Environ. Sci. Technol. 2001, 35, 923-927
Influence of Current Velocity on Cadmium Accumulation by an Aquatic Moss and the Consequences for Its Use as a Biomonitor L O U I S C R O I S E T I EÅ R E , † , ‡ L A N D I S H A R E , * , § A N D A N D R EÄ T E S S I E R § De´partement de Chimie-Biologie, Universite´ du Que´bec a` Trois-Rivie`res, C.P. 500, Trois-Rivie`res, Que´bec, Canada G9A 5H7, and Institut National de la Recherche Scientifique-Eau (INRS-Eau), Universite´ du Que´bec, C.P. 7500, Sainte-Foy, Que´bec, Canada G1V 4C7
Aquatic mosses are widely used as biomonitors of contaminant concentrations in running waters. The results of several previous studies suggest that metal concentrations in mosses are influenced by current velocity and that this variable should be taken into account when mosses are used as metal biomonitors. However, in these studies, the purported influence of water velocity was confounded by other uncontrolled variables. We conducted our study to test the influence of current velocity on Cd accumulation by the riverine moss Fontinalis dalecarlica. We found no difference in Cd accumulation over 2 weeks by moss exposed in the laboratory to a constant Cd concentration over a wide range of current velocities (0.01-0.70 m s-1) that occur in the field. Similarly, the results of a field experiment, in which we exposed F. dalecarlica in a Cdcontaminated creek to four current velocities (0.05-0.50 m s-1), confirmed that in nature Cd accumulation by this moss is not influenced by current velocity. We show that a bioaccumulation model that ignores current velocity describes Cd accumulation by F. dalecarlica very well. Our results suggest that current velocity does not have to be considered when using aquatic mosses as metal biomonitors.
Introduction Trace metals can be present in natural waters at concentrations that are difficult to measure (1) and yet could be toxic to organisms. However, even accurate measurements of trace metal concentrations in water are of limited use on their own because they ignore the metal’s availability to organisms. In contrast, measurements of metal concentrations in aquatic organisms can provide biologically meaningful estimates of metal contamination in nature (2, 3). In freshwaters, mosses are used as biomonitors because (i) they can accumulate trace metals to easily measurable levels (4, 5); (ii) their metal concentrations respond rapidly (days to weeks) to a change in ambient metal concentration (6-8); (iii) they are widespread in running waters (9) and, if not present, can be easily * Corresponding author phone: (418)654-2640; fax: (418)654-2600; e-mail:
[email protected]. † Universite ´ du Que´bec a` Trois-Rivie`res. ‡ Present address: INRS-Eau. § INRS-Eau. 10.1021/es001435p CCC: $20.00 Published on Web 02/02/2001
2001 American Chemical Society
transplanted to sites under evaluation (10). Despite their long history as biomonitors (10, 11), there has been little attempt to develop theoretically based models to rigorously relate metal concentrations in mosses to those in water. Presently, measurements of trace metal concentrations in mosses exposed to polluted waters are often simply compared to background metal concentrations to classify water systems as to their relative degree of metal pollution (4, 12). There is a need to put biomonitoring studies using mosses on a firmer theoretical foundation by quantifying the influence of various environmental variables on metal accumulation by these plants. Several researchers have suggested that mosses accumulate more Cd, Pb, Cr, and Zn at higher current velocities (4, 13, 14). Such increases could be explained by a reduction in the diffusive boundary layer (DBL) thickness at the plant surface and a concomitant increase in metal transport to the plant. Diffusive transport of metal across the DBL should follow Fick’s law, that is, F ) D dC/dx, where F is the flux (mol cm-2 s-1), D is the diffusion coefficient of the metal, and dC/dx is the concentration gradient across the DBL. Because current velocity has usually been studied in conjunction with other confounding factors, available data do not allow us to draw a clear conclusion about the influence of this parameter on metal accumulation by mosses. We conducted experiments in both the laboratory and the field to unambiguously determine if current speed influences the accumulation of Cd by the aquatic moss Fontinalis dalecarlica Schimp ex. B.S.G. The results of our study allow us to determine whether current velocity should be included in metal accumulation models for mosses.
Methods We collected F. dalecarlica for our laboratory and field experiments from a small creek located in the Mastigouche Reserve, Quebec, Canada (46°50′41′′ N, 73°19′52′′ W). Mosses were maintained near field temperature during their transport to the laboratory where they were cleaned with deionized water to remove adhering particles. Leafy stems were selected and held for less than 15 d in dechlorinated tap water at 14 °C under a light regime similar to that in the field. We conducted laboratory experiments to determine the influence of current velocity on Cd uptake by F. dalecarlica in the custom-made apparatus shown in Figure 1. Exposure medium was prepared continuously in a reservoir fed with appropriate volumes of CdCl2 stock solution and tap water (dechlorinated on active charcoal) to attain a nominal Cd concentration of 1 µg L-1 (8.9 nM, Table 1). This Cd concentration can be found in Cd-contaminated natural waters (2). Given the low concentrations of ligands present in the dechlorinated tap water, we calculated using the Windermere Humic Aqueous Model (WHAM; 18) that ≈97% of the total Cd added was present as the free ion, Cd2+ (Table 1). Fresh exposure medium was added at the rate of 6 L min-1 into a large fiberglass basin holding the 8 Cd-exposure units (2 units are shown as part 8 in Figure 1). From this basin, exposure medium was fed to two nonmetallic pumps (Jacuzzi model 15 RTC, part 6 in Figure 1) having a total output of ≈900 L min-1. Current velocity in each exposure unit (Table 2) was controlled by a plastic valve (part 7 in Figure 1) that regulated flow from these pumps. All of our current velocities were within the range measured in the field. Into each Cd-exposure unit, we placed a large number of leafy stems of F. dalecarlica that were held in place by a stainless steel clip (part 9 in Figure 1). Water samples for the measurement of Cd, Ca, Mg, Na, and K were collected daily VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Apparatus used for our laboratory experiment consisting of the following: (1) container with Cd stock solution, (2) peristaltic pump, (3) activated charcoal bed, (4) tap water intake, (5) reservoir for Cd-contaminated water, (6) high-output pumps, (7) flow-regulation valves, (8) two of eight moss exposure units made of semicircular sections of PVC piping, (9) stainless steel clip to hold moss stems in each exposure unit, (10) large common basin holding exposure units, and (11) system overflow.
TABLE 1. Temperature, Conductivity, pH, Cd, and Total Dissolved (TD) Concentrations of Humic Substances and Major Ions Measured during Our Laboratory and Field Experiments mean ( SD; n parameter
laboratory exp
field exp
pH temp (°C) [CdTD] (nM) [Cd2+] (nM) [Ca] (µM) [Mg] (µM) [Na] (µM) [K] (µM) [Cl] (µM) [SO4] (µM) [humic substances] (mg L-1) conductivity (µS cm-1)
7.54 ( 0.05; 15 16.9 ( 0.6; 15 8.6a ( 0.8; 125 8.3 151 ( 8; 15 23.8 ( 4.5; 15 151 ( 7; 15 12.0 ( 2.8; 15 110 ( 6; 7 127 ( 5; 7 0c 63.8 ( 4.2; 15
6.80 ( 0.11; 6 20.3 ( 1.4; 6 1.4b ( 0.3; 10 0.16 224 ( 37; 6 88.3 ( 15.6; 6 209 ( 80; 6 9.8 ( 2.8; 6 194 ( 86; 4 26.8 ( 4.5; 4 78.6 ( 0.2; 3 61.3 ( 15.2; 6
a Mean for all treatment levels. b Mean for water samples collected at 3, 6, 10, and 14 d. c [Humic substances] assumed to be negligible in dechlorinated tap water.
with a disposable pipet from each Cd-exposure unit and transferred to pre-acidified (HNO3; pH
