A comparison of nutrient-limited productivity in macroalgae from a Caribbean barrier reef and from a mangrove ecosystem

Abstract

Productivity-nutrient enrichment assays and tissue analysis of three dominant macroalgae at Carrie Bow Cay, Belize, suggest that the degree and type of nutrient limitation in tropical marine macroalgae are highly ecosystem and species dependent. In the frondose algae, Dictyota divaricata L'amouroux and Acanthophora spicifera Lamouroux, collected from a pristine barrier reef site, productivity increased significantly with enrichment by dissolved inorganic nitrogen (N, as NH 4 +) and phosphorus (P i, which accounted for more of the enhanced productivity), compared to insignificant or reduced effects of enrichment of these algae collected at a detritus-rich mangrove island. Tissue levels of total nitrogen and phosphorus were also generally higher in D. divaricata and A. spicifera collected at the mangrove site, compared to these species collected at the barrier reef site. Higher concentrations of N and P i in seawater at the mangrove site, compared to the barrier reef site, appeared responsible for the above pattern. These results indicate that macroalgae in mangrove fringe areas may not always experience the severe nutrient limitation often associated with oligotrophic tropical reef ecosystems. In contrast to the above ecosystem-dependent pattern of nutrient limitation in the frondose algal forms, productivity of the calcareous Halimeda opuntia (L.) Lamouroux was enhanced at both sites only by N enrichment and the effects of P i enrichment were insignificant. These different types of nutrient limitation between two frondose and one calcareous macroalga suggest that algal community structure in tropical marine systems may be mediated by different resource limitations that are imposed on different populations within the community.