Article pubs.acs.org/est
Diel Changes in Trace Metal Concentration and Distribution in Coastal Waters: Catalina Island As a Study Case Paulina Pinedo-Gonzalez,*,† A. Joshua West,† Ignacio Rivera-Duarte,‡ and Sergio A. Sañudo-Wilhelmy§ †
Department of Earth Sciences and §Department of Biological Sciences and Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, United States ‡ SPAWAR Systems Center Pacific, Code 71750, Environmental Analysis and Compliance, San Diego, California 92152-6343, United States S Supporting Information *
ABSTRACT: Understanding biogeochemical cycling of trace metals in the ocean requires information about variability in metal concentrations and distribution over short, e.g., diel, time scales. Such variability and the factors that influence it are poorly characterized. To address this shortcoming, we measured trace metal concentrations in the total dissolved, colloidal, and soluble fractions every 3−4 h for several consecutive days and nights in surface waters from a coastal station. Our results show that both the concentration and the size partitioning of some biologically essential (Fe, Cu, Co, and Cd) and anthropogenic (Pb) metals are subjected to diel variations that may be related to both inorganic and biological processes (e.g., photolysis of high-molecularweight dissolved organic matter, photoinduced reduction/oxidation of metal(hydrous)oxides, uptake by growing phytoplankton, degradation of organic matter, lysis, and grazing). The largest fluctuations were observed in the soluble and colloidal pools. Soluble Fe varied during the day-night cycle by a factor of 40, and the contribution of colloidal Pb to the total dissolved fraction increased from 6 ± 3% during the day to as much as 70−80% during the night. Our results suggest that changes occurring over time scales of hours need to be considered when collecting and interpreting trace metal data from the surface ocean.
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INTRODUCTION Day and night cycles resulting from Earth’s rotation are a primary source of biological and physicochemical variability in the surface ocean. This diurnal cycle produces regular and dynamic changes in numerous physical and biogeochemical parameters and processes (i.e., surface temperature, solar radiation and associated photochemical reactions, photosynthesis and respiration), and thus it is expected to influence the concentrations of photochemical and bioactive chemical constituents in the surface ocean, including some trace metals. Trace metals are important elements in the biogeochemistry of marine ecosystems, since some are required by aquatic organisms for various metabolic functions.1 The role of trace metal concentrations in the productivity of the world’s ocean has stimulated several research programs (e.g., GEOTRACES) to acquire accurate data on metal concentration, chemical form, and distribution. However, these research programs, due to the challenges of sampling around-the-clock at a single location, do not evaluate the patterns that might occur in metal concentrations over diel time scales. It is not just variability in total metal concentrations that are important to understand; numerous studies have shown that the traditional subdivision of trace metals into two fractions, “dissolved” (typically defined as 0.2−0.45 μm), provides limited © 2014 American Chemical Society
information about the actual behavior of several elements in the ocean.2−6 This is because the dissolved fraction of many metals not only includes the soluble metal pool but also contains colloids with sizes operationally defined as ranging from 1.0 or 10 kDa to 0.2−1.0 μm.7−10 These colloids can have a significant effect on metal transport, distribution, bioavailability, and toxicity.2,11,12 Thus, it is important to evaluate not only changes in total dissolved metal concentration over diel time scales but also changes in their distribution between different size fractions, referred to as their size fractionation. Comprehensive studies in streams13−16 and lakes17,18 have demonstrated that dissolved concentrations and size fractionation of many trace elements change substantially during 24-h periods. These variations have been attributed to changes in temperature, biological activity, photoinduced metal reduction/oxidation, and adsorption/desorption processes. In the ocean, diurnal fluctuations in the concentration and size fractionation of trace metals along with the main factors that control these variations remain poorly investigated. To address the extent to which trace metal concentrations and Received: Revised: Accepted: Published: 7730
September 27, 2013 June 13, 2014 June 22, 2014 June 23, 2014 dx.doi.org/10.1021/es5019515 | Environ. Sci. Technol. 2014, 48, 7730−7737
Environmental Science & Technology
Article
partitioning between the traditionally defined total dissolved (
