Characterization of a large-scale ephemeral bloom of the green alga Cladophora sericea on the coral reefs of West Maui, Hawai'i

The filamentous green alga Cladophora sericea G. Hudson (Kutzing) has formed episodic and ephemeral nuisance blooms on West Maui's coral reefs over the past 2 decades. Despite a paucity of evidence, nutrient-rich runoff, groundwater seepage, and upwelling have all been suggested as the cause of these blooms. The goals of this study were to characterize a number of physical and biological variables during a bloom event that occurred during the summer of 2001. We quantified the nutrient environment (water column and sediment porewater), benthic community structure, and herbivore abundance along a depth gradient in an effort to identify factors that may influence bloom dynamics. Further nutrient enrichment and growth experiments were conducted with C. sericea in both the field and the laboratory to determine the response of this alga to enhanced nutrient concentrations. Sediment porewater sampled 0.25 m into the substrate had high concentrations of ammonium, nitrate, and silicate, and low salinity relative to overlying ambient water, suggesting groundwater intrusion was occurring into the sediment interstices. Tissue samples of C. sericea showed elevated nitrogen and delta N-15 at shallow sites, and these values declined with depth. In situ nutrient enrichments and subsequent determinations of relative electron transport rates (rETR) for C. sericea using chlorophyll fluorescence revealed that some samples were nutrient limited and maximum rates of rETR generally increased with enrichment. Further, algae located at sites with the highest concentrations of dissolved inorganic nitrogen did not respond to nutrient enrichment. Growth rates of up to 25% increase in wet weight d(-1) were observed for C. sericea in the laboratory when enriched with commercial fertilizer. The results of this study suggest that the bloom of C. sericea may have been influenced by land-based nutrients via groundwater seepage, but other physical factors are also likely to be involved in the development, persistence and senescence of this dynamic species.