Sampling techniques for reliable determination of trace metals in macrophytes and periphyton

Abstract

Sampling techniques were developed to measure trace-metal concentrations reliably in the shoots of a submerged macrophyte, Najas tenuifolia, and its attached periphyton growing in the Magela Creek flood plain in monsoonal northern Australia. At each site, 30 samples were collected within an 8-m by 8-m quadrat in a pattern of five samples from each of six irregularly spaced points. Samples were collected underwater to include all periphyton present, and precautions were taken at all stages of processing to avoid contamination. Periphyton was dislodged for separate chemical analysis by sonication followed by vigorous shaking. A significant aspect was the large amount of periphyton, up to 35% of the total sample dry weight, associated with the Najas sp. Only a proportion, up to 43%, of this periphyton could be dislodged by the physical procedures used.

Trace metals were generally not present in the same concentrations in both the Najas sp. and the periphyton. The average trace-metal concentrations (μg g^-1 dry weight) and standard errors measured in the Najas sp. (and periphyton) were cadmium 0.24 ± 0.013 (0.093 ± 0.012), copper 5.2± 0.25 (12 ± 0.74), lead 0.93 ± 0.084 (6.5 ± 0.43), manganese 2500 ± 110 (1500 ± 110), uranium 0.24 ± 0.023 (0.27 ± 0.030), and zinc 38 ± 1.0 (23 ± 1.5). The significant differences in Najas sp. and periphyton trace-metal concentrations show the need to consider the effect of periphyton when sampling and reporting trace-metal concentrations in submerged macrophytes.