DO BENTHIC FORAMINIFERAL ASSEMBLAGES REFLECT HEAVY METAL CONTAMINATION IN SEDIMENTS OF BISCAYNE BAY, FLORIDA, USA?

Heavy-metal pollution is an issue of concern in estuaries like Biscayne Bay that are heavily influenced by urban runoff and boat traffic. The goal of this study was to determine if benthic foraminiferal assemblages in Biscayne Bay reflect heavy-metal contamination in sediments. Surficial samples were collected at 38 sites along transects in north and central Biscayne Bay. Geochemical analyses revealed elevated concentrations of Ag, As, Cu, Hg, Ni, Pb, or Zn at 27 sites, most from the northern bay transects. Zn, Cu or Ni concentrations at five nearshore sites exceeded concentrations that previous studies have shown to be associated with biological effects in 50% of the test cases (i.e., effects range-median or ER-M limits). At 13 sites, four or more metals exceeded concentrations that previous studies have shown to be associated with biological effects in 10% of the test cases (i.e., effects range-low or ER-L limits). Cluster analysis and multi-dimensional scaling identified two distinct foraminiferal assemblages. In one assemblage, approximately 60% of the specimens were identified as Ammonia, Cribroelphidium, Nonion, Bolivina or Elphidium, all of which are considered to be opportunistic genera that tolerate environmental stresses. This assemblage was found only in samples from northern bay transects. Abundances of most of these opportunistic genera were strongly correlated with concentrations of Cu, Pb, Zn and Ag in the sediments, indicating their tolerance of these metals. The smaller miliolid genera Quinqueloculina and Triloculina accounted for about 60% of the second assemblage. Abundances of these smaller miliolids and of Archaias, which host algal endosymbionts, were negatively correlated with concentrations of Cu, Pb, Zn and Ag, indicating sensitivity either to these metals or to other environmental stresses that co-occurred with the metals. Heavy-metal concentrations tended to be higher in finer-grained sediments. However, abundances of only two foraminiferal genera significantly correlated with sediment texture, indicating that foraminiferal assemblages overall are responding to other environmental factors. Symbiont-bearing foraminifera, notably Archaias and Peneroplis, were rare in the northern bay and more common in the central bay, farther from urban Miami.