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 <1g wet weight).
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.
