Pollutants and Foraminiferal Assemblages in Torrecillas Lagoon: An Environmental Micropaleontology Approach

Abstract

Torrecillas Lagoon in the North Coast of Puerto Rico has experienced extensive anthropogenic influence over the past 400 years. Elevated concentrations of Potential Toxic Elements (PTEs) have been reported in surficial sediments. The main goal of this dissertation was to implement in Puerto Rico the use of benthic foraminifers as a bioindicators of PTEs and to compare the impact of Cu(II) on field samples with results of experimental work using cultures. Analyses included geochemical assessment for bulk and carbonate- soluble bioavailable concentrations of PTEs in surface, core and pore-water samples, as well as analyses of grain-size, Percent Total Organic Carbon (%TOC), Percent Carbonate (%CO3), foraminiferal assemblages and distribution, and ecological indices. PTEs of concern (Cu, Zn, Ni, Pb, Cr, As, Li, Se, Fe, Mn, V, Se) have relatively uniform spatial distributions. Areas with higher concentrations are associated with higher %TOC and %mud, as well as with anoxic conditions. Temporal distributions show limited variability although an overall decrease in enrichment indicates improvement of environmental conditions in the 20th century. Ammonia beccarii, Quinquloculina rhodiensis, Q. seminulum, and Ammobaculites agglutinans are the dominant foraminifers in the lagoon and are characteristic of stressed coastal environments. Several PTEs, including bioavailable Cu and Zn are negatively correlated with the dominant foraminiferal taxa and with diversity indices, indicating that these pollutants are influencing the spatial and temporal distributions of foraminiferal assemblages. Ammonia beccarii abundance negatively correlates with bulk concentration of Cu(II) and exhibits no correlation with its bioavailable fraction. These observations suggest that fractionation and bioavailability of PTEs need to be considered more in depth as influences on ecological indices and foraminiferal behavior. Exchangeable and oxidizable fractions are considered the most likely to influence the ecology of foraminifers under most circumstances. Ammonia aomoriensi was exposed to Cu(II) concentrations (0–0.32 µmol/L) under controlled conditions (25ºC, 35PSU and pH= 7.8). After a 23-day treatment, foraminifers exposed to 0.22 and 0.32 µmol/L concentrations exhibited reduced growth and morphological deformities in which the long axis of the chamber extended ventrally, increasing the height of the trochospire. The waters in Torrecillas lagoon show strong stratification, with hypoxic/anoxic (DO/L) and corrosive (pH\u3c 7.4) conditions below 4 m depth. The presence of such strong gradients in very shallow water represents a dynamic chemical environment, with changes occurring on day-night cycles, tidal cycles, and especially with storm activity that induces mixing of otherwise highly stratified, very localized waters. Recognizing the potential for sequestered PTEs to be mobilized is essential insight for coastal management agencies that must assess the risks of existing PTEs during coastal engineering activities (e.g., dredge and fill activities) and major storm events