) and phosphorus (Lachat QC8000 flow injection analyser).

Porewater was drawn from cores with a modified syringe (pasteur tip on the end of a 10 mL syringe) from a depth of 4.5 cm from sediment surface, a total of four times for each core, with samples spread throughout the duration of the experiment. Porewater was filtered through Whatman GF/C syringe filters, and stored at -20 °C until analysis.
Porewater was analysed on a Skalar autoanalyser for ammonia (modified Berthelot reaction) and soluble reactive phosphate (SRP) was determined colourimetrically (Murphy & Riley 1962).

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)