SHORT-TERM NUTRIENT PULSES ENHANCE GROWTH AND PHOTOSYNTHESIS OF THE CORAL-REEF MACROALGA SARGASSUM-BACCULARIA

Due to their proximity to the mainland, nearshore reefs of the Great B arrier Reef (GBR), Australia, are directly subjected to land run-off. It is assumed that enhanced nutrient inputs to coastal waters are like ly to lead to enhanced growth of primary producers. We simulated the e ffect of enhanced nutrient inputs on the growth and productivity of Sa rgassum baccularia (Mertens) C. Agardh, a large fucoid seaweed with a very high abundance on a number of nearshore reefs in the GBR. Nutrien ts were added as short-term pulses (24 h or 1 h duration) of ammonium and phosphate in addition to the natural background nutrients. Pulses of 8 mu mol ammonium and 1 pmol phosphate, or higher, were taken up ra pidly and significantly increased the tissue nutrient content in S. ba ccularia shoots. These nutrient stores were used to sustain enhanced g rowth and net-photosynthesis rates for about 1 wk. The strongest growt h enhancement was obtained when ammonium and phosphate were applied to gether. The magnitude of the growth response was strongly dependent on the initial levels of tissue nutrients. In general, S. baccularia was highly responsive, underlining the nutrient limitation of this specie s at the field site. Nutrients are imported into the coastal zone of t he GBR mainly by rain and riverine input, predominantly during the aus tral summer wet season. This is also the main growth period with the h ighest nutrient demand of the large Sargassum species. Our data sugges t that an enhanced nutrient input during this season will significantl y increase the productivity of these algae.