Students: Riley Clagett, Natalie Ma, & Casey Sheridan

Independent Project

Advisor: Dr. Peggy Fong

Anthropogenic nutrient input is an ever increasing problem in tropical coastal marine ecosystems. Long term nutrient enrichment of the water column leads to enrichment of benthic sediment, which could in turn affect the algal community. In our study we investigated the effect of water column nutrient enrichment and sediment nutrient enrichment on the rhizophytic algal species Halimeda incrassata and the non-rhizophytic species Halimeda opuntia. We initially proposed that sediment enrichment would lead to greater growth in H. incrassata than H. opuntia, while water column enrichment would lead to greater growth in H. opuntia than in H. incrassata. A field experiment was performed in Cook’s Bay in Moorea, French Polynesia, where both Halimeda species were exposed to no enrichment, sediment enrichment, water enrichment, or sediment and water enrichment and growth was measured by segments. A corresponding microcosm experiment with the same nutrient treatments was also done, but with each algal species either alone or with the other species to determine effect of competition on growth. We found that sediment had a negative effect on H. incrassata in both experiments, resulting in reduced growth compared to unenriched algae. In the field experiment, sediment enrichment had a positive effect on H. opuntia’s growth, but only when water enrichment was not present. In the lab experiment, however, sediment enrichment had a negative effect on H. opuntia’s growth. Overall, our results indicate that many of our sediment and sediment/water enriched samples may have become overloaded with nutrients, which resulted in death of the algae. This provides evidence that sediment of Cook’s Bay is already high in nutrients, and may be undergoing a community shift as a result continuous anthropogenic nutrient influx.