SEASONALITY IN ESTUARINE SOURCES OF METHYLATED ARSENIC

The effect of seasonal temperature change on the release of methylated arsenic from macroalgae, phytoplankton and sediment porewaters has be en investigated by a series of controlled laboratory experiments. The appearance of dissolved arsenic species in the overlying waters was mo nitored using a coupled hydride generation/GC AA analytical technique. The liberation of dissolved arsenic species by the macroalgae Ascophy llum nodosum was examined under estuarine conditions at 5-degrees-C an d 15-degrees-C. At the lower temperature the release rates were 0.2 mu g kg-1 h-1 (wet weight of material) for monomethylarsenic (MMA) and 0. 5 mug kg-1 h-1 for dimethylarsenic (DMA), whereas at 15-degrees-C the rates were 0.4 mug kg-1 h-1 and 3.2 mug kg-1 h-1, respectively. Incuba tion experiments were also carried out at 15-degrees-C using the diato m Skeletonema costatum. During the log growth phase, when chlorophyll a concentrations were in the range 1-5 mug dm-3, the rate of appearanc e of DMA in the water was approximately 3 ng dm-3 h-1. Sediment sample s from the freshwater and seawater end-members of the Tamar Estuary, U K, were incubated under natural conditions at 5-degrees-C and 15-degre es-C. The freshwater sediments released DMA in preference to MMA; the concentrations of both species increased exponentially and reached a s teady state in the overlying water after 250 h. Considerably more DMA was produced at 15-degrees-C than at 5-degrees-C, whilst the amount of MMA produced appeared to be insensitive to the temperature increase. In contrast, the seawater sediments always produced more MMA than DMA and the increase in temperature had little effect on the production of either MMA or DMA. The results of the laboratory experiments were com pared with field observations in temperate estuaries, including the Ta mar Estuary. The implications of changes of water temperature on the f ate of arsenic in estuaries is discussed and modifications to the estu arine arsenic cycle are proposed.