The Hawaiian coastal zone is influenced by a variety of anthropogenic stressors that may alter biogeochemical cycling, including mangrove invasion and fish-cage aquaculture. Mangroves produce tannin-rich detritus and a root-fiber mat that may shift energy flow to microbial pathways, causing a reduction in food availability for commercial and native Hawaiian species. Pulse-chase experiments and measurements of natural abundance stable isotopes new and powerful approaches to evaluating biogeochemical recycling and ecosystem function in mangrove sediments in Hawaii.

We are employing pulse-chase experiments, using 13C and 15N labeled algal detritus, to evaluate the impact of mangrove invasion on benthic ecosystem function. We are testing the following hypotheses: (1) Biogeochemical cycling in mangrove sediments favors microbial pathways, yielding greater nitrogen storage and lower macrofaunal assimilation of organic matter than in open sediments. (2) Due to the persistence of a dense root-fiber mat, microbial metabolism is enhanced, and macrofaunal assimilation of organic material reduced, in sediments for years following mangrove removal.

These hypotheses are being addressed by adding 13C and 15N labeled organic matter to sediment communities from healthy mangroves, mangrove removal sites of varying ages and control sites, and tracing assimilation and mineralization of this material over 4-day periods. The partitioning of 13C into bacterial fatty acids and porewater CO2, and 13C and 15N into faunal biomass, is being measured using state-of-the-art geochemical and molecular techniques. In addition, foodweb structure in these different communities is being assessed using measurements of natural abundance 13C and 15N in meiofauna and macrofauna.