Short-term nutrient pulses enhance growth and photosynthesis of the coral reef macroalga Sargassum baccularia

Abstract

Due to their proximity to the mainland, nearshore reefs of the Great Barrier Reef (GBR), Australia, are directly subjected to land run-off. It is assumed that enhanced nutrient inputs to coastal waters are likely to lead to enhanced growth of primary producers. We simulated the effect of enhanced nutrient inputs on the growth and productivity of Sargassum baccularia (Mertens) C. Agardh, a large fucoid seaweed with a very high abundance on a number of nearshore reefs in the GBR. Nutrients 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 μmol ammonium and 1 μmol phosphate, or higher, were taken up rapidly and significantly increased the tissue nutrient content in S. baccularia shoots. These nutrient stores were used to sustain enhanced growth and net-photosynthesis rates for about 1 wk. The strongest growth enhancement was obtained when ammonium and phosphate were applied together. 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 species at the field site. Nutrients are imported into the coastal zone of the GBR mainly by rain and riverine input, predominantly during the austral summer wet season. This is also the main growth period with the highest nutrient demand of the large Sargassum species. Our data suggest that an enhanced nutrient input during this season will significantly increase the productivity of these algae.