Temporal and spatial variation in metal concentrations
The 2-week sample values of metal concentrations (averages of the three replicate samples) are presented for each location in Table 1. Different lowercase letters indicate per location (row) statistically significant differences between metal concentrations in samples taken at different dates. For nine of the metals included in our study, no differences between 2-week sample values were found, and the concentrations were apparently constant over time. For the other nine metals (Al, Cd, Co, Cr, Cu, Mn, Sr, Ti and V), significant differences between 2-week sample values were found in at least one of the study locations. The fluctuations in concentration indicate a significant variation in exposure of honeybees to these metals in the environment.
For all but three metals, no significant differences in mean concentration (over the entire study period) between locations could be detected. This indicates that the overall environmental exposure of honeybees to the metals Al, As, Cd, Cr, Cu, Li, Mn, Ni, Pb, Sb, Se, Sn, Ti and Zn during the study period was comparable in Maastricht, Buggenum and Hoek van Holland. The overall mean concentrations of Co, Sr and V, however, differed significantly between the study locations (Table 2). These spatial differences might be caused by differences in industrial activity near these locations. Markedly more significant temporal (nine) than spatial (three) differences were found. This probably indicates that the temporal fluctuations in source strength over a 3-month period are greater than the more structural differences between locations. In a small and densely populated country as the Netherlands, spatial differences may be expected to be limited. However, as our method does not provide information on sources and mechanisms, any explanation of the observed differences, spatial as well as temporal, will remain speculative. In case the variations in metal concentrations in time and space detected with honeybees are considered to be a reason of concern, other, more specific methods will have to be used to investigate the causal mechanisms. For example, use could be made of the Enrichment Factor (Chester et al. 1999), to determine whether trace metals in the air have significant non-crustal sources.
Comparison with previously reported concentrations
For Al, Co, Li, Mo, Sb, Sn, Sr, Ti and V, no previous reports on their concentrations in adult honeybees have been published. The ranges of the concentrations of these metals as found in our study are as follows: Al, 4.6–15.52 μg g−1; Co, 0.08–0.33 μg g−1; Li, 0.01–0.05 μg g−1; Mo, 0.36–1.16 μg g−1; Sb, 0.07–0.19 μg g−1; Sn, 0.44–0.76 μg g−1; Sr, 0.70–2.18 μg g−1; Ti, 0.09–0.55 μg g−1; and V, 0.006–0.31 μg g−1. For As, Cd, Cr, Cu, Mn, Ni, Pb, Se and Zn, published reports on concentrations in adult honeybees are available from a wide variety of sampling locations. These values are presented in Table 3, together with the ranges of concentrations found in our study. The method of analysis to determine metal concentrations in bees was either ICP-AES, as in our study, or atomic absorption spectrometry. Kump et al. (1996) compared both methods for Cr, Cu, Mn, Ni, Pb and Zn and found only significant differences between these methods for Cr. The concentrations of the metals measured in our study are all within the bandwidth of the values reported in the literature, with the exception of Mn and Se. The concentrations we found for these two metals are lower than reported from other studies, but in the same order of magnitude. In general, the metal concentrations in the present study were at the lower or middle-lower end of the spectrum of concentration values found in other studies and often match the range of concentrations reported for supposedly relatively ‘clean’ locations. This indicates that the level of metal pollution at our three study locations was relatively low. The high concentrations, in absolute terms, of Cu, Mn and Zn are comparable to values found in other studies and are most likely due to the relatively high natural concentrations of these metals in pollen (Lambers et al. 1998), on which the bees feed.
Sources of metals in the hive environment
The commercial-type beehives used in our experiment have metal or metal-based components, such as stainless steel frame holders and wood-preserving coatings. ICP-AES analysis of samples from the frame holders and from paint of the landing board at the hive entrance revealed traces of As, Cr, Cu and Ni in both types of material (L. Goessen, personal communication). Thus, we cannot exclude that at least part of the load of these metals in the sampled bees originated from hive-associated sources. Kalnins and Detroy (1984) studied the effect of the use of the wood preservative chromated copper arsenate (CCA) in hives on the concentrations of As, Cr and Cu in bees. They found that the use of CCA significantly enhanced the concentrations of As and Cr in bees, but the concentrations of Cu were not significantly affected (Table 3). The lack of effect on Cu is probably due to the much greater importance of pollen as a source of this metal in bees. The concentrations of As found in our study correspond with those from hives treated with CCA in Kalnins and Detroy’s study (1984) and are therefore probably the consequence of exposure to hive-associated sources rather than to sources in the external environment. As for Cr, the concentrations found in our study correspond with those from hives not treated with CCA, suggesting that the Cr-containing materials of the hive were not an important source of contamination in this case.