Macrobenthic Community Structure, Secondary Production, and Rates of Bioturbation and Sedimentation at the Kiine'ohe Bay Lagoon Floor!

The Kane'ohe Bay lagoon floor is one of the largest shallowwater, muddy habitats in Hawai'i and is a major repository for sediments and, possibly, pollutants from the Kane'ohe watershed. Nonetheless, macrobenthic community structure, secondary production, and particle-mixing rates at the lagoon floor remain largely unstudied. During 1990-1991, we surveyed macrobenthic community structure at four stations 12 m deep at the lagoon floor and evaluated macrobenthic secondary production, as well as particle mixing and sedimentation, at one representative station. Macrobenthic abundance in the lagoon during our survey was high (44,000-100,000 individuals m), with very small deposit-feeding polychaetes dominating the community. This lowdiversity assemblage was relatively similar throughout the bay and resembled the communities found in highly depositional environments (e.g., river deltas, and zones of active erosion and redeposition). Macrobenthic secondary production at the representative station was low, with a best estimate of 4.9 g m2 yr1 ash-free dry weight (reasonable range 1.2-20 g m2 yr1); this appeared to be enough production to support <2% of the annual fish yield in Kane'ohe Bay. Tracer-particle experiments at the representative station, sampled after 7 months and 1 yr, indicated low sediment-mixing rates (diffusive mixing coefficient ~0.9 cm yr), little size dependence in particle mixing, and relatively high short-term rates of sedimentation (6-7 cm yr). After corrections for sediment compaction, these short-term sedimentation rates (2.7-3.7 cm yr) are about three-fold higher than longer-term (decadal) sedimentation rates (~1.0 cm yr1) estimated using Pb-21O geochronology at a nearby site; the discrepancy may be caused by sediment transport from nearby fringing reefs, resuspension of bottom sediments by alpheid shrimp, or interannual variability of sediment flux into the bay. We conclude that the Kane'ohe Bay lagoon harbors a low-diversity, low-productivity macrobenthic assemblage largely structured by high gross sedimentation rates. In addition, we conclude that sandsized particles entering the bay are rapidly (within months) sequestered below the sediment-water interface, where they remain for at least l-yr time scales. and provides diverse resources for boating, diving, and scientific research (e.g., S. V. Smith et al. 1973, 1981, Everson 1994). In addition, the bay acts as a repository for terrestrial sediments and pollutants running off the Kane'ohe watershed, as well as carbonate sediments derived from barrier and fringing reefs (Roy 1970, S. V. Smith et al. 1973, Hollett 1977, Mackenzie et al. 1981). Because the bay supports a broad range of human activities, its environmental quality is of prime interest to the human population of