TRACE-METAL NUTRIENT RELATIONSHIPS IN ESTUARIES

The distributions of cadmium, copper, zinc and nutrients were determined in the estuaries of the Medway (Canada), Savannah (U.S.A.) and Maeklong (Thailand) rivers to assess the influence of biological processes on the trace metal estuarine behavior. These metals were chosen because they show nutrient-type behavior in the ocean. Three types of metal-nutrient relationships were considered: those resulting from biological uptake, regeneration and abiotic processes. Biological uptake was not observed to be an important process affecting trace metal behavior in estuaries. Biological regeneration appears to be an important process only for cadmium, based on its relationship with phosphate or nitrogen. The enrichment of nickel in estuarine waters is greater than can be explained by regeneration of organic material, which suggests that abiotic processes are more important. Zinc is generally removed from solution during estuarine mixing, perhaps associated with iron removal. Copper covaries closely with silica and is suggested to be the result of additional weathering of silicate minerals within the estuarine environment. Results from all the estuarine studies suggest that copper and silica are not fractionated during weathering and transport from the continents to the ocean. Iron-phosphate covariance in the Savannah Estuary is attributed to their simultaneous removal by the formation of FePO4.