Role of Ecklonia radiata (C. Ag.) J. Agardh in determining trace metal availability in coastal waters. II. Trace metal speciation

Abstract

Detrital decomposition of the kelp E. radiata leads to the leaching of substantial amounts of trace metals and dissolved organic carbon (DOC). About 15% of the total Fe, 30% of Cu and Mn, 80% of Zn and Cd, and 25% of the DOC were released. The holdfast was more resistant to decomposition than the other tissues. The high absolute concentration of Fe in E. radiata ensured that it was the predominant metal in the leachates.

The DOC released contained organic ligands that were capable of forming strong complexes with Cu, Fe and Zn, and the complexing capacity of the DOC exceeded the total amount of these metals released during leaching. The copper complexing capacity of the leachate (15 nmol mg^-1 DOC) is comparable to the copper complexing capacity of Australian coastal waters. Exudation of DOC by living E. radiata plants also results in the release by ligands with strong metal complexing abliity.

Fractionation of the leachates by reverse-phase chromatography confirmed that Cu, Fe and Zn were present as metal-organic complexes. These complexes were generally of high polarity. Polyphenolic compounds were an important component of the exudate but HPLC analysis indicated that other complexing agents also occurred. E. radiata beds may play a major role in regulating both the concentration and speciation of heavy metals in nearshore environments.