Aspects of the dynamics of mercury cycling in a small Hawaiian estuary

Abstract

The environmental dynamics of mercury were studied in Ala Wai Canal, a small Hawaiian estuary. Three indicator species were used in the study - the detritus feeding polychaete Nereis succinea and shrimp Palaemon debilis and the predaceous decapod Thalamita crenata. The study employed laboratory experiments, utilizing 203-HgCl2, and analyses of total mercury concentrations in sediment and biota samples collected periodically from the estuary. The two detritus feeders concentrated dissolved 203-Hg from 160 to 310 times over the concentration in seawater. Reduced salinity inhibited 203-Hg uptake by the worm but had little effect on uptake by the shrimp. Dissolved organic material also reduced 203-Hg accumulation. Little 203-Hg was accumulated from labeled estuarine sediment by either species - the steady state concentrations of 203-Hg in the animals ranged from only 0.25% to 1.5% of the concentration in the sediment. However, 203-Hg bound to sediment originating from urban street runoff was more available to both species than was 203-Hg bound to estuarine sediment. The availability of 203-Hg from runoff sediment declined as the sediment aged in seawater. Net loss of mercury occurred by way of two mono-exponential processes in both organisms and was slow relative to net accumulation. Total mercury analyses illustrated that mercury concentrations in sediment in Ala Wai Canal showed no obvious pattern of temporal variation. However, there was an obvious decrease in total mercury in the two detritus feeders between the rainy season and the dry summer months. Samples of P. debilis were collected at five day intervals during the rainy season. The equations describing accumulation and loss of mercury in this species were used with these samples and rainfall data to construct a simple model of biotic mercury dynamics in the estuary. The model illustrated a strong correlation between rainfall and biotic mercury concentrations, and indicated that the primary source of mercury for the shrimp, and probably the worm, occurred in dissolved form. To further study the trophic transfer of mercury the predator, T. crenata, was fed 203-Hg labeled N. succinea over a two week period. The activity of 203-Hg in the crab bodies appeared to reach a steady state after four to seven days. At ingestion rates of up to 3% of body weight per day the concentration of 203-Hg in the muscle of this species never exceeded 26% of the concentration of 203-Hg in the food of the crab. Total mercury and organic mercury were also determined in crabs collected from Ala Wai Canal. Of the total mercury in crab muscle 25% was in organic form. It appeared that dissolved mercury was an important source of this ion for T. crenata and that no food chain biomagnification of mercury occurred at this trophic level. In their natural habitat the shrimp and the worm are exposed to mercury concentrations in their food several times greater than the minimum lethal dose of dissolved mercury. However, neither these species nor the predator T. crenata concentrated mercury to whole body concentrations greater than 5% of that in their food. It is hypothesized that natural selection might favor processes which result in inefficient assimilation of cations from food by aquatic species. Toxic cations occur in much higher concentrations in the food of aquatic species than in solution. In ecosystems where a sufficient supply of essential cations exists in solution, an inefficiency in removing cations from food might reduce assimilation of toxic cations from their most concentrated pool at no expense to essential cation regulation.