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Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos
Environmental Pollution 166 (2012) 196e207
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Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos Philip S. Rainbow a, *, Alan G. Hildrew b, c, Brian D. Smith a, Tim Geatches d, Samuel N. Luoma a, e a
Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD, UK School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, UK c Freshwater Biological Association, The Ferry Landing, Ambleside, Cumbria LA22 OLP, UK d Environment Agency, Sir John Moore House, Victoria Square, Bodmin, Cornwall PL31 1EB, UK e John Muir Institute of the Environment, University of California at Davis, Davis, CA 95616, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2012 Received in revised form 8 March 2012 Accepted 11 March 2012
It has been proposed that bioaccumulated concentrations of toxic metals in tolerant biomonitors be used as indicators of metal bioavailability that could be calibrated against the ecological response to metals of sensitive biotic assemblages. Our hypothesis was that metal concentrations in caddisfly larvae Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, indicate metal bioavailability in contaminated streams, and can be calibrated against metal-specific ecological responses of mayflies. Bioaccumulated concentrations of Cu, As, Zn and Pb in H. siltalai from SW English streams were related to the mayfly assemblage. Mayflies were always sparse where bioavailabilities were high and were abundant and diverse where bioavailabilities of all metals were low, a pattern particularly evident when the combined abundance of heptageniid and ephemerellid mayflies was the response variable. The results offer promise that bioaccumulated concentrations of metals in tolerant biomonitors can be used to diagnose ecological impacts on stream benthos from metal stressors. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Trace metal bioavailabilities Freshwater streams Ecotoxicological effects Benthic insect communities Mayflies Caddisflies
1. Introduction It is difficult to demonstrate and isolate the effect of particular factors on biodiversity in natural systems, and this is especially true for the effect of toxic metals, as in catchments affected by mining activity (Luoma and Rainbow, 2008, 2010). Doseeresponse curves that define metal toxicity in laboratory tests are difficult to translate to the field, where the demonstration of cause and effect requires knowledge of ecological responses diagnostic of metal stress (Luoma et al., 2010). Luoma and Rainbow (2008, 2010) advocated the use of risk assessment approaches with a greater emphasis on biological and ecological principles, and a stronger integration of field observations into decision making. Such a ‘lateral risk assessment’ and risk management process, encompassing hitherto separate approaches and using several lines of evidence, would not only be interdisciplinary but would recognise the strong potential contribution of observational data from nature in such a process (Luoma and Rainbow, 2008). Some species of hydropsychid caddisflies (Trichoptera) are widespread in metal-contaminated streams and are excellent * Corresponding author. E-mail address: [email protected] (P.S. Rainbow).
biomonitors of toxic metals like copper which they accumulate to high body concentrations (Luoma and Rainbow, 2008). Their larvae are hardy and tolerate raised local trace metal availabilities to greater extent than more metal-sensitive stream insects such as mayflies and, in particular, members of the Heptageniidae and Ephemerellidae (Winner et al., 1980; Clements, 1991, 2000; Gower et al., 1994; Buchwalter et al., 2007; Cain et al., 2004; Luoma et al., 2010). Thus heptageniid and ephemerellid larvae are eliminated from streams at trace metal bioavailabilities tolerated by the caddisfly larvae. It also follows that the threshold bioavailability that corresponds to the loss of particular mayflies should be correlated with a particular concentration of an accumulated metal in the caddisfly larvae, given that this accumulated concentration is an integrated measures of the metal bioavailability to which the larvae have been exposed (Luoma and Rainbow, 2008; Luoma et al., 2010). This study addressed the hypothesis of Luoma et al. (2010) that there will be a correlation between the bioaccumulated metal concentrations in caddisfly larvae and the presence and abundance of heptageniid and ephemerellid mayflies in streams. Luoma et al. (2010) provided initial evidence supporting this hypothesis in the case of the copper-contaminated Clark Fork River system in the USA, and here we seek to establish whether
0269-7491/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2012.03.017
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
the hypothesis holds in another mine-affected catchment system, in this case on another continent. The methodology used is an example of lateral risk assessment as proposed by Luoma and Rainbow (2008). It encompasses two established approaches to bioassessment traditionally employed independently e (a) measurement of exposure to bioavailable metals in terms of a bioaccumulated concentration in a widespread biomonitoring species, and (b) conventional bioassessment based on the distribution and abundance of biotic assemblages, here benthic macroinvertebrates, in relation to general environmental stress. We combine these two by calibrating the measurement of bioavailable metal exposure against the response of a sensitive benthic assemblage, hypothesising that bioavailable metal exposure would be correlated with the assemblage-level data (Luoma et al., 2010). A combination of these methods offers the potential of diagnosing the particular effects of toxic metals on biological communities in natural systems. We chose seven metal-rich stream systems in Cornwall (SW England) and, as a biomonitor, used primarily the locally abundant net-spinning caddisfly Hydropsyche siltalai. We collected a smaller dataset for another net-spinning caddisfly, the highly metal- and acid-tolerant Plectrocnemia conspersa (Polycentropodidae). These streams show a range of concentrations of toxic metals/metalloids including very high bioavailabilities of copper, arsenic, zinc and lead in some, but not all, systems. Raised copper bioavailability in particular is a feature of many of the local catchments (Gower et al., 1994), including that of the Hayle (Brown, 1977). The Carnon River drains a region with a long history of mining for metals, and correspondingly has delivered sediments to its estuary with extraordinarily high loads of As, Cu, Fe, Mn and Zn, while the Gannel River is associated with high bioavailabilities of Pb and Zn, but not Cu (Bryan et al., 1980; Bryan and Gibbs, 1983). All the streams are physically suitable as mayfly habitat (they are medium
197
to small fast-flowing stony streams of orders 1e3), are relatively free from other obvious causes of stress, and mayflies are abundant in reaches not contaminated with metals. The specific hypothesis addressed in this study, therefore, was that bioaccumulated concentrations of toxic metals in Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, can be used as indicators of metal bioavailabilities and calibrated against the metal-specific ecological responses of mayflies. 2. Methods 2.1. Field collection We made field collections in late April 2009, when caddisfly larvae would be large enough (just before emergence) for individual metal analysis, and most mayfly species would still be present before pupation and emergence and of a size enabling specific identification. Samples were taken from 24 sites on 7 stream systems in Cornwall (Table 1, Fig. 1). Very high flow at one site (15, Bissoe) prevented collection of any insect larvae, and it was revisited and sampled on 12 June 2009. Larvae of the hydropsychid caddisfly Hydropsyche siltalai were found in 6 catchments (and at 16 sites), and of another hydropsychid, Diplectrona felix, from 2 catchments (2 sites). Larvae of the polycentropodid Plectrocnemia conspersa were taken from from 5 catchments (and 10 sites) (Table 1). A few (<5) larvae of another polycentropodid Polycentropus flavomaculatus were collected from each of sites 10, 12, 13, 14 and 22 (Table 1), although they were too small for subsequent metal analysis (see below). All specimens of the other 3 caddisfly species collected from each site were individually bagged in small polythene bags, and kept on ice in a cooler until freezing at 20 C. Three standard 1 min kick samples (net mesh 300 mm) were also taken from stony riffles and runs at each site, and preserved on site in 80% alcohol. 2.2. Laboratory analysis 2.2.1. Caddisflies In the laboratory, identifications of caddisfly larvae H. siltalai, D. felix and P. conspersa made in the field were verified under a binocular microscope before individual larvae were rinsed in double-distilled water and dried to constant mass in acid-washed Pyrex tubes. The caddisfly larvae were in the final instar and of
Table 1 Details of collection sites in 7 catchments and numbers of each of 3 species of caddisfly larvae collected for metal analysis. Site Lynher catchment (1) Darleyford Darleyford stream Lake Farm Lower Addicroft Marke Valley Upton Cross Hayle catchment (4) Drym Farm Godolphin Relubbus St Erth gauging station Red River catchment (5) CoombeeTehidy stream Roscroggan Bridge Upstream South Crofty Mine Porthtowan catchment (6) Menagissey Bridge Carnon catchment (3) Chacewater Viaduct Downstream Chacewater STW Bissoe, below County and Wellington Adit Hicks Mill Gannel catchment (2) East Wheel Rose Bridge Mine stream East Wheel Rose Bridge Control stream Trewerry Mill, Benny stream Benny Bridge Kestle Mill Bridge, Gannel GWills, Gannel Seaton catchment (7) Courtney’s Mill Bridge Crows Nest
Site no
Date 2009
Co-ordinates
Hydropsyche siltalai
1 2 3 4
27 27 27 27
April April April April
N N N N
50 50 50 50
31.881, 32.154, 31.701, 31.325,
W W W W
004 004 004 004
26.264 25.211 23.849 25.106
5 6 7 8
28 28 28 28
April April April April
N N N N
50 50 50 50
09.413, 08.564, 08.279, 09.422,
W W W W
005 005 005 005
19.874 21.730 24.472 25.994
10 11 19 16
3
9 10 11
28 April 28 April 28 April
N 50 14.034, W 005 19.492 N 50 13.873, W 005 17.779 N 50 13.315, W 005 16.819
14 5 15
12
28 April
N 50 16.361, W 005 13.058
16
13 14 15 16
29 29 12 29
April April June April
N N N N
50 50 50 50
15.836, 14.803, 13.523, 13.678,
W W W W
005 005 005 005
10.001 09.100 07.56 07.917
6 17
17 18 19 20 21 22
29 29 29 29 29 29
April April April April April April
N N N N N N
50 50 50 50 50 50
21.415, 21.415, 22.905, 22.614, 23.641, 23.587,
W W W W W W
005 005 005 005 005 005
02.750 02.750 02.602 02.336 01.532 03.317
23 24
30 April 30 April
N 50 25.769, W 004 24.724 N 50 29.964, W 004 26.956
Diplectrona felix
Plectrocnemia conspersa 14 10 12 8
4
2
1 10 3 8
19 10 8 15 5 27 2
198
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Fig. 1. Collection sites (n ¼ 24) in 7 river catchments in Cornwall, UK in April 2009.
a limited size range. Nevertheless any relatively small larvae of the same species of approximately equal size (typically <0.005 g but site-dependent) were combined to give pooled samples with sufficient biomass for analysis. Larvae were digested in Aristar grade (Merck) concentrated nitric acid at 100 C, made up to volume with double-distilled water and analysed for metals on a Vista-Pro CCD Simultaneous ICPOES. In the light of previous observations in the area (e.g. Gower et al., 1994), we were particularly interested in Cu, As, Zn and Pb. A wide range of other accumulated metal concentrations in the caddisflies was also available from the ICP analysis, although none proved atypically high and relevant to our study. Accumulated metal concentrations are expressed as mg g1 dry weight. Simultaneous analyses of standard reference materials confirmed the acceptability of the analytical procedure (Supplementary Table S1). Datasets for each metal at each site were checked (regression analysis) for effects of larval body size (individual dry weight) on accumulated metal concentration, the likelihood of any size effects having already been reduced by the collection of larvae of limited size ranges. This procedure led to the removal from statistical comparisons of a single measured Zn concentration for a pooled sample of the smallest H. siltalai larvae at site 18. Thereafter there was no effect of size in any metal concentration dataset for any site, and accumulated metal concentration data were therefore expressed as means and standard deviations. The definitions of trace metals and heavy metals are fraught with controversy and lack of precision, with great variability between authors. Here we simply refer to all metals and metalloids (As) in this paper as ‘trace metals’, after Luoma and Rainbow (2008). 2.2.2. Mayfly assemblage structure All mayfly larvae were sorted from each of the 3 kick samples per site and subsequently identified to species (after Elliott and Humpesch, 2010) and counted to calculate various simple measures of assemblage structure and species abundance (Table 2). Abundance is expressed as the mean number of mayfly larvae per 1 min kick sample.
3. Results Eleven species of mayflies were represented overall, distributed among six families (Table 2). All catchments had at least two species, while all 11 species were found in the Gannel system. Two species of Baetis (B. vernus and B. rhodani) were the most widespread and abundant overall. Measurable concentrations of As, Cu, Pb and Zn were found in all of the three species, H. siltalai, D. felix and P. conspersa (Table 3). In some cases accumulated concentrations were below detection limits, and the data are then presented in terms of a range of ‘less than’ concentrations (inevitable given the differences in masses of individual or pooled samples analysed), and are not included in further analysis. Furthermore there were insufficient data available
for Diplectrona felix to be compared against mayfly assemblage data, and these accumulated metal concentrations are presented simply for future reference. The most comprehensive dataset is that for Hydropsyche siltalai. 3.1. Hydropsyche siltalai Mayflies were never more abundant than 50 larvae per sample when Cu in H. siltalai exceeded 400 mg Cu g1. Moderate mayfly abundances (50e150 per sample) occurred between 100 and 400 mg Cu g1 and the highest abundances occurred when Cu bioavailability resulted in <100 mg Cu g1 in H. siltalai. There were also sites where mayflies were scarce when caddisfly-accumulated Cu concentrations were low (Fig. 2a). For example, mayfly abundance ranged from zero to more than 250 at low caddisfly Cu concentrations. This pattern is typical of circumstances where the independent variable (Cu in H. siltalai) limits a response (mayfly abundance) at stressful concentrations, but factors other than Cu can limit the response when Cu is not a stressor (Cade and Noon, 2003; Ramsey et al., 2005; Luoma et al., 2010). The total abundance of heptageniid and ephemerellid larvae shows a similar pattern, but a sharper threshold (Fig. 2b). Heptageniids and ephemerellids were absent or very scarce at all sites where Cu concentrations in caddisflies exceeded 170 mg Cu g1. It is possible that the stressors that affect mayflies at low Cu concentrations could be other metals. There was no statistically significant relationship between the concentrations of arsenic, zinc or lead versus copper in H. siltalai, thus reducing the risk of confounding effects (Supplementary Fig. S1). Thus any influence of other metals on the Cu versus mayfly relationships would be additive rather than a result of co-variation. A plot of Cu concentration against Pb concentration in H. siltalai for the whole dataset (Fig. 3) shows that caddisflies from some sites in the Gannel River have low accumulated Cu concentrations but high Pb concentrations (Table 3) (including all caddisflies with more than 50 mg Pb g1 in Fig. 3). Fig. 3 also shows that there are no mayflies present when accumulated Pb concentrations in H. siltalai exceed 300 mg Pb g1 even though accumulated Cu concentrations are low (see also Fig. S1). The sites with the greatest abundance of
Table 2 Details of mayfly larvae collected at up to 24 sites from 7 catchments (see Table 1). Numbers against species names represent the mean numbers of individual mayfly larvae in 3 1 min kick samples (also expressed as abundance). 1
1
1
1
4
4
4
4
5
5
5
6
3
3
3
3
2
2
2
2
2
2
7
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.7
0.7
3.3
14.7
134.7
10.0
16.3
18.7
13.7
43.3
Ephemerellidae Seratella ignite Caenidae Caenis rivulorum Ephemeridae Ephemera danica Heptageniidae Rithrogena semicolorata Ecdyonurus torrentis Baetidae Alainites muticus Baetis vernus Baetis rhodani Baetis scambus/fuscatus Leptophlebiidae Habrophlebia fusca Paraleptophlebia cincta No of mayfly species No of heptageniid species No of ephemerellid species No of heptageniid þ ephemerellid species No of mayfly species except baetids Mayfly abundance Heptageniid abundance Ephemerellid abundance Heptageniid þ ephemerellid abundance
42.0
1.0 9.3
25.0 9.7
26.3
47.7
8.0
35.0
24.7
40.0 2.3
46.3
4.0
8.3
33.0 10.7 47.0
34.3 10.0
0.3 1.3 120.7
33.0 1.3 36.0
7.0
0.7
2.0 0.3
0.3
2 0 0 0 0 34.7 0.0 0.0 0.0
1 0 0 0 0 26.3 0.0 0.0 0.0
0 0 0 0 0 0.0 0.0 0.0 0.0
2 1 0 1 1 34.0 9.3 0.0 9.3
1 0 0 0 0 8.0 0.0 0.0 0.0
1 0 0 0 0 35.0 0.0 0.0 0.0
2 0 0 0 0 42.3 0.0 0.0 0.0
2 0 1 1 1 50.3 0.0 42.0 42.0
1 0 0 0 0 46.3 0.0 0.0 0.0
1 0 0 0 0 4.0 0.0 0.0 0.0
4 0 1 1 1 91.3 0.0 0.7 0.7
3 0 1 1 1 45.0 0.0 0.7 0.7
4 0 1 1 1 125.7 0.0 3.3 3.3
0 0 0 0 0 0.0 0.0 0.0 0.0
1 0 0 0 0 7.0 0.0 0.0 0.0
0 0 0 0 0 0.0 0.0 0.0 0.0
5 1 0 1 2 118.3 47.7 0.0 47.7
0.7
109.7 3.0
5.7
9.7
1.0 1.0
1.3 2.0
8 1 1 2 5 266.3 0.3 134.7 135.0
7 1 1 2 6 63.7 0.7 10.0 10.7
3.0 6.3 0.7
2.0 2.0
0.3
0 0 0 0 0 0.0 0.0 0.0 0.0
0.7
7 2 1 3 5 43.7 2.3 14.7 17.0
2 0 0 0 0 4.0 0.0 0.0 0.0
3 1 1 2 2 26.7 0.7 16.3 17.0
0 0 0 0 0 0.0 0.0 0.0 0.0
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Catchment Site number
199
200
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Table 3 Mean concentrations (mg g1 dry wt, 1 standard deviation) of the trace metals As, Cu, Pb and Zn in three species of caddisfly larvae (Diplectrona felix, Plectrocnemia conspersa, Hydropsyche siltalai) collected from 24 sites in SW England (n is the number of individuals sampled plus any pooled samples analysed for each site, perhaps varying between metal analyses with different detection limits). Species
Site
n
As
Cu
Pb
Zn
Diplectrona felix
2 16
3 1
<64e<82 97.1
1102 226 1011
242 33 <24
211 13 528
Plectrocnemia conspersa
1 2 3 4 12 14 15 16 17 24
1e9 5 4 1e7 2 1 8 2 1e4 2
<12e<55 <10e< 28 <27e<59 38.3 <31e<32 <49 181 128 <31e<43 <36e<48 <25e<50
328 188 425 137 259 61 371 181 98.4 13.4 94.6 291 173 138 52 131 684 530
<11e<49 99.8 59.1 <24e<52 <13e<25 <27e<28 <43 <29e<38 <28e<38 204 53 <22e<44
187 178 232 200 426 452 259 306 180 191
5 6 7 8 9 10 11 12 13 14 18 19 20 21 22 23
1e6 5 2e8 5 6 2 11 11 4 9 1e9 4 6 10 3 1e16
41.6 110 19 151 54 67.2 25.8 49.6 26.3 65.2 2.3 120 51 172 45 <25e<35 82.3 10.7 34.6 49.3 22.1 467 94 <8e<30 <11e<27 20.6
41.0 13.1 844 204 831 262 575 120 102 52 433 52 1204 454 169 43 247 57 315 48 19.3 1.7 35.5 5.4 44.5 4.7 25.0 4.1 26.5 2.9 835 161
14.7 17.9 2.8 11.4 1.9 <7e<22 <12e<26 <11e<18 <17e<30 <11e<30 <22e<30 48.4 4.2 32.4 5.3 209 55 441 68 <7e<26 90.3 22.8 <10e<25
218 508 444 475 274 214 246 313 353 414 227 562 600 200 636 203
Hydropsyche siltalai
mayflies have a combination of caddisfly Pb < 300 mg Pb g1 and Cu < 100 mg Cu g1. Six individual caddisfly samples had high arsenic concentrations (above 300 mg As g1), all from a single site (20, Benny Bridge) on the Gannel, where the Cu concentration of the same larvae was low (Fig. S1a). The relationship between mayfly abundance and the mean As concentration accumulated by H. siltalai at each site was weak (Fig. 4a). However, the baetids amongst the mayflies showed a relative insensitivity to arsenic. Removal of the baetid data for As (Fig. 4b), or plotting the combined abundance of Heptageniidae and Ephemerellidae only (Fig. 4c), indicates that more sensitive mayflies are limited where H. siltalai has accumulated more than 85 mg As g1. Hydropsyche siltalai larvae apparently contain a ‘background’ concentration of about 200 mg Zn g1, irrespective of site of collection, and so any threshold accumulated concentration correlating with an ecotoxicological effect on mayfly assemblages occurs at concentrations above this baseline (Fig. 5). Again, the relationship between the mayfly assemblage and caddisfly bioaccumulated Zn was much more evident when the combined abundance of the metal-sensitive Heptageniidae and Ephemerellidae was plotted (Fig. 5b) than when other mayflies were included (Fig. 5a). A plot of mean Pb concentrations in H. siltalai against mayfly abundance for all sites for which Pb concentrations were measurable (Fig. 6a) does show the expected pattern but includes some low abundances where Pb availability is low. The explanation is as in the case of copper. These low mayfly abundances occur in streams with high Cu bioavailability while Pb bioavailability is low. Thus the more limited dataset just for the Gannel (Fig. S2a, b) eliminates these high Cu/low Pb sites and the expected pattern is restored, albeit with few data points. The same effect and
61 16 52 55 107
58 40 60 20 49 101 94 40 148 26 62 85 109 74 42 317 87 31 525 49
interpretation apply when data are expressed as numbers of mayfly species (data not shown), and abundances of heptageniid and ephemerellid mayflies (Fig. 6b, Fig. S2b). A pattern of low mayfly abundance and diversity with high accumulated metal concentrations in Hydropsyche siltalai is apparent, with differing degrees of confidence, for the four metals Cu, As, Zn and Pb (Figs. 2e6). The effects are especially clear when the only Heptageniidae and Ephemerellidae are considered. We can then test directly whether there is any apparent combined ecotoxicological effect of the metals on the abundance and diversity of mayflies in these streams. Assessing the additive effects of metals raises the question of choice of units to summate across the metals. One option is a variation of the ‘toxic unit’ concept well established in mixed toxicity testing (Adams and Rowland, 2003), that can be extended into the field, for example as a cumulative criterion unit (Clements, 2004). Our data allow us to define the thresholds of accumulated concentrations of Cu, As and Zn in H. siltalai that have an ecotoxicological effect on the mayfly assemblage, and most clearly on the combined abundance of Heptageniidae and Ephemerellidae (Figs. 2b, 4c and 5b). Appropriate thresholds were estimated from these data to be 170 mg Cu g1 (Fig. 2b), 85 mg As g1 (Fig. 4c) and 300 mg Zn g1 (Fig. 5b) in H. siltalai. For lead a more approximate threshold accumulated concentration of 300 mg Pb g1 has also been estimated from the smaller dataset for that metal (Table 3; Figs. 3and 6c). The mean accumulated concentration in H. siltalai at each site is divided by this threshold concentration for the relevant metal, the resulting number being now expressed in toxic units for that metal. Furthermore toxic units for different metals can now be summed for the H. siltalai at each site. Fig. 7 shows the patterns obtained for all sites, with mayfly assemblage data expressed as total mayfly abundance or
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Mayfly abundance
a
201
Cornwall Streams
300 250 200 150 100 50 0 0
200
400
600
800
1000
1200
1400
Hydropsyche Cu (µg/g)
b Heptageniid + Ephemerellid abundance
Cornwall Streams 160 140 120 100 80 60 40 20 0 0
200
400
600
800
1000
1200
1400
Hydropsyche Cu (µg/g) 1
Fig. 2. Plots of mean accumulated Cu concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
abundance of heptageniid and ephemerellid mayflies, plotted against combined toxic unit scores for all four of Cu, As, Zn and Pb (Fig. 7a, b) or just for Cu and As combined (Fig. 7c). The patterns expected from the hypothesis are present when all four metals are considered (Fig. 7a, b). Again the pattern is clearest for the most sensitive metric, the combined abundance of heptageniids and ephemerellids (Fig. 7b). There are no sites with abundant populations of heptageniids and ephemerellids when the summed metal toxic units exceed 2. There is also only one site with a toxic unit score <2 and a low abundance of heptageniids and ephemerellids. The expected pattern is still clear when only Cu and As data are considered (Fig. 7c), although there are 4 sites in this plot where
Pb concentration (µg/g)
600
“other stressors” are apparent (low toxic unit score, low abundance), for this plot does not allow for the high lead bioavailability of the Gannel. The paucity of sites with a low metals toxic unit score and low abundance of heptageniids plus ephemerellids indicates that metals are the most important factor limiting these groups of mayflies among this suite of streams. 3.2. Plectrocnemia conspersa A smaller but potentially useful corresponding dataset is available for the caddisfly Plectrocnemia conspersa, which typically occurs nearer the source of streams than Hydropsyche siltalai
Hydropsyche siltalai
zero
500 400 300
10<>20
200
<10
100
>20
0 0
zero 200
400
600
800
1000
1200
Cu concentration (µg/g)
Fig. 3. Plot of accumulated Cu concentrations (mg g1) against accumulated Pb concentrations (mg g1) in individual samples of larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers. Increasing sizes of symbols indicate increased abundances of heptageniid and ephemerellid mayfly larvae at the associated sites (zero, <10, 10<>20, >20). Sites in the catchment of the Gannel are unfilled.
202
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Mayfly abundance
a
Cornwall Streams 150
100
50
0 0
50
100
150
200
250
300
350
400
450
500
Hydropsyche As (µg/g)
Mayfly abundance, not Baetids
b
Cornwall Streams
150
100
50
0 0
50
100
150
200
250
300
350
400
450
500
350
400
450
500
Hydropsyche As (µg/g)
Heptageniid + Ephemerellid abundance
c
Cornwall Streams 50 45 40 35 30 25 20 15 10 5 0 0
50
100
150
200
250
300
Hydropsyche As (µg/g) 1
Fig. 4. Plots of mean accumulated As concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
(Edington and Hildrew, 1995). As for H. siltalai, the postulated pattern of low mayfly abundance where caddisfly-accumulated copper concentrations are highest is recognisable (Fig. 8a). Also as for H. siltalai, there are two sites with low mayfly abundances where accumulated Cu concentrations are <200 mg Cu g1. One such site (17) is in the Gannel catchment where Cu bioavailability is low but lead bioavailability high (Table 3). For the purposes of our analysis, there were insufficient sites with P. conspersa where accumulated concentrations of As and Pb were measurable. However, we can test our hypothesis with respect to accumulated zinc concentrations in P. conspersa and mayfly abundance. As for H. siltalai, there was no significant covariance of individual accumulated Cu and Zn concentrations
across the dataset (data not shown). The accumulated Zn concentrations in P. conspersa did not show the expected relationship with mayfly abundance (Fig. 8b) although there were no sites with very high Zn concentrations in P. conspersa and inadequate data were available to consider the heptageniids and ephemerellids separately from other mayflies (Table 2). 4. Discussion Our results confirm that bioaccumulated metal concentrations (reflecting the local bioavailabilities of the metal) in a tolerant biomonitor, Hydropsyche siltalai, can be calibrated against metaldriven ecotoxicological effects on populations of other more
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Mayfly abundance
a
203
Cornwall Streams 300 250 200 150 100 50 0 0
100
200
300
400
500
600
700
600
700
Hydropsyche Zn (µg/g)
Heptageniid + Ephemerellid abundance
b
Cornwall Streams 160 140 120 100 80 60 40 20 0 0
100
200
300
400
500
Hydropsyche Zn (µg/g) 1
Fig. 5. Plots of mean accumulated Zn concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
sensitive members of the biota, in this case mayfly larvae. Among the mayflies, the abundance of heptageniids and ephemerellids appeared to be the most responsive to the metals (as originally suggested by Clements, 2000). Baetid mayflies were more tolerant, at least of some metals. For copper (Fig. 2), heptageniid and ephemerellid mayflies were essentially absent in all streams with greater than 170 mg Cu g1 in H. siltalai. Recall that the whole dataset covers streams that might have high bioavailabilities of other toxic metals in the absence of Cu, such as lead in the case of the Gannel. Nevertheless, the postulated pattern was sufficiently strong to be evident for copper over the whole suite of streams and sites, despite catchment heterogeneity, although it became clearer after accounting for variation in lead (Fig. 3). As for Cu, accumulated As concentrations in H. siltalai, reflecting local As bioavailabilities, also fitted the expected pattern when plotted against mayfly abundance, more clearly in this case after the selective removal of data for baetid mayflies (Fig. 4). This is field evidence that baetids appear to be relatively insensitive to arsenic. Patterns for zinc (Fig. 5) and lead (Fig. 6) fitted the same expected scenario. Lead data were limited but the expected pattern was apparent in the most lead contaminated catchment, the Gannel (Fig. S2). Overall, the metals of most ecotoxicological significance in these Cornish streams appear to be copper and arsenic, in agreement with Gower et al. (1994) who recognised the importance of copper in this area affected by historical mining activity. The use of toxic units allowed consideration of the effects of more than one toxic
metal at a time (Fig. 7), however, thus overcoming the effect in these sites of the inversely related Cu and Pb bioavailabilities. The patterns seen when all four metals (Cu, As, Zn, Pb) were included in the analysis remained clear even when only Cu and As data were considered (Fig. 7). High Cu þ As toxic unit scores always coincided with the scarcity or absence of heptageniid and ephemerellid mayflies at 12 different sites. At a further four sites low abundances coincided with a Cu þ As toxic unit score <1. But these were sites with extremely high lead or zinc availabilities. Thus copper and arsenic have more general importance as drivers of reduced mayfly abundance and diversity across the Cornish catchments investigated than Zn and Pb, based upon the number of sites affected. More importantly, however, wherever heptageniids and ephemerellids were scarce or absent, sufficient metal was present to explain those absences. In this suite of sites, metals were apparently the main stressor limiting the natural abundance of these organisms. From the more limited dataset available for the polycentropid caddisfly Plectrocnemia conspersa, bioaccumulated concentrations of copper can be used as an indicator Cu bioavailability in metalcontaminated streams in southwest England, and calibrated against associated population level ecotoxicological responses in mayflies (Fig. 8a). There were insufficient data available to draw conclusions for As and Pb, and in fact there was not the expected pattern for Zn in P. conspersa (Fig. 8b). The data points in Fig. 8b that cause the deviation from the expected pattern for Zn are from two sites with relatively low accumulated Cu concentrations in the caddisfly, indicating that, as for the sites containing H. siltalai, it is
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Hydropsyche Pb (µg/g) Fig. 6. Plots of mean accumulated Pb concentrations (mg g1) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
again copper that is the driving factor affecting mayfly abundance and species number at the sites where P. conspersa is present. Our data provide evidence of the typical background concentrations of metals in two caddisflies in these stream systems in the absence of raised local bioavailabilities (Table 3). For H. siltalai these are 20e25 mg Cu g1, 200 mg Zn g1, 20e40 mg As g1 and 10e20 mg Pb g1; corresponding concentrations for P. conspersa from our database are 95e140 mg Cu g1 and 180e190 mg Zn g1 for the two metals with sufficient data (Table 3). Two points arise. First, it is not unusual for there to be higher background bioaccumulated concentrations of Zn than Cu, as seen here in H. siltalai, as dramatically shown for example by barnacles (Rainbow, 2002, 2007; Luoma and Rainbow, 2008). Secondly, nor is it unusual for two relatively closely related animals to have very different typical accumulated concentrations of a metal, as seen here for copper. The reason in both cases is attributable to the different means of detoxifying and storing metals (as opposed to their excretion) between taxa (Rainbow, 2002, 2007; Luoma and Rainbow, 2008). Plectrocnemia conspersa larvae store accumulated copper in copper and sulphur-rich granules (presumably derived from lysosomal breakdown of metallothionein binding copper e Luoma and Rainbow, 2008), in malpighian tubules and under the cuticle (Darlington and Gower, 1990), highlighting the role of detoxified
storage in the accumulation of copper by this species. On the other hand, Gower and Darlington (1990) found a mean copper concentration of only 29 mg Cu g1 in P. conspersa from a non-mineralised stream in nearby East Devon, closer to the H. siltalai background copper concentration measured in the mineralised streams sampled here. It is this ability to detoxify metals that allows the biomonitor (in this instance, caddisfly larvae) to survive contamination that has clear effects on the more sensitive mayflies. We have chosen two caddisflies as the metal-tolerant biomonitors in this study. The principle of using a metal biomonitor in this way can be extended to other benthic species that are net accumulators of the metal in question. Hydropsychid caddisflies have much appeal in this context e typically they are strong trace metal accumulators (Cain et al., 2004; Luoma and Rainbow, 2008; but see Buchwalter et al., 2008). Detailed knowledge of the bioaccumulation kinetics of a particular metal in a particular hydropsychid species would allow estimation of the time period of recent exposure reflected in the accumulated concentration in the caddisfly. The choice of more than one metal biomonitor with different kinetics of metal bioaccumulation would provide further relevant information in a field ecotoxicology study. Our results thus strongly support the proposal of Luoma et al. (2010) that bioaccumulated concentrations of toxic metals in
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
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Hydropsyche Cu As Toxic Units
Fig. 7. Plots of combined mean accumulated metal concentrations, expressed as toxic units (see text), in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
tolerant biomonitors can be used as indicators of metal bioavailabilities and be calibrated against metal-specific assemblage-level responses in sensitive species. The thresholds for adverse effects of Cu reported by Luoma et al. (2010) from both Montana, USA and the Philippines are also in reasonable agreement with these seen in southwest England. The three studies together therefore suggest that when caddisflies have accumulated more than 100e200 mg Cu g1 at a site, it can be expected that heptageniid and ephemerellid mayflies will be scarce or absent. The data of Luoma et al. (2010) for copper in the Clark Fork River in Montana, USA have been augmented by the data for copper and three other trace metals in the mining contaminated streams of southwest England. We recommend that the proposal be tested in other metalcontaminated contaminated freshwater systems across the world. Interestingly, using a similar approach, Bervoets et al. (2005) were
able to relate parameters of fish community structure to summated toxic units reflecting trace metal accumulation in fish liver in metal-contaminated river systems in Flanders, Belgium. This specific approach, however, lacks the option of obtaining a measure of high metal bioavailabilities in the absence of the species directly affected by those high bioavailabilities. Bioassessment of streams based on assemblages of macroinvertebrates is almost universal (Friberg et al., 2011), and can readily detect impairments to ecological status, yet the challenge remains to disentangle the many possible stressors at work and to diagnose problems in particular systems. The use of a metaltolerant biomonitor in conjunction with assemblage-based metrics, as demonstrated here, offers the ability to identify metal stressors in situations where they may be significant, even in situations where other stressors may be present.
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Plectrocnemia Zn (µg/g) Fig. 8. Plots of (a) mean accumulated Cu concentrations and (b) mean accumulated Zn concentrations (mg g1) in larvae of the caddisfly Plectrocnemia conspersa from sites in Cornish rivers against mayfly abundance (mean number of mayfly larvae in 1 min kick samples) at the site.
Acknowledgements We thank Tony Gower for provision of original data and his interest in the study, Rob Knell and Steve Le Comber for advice on applicability of statistical methods, Stuart Orton for mayfly identifications, Malcolm Penn for help with GIS and Claus Svendsen for his advice on toxic units. We are also grateful to staff of the NHM EMMA facility for metal analysis. Funds for field collection were provided by the NHM Department of Zoology Investment Fund. Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.envpol.2012.03.017. References Adams, W.J., Rowland, C.D., 2003. Aquatic toxicology test methods. In: Hoffman, D.J., Rattner, B.A., Burton, G.A., Cairns, J. (Eds.), Handbook of Ecotoxicology. CRC Press, Boca Raton, pp. 19e45. Bervoets, L., Knaepkens, G., Eens, M., Blust, R., 2005. Fish community responses to metal pollution. Environmental Pollution 138, 338e349. Brown, B.E., 1977. Effects of mine drainage on the River Hayle, Cornwall. a) factors affecting concentrations of copper, zinc and iron in water, sediments and dominant invertebrate fauna. Hydrobiologia 52, 221e233. Bryan, G.W., Langston, W.J., Hummerstone, L.G., 1980. The Use of Biological Indicators of Heavy Metal Contamination in Estuaries, vol. 1. Occasional Publications of the Marine Biological Association of the United Kingdom, 1e73 pp. Bryan, G.W., Gibbs, P.E., 1983. Heavy Metals in the Fal Estuary, Cornwall: A Study of Long-Term Contamination by Mining Waste and Its Effects on Estuarine Organisms, vol. 2. Occasional Publications of the Marine Biological Association of the United Kingdom, 1e112 pp. Buchwalter, D.B., Cain, D.J., Clements, W.H., Luoma, S.N., 2007. Using biodynamic models to reconcile differences between laboratory toxicity and field biomonitoring with aquatic insects. Environmental Science and Technology 41, 4821e4828.
Buchwalter, D.B., Cain, D.J., Martin, C.A., Kie, L., Luoma, S.N., Garland, T., 2008. Aquatic insect ecophysiological traits reveal phylogenetically based differences in dissolved cadmium susceptibility. Proceedings of the National Academy of Sciences 105, 8321e8326. Cade, B.S., Noon, B.R., 2003. A gentle introduction to quantile regression for ecologists. Frontiers in Ecology and the Environment 1, 412e421. Cain, D.J., Luoma, S.N., Wallace, W.G., 2004. Linking metal bioaccumulation of aquatic insects to their distribution patterns in a mining-impacted river. Environmental Toxicology and Chemistry 23, 1463e1473. Clements, W.H., 1991. Community responses of stream organisms to heavy metals: a review of observational and experimental approaches. In: Newman, M.C., McIntosh, A.W. (Eds.), Metal Ecotoxicology. Lewis Publishers, Chelsea, MI, pp. 363e391. Clements, W.H., 2000. Integrating effects of contaminants across levels of biological organization: an overview. Journal of Aquatic Ecosystem Stress and Recovery 7, 113e116. Clements, W.H., 2004. Small-scale experiments support causal relationships between metal contamination and macroinvertebrate community responses. Ecological Applications 14, 954e957. Darlington, S.T., Gower, A.M., 1990. Location of copper in larvae of Plectrocnemia conspersa (Curtis) (Trichoptera) exposed to elevated metal concentrations in a mine drainage stream. Hydrobiologia 196, 91e100. Edington, J.M., Hildrew, A.G., 1995. Caseless Caddis Larvae of the British Isles. Freshwater Biological Association, Ambleside, UK, Scientific Publications of the Freshwater Biological Association No. 53, 1e134 pp. Elliott, J.M., Humpesch, U.H., 2010. Mayfly Larvae (Ephemeroptera) of Britain and Ireland: Keys and a Review of Their Ecology. Freshwater Biological Association, Ambleside, UK, Scientific Publications of the Freshwater Biological Association No. 66, 1e152 pp. Friberg, N., Bonada, N., Bradley, D.C., Dunbar, M.J., Edwards, F.K., Grey, J., Hayes, R.B., Hildrew, A.G., Lamouroux, N., Trimmer, M., Woodward, G., 2011. Biomonitoring of human impacts in natural ecosystems: the good, the bad and the ugly. Advances in Ecological Research 44, 1e68. Gower, A.M., Darlington, S.T., 1990. Relationships between copper concentrations in larvae of Plectrocnemia conspersa (Curtis) (Trichoptera) and in mine drainage streams. Environmental Pollution 65, 155e168. Gower, A.M., Myers, G., Kent, M., Foulkes, M.E., 1994. Relationships between macroinvertebrate communities and environmental variables in metalcontaminated streams in south-west England. Freshwater Biology 32, 199e221. Luoma, S.N., Rainbow, P.S., 2008. Metal Contamination in Aquatic Environments: Science and Lateral Management. Cambridge University Press, Cambridge, UK.
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207 Luoma, S.N., Rainbow, P.S., 2010. Linking new science frontiers in metal ecotoxicology to better risk management: lateral thinking. In: Bury, N.R., Handy, R.D. (Eds.), 2010. Surface Chemistry, Bioavailability and Metal Homeostasis in Aquatic Organisms: An Integrated Approach, vol. 2. Society for Experimental Biology Essential Reviews in Experimental Biology, London, UK, pp. 1e28. Luoma, S.N., Cain, D.J., Rainbow, P.S., 2010. Calibrating biomonitors to ecological disturbance: a new technique for explaining metal effects in natural waters. Integrated Environmental Assessment and Management 6, 199e209.
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Rainbow, P.S., 2002. Trace metal concentrations in aquatic invertebrates: why and so what? Environmental Pollution 120, 497e507. Rainbow, P.S., 2007. Trace metal bioaccumulation: models, metabolic availability and toxicity. Environment International 33, 576e582. Ramsey, P.W., Rillig, M.C., Feris, K.P., Moore, J.N., Gannon, J.E., 2005. Mine waste contamination limits soil respiration rates: a case study using quantile regression. Soil Biology and Biochemistry 37, 1177e1183. Winner, R.W., Boesel, M.W., Farrel, M.P., 1980. Insect community structure as an index of heavy-metal pollution in lotic ecosystems. Canadian Journal of Fisheries and Aquatic Sciences 37, 647e655.
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos Philip S. Rainbow a, *, Alan G. Hildrew b, c, Brian D. Smith a, Tim Geatches d, Samuel N. Luoma a, e a
Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD, UK School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, UK c Freshwater Biological Association, The Ferry Landing, Ambleside, Cumbria LA22 OLP, UK d Environment Agency, Sir John Moore House, Victoria Square, Bodmin, Cornwall PL31 1EB, UK e John Muir Institute of the Environment, University of California at Davis, Davis, CA 95616, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2012 Received in revised form 8 March 2012 Accepted 11 March 2012
It has been proposed that bioaccumulated concentrations of toxic metals in tolerant biomonitors be used as indicators of metal bioavailability that could be calibrated against the ecological response to metals of sensitive biotic assemblages. Our hypothesis was that metal concentrations in caddisfly larvae Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, indicate metal bioavailability in contaminated streams, and can be calibrated against metal-specific ecological responses of mayflies. Bioaccumulated concentrations of Cu, As, Zn and Pb in H. siltalai from SW English streams were related to the mayfly assemblage. Mayflies were always sparse where bioavailabilities were high and were abundant and diverse where bioavailabilities of all metals were low, a pattern particularly evident when the combined abundance of heptageniid and ephemerellid mayflies was the response variable. The results offer promise that bioaccumulated concentrations of metals in tolerant biomonitors can be used to diagnose ecological impacts on stream benthos from metal stressors. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Trace metal bioavailabilities Freshwater streams Ecotoxicological effects Benthic insect communities Mayflies Caddisflies
1. Introduction It is difficult to demonstrate and isolate the effect of particular factors on biodiversity in natural systems, and this is especially true for the effect of toxic metals, as in catchments affected by mining activity (Luoma and Rainbow, 2008, 2010). Doseeresponse curves that define metal toxicity in laboratory tests are difficult to translate to the field, where the demonstration of cause and effect requires knowledge of ecological responses diagnostic of metal stress (Luoma et al., 2010). Luoma and Rainbow (2008, 2010) advocated the use of risk assessment approaches with a greater emphasis on biological and ecological principles, and a stronger integration of field observations into decision making. Such a ‘lateral risk assessment’ and risk management process, encompassing hitherto separate approaches and using several lines of evidence, would not only be interdisciplinary but would recognise the strong potential contribution of observational data from nature in such a process (Luoma and Rainbow, 2008). Some species of hydropsychid caddisflies (Trichoptera) are widespread in metal-contaminated streams and are excellent * Corresponding author. E-mail address: [email protected] (P.S. Rainbow).
biomonitors of toxic metals like copper which they accumulate to high body concentrations (Luoma and Rainbow, 2008). Their larvae are hardy and tolerate raised local trace metal availabilities to greater extent than more metal-sensitive stream insects such as mayflies and, in particular, members of the Heptageniidae and Ephemerellidae (Winner et al., 1980; Clements, 1991, 2000; Gower et al., 1994; Buchwalter et al., 2007; Cain et al., 2004; Luoma et al., 2010). Thus heptageniid and ephemerellid larvae are eliminated from streams at trace metal bioavailabilities tolerated by the caddisfly larvae. It also follows that the threshold bioavailability that corresponds to the loss of particular mayflies should be correlated with a particular concentration of an accumulated metal in the caddisfly larvae, given that this accumulated concentration is an integrated measures of the metal bioavailability to which the larvae have been exposed (Luoma and Rainbow, 2008; Luoma et al., 2010). This study addressed the hypothesis of Luoma et al. (2010) that there will be a correlation between the bioaccumulated metal concentrations in caddisfly larvae and the presence and abundance of heptageniid and ephemerellid mayflies in streams. Luoma et al. (2010) provided initial evidence supporting this hypothesis in the case of the copper-contaminated Clark Fork River system in the USA, and here we seek to establish whether
0269-7491/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2012.03.017
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
the hypothesis holds in another mine-affected catchment system, in this case on another continent. The methodology used is an example of lateral risk assessment as proposed by Luoma and Rainbow (2008). It encompasses two established approaches to bioassessment traditionally employed independently e (a) measurement of exposure to bioavailable metals in terms of a bioaccumulated concentration in a widespread biomonitoring species, and (b) conventional bioassessment based on the distribution and abundance of biotic assemblages, here benthic macroinvertebrates, in relation to general environmental stress. We combine these two by calibrating the measurement of bioavailable metal exposure against the response of a sensitive benthic assemblage, hypothesising that bioavailable metal exposure would be correlated with the assemblage-level data (Luoma et al., 2010). A combination of these methods offers the potential of diagnosing the particular effects of toxic metals on biological communities in natural systems. We chose seven metal-rich stream systems in Cornwall (SW England) and, as a biomonitor, used primarily the locally abundant net-spinning caddisfly Hydropsyche siltalai. We collected a smaller dataset for another net-spinning caddisfly, the highly metal- and acid-tolerant Plectrocnemia conspersa (Polycentropodidae). These streams show a range of concentrations of toxic metals/metalloids including very high bioavailabilities of copper, arsenic, zinc and lead in some, but not all, systems. Raised copper bioavailability in particular is a feature of many of the local catchments (Gower et al., 1994), including that of the Hayle (Brown, 1977). The Carnon River drains a region with a long history of mining for metals, and correspondingly has delivered sediments to its estuary with extraordinarily high loads of As, Cu, Fe, Mn and Zn, while the Gannel River is associated with high bioavailabilities of Pb and Zn, but not Cu (Bryan et al., 1980; Bryan and Gibbs, 1983). All the streams are physically suitable as mayfly habitat (they are medium
197
to small fast-flowing stony streams of orders 1e3), are relatively free from other obvious causes of stress, and mayflies are abundant in reaches not contaminated with metals. The specific hypothesis addressed in this study, therefore, was that bioaccumulated concentrations of toxic metals in Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, can be used as indicators of metal bioavailabilities and calibrated against the metal-specific ecological responses of mayflies. 2. Methods 2.1. Field collection We made field collections in late April 2009, when caddisfly larvae would be large enough (just before emergence) for individual metal analysis, and most mayfly species would still be present before pupation and emergence and of a size enabling specific identification. Samples were taken from 24 sites on 7 stream systems in Cornwall (Table 1, Fig. 1). Very high flow at one site (15, Bissoe) prevented collection of any insect larvae, and it was revisited and sampled on 12 June 2009. Larvae of the hydropsychid caddisfly Hydropsyche siltalai were found in 6 catchments (and at 16 sites), and of another hydropsychid, Diplectrona felix, from 2 catchments (2 sites). Larvae of the polycentropodid Plectrocnemia conspersa were taken from from 5 catchments (and 10 sites) (Table 1). A few (<5) larvae of another polycentropodid Polycentropus flavomaculatus were collected from each of sites 10, 12, 13, 14 and 22 (Table 1), although they were too small for subsequent metal analysis (see below). All specimens of the other 3 caddisfly species collected from each site were individually bagged in small polythene bags, and kept on ice in a cooler until freezing at 20 C. Three standard 1 min kick samples (net mesh 300 mm) were also taken from stony riffles and runs at each site, and preserved on site in 80% alcohol. 2.2. Laboratory analysis 2.2.1. Caddisflies In the laboratory, identifications of caddisfly larvae H. siltalai, D. felix and P. conspersa made in the field were verified under a binocular microscope before individual larvae were rinsed in double-distilled water and dried to constant mass in acid-washed Pyrex tubes. The caddisfly larvae were in the final instar and of
Table 1 Details of collection sites in 7 catchments and numbers of each of 3 species of caddisfly larvae collected for metal analysis. Site Lynher catchment (1) Darleyford Darleyford stream Lake Farm Lower Addicroft Marke Valley Upton Cross Hayle catchment (4) Drym Farm Godolphin Relubbus St Erth gauging station Red River catchment (5) CoombeeTehidy stream Roscroggan Bridge Upstream South Crofty Mine Porthtowan catchment (6) Menagissey Bridge Carnon catchment (3) Chacewater Viaduct Downstream Chacewater STW Bissoe, below County and Wellington Adit Hicks Mill Gannel catchment (2) East Wheel Rose Bridge Mine stream East Wheel Rose Bridge Control stream Trewerry Mill, Benny stream Benny Bridge Kestle Mill Bridge, Gannel GWills, Gannel Seaton catchment (7) Courtney’s Mill Bridge Crows Nest
Site no
Date 2009
Co-ordinates
Hydropsyche siltalai
1 2 3 4
27 27 27 27
April April April April
N N N N
50 50 50 50
31.881, 32.154, 31.701, 31.325,
W W W W
004 004 004 004
26.264 25.211 23.849 25.106
5 6 7 8
28 28 28 28
April April April April
N N N N
50 50 50 50
09.413, 08.564, 08.279, 09.422,
W W W W
005 005 005 005
19.874 21.730 24.472 25.994
10 11 19 16
3
9 10 11
28 April 28 April 28 April
N 50 14.034, W 005 19.492 N 50 13.873, W 005 17.779 N 50 13.315, W 005 16.819
14 5 15
12
28 April
N 50 16.361, W 005 13.058
16
13 14 15 16
29 29 12 29
April April June April
N N N N
50 50 50 50
15.836, 14.803, 13.523, 13.678,
W W W W
005 005 005 005
10.001 09.100 07.56 07.917
6 17
17 18 19 20 21 22
29 29 29 29 29 29
April April April April April April
N N N N N N
50 50 50 50 50 50
21.415, 21.415, 22.905, 22.614, 23.641, 23.587,
W W W W W W
005 005 005 005 005 005
02.750 02.750 02.602 02.336 01.532 03.317
23 24
30 April 30 April
N 50 25.769, W 004 24.724 N 50 29.964, W 004 26.956
Diplectrona felix
Plectrocnemia conspersa 14 10 12 8
4
2
1 10 3 8
19 10 8 15 5 27 2
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Fig. 1. Collection sites (n ¼ 24) in 7 river catchments in Cornwall, UK in April 2009.
a limited size range. Nevertheless any relatively small larvae of the same species of approximately equal size (typically <0.005 g but site-dependent) were combined to give pooled samples with sufficient biomass for analysis. Larvae were digested in Aristar grade (Merck) concentrated nitric acid at 100 C, made up to volume with double-distilled water and analysed for metals on a Vista-Pro CCD Simultaneous ICPOES. In the light of previous observations in the area (e.g. Gower et al., 1994), we were particularly interested in Cu, As, Zn and Pb. A wide range of other accumulated metal concentrations in the caddisflies was also available from the ICP analysis, although none proved atypically high and relevant to our study. Accumulated metal concentrations are expressed as mg g1 dry weight. Simultaneous analyses of standard reference materials confirmed the acceptability of the analytical procedure (Supplementary Table S1). Datasets for each metal at each site were checked (regression analysis) for effects of larval body size (individual dry weight) on accumulated metal concentration, the likelihood of any size effects having already been reduced by the collection of larvae of limited size ranges. This procedure led to the removal from statistical comparisons of a single measured Zn concentration for a pooled sample of the smallest H. siltalai larvae at site 18. Thereafter there was no effect of size in any metal concentration dataset for any site, and accumulated metal concentration data were therefore expressed as means and standard deviations. The definitions of trace metals and heavy metals are fraught with controversy and lack of precision, with great variability between authors. Here we simply refer to all metals and metalloids (As) in this paper as ‘trace metals’, after Luoma and Rainbow (2008). 2.2.2. Mayfly assemblage structure All mayfly larvae were sorted from each of the 3 kick samples per site and subsequently identified to species (after Elliott and Humpesch, 2010) and counted to calculate various simple measures of assemblage structure and species abundance (Table 2). Abundance is expressed as the mean number of mayfly larvae per 1 min kick sample.
3. Results Eleven species of mayflies were represented overall, distributed among six families (Table 2). All catchments had at least two species, while all 11 species were found in the Gannel system. Two species of Baetis (B. vernus and B. rhodani) were the most widespread and abundant overall. Measurable concentrations of As, Cu, Pb and Zn were found in all of the three species, H. siltalai, D. felix and P. conspersa (Table 3). In some cases accumulated concentrations were below detection limits, and the data are then presented in terms of a range of ‘less than’ concentrations (inevitable given the differences in masses of individual or pooled samples analysed), and are not included in further analysis. Furthermore there were insufficient data available
for Diplectrona felix to be compared against mayfly assemblage data, and these accumulated metal concentrations are presented simply for future reference. The most comprehensive dataset is that for Hydropsyche siltalai. 3.1. Hydropsyche siltalai Mayflies were never more abundant than 50 larvae per sample when Cu in H. siltalai exceeded 400 mg Cu g1. Moderate mayfly abundances (50e150 per sample) occurred between 100 and 400 mg Cu g1 and the highest abundances occurred when Cu bioavailability resulted in <100 mg Cu g1 in H. siltalai. There were also sites where mayflies were scarce when caddisfly-accumulated Cu concentrations were low (Fig. 2a). For example, mayfly abundance ranged from zero to more than 250 at low caddisfly Cu concentrations. This pattern is typical of circumstances where the independent variable (Cu in H. siltalai) limits a response (mayfly abundance) at stressful concentrations, but factors other than Cu can limit the response when Cu is not a stressor (Cade and Noon, 2003; Ramsey et al., 2005; Luoma et al., 2010). The total abundance of heptageniid and ephemerellid larvae shows a similar pattern, but a sharper threshold (Fig. 2b). Heptageniids and ephemerellids were absent or very scarce at all sites where Cu concentrations in caddisflies exceeded 170 mg Cu g1. It is possible that the stressors that affect mayflies at low Cu concentrations could be other metals. There was no statistically significant relationship between the concentrations of arsenic, zinc or lead versus copper in H. siltalai, thus reducing the risk of confounding effects (Supplementary Fig. S1). Thus any influence of other metals on the Cu versus mayfly relationships would be additive rather than a result of co-variation. A plot of Cu concentration against Pb concentration in H. siltalai for the whole dataset (Fig. 3) shows that caddisflies from some sites in the Gannel River have low accumulated Cu concentrations but high Pb concentrations (Table 3) (including all caddisflies with more than 50 mg Pb g1 in Fig. 3). Fig. 3 also shows that there are no mayflies present when accumulated Pb concentrations in H. siltalai exceed 300 mg Pb g1 even though accumulated Cu concentrations are low (see also Fig. S1). The sites with the greatest abundance of
Table 2 Details of mayfly larvae collected at up to 24 sites from 7 catchments (see Table 1). Numbers against species names represent the mean numbers of individual mayfly larvae in 3 1 min kick samples (also expressed as abundance). 1
1
1
1
4
4
4
4
5
5
5
6
3
3
3
3
2
2
2
2
2
2
7
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.7
0.7
3.3
14.7
134.7
10.0
16.3
18.7
13.7
43.3
Ephemerellidae Seratella ignite Caenidae Caenis rivulorum Ephemeridae Ephemera danica Heptageniidae Rithrogena semicolorata Ecdyonurus torrentis Baetidae Alainites muticus Baetis vernus Baetis rhodani Baetis scambus/fuscatus Leptophlebiidae Habrophlebia fusca Paraleptophlebia cincta No of mayfly species No of heptageniid species No of ephemerellid species No of heptageniid þ ephemerellid species No of mayfly species except baetids Mayfly abundance Heptageniid abundance Ephemerellid abundance Heptageniid þ ephemerellid abundance
42.0
1.0 9.3
25.0 9.7
26.3
47.7
8.0
35.0
24.7
40.0 2.3
46.3
4.0
8.3
33.0 10.7 47.0
34.3 10.0
0.3 1.3 120.7
33.0 1.3 36.0
7.0
0.7
2.0 0.3
0.3
2 0 0 0 0 34.7 0.0 0.0 0.0
1 0 0 0 0 26.3 0.0 0.0 0.0
0 0 0 0 0 0.0 0.0 0.0 0.0
2 1 0 1 1 34.0 9.3 0.0 9.3
1 0 0 0 0 8.0 0.0 0.0 0.0
1 0 0 0 0 35.0 0.0 0.0 0.0
2 0 0 0 0 42.3 0.0 0.0 0.0
2 0 1 1 1 50.3 0.0 42.0 42.0
1 0 0 0 0 46.3 0.0 0.0 0.0
1 0 0 0 0 4.0 0.0 0.0 0.0
4 0 1 1 1 91.3 0.0 0.7 0.7
3 0 1 1 1 45.0 0.0 0.7 0.7
4 0 1 1 1 125.7 0.0 3.3 3.3
0 0 0 0 0 0.0 0.0 0.0 0.0
1 0 0 0 0 7.0 0.0 0.0 0.0
0 0 0 0 0 0.0 0.0 0.0 0.0
5 1 0 1 2 118.3 47.7 0.0 47.7
0.7
109.7 3.0
5.7
9.7
1.0 1.0
1.3 2.0
8 1 1 2 5 266.3 0.3 134.7 135.0
7 1 1 2 6 63.7 0.7 10.0 10.7
3.0 6.3 0.7
2.0 2.0
0.3
0 0 0 0 0 0.0 0.0 0.0 0.0
0.7
7 2 1 3 5 43.7 2.3 14.7 17.0
2 0 0 0 0 4.0 0.0 0.0 0.0
3 1 1 2 2 26.7 0.7 16.3 17.0
0 0 0 0 0 0.0 0.0 0.0 0.0
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Catchment Site number
199
200
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Table 3 Mean concentrations (mg g1 dry wt, 1 standard deviation) of the trace metals As, Cu, Pb and Zn in three species of caddisfly larvae (Diplectrona felix, Plectrocnemia conspersa, Hydropsyche siltalai) collected from 24 sites in SW England (n is the number of individuals sampled plus any pooled samples analysed for each site, perhaps varying between metal analyses with different detection limits). Species
Site
n
As
Cu
Pb
Zn
Diplectrona felix
2 16
3 1
<64e<82 97.1
1102 226 1011
242 33 <24
211 13 528
Plectrocnemia conspersa
1 2 3 4 12 14 15 16 17 24
1e9 5 4 1e7 2 1 8 2 1e4 2
<12e<55 <10e< 28 <27e<59 38.3 <31e<32 <49 181 128 <31e<43 <36e<48 <25e<50
328 188 425 137 259 61 371 181 98.4 13.4 94.6 291 173 138 52 131 684 530
<11e<49 99.8 59.1 <24e<52 <13e<25 <27e<28 <43 <29e<38 <28e<38 204 53 <22e<44
187 178 232 200 426 452 259 306 180 191
5 6 7 8 9 10 11 12 13 14 18 19 20 21 22 23
1e6 5 2e8 5 6 2 11 11 4 9 1e9 4 6 10 3 1e16
41.6 110 19 151 54 67.2 25.8 49.6 26.3 65.2 2.3 120 51 172 45 <25e<35 82.3 10.7 34.6 49.3 22.1 467 94 <8e<30 <11e<27 20.6
41.0 13.1 844 204 831 262 575 120 102 52 433 52 1204 454 169 43 247 57 315 48 19.3 1.7 35.5 5.4 44.5 4.7 25.0 4.1 26.5 2.9 835 161
14.7 17.9 2.8 11.4 1.9 <7e<22 <12e<26 <11e<18 <17e<30 <11e<30 <22e<30 48.4 4.2 32.4 5.3 209 55 441 68 <7e<26 90.3 22.8 <10e<25
218 508 444 475 274 214 246 313 353 414 227 562 600 200 636 203
Hydropsyche siltalai
mayflies have a combination of caddisfly Pb < 300 mg Pb g1 and Cu < 100 mg Cu g1. Six individual caddisfly samples had high arsenic concentrations (above 300 mg As g1), all from a single site (20, Benny Bridge) on the Gannel, where the Cu concentration of the same larvae was low (Fig. S1a). The relationship between mayfly abundance and the mean As concentration accumulated by H. siltalai at each site was weak (Fig. 4a). However, the baetids amongst the mayflies showed a relative insensitivity to arsenic. Removal of the baetid data for As (Fig. 4b), or plotting the combined abundance of Heptageniidae and Ephemerellidae only (Fig. 4c), indicates that more sensitive mayflies are limited where H. siltalai has accumulated more than 85 mg As g1. Hydropsyche siltalai larvae apparently contain a ‘background’ concentration of about 200 mg Zn g1, irrespective of site of collection, and so any threshold accumulated concentration correlating with an ecotoxicological effect on mayfly assemblages occurs at concentrations above this baseline (Fig. 5). Again, the relationship between the mayfly assemblage and caddisfly bioaccumulated Zn was much more evident when the combined abundance of the metal-sensitive Heptageniidae and Ephemerellidae was plotted (Fig. 5b) than when other mayflies were included (Fig. 5a). A plot of mean Pb concentrations in H. siltalai against mayfly abundance for all sites for which Pb concentrations were measurable (Fig. 6a) does show the expected pattern but includes some low abundances where Pb availability is low. The explanation is as in the case of copper. These low mayfly abundances occur in streams with high Cu bioavailability while Pb bioavailability is low. Thus the more limited dataset just for the Gannel (Fig. S2a, b) eliminates these high Cu/low Pb sites and the expected pattern is restored, albeit with few data points. The same effect and
61 16 52 55 107
58 40 60 20 49 101 94 40 148 26 62 85 109 74 42 317 87 31 525 49
interpretation apply when data are expressed as numbers of mayfly species (data not shown), and abundances of heptageniid and ephemerellid mayflies (Fig. 6b, Fig. S2b). A pattern of low mayfly abundance and diversity with high accumulated metal concentrations in Hydropsyche siltalai is apparent, with differing degrees of confidence, for the four metals Cu, As, Zn and Pb (Figs. 2e6). The effects are especially clear when the only Heptageniidae and Ephemerellidae are considered. We can then test directly whether there is any apparent combined ecotoxicological effect of the metals on the abundance and diversity of mayflies in these streams. Assessing the additive effects of metals raises the question of choice of units to summate across the metals. One option is a variation of the ‘toxic unit’ concept well established in mixed toxicity testing (Adams and Rowland, 2003), that can be extended into the field, for example as a cumulative criterion unit (Clements, 2004). Our data allow us to define the thresholds of accumulated concentrations of Cu, As and Zn in H. siltalai that have an ecotoxicological effect on the mayfly assemblage, and most clearly on the combined abundance of Heptageniidae and Ephemerellidae (Figs. 2b, 4c and 5b). Appropriate thresholds were estimated from these data to be 170 mg Cu g1 (Fig. 2b), 85 mg As g1 (Fig. 4c) and 300 mg Zn g1 (Fig. 5b) in H. siltalai. For lead a more approximate threshold accumulated concentration of 300 mg Pb g1 has also been estimated from the smaller dataset for that metal (Table 3; Figs. 3and 6c). The mean accumulated concentration in H. siltalai at each site is divided by this threshold concentration for the relevant metal, the resulting number being now expressed in toxic units for that metal. Furthermore toxic units for different metals can now be summed for the H. siltalai at each site. Fig. 7 shows the patterns obtained for all sites, with mayfly assemblage data expressed as total mayfly abundance or
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Mayfly abundance
a
201
Cornwall Streams
300 250 200 150 100 50 0 0
200
400
600
800
1000
1200
1400
Hydropsyche Cu (µg/g)
b Heptageniid + Ephemerellid abundance
Cornwall Streams 160 140 120 100 80 60 40 20 0 0
200
400
600
800
1000
1200
1400
Hydropsyche Cu (µg/g) 1
Fig. 2. Plots of mean accumulated Cu concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
abundance of heptageniid and ephemerellid mayflies, plotted against combined toxic unit scores for all four of Cu, As, Zn and Pb (Fig. 7a, b) or just for Cu and As combined (Fig. 7c). The patterns expected from the hypothesis are present when all four metals are considered (Fig. 7a, b). Again the pattern is clearest for the most sensitive metric, the combined abundance of heptageniids and ephemerellids (Fig. 7b). There are no sites with abundant populations of heptageniids and ephemerellids when the summed metal toxic units exceed 2. There is also only one site with a toxic unit score <2 and a low abundance of heptageniids and ephemerellids. The expected pattern is still clear when only Cu and As data are considered (Fig. 7c), although there are 4 sites in this plot where
Pb concentration (µg/g)
600
“other stressors” are apparent (low toxic unit score, low abundance), for this plot does not allow for the high lead bioavailability of the Gannel. The paucity of sites with a low metals toxic unit score and low abundance of heptageniids plus ephemerellids indicates that metals are the most important factor limiting these groups of mayflies among this suite of streams. 3.2. Plectrocnemia conspersa A smaller but potentially useful corresponding dataset is available for the caddisfly Plectrocnemia conspersa, which typically occurs nearer the source of streams than Hydropsyche siltalai
Hydropsyche siltalai
zero
500 400 300
10<>20
200
<10
100
>20
0 0
zero 200
400
600
800
1000
1200
Cu concentration (µg/g)
Fig. 3. Plot of accumulated Cu concentrations (mg g1) against accumulated Pb concentrations (mg g1) in individual samples of larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers. Increasing sizes of symbols indicate increased abundances of heptageniid and ephemerellid mayfly larvae at the associated sites (zero, <10, 10<>20, >20). Sites in the catchment of the Gannel are unfilled.
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Mayfly abundance
a
Cornwall Streams 150
100
50
0 0
50
100
150
200
250
300
350
400
450
500
Hydropsyche As (µg/g)
Mayfly abundance, not Baetids
b
Cornwall Streams
150
100
50
0 0
50
100
150
200
250
300
350
400
450
500
350
400
450
500
Hydropsyche As (µg/g)
Heptageniid + Ephemerellid abundance
c
Cornwall Streams 50 45 40 35 30 25 20 15 10 5 0 0
50
100
150
200
250
300
Hydropsyche As (µg/g) 1
Fig. 4. Plots of mean accumulated As concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
(Edington and Hildrew, 1995). As for H. siltalai, the postulated pattern of low mayfly abundance where caddisfly-accumulated copper concentrations are highest is recognisable (Fig. 8a). Also as for H. siltalai, there are two sites with low mayfly abundances where accumulated Cu concentrations are <200 mg Cu g1. One such site (17) is in the Gannel catchment where Cu bioavailability is low but lead bioavailability high (Table 3). For the purposes of our analysis, there were insufficient sites with P. conspersa where accumulated concentrations of As and Pb were measurable. However, we can test our hypothesis with respect to accumulated zinc concentrations in P. conspersa and mayfly abundance. As for H. siltalai, there was no significant covariance of individual accumulated Cu and Zn concentrations
across the dataset (data not shown). The accumulated Zn concentrations in P. conspersa did not show the expected relationship with mayfly abundance (Fig. 8b) although there were no sites with very high Zn concentrations in P. conspersa and inadequate data were available to consider the heptageniids and ephemerellids separately from other mayflies (Table 2). 4. Discussion Our results confirm that bioaccumulated metal concentrations (reflecting the local bioavailabilities of the metal) in a tolerant biomonitor, Hydropsyche siltalai, can be calibrated against metaldriven ecotoxicological effects on populations of other more
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
Mayfly abundance
a
203
Cornwall Streams 300 250 200 150 100 50 0 0
100
200
300
400
500
600
700
600
700
Hydropsyche Zn (µg/g)
Heptageniid + Ephemerellid abundance
b
Cornwall Streams 160 140 120 100 80 60 40 20 0 0
100
200
300
400
500
Hydropsyche Zn (µg/g) 1
Fig. 5. Plots of mean accumulated Zn concentrations (mg g ) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
sensitive members of the biota, in this case mayfly larvae. Among the mayflies, the abundance of heptageniids and ephemerellids appeared to be the most responsive to the metals (as originally suggested by Clements, 2000). Baetid mayflies were more tolerant, at least of some metals. For copper (Fig. 2), heptageniid and ephemerellid mayflies were essentially absent in all streams with greater than 170 mg Cu g1 in H. siltalai. Recall that the whole dataset covers streams that might have high bioavailabilities of other toxic metals in the absence of Cu, such as lead in the case of the Gannel. Nevertheless, the postulated pattern was sufficiently strong to be evident for copper over the whole suite of streams and sites, despite catchment heterogeneity, although it became clearer after accounting for variation in lead (Fig. 3). As for Cu, accumulated As concentrations in H. siltalai, reflecting local As bioavailabilities, also fitted the expected pattern when plotted against mayfly abundance, more clearly in this case after the selective removal of data for baetid mayflies (Fig. 4). This is field evidence that baetids appear to be relatively insensitive to arsenic. Patterns for zinc (Fig. 5) and lead (Fig. 6) fitted the same expected scenario. Lead data were limited but the expected pattern was apparent in the most lead contaminated catchment, the Gannel (Fig. S2). Overall, the metals of most ecotoxicological significance in these Cornish streams appear to be copper and arsenic, in agreement with Gower et al. (1994) who recognised the importance of copper in this area affected by historical mining activity. The use of toxic units allowed consideration of the effects of more than one toxic
metal at a time (Fig. 7), however, thus overcoming the effect in these sites of the inversely related Cu and Pb bioavailabilities. The patterns seen when all four metals (Cu, As, Zn, Pb) were included in the analysis remained clear even when only Cu and As data were considered (Fig. 7). High Cu þ As toxic unit scores always coincided with the scarcity or absence of heptageniid and ephemerellid mayflies at 12 different sites. At a further four sites low abundances coincided with a Cu þ As toxic unit score <1. But these were sites with extremely high lead or zinc availabilities. Thus copper and arsenic have more general importance as drivers of reduced mayfly abundance and diversity across the Cornish catchments investigated than Zn and Pb, based upon the number of sites affected. More importantly, however, wherever heptageniids and ephemerellids were scarce or absent, sufficient metal was present to explain those absences. In this suite of sites, metals were apparently the main stressor limiting the natural abundance of these organisms. From the more limited dataset available for the polycentropid caddisfly Plectrocnemia conspersa, bioaccumulated concentrations of copper can be used as an indicator Cu bioavailability in metalcontaminated streams in southwest England, and calibrated against associated population level ecotoxicological responses in mayflies (Fig. 8a). There were insufficient data available to draw conclusions for As and Pb, and in fact there was not the expected pattern for Zn in P. conspersa (Fig. 8b). The data points in Fig. 8b that cause the deviation from the expected pattern for Zn are from two sites with relatively low accumulated Cu concentrations in the caddisfly, indicating that, as for the sites containing H. siltalai, it is
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P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
a
Cornwall Streams
200
Mayfly abundance
180 160 140 120 100 80 60 40 20 0 0
50
100
150
200
250
300
350
400
450
500
350
400
450
500
Hydropsyche Pb (µg/g)
Heptageniid + Ephemerellid abundance
b
Cornwall Streams
75
50
25
0 0
50
100
150
200
250
300
Hydropsyche Pb (µg/g) Fig. 6. Plots of mean accumulated Pb concentrations (mg g1) in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
again copper that is the driving factor affecting mayfly abundance and species number at the sites where P. conspersa is present. Our data provide evidence of the typical background concentrations of metals in two caddisflies in these stream systems in the absence of raised local bioavailabilities (Table 3). For H. siltalai these are 20e25 mg Cu g1, 200 mg Zn g1, 20e40 mg As g1 and 10e20 mg Pb g1; corresponding concentrations for P. conspersa from our database are 95e140 mg Cu g1 and 180e190 mg Zn g1 for the two metals with sufficient data (Table 3). Two points arise. First, it is not unusual for there to be higher background bioaccumulated concentrations of Zn than Cu, as seen here in H. siltalai, as dramatically shown for example by barnacles (Rainbow, 2002, 2007; Luoma and Rainbow, 2008). Secondly, nor is it unusual for two relatively closely related animals to have very different typical accumulated concentrations of a metal, as seen here for copper. The reason in both cases is attributable to the different means of detoxifying and storing metals (as opposed to their excretion) between taxa (Rainbow, 2002, 2007; Luoma and Rainbow, 2008). Plectrocnemia conspersa larvae store accumulated copper in copper and sulphur-rich granules (presumably derived from lysosomal breakdown of metallothionein binding copper e Luoma and Rainbow, 2008), in malpighian tubules and under the cuticle (Darlington and Gower, 1990), highlighting the role of detoxified
storage in the accumulation of copper by this species. On the other hand, Gower and Darlington (1990) found a mean copper concentration of only 29 mg Cu g1 in P. conspersa from a non-mineralised stream in nearby East Devon, closer to the H. siltalai background copper concentration measured in the mineralised streams sampled here. It is this ability to detoxify metals that allows the biomonitor (in this instance, caddisfly larvae) to survive contamination that has clear effects on the more sensitive mayflies. We have chosen two caddisflies as the metal-tolerant biomonitors in this study. The principle of using a metal biomonitor in this way can be extended to other benthic species that are net accumulators of the metal in question. Hydropsychid caddisflies have much appeal in this context e typically they are strong trace metal accumulators (Cain et al., 2004; Luoma and Rainbow, 2008; but see Buchwalter et al., 2008). Detailed knowledge of the bioaccumulation kinetics of a particular metal in a particular hydropsychid species would allow estimation of the time period of recent exposure reflected in the accumulated concentration in the caddisfly. The choice of more than one metal biomonitor with different kinetics of metal bioaccumulation would provide further relevant information in a field ecotoxicology study. Our results thus strongly support the proposal of Luoma et al. (2010) that bioaccumulated concentrations of toxic metals in
P.S. Rainbow et al. / Environmental Pollution 166 (2012) 196e207
a
205
Cornwall Streams
300
Mayfly abundance
250 200 150 100 50 0 0
1
2
3
4
5
6
7
8
9
10
7
8
9
10
Hydropsyche Cu As Zn Pb Toxic Units
Heptageniid + Ephemerellid abundance
b
Cornwall Streams
160 140 120 100 80 60 40 20 0 0
1
2
3
4
5
6
Hydropsyche Cu As Zn Pb Toxic Units
Heptageniid + Ephemerellid abundance
c
Cornwall Streams
160 140 120 100 80 60 40 20 0 0
1
2
3
4
5
6
7
8
9
Hydropsyche Cu As Toxic Units
Fig. 7. Plots of combined mean accumulated metal concentrations, expressed as toxic units (see text), in larvae of the caddisfly Hydropsyche siltalai from sites in Cornish rivers against selected parameters of the assemblage of mayfly larvae at the site. Abundance data are the mean number of mayfly larvae in 1 min kick samples.
tolerant biomonitors can be used as indicators of metal bioavailabilities and be calibrated against metal-specific assemblage-level responses in sensitive species. The thresholds for adverse effects of Cu reported by Luoma et al. (2010) from both Montana, USA and the Philippines are also in reasonable agreement with these seen in southwest England. The three studies together therefore suggest that when caddisflies have accumulated more than 100e200 mg Cu g1 at a site, it can be expected that heptageniid and ephemerellid mayflies will be scarce or absent. The data of Luoma et al. (2010) for copper in the Clark Fork River in Montana, USA have been augmented by the data for copper and three other trace metals in the mining contaminated streams of southwest England. We recommend that the proposal be tested in other metalcontaminated contaminated freshwater systems across the world. Interestingly, using a similar approach, Bervoets et al. (2005) were
able to relate parameters of fish community structure to summated toxic units reflecting trace metal accumulation in fish liver in metal-contaminated river systems in Flanders, Belgium. This specific approach, however, lacks the option of obtaining a measure of high metal bioavailabilities in the absence of the species directly affected by those high bioavailabilities. Bioassessment of streams based on assemblages of macroinvertebrates is almost universal (Friberg et al., 2011), and can readily detect impairments to ecological status, yet the challenge remains to disentangle the many possible stressors at work and to diagnose problems in particular systems. The use of a metaltolerant biomonitor in conjunction with assemblage-based metrics, as demonstrated here, offers the ability to identify metal stressors in situations where they may be significant, even in situations where other stressors may be present.
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Mayfly abundance
a
Cornwall Streams
150
100
50
0 0
100
200
300
400
500
600
700
Plectrocnemia Cu (µg/g)
Mayfly abundance
b
Cornwall Streams
150
100
50
0 0
50
100
150
200
250
300
350
400
450
500
Plectrocnemia Zn (µg/g) Fig. 8. Plots of (a) mean accumulated Cu concentrations and (b) mean accumulated Zn concentrations (mg g1) in larvae of the caddisfly Plectrocnemia conspersa from sites in Cornish rivers against mayfly abundance (mean number of mayfly larvae in 1 min kick samples) at the site.
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