Biomass and productivity of tropical macroalgae in the inshore Great Barrier Reef

Brief description

Field and laboratory culture experiments were conducted to determine limiting and optimum concentrations of available nutrients (nitrogen and phosphorus) for growth of Sargassum baccularia. The macroalgal communities at each of Brook (Oct 1995, May 1996), Fantome (Nov 1995, Mar 1996) and Great Palm (Oct, Nov 1995; Feb, May, July Aug 1996) Islands were examined. Critical and subsistence levels of available nutrients in algal tissues were determined in the field and compared with experimental values. A 'mini budget' estimated nutrient requirements with nutrient supply.Samples were sorted to genus or species level with identifications comprising Chlorophyta (17 taxa), Phaeophyta (21), Rhodophyta (16) and seagrasses (5). Categories for biomass sampling were: dominant, ephemeral (seasonally occurring), minor/present (biomass Tissue nutrient analyses were conducted on young basal shoots (from holdfast tissue and distal parts of older shoots); bearing branches; and vesicles. Total carbon and nitrogen, total phosphorus and water soluble phosphorus (assumed to consist of both phosphate and polyphosphates, 'storage P') were analysed. Growth rates were recorded for 20 Sargassum baccularia thalli in each locality every 4-6 weeks over a period of 15 months: wet weight, maximum length (to the nearest 5mm, and for excised young shoots to the nearest mm). Although loss or damage to shoots occurred, at least 10 shoots (20 originally) were always present. For the calculation of mass-specific growth rates, length (in cm) were converted to fresh weight data (in g).Productivity measurements were carried out on 8 replicates of excised young shoots at Great Palm Island using a data logging respirometer to measure photosynthesis and respiration rates. In some cases the oxygen probe failed and an additional run was made. The respirometer simultaneously monitored water temperature (averaged over the 24 hour period of each run) and underwater light (integrated to total daily sums).The fresh weight of each basal shoot was determined at the start and at the end of the 3 wk experimental culture period. Growth was determined as mass-specific growth rate per day. The concentration of nitrogen and phosphorus in tissue was analysed in samples taken at the start and end of each experiment. Uptake rates were calculated from the differences in nutrient concentrations of the in- and out- flowing seawater. At the end of Weeks 1, 2, and 3 of each experimental run, triplicate water samples were taken from each culture flask for both ammonium and phosphate analyses. To determine subsistence concentrations of nitrogen and phosphorus (tissue nutrient levels at zero growth rates), shoots were kept under conditions with minimal nutrient supply - natural seawater without detectable inorganic nutrients but with organic nutrients. This also provided an estimate of the nutrient storage capacity (the length of time that growth can be sustained without external inorganic nutrient supply). To obtain mass-specific growth rates, the fresh weights of the experimental shoots were determined every 5 days for 30 days. Water samples for analyses of standing concentrations of nutrients were taken in triplicate at high tide at each of 3 Great Palm Island stations.The nutrient 'mini budget' calculations used nitrite and nitrate (DIN) and phosphate measurements from 9 samples each taken on 8 occasions between June 1995 and June 1996 at Great Palm Island. To estimate the limiting and optimum nutrient concentrations for the growth of Sargassum baccularia under continuous supply of ammonium and phosphate.To assess the nutrient situation in the field by estimating a 'mini budget' of nutrient requirements and supply by comparing levels determined in cultures with those in field tissues.To provide data on the biomass and genera/species distribution of macrophytes on 3 fringing reef sites over the course of 15 months.To assess the growth and productivity of Sargassum baccularia, the dominant macroalgal biomass on the 3 sites. Algal species identified: Acanthophora spicifera, Amphiroa crassa, Anadyomene sp., Caulerpa taxifola, C. sertularioides, C. serrulata, C. racemosa, Chaetomorpha spiralis, Chladophora sp., Chlorodesmis fastigiata, Chnoospora implexa, Chondria sp., Codium sp., Colpomenia sinuosa, Cymodocea rotunda, C. serrulata, Cystoseria trinodis, Dictyosphaeria cavernosa, D. versluysii, Dictyota spp., Digenea simplex, Gracilaria arcuata, G. edulis, G. salicornia, G. textorii, Halimeda cylindracea, H. optunia, H. tuna, Halodule uninervis, Halophila ovalis, Hormophysa cuneiformis, Hydroclathrus clathratus, Jania crassa, J. adhaerens, Laurencia filiformis, L. intricata, L. papillosa, Leveilla jungermannioides, Liagora sp., Lobophora variegata, Neomeris van-bosseae, Padina spp., Peyssonnelia sp., Rosevingea orientalis, R. intricarta, Sargassum baccularia, S. decurrens, S. fissifolium, S. linnearifolium, S. oligocystum, S. opacum, S. polycystum, S. siliquosum, Spatoglossum sp., Thalassia hemprichii, Turbinaria ornata, Turbinaria sp., Valonia sp., Ventricaria ventricosa.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: The model outlined by Wilkinson (1961) was used because the growth followed a distinct optimum curve and not a saturable hyperbolic function as required for other nutrient kinetic models.Several assumptions were necessary for calculations of the 'mini budget'. The nutrient requirements of S. baccularia were calculated using the in situ growth rates and the corresponding tissue nitrogen and phosphorus concentrations. The nutrient supply was estimated as the daily nutrient uptake of field plants. This was calculated from the in situ nutrient concentrations of the water column and the initial slope, a, of the ammonium or phosphate uptake kinetics obtained in the laboratory. For this calculation the 'snapshot' nutrient data obtained for 1 day in each sampling month was assumed to be representative for the whole month. Since we only had data for the uptake of ammonium, we assumed the 'mini budget' similar uptake rates for all DIN species. A further assumption was that the nutrient uptake rates were similar during day and night.Source Description: Wilkinson GN (1961) Statistcal estimations in enzyme kinetics. Biochem J 80:324-332

Notes

Credit
Schaffelke, Britta, Dr (Principal Investigator)