Potential effects of terrestrial nutrients in submarine groundwater discharge on macroalgal blooms in a fringing reef ecosystem

To investigate in situ nitrogen or phosphor-us limitation and the effects of terrestrial nutrients on blooms of invasive macroalgae, the physiological responses of the red macroalga Hypnea musciformis to N- and/or P-enrichment were examined at a fringing coral reef in South Maui, Kihei, Hawaii, in January 2006. N- and/or P-enriched algae were cultured outdoors for several days. These manipulated algae were then put into separate mesh bags and placed in several locations along a shore-normal transect from the shore, where groundwater seepage is observed. Algal growth rates and several chemical components (i.e. stable N isotope ratio [delta N-15], N content, and P content) of algal tissues were monitored during 1- and 3-d in suit incubations. Relatively higher growth rates and N content at locations close to the shoreline suggested that terrestrial nutrients via groundwater seepage enhanced algal growth rates. We only observed increases in the delta N-15 of algae growing near the shoreline, indicating that these algae used anthropogenic N with relatively higher delta N-15. In contrast, consistently lower P content in most algal samples suggested that bioavailable P supply to macroalgae was often limited throughout the reef. An increase of bioavailable P supplies may further stimulate algal blooms in this area.