FORAMINIFERAL ASSEMBLAGES AND GEOCHEMICAL PROPERTIES OF SEDIMENTS IN BISCAYNE BAY, FLORIDA

Heavy-metal pollution is a concern in estuaries that are heavily influenced by agricultural, urban, and harbor activities.  The goals of this study were to (1) provide a “state of the bay” assessment relative to changes that have occurred over the past 60 years in Biscayne Bay, Florida, (2) provide a current baseline to compare changes in the ecosystems during and after execution of the Comprehensive Everglades Restorations Plan (CERP), and (3) determine if benthic foraminiferal assemblages in the bay reflect heavy-metal contamination in surface sediments. Surficial samples were collected at 147 sites throughout the bay.  Analyses included geochemical assessment for 32 elements, grain-size analysis, and assessment of foraminiferal assemblages at the genus level.  Geochemical analyses revealed elevated concentrations of a suite of heavy metals in the sediments of the northern bay, between Miami and Key Biscayne, and the periphery of the southern bay from Black Creek Canal south to Turkey Point. Heavy metals of concern included Cu, Zn, Pb, Ag, Sn, Hg, Cr, and Ni. Cluster analysis, multi-dimensional scaling, and multivariate-correlation analyses revealed three distinct foraminiferal assemblages.  One assemblage, characteristic of the northern bay, was defined by stress-tolerant taxa including Ammonia, Cribroelphidium, Nonion, and Haynesina, which were present in low abundances. The stress-tolerant assemblage correlated with the most elevated concentrations of heavy metals.  The assemblage that defined the southwestern margin of the bay was dominated by Ammonia and Quinqueloculina. This assemblage is characterized by the lowest diversities and highest abundances, and is likely influenced by both reduced salinity and elevated organic-carbon concentrations. A diverse assemblage of smaller miliolids and rotaliids characterized the open-bay assemblage.  The open-bay assemblage is the only assemblage that contains a significant component (~10%) of symbiont-bearing foraminifers. Comparison with historic studies of the past 60 years shows that populations of symbiont-bearing taxa, particularly Archaias and Sorites, which are indicators of normal, marine conditions, have decreased in Biscayne Bay, while populations of stress-tolerant taxa have increased.