Environ. Sci. Technol. 2007, 41, 6103-6108
Impact of Water Temperature and Dissolved Oxygen on Copper Cycling in an Urban Estuary A A R O N J . B E C K * ,† A N D SERGIO A. SAN ˜ UDO-WILHELMY‡ Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794-5000, and Department of Biological Sciences and Department of Earth Sciences, University of Southern California, Marine and Environmental Biology, Los Angeles, California 90089-0371
An increasing body of evidence suggests that much of the trace metal contamination observed in coastal waters is no longer derived from point-source inputs, but instead originates from diffuse, non-point sources. Previous research has shown that water temperature and dissolved oxygen regulate non-point source processes such as sediment diagenesis; however, limited information is available regarding the effect of these variables on toxic trace metal cycling and speciation in natural waters. Here, we present data on the seasonal variation of dissolved Cu cycling in the Long Island Sound, an urban estuary adjacent to New York City. An operationally defined chemical speciation technique based on kinetic lability and organic complexation has been applied to examine the most ecologically relevant metal fraction. In contrast to the decrease from spring to summer observed in the total dissolved Cu pool (average ( SD: 15.1 ( 4.4 nM in spring and 11.8 ( 3.5 nM in summer), our results revealed that in the highly impacted western LIS, levels of labile Cu reached higher levels in summer (range 3.6-7.7 nM) than in spring (range 1.53.9 nM). Labile Cu in surface waters of the western Sound appeared to have a wastewater source during spring high flow conditions, coinciding with elevated levels of sewage-derived Ag. Labile Cu elsewhere in the LIS during spring apparently resulted from fluvial input and mixing. During summer, labile Cu increased in bottom waters (at one site, bottom water labile Cu increased from 1.5 nM in spring to 7.7 nM in summer), and covariance with tracers of diagenetic remobilization (e.g., Mn) revealed a sedimentary source. Although total dissolved Cu showed no consistent trends with water quality parameters, labile Cu in bottom waters showed an inverse correlation with dissolved oxygen and a positive, exponential correlation with water temperature. These results suggest that future increases in coastal water temperatures may cause the benthic source of labile Cu to become proportionally more significant.
Introduction During the several decades following the passage of the Clean Water Act in the 1970s, billions of dollars have been spent * Corresponding author e-mail:
[email protected]. † Stony Brook University. ‡ University of Southern California. 10.1021/es062719y CCC: $37.00 Published on Web 07/24/2007
2007 American Chemical Society
reducing inputs of toxic contaminants from point sources into United States coastal waters. Despite such economic effort, the effect of these reductions is often not reflected in a decline in dissolved metal concentrations (1). The reason for this limited decline in water column metal levels is that sediments are now the major reservoir of toxic metals in coastal environments, and diffusive inputs are continuously modifying the dissolved composition of coastal waters (2, 3). An important issue that has not been addressed is how dissolved metal concentrations in coastal environments will be affected by climate change (4). Due to the increase in microbial activity with temperature (5), benthic remobilization of metals from contaminated sediments increases almost exponentially as water temperatures approach or exceed 20 °C (6, 7). If this is the case, the predicted increases in temperature associated with global warming may intensify benthic remobilization of toxic metals in the future, and efforts to control metal inputs from wastewater discharges may have a limited impact on water column metal levels. Dissolved oxygen levels can also affect metal cycling in coastal waters, as metal remobilization from sediments is controlled by the redox conditions prevailing at the sediment-water interface. For example, shallow oxygen penetration into surficial sediments results in formation of an oxide-rich layer which can adsorb dissolved metals, preventing their transport into the overlying water (8-10). As oxygen levels in the overlying water decrease, this surface oxic layer thins, allowing greater movement of dissolved constituents out of the sediment (11). However, under anoxic-hypoxic conditions, many metals (e.g., Cu) are in a reduced, insoluble form, and may be preserved in particulate phases and sediments in non-bioavailable chemical forms (12, 13). While the dependence of metal bioavailability and toxicity on chemical speciation is well established (e.g., ref 14, and references therein), it is yet unclear how important water temperature and dissolved oxygen are in defining species distribution and their concentrations within the dissolved metal pool. Therefore, the main objective of this study is to establish the impact of water temperature and dissolved oxygen levels on the metal speciation of a potentially toxic trace metal (Cu) in a major urban estuary, the Long Island Sound (LIS). We hypothesize that seasonal changes in those two environmental variables in the LIS will cause significant increases in dissolved Cu concentrations, particularly in bottom waters, and that increases in the labile (potentially bioavailable) Cu fraction will be most pronounced. We focused our study on the LIS because it is one of the most contaminated ecosystems in the nation (ref 15, and references therein) and recent mass balance estimates have suggested that remobilization from sediments is probably the major source of some toxic trace metals to the water column (15). Because regions within the LIS are hypoxic part of the year (16), and bottom water temperatures have steadily increased by about 0.2 °C per year during the past decade (temperatures in excess of 23 °C are now often observed in some areas of the Sound; ref 17), the LIS is an environment well-suited to study the impact of those two variables on metal cycling. Furthermore, recent punctuated and catastrophic declines in lobster populations have resulted in major economic losses, with tens of millions of dollars spent on disaster and economic relief (18). Additional millions of dollars spent on research to address the issue have failed to conclusively identify the cause of the massive lobster mortality. Given the high levels of metals in LIS sediments (19), a benthic source of toxic metals could pose a significant threat to bottom-dwelling lobster populations (20). Despite VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map of Long Island Sound with sampling stations indicated. this, no measurements of dissolved trace metals have been published from bottom waters of the Sound, and metal speciation measurements have been limited to Hg (21). Because crustaceans are particularly susceptible to Cu toxicity (22, 23), Cu was also chosen as the contaminant of focus for this study.
Materials and Methods Description of the Study Site. The Long Island Sound (LIS, Figure 1) is one of the largest estuaries in the United States, comprising a surface area of 3280 km2 and an average depth of 20 m. The LIS extends roughly east-west, and at 200 km long, is about four times longer than its widest point. Fresh water enters the LIS through the East River as well as four Connecticut rivers (Thames, Housatonic, Quinnipiac, and Connecticut Rivers), which provide approximately 70% of the freshwater input. In the LIS, dissolved oxygen and temperature follow extreme, regular seasonal variations. Dissolved oxygen is generally supersaturated during cool months, but declines during summer months to hypoxic levels of less than 0.19 mM in response to increased biological respiration and thermohaline stratification (24). Hypoxia is primarily restricted to the nutrient-contaminated western reaches of the LIS (16), and is at a maximum during late summer. Water temperatures vary by over 20 °C throughout the year, peaking at nearly 24 °C in summer months (17). Sampling Cruises. Two cruises were conducted during 2005 to examine differences in metal speciation associated with spring (April 26-29) and summer (September 6-9) conditions. During the spring cruise, water temperatures ranged from 5 to 11 °C, with highest temperatures in the western Sound. The water column of the LIS was well oxygenated at that time, with concentrations ranging between 0.44 and 0.69 mM. During the summer cruise, water temperatures ranged between 19 and 24.5 °C. Dissolved oxygen was generally e0.44 mM, with nearly hypoxic levels (
