Brian H. Hill,"* James M. Lazorchak, a Frank H. McCormick a & W. Thomas Willingham b. aNational Exposure Research ... Dudley, 1981). While structural-based ...
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(96)00123-6
Environmental Pollution, Vol. 95, No. 2, pp. 183 190, 1997 Published by Elsevier Science Ltd Printed in Great Britain 0269-7491/'97 $17.00 + 0.00
ELSEVIER
THE EFFECTS OF ELEVATED METALS ON BENTHIC C O M M U N I T Y METABOLISM IN A ROCKY M O U N T A I N STREAM
B r i a n H . Hill,"* J a m e s M . L a z o r c h a k , a F r a n k H . M c C o r m i c k a & W. T h o m a s W i l l i n g h a m b aNational Exposure Research Laboratory, US Environmental Protection Agency, 26 W Martin Luther King Drive, Cincinnati, OH 45268, USA bRegion 8 Water Management Division, US Environmental Protection Agency, Denver Federal Center, Denver, CO 80225, USA
(Received 14 February 1996; accepted 18 September 1996)
Abstract The effects of elevated metals (dissolved Zn, Mn and/or Fe) in a Rocky Mountain stream were assessed using measures of primary productivity, community respiration and watercolumn toxicity. Primary productivity was measured as rates of 02 evolution from natural substrates incubated in situ in closed chambers. Oxygen depletion within these chambers, when incubated in the dark, provided estimates of periphyton community respiration. Sediment community respiration on fine-grained sediments, collected and composited along each stream study reach, was measured on-site by incubating these sediments in closed chambers and measuring 02 depletion. Toxicity was measured as percent mortality of Ceriodaphnia dubia during 48 h acute tests. Gross (GPP) and net primary productivity (NPP) decreased significantly with increasing metal concentrations, from 10.88+1.46g 0 2 m -2day -1 to 0.83+0.20g 02 m -2 day -1 and 9.85+1.43 g 02 m -2 day -1 to 0.814-0.20 g 02 m -2 day -1, respectively for the reference and most impacted site. Community respiration (CR) declinedfrom 0.65 4- 0.08 g 02 m -2 day -! to 0.02 4- 0.01 g 02 m -2 day -1 with increasing metal concentrations. Sediment community respiration (SCR) decreased from 0.264-0.02 g 02 m -2 day -1 to 0.01 +0.01 g 02 m -2 day -1 at these same sites. Ceriodaphnia dubia mortality increased from 0% at the reference site to 95 ± 5 % at the most impacted sites. Net daily metabolism, quantum yield and assimilation ratio all decreased with increasing metal concentrations, suggesting that both autotrophic and heterotrophic components of the periphyton community were impaired. Overall, functional measures were able to discern sites receiving greater metal impacts from lessimpacted sites, with combinations of dissolved metals explaining between 25 and 92% of the variance in the regression models. Using these regression models we were able to calculate lethal and inhibition concentrations of dissolved Zn in the Eagle River. The lethal concentration (LCso) of Zn for Ceriodaphnia dubia is 123 mg liter -1. The concentrations of Zn which inhibited respiration (ICso) were 177 mg liter-! for CR and 199 mg liter -1
for SCR. These results indicate functional measures may be as sensitive to metal concentrations as acute toxicity tests. Published by Elsevier Science Ltd Keywords: Benthic metabolism, Ceriodaphnia toxicity.
chambers,
metals,
INTRODUCTION Biological integrity, as related to the Clean Water Act, has been defined as the ability of an aquatic ecosystem to support and maintain a balanced, integrated, adaptive community of organisms having a species composition, diversity and functional organization comparable to that of the natural habitats within a region (Karr & Dudley, 1981). While structural-based criteria (e.g. community composition, species diversity) are well established, development of functional criteria has lagged far behind. Ecosystem function has been a major focus of stream research over the past decade. In spite of advances in stream ecology, regulatory assessment and monitoring of streams is still generally limited to collection of structural data (Cairns & Pratt, 1986; Cummins, 1991). While structural indicators are relatively easy to quantify and employ standardized methods, within-community variability may hamper their ability to assess stream ecosystem integrity (Cairns & Pratt, 1986). Functional metrics, such as community metabolism or nutrient spiralling, are also variable; however their ability to integrate diverse communities into a single attribute allows easier comparisons within a system through time (temporal heterogeneity) and among diverse systems (spatial heterogeneity) (O'Neill et al., 1986; Cummins, 1988). Functional indicators are less likely to be constrained by regionally restricted biota. Thus, functional approaches lead to a broader view of stream ecosystems, a view that is much less variable than one based only on taxa inhabiting streams. Hunsaker et al. (1990) have argued that for regional ecological risk assessments to be effective, the system must be functionally defined, with the
*To whom correspondence should be addressed. 183
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spatio-temporal boundaries of the system set by functional attributes of the communities inhabiting the system. Assessments that are functionally based are likely to have greater applicability across regions. The most commonly measured functional attributes of ecosystems are gross or net primary productivity (GPP or NPP) and community respiration (CR24 or R). These two metrics are termed community metabolism when considered together and have been shown to be sensitive indicators of ecosystem stress (Matthews et al., 1982; Bott et al., 1985; Hill & Gardner, 1987). Niemi et al. (1993) analyzed the ability of several measures of chemical and biological structure and function to detect impact and recovery in stream ecosystems. They found that GPP and respiration were able to detect impacts at lower levels than most structural measures. Normalization of primary productivity to chlorophyll concentration, termed the assimilative ratio (AR), has been useful in reducing the variance of these measures. While Crossey and LaPointe (1988) were unable to detect differences in GPP between metal impacted and control sites, they were able to measure significant differences in AR, with higher values at the control site. Functional measures have not been extensively used in environmental monitoring, but several researchers have demonstrated the utility of functional assessments of perturbations. Stressors such as metals, chlorine, pesticides, oil and channel desiccation have been shown to depress GPP, NPP and/or R (Bott & Rogenmuser, 1978; Rodgers et al., 1979; Matthews et al., 1982; Hill & Gardner, 1987; Crossey & LaPointe, 1988). This study was undertaken to test the use of functional measures of ecosystem integrity for detecting and assessing the impacts of metal contamination from unmitigated mining operations on stream ecosystems. Two questions were addressed. First, is the variance associated with functional measures low enough so that changes in responses will be detectable from background noise? Second, can functional measures detect changes in metal concentrations found in streams impacted by mining activities, and how sensitive are these measures to changes in metal concentrations with respect to a representative, standard invertebrate toxicity test?
METHODS Study area The Eagle River in Colorado is one of the principal drainages of the Southern Rocky Mountain eco-region (Omernik, 1987) (Fig. 1). Within this 612 km 2 study area the river drops from 2600 to 2475 m elevation above mean sea level, producing an average gradient of 0.026 m/m. The region is primarily igneous and metamorphic rock, and the fiver bed is characterized by small (0.3 m diameter) to larger boulders (> 2.0 m diameter). Five sites (ER-lr, ER-3, ER-5, ER-12 and ER12a) along the upper Eagle River, Colorado, representing conditions upstream of, and adjacent to, a closed mining operation, were selected for study. Stream flow
at these third and fourth order (Strahler, 1957) sites in mid- to late September 1993 was shallow (< 1.0 m) and swift (average velocity >0.5 m s-1) (Table 1). The stream at these sites was cold (8.8-10. I°C), well aerated, alkaline (pH = 8.09-8.64) and nutrient poor, with metal chemistries ranging from near detection limits to orders of magnitude higher (Table 2). Stream chemistry and toxicity Grab samples of stream water for chemistry and toxicity assessment were collected on 26-27 September 1993 from 11 Eagle River locations, including the five sites used to study benthic community metabolism. A 500 ml aliquot of stream water was filtered (glass fiber, 1.0 mm average pore size) in the field and acidified (2 ml HNO3) before shipping to the laboratory for ICP analyses of dissolved metals (As, Cd, Cu, Fe, Mn, Pb and Zn) (USEPA, 1983). A second 1 litre subsample of stream water was collected without filtering and stored on ice (holding time < 7 days) for nutrient (NO2/NO3, SO4, total dissolved P) analyses on a Technicon Autoanalyzer (USEPA, 1983). Samples for toxicity testing were collected at the same time in 4 litre cubitainers and transported on ice to mobile laboratories near Leadville, CO, where 48 h acute toxicity tests were conducted using Ceriodaphnia dubia (USEPA, 1993a). Samples from ER-lr, ER-3 and ER-12 were tested using profile analysis protocols (100% site water), whereas samples from the remaining sites were tested using the modified definitive analysis protocols (100, 75, 50 and 25% site water diluted with moderately hard reconstituted water at ER-12a, and 100, 75 and 50% site water at ER-5). Inhibition (IC50) and lethal (LCs0) concentrations were calculated by regressing dependent variables against Zn concentrations. IC50 is the concentration of Zn that elicits the median response value from the metabolic variables. LCso, the concentration of Zn that causes 50% Ceriodaphnia mortality, was calculated using US Environmental Protection Agency toxicity data analyses software (USEPA, 1994) Benthic community metabolism Benthic community metabolism was measured in situ on 24-27 September 1993, using static, 7 litre clear acrylic chambers. While static chambers have been shown to underestimate metabolism by as much as 65% during 4h incubations (Rodgers & Harvey, 1976; Rodgers et al., 1978), they are suitable for incubations where the likelihood of nutrient depletion or inhibition by metabolic wastes is limited. Three rocks at each site were collected from the streambed (depth ca 50 cm) and placed individually in chambers. The chambers were filled with stream water and initial dissolved oxygen (DO) concentration measured with a YSI Model 58 meter equipped with a vibrating stirrer. After initial DO measurement, chambers were sealed and incubated for 1 h in the dark (heavy black plastic, PhAR < 200/zmol cm -2 s -1, Li-Cor Model LI-189 Quantum Photometer). Dissolved oxygen was re-measured following the dark incubation to determine community respiration (CR). Chambers
Effects of metals on stream metabolism
185
Vail
Gore Creek
Two Elk Creek
ER-12a ER-12
iiiiiiii~
Turkey Crcek ER-5 ER-Ir
Cross Creek ER-3
Red Cliff
Fall Creek
!
Homestakecreek t
5kin
'
/ iiiiii M,0eTa,,iog
Fig. 1. Location of the Eagle River sampling sites, September 1993. Stippled areas are the locations of mines and mine tailings piles. The direction of flow is from ER-lr to ER-12a.
Table 1. Physical characteristics of the Eagle River study sites, September 1993 Site
River Elevation (m) Distance a (km)
ER-lr ER-3 ER-5 ER-12 ER-12a
2600 2550 2525 2485 2475
aDistance from mouth.
79.8 77.3 76.2 73.7 72.8
Stream order 3 4 4 4 4
Watershed area (km 2)
Relative discharge (m 3 s-t)
Stream temperature (°C)
Light intensity (~mol s-~ m -2)
245 426 429 468 469
0.515 0.980 1.058 1.126 1.126
10.1 8.8 8.0 9.9 9.9
1400 1458 1676 1751 1629
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were re-sealed and incubated for 1 h in ambient light (1400-1751 /zmols -1 m -2) before re-measuring DO to determine net primary productivity (NPP). Gross primary productivity (GPP) was estimated as the sum of CR and NPP (Bott et al., 1985; Hill & Gardner, 1987). Following the light incubation, rocks were removed from chambers for surface area measurement and subsampling for chlorophyll a and ash free dry mass (AFDM). Upper rock surface area was determined by gravimetric analysis using foil to delineate the surface area (Bott et al., 1985). Foil templates of the rock surfaces were weighed (+0.01 g) and compared to the regression of weights of foil of known surface area. All metabolic rates were normalized to a common unit of area (m2) before statistical comparisons. To facilitate comparison of our results with other published studies, we converted hourly estimates of productivity and respiration to daily values. Daily gross and net primary productivity were estimated by multiplying hourly GPP and NPP by day length (12 h). This may overestimate GPP and NPP but, because of the low light saturation levels of benthic algae (Gray & Hill, 1995), it was assumed that the magnitude of error would be relatively low. Community respiration was estimated by multiplying hourly oxygen uptake within darkened chambers by 24. Daily CR was subtracted from daily NPP to estimate net daily metabolism (NDM), a measure of the degree of auto- or heterotrophy of the stream community (Bott et al., 1985). Quantum yield (QY), a measure of the ability of the producer community to convert radiant energy into photosynthetic products, was calculated by converting 02 production rates from mg 02 g-i h-i to mol 02 m -2 h -1 and dividing by radiant energy from the sun (mol m -2 h -I) (Sestak et al., 1971). Chlorophyll a and AFDM were subsampled by brushing a delineated area (12 cm 2) with a mediumbristled toothbrush and washing loosened material into a collection bottle. The total volume of collected material was noted and 25 ml subsamples were removed and filtered onto pre-leached, pre-ashed, pre-weighed glass fiber filters (1.0 #m average pore size) for chlorophyll and AFDM analyses. Filters were wrapped in foil and frozen until the analyses were performed (USEPA, 1993b). Chlorophyll a was removed from each filter by leaching in cold (4°C), 90% (v/v) acetone for 18 h. The light absorption by leachate was measured in a spectrophotometer at 750 and 664 nm and again after the addition of 1 N HCl at 750 and 665 nm. Chlorophyll a concen-
tration was normalized for subsample area and AFDM (APHA, 1995). Assimilation ratio (AR, mg 02 mg -1 Chl h-l), a measure of the relative proportion of periphyton biomass that is photosynthetically active, was calculated by dividing net hourly primary productivity within a chamber by the amount of chlorophyll in that sample (Crossey & LaPointe, 1988). Sediment community respiration Sediment community respiration (SCR) was measured at all sites except ER-12a. Fine-grained sediments (top 2 cm) were collected from depositional areas along the stream study reach (150-500 m, based on 40x average channel width). These sediments were composited, mixed and subsampled. Approximately 10 ml of sediment were placed into each of five labelled, 50 ml screwtop centrifuge tubes. Each tube was filled to the top (no head space) with stream water (known DO concentration and temperature), sealed and incubated in the dark at ambient temperature for 2 h (closed ice chest 1/2 filled with stream water). Following incubation, the DO concentration in each tube was re-measured, the overlying water in each tube decanted and the remaining sediment saved for AFDM analysis. Sediment samples were frozen until analyses were performed, according to methods given in USEPA (1993b). Statistical analyses Statistical analyses were performed using SAS (SAS, 1985). One-way ANOVA and Scheffe's multiple range test were used to analyze the data for significant differences between stations. Metal effects on metabolic variables were also evaluated using stepwise multiple regression with a forward selection procedure with a 0.05 significance level for variable entry and removal from the model. Linear regression of metabolic variables against Zn were used to calculate lethal and inhibition concentrations (described below). Correlation analysis (Pearson product-moment) was used to measure the degree of association among the biotic variables.
RESULTS Stream chemistry and toxicity Analysis of Eagle River water indicated that Zn, Mn and Fe increased dramatically adjacent to, and downstream from, the mine and mine tailings piles (Table 2).
Table 2. Nutrient and metal chemistry at the Eagle River study sites, September 1993 Dissolved metals
Dissolved nutrients Site
pH
NO2/NO3 (mg liter-1 )
TDP (mg litre-I)
SO4 (mg liter-1)
Zn (/zg liter-1)
Mn (/~g liter-1)
Fe (/zg liter-1)
ER-Ir ER-3 ER-5 ER- 12 ER-12a
8.64 8.33 8.59 8.32 8.09
< 0.05 < 0.05 < 0.05 < 0.05 < 0.05
0.0l 0.01 < 0.01 < 0.01 < 0.03
11.5 7.9 9.7 13.3 17.9
4 6 162 263 225
6 20 100 186 422
24 32 140 171 359
Effects of metals on stream metabolism Zinc concentrations increased from 4/zg liter -~ at ERlr to 263/zg liter -1 at ER-12, manganese increased from 6/~g liter -1 at ER-lr to 353/zg liter -~ at ER-12a and iron increased from 24/xg liter -1 at ER-lr to 359/zg liter -1 at ER-12a. No other metals or nutrients displayed any spatial trends. This may be partially due to the fact that concentrations were often at or below our instrument detection limits. The toxicity of Eagle River waters increased significantly downstream from sites ER-lr and ER-3 (ANOVA, F4,15= 68.4, p < 0.0001, Scheffe). Ceriodaphnia dubia mortality ranged from 0% at ER-lr to 95 ± 5% at ER-5 and ER-12 (Table 3). Stepwise multiple regression of mortality against dissolved metals was significant (r2=0.92, p
