Gracilaria edulis (Rhodophyta) as a biological indicator of pulsed nutrients in oligotrophic waters

The response of the marine macroalga Gracilaria edulis (Gmelin) Silva to nu trient pulses of varying magnitude was investigated to test its applicabili ty as a marine bioindicator at two oligotrophic locations. After exposure t o nutrient pulses, algal amino acid, tissue nitrogen, and chlorophyll a con tent were assessed relative to algae incubated under control conditions (no nutrient enrichment), The smallest nutrient pulse involved a nutrient enri chment experiment conducted within a coral atoll, whereas two larger pulses resulted from sewage discharge to a tropical coastal bay. After exposure t o the smallest nutrient pulse (10 x ambient), only changes in macroalgal am ino acid concentration and composition were detected (mainly as increases i n citrulline), At 100 x ambient concentrations, increases in tissue % nitro gen of the macroalgae were detected, in addition to responses in amino acid s, Macroalgae exposed to the highest nutrient pulse (1000 x ambient) respon ded with increased chlorophyll a, tissue nitrogen, and amino acids within t he three day incubation period, In contrast to these algal responses, analy tical water sampling techniques failed to detect elevated nutrients when nu trient pulses were not occurring, The responses of this algal bioindicator to variable nutrient pulses may provide a useful tool for investigating the source and geographical extent of nutrients entering oligotrophic coastal waters.