Seasonal Metal Accumulation Patterns in the Red Wood ant Formica pratensis (Hymenoptera) at Contaminated and Reference Sites

Abstract

Elucidating details of the ecophysiology of metals in ants and assessing their potential application in biomonitoring programmes of environmental metal contamination requires knowledge of the general variability of metal accumulation in these organisms at different times and sites. Temporal fluctuations of metal concentrations were measured in worker ants of Formica pratensis, collected at sites with different contamination histories. The metal concentrations in the ants were found to differ significantly between the measured months (maximally 6-fold for lead) as well as between the sites (maximally 350-fold for lead). At all sites, a common seasonal pattern was found. The pattern revealed lowest metal concentrations in spring and autumn, resulting in an inverse U-shaped distribution. Significant weight changes in similarly sized workers occurred over the year, following a U-shaped distribution. with the minima in summer. In some cases, weight was negatively correlated with the metal concentrations, indicating that seasonal weight changes contribute to the temporal fluctuations of metal concentrations. The weight increase of workers towards winter may be associated with the storage of nutrient reserves in their fat body for the production of sexuals in early spring and also contributes to the temporal metal accumulation pattern. Other cases showed weight-independence and indicated a regulative capacity, e.g. for copper. Monthly maximum/minimum ratios were higher for the non-essential metals lead and cadmium, than for the essential iron, copper, zinc and manganese. While fluctuations of zinc increased with the level of site contamination, the opposite was true for lead and cadmium, demonstrating the impact of contamination history on temporal accumulation patterns. When individuals of mound-building ant species are included in biomonitoring studies, it should be borne in mind that not only ant activity and nutrition, but also their weight follows seasonal patterns and contributes to the metal dynamics of the colony. If between-site comparisons are conducted, identical collecting intervals are recommended.