Biomonitoring heavy metals in estuaries: a field comparison of two brown algae species inhabiting upper estuarine reaches.

Biomonitoring dissolved heavy metals within estuaries, particularly at their upper reaches, frequently has to rely on several biomonitors; rarely a single species thrives all along the salinity gradient. To properly do so, it must be established whether those biomonitors actually accumulate heavy metals alike. In this study, two brown seaweeds from the upper section of three NW Spain estuaries--the widely-known Fucus vesiculosus and the estuarine Fucus ceranoides--were compared as metal biomonitors. Both species were collected at five locations where they either coexist or live close to each other and their heavy metal content (Cu, Cr, Mn, Zn, Fe, Al) was measured. Analyses were appropriately replicated for each species x location combination to allow a statistically reliable detection of differences in bioaccumulation, with particular emphasis on the magnitude of interspecific differences. The lack of significant differences for Cu, Mn, and Zn contents in F. ceranoides and F. vesiculosus supports the feasibility of their joint use to monitor these metals along the estuaries. Conversely, F. ceranoides concentrated significantly higher levels of Cr, Fe, and Al than F. vesiculosus and hence combining data for both fucoids to monitor these elements seems impractical. The correlation of species differences together with a similar Al:Fe ratio in both weed tissue and sediment suggest that Cr, Fe, and Al tissue-burdens might be considerably biased by sediment retained on the surface of the weed. Parallel analyses of Al and/or Fe in seaweeds and sediments could serve to keep track of this interference and may help to combine data from both fucoids for monitoring elements like Cr.