Over 100,000 chemicals are in use around the world today and many more are added each year. Many of these will be released either accidentally or deliberately into the environment, but the scale and extent of the threat they pose to ecosystems remains unclear.
Lakes act as natural sinks for contaminants both deposited from the atmosphere and transported from upstream sources. As a result, lake-dwelling organisms are exposed to a cocktail of contaminants, usually at low concentrations, but extending over the whole of the organism’s life-time. This cocktail includes a wide range of chemicals including trace metals (such as mercury and lead) and also persistent organic pollutants (POPs) from industrial, agricultural and domestic sources. These contaminants are now largely regarded as ubiquitous and a number of studies have explored the scale of pollutant burden in both lakes and rivers (Figure 1).
Figure 1: The concentrations of mercury (Hg) and a flame retardant (TBBP-A) in fish, water and sediment from Chapman’s Pond in Yorkshire, UK, undertaken as part of the OPAL project
In order to determine the risk that contaminants in lake sediments pose to freshwater organisms, Sediment Quality Guidelines (SQGs) have been developed. These generally comprise two levels: Threshold Effects Concentrations (TECs) which are defined as contaminant concentrations below which harmful effects on sediment-dwelling organisms would not be expected, and Probable Effects Concentrations (PECs) above which harmful effects would be expected to occur frequently due to that pollutant alone. However, SQGs only consider the impact from individual pollutants on their own and so, because a pollutant is only ever likely to be present as part of a mixture, predictions of an organism’s exposure are usually underestimated.
What is also important is to know whether the overall pollutant burden for aquatic organisms is getting better or worse, and the rate at which any change is occurring. The lake sediment record can provide a natural archive of contaminant inputs to lakes over decades and centuries and so, by measuring the concentrations of different pollutants in sediment cores and comparing these with SQGs to get a “relative potency factor”, we can reconstruct their combined effects through time, what we have called the lake’s ‘palaeotoxicity’.
We presented our first results on palaeotoxicity at the SIL Conference in Turin last year, with data from a range of rural and urban lakes across the UK. Using an approach called Probable Effects Concentration Quotients (PEC-Qs) which assesses the relative potency of each pollutant by comparing its measured concentration to its PEC we were able to track the likely impacts of a wide range of pollutants through time at each lake. Intriguingly, this shows how the impacts of trace metals are declining as a result of emissions reductions, but that the rapid increase in concentrations of POPs maybe compensating for this such that detrimental effects to aquatic biota are now increasing once again (Figure 2). Similar patterns were observed in a number of our study lakes.
Figure 2: Reconstructed palaeotoxicity for Edgbaston Pool in central Birmingham, UK. The detrimental effects of metals peaked in the 1970s and are now declining, but rapid increases in POPs mean that the overall likelihood of toxicity is increasing once again. The red line represents a PEC-Q of 2.0 considered a harmful threshold for aquatic organisms.
Now, in the NERC-funded Hydroscape project we are measuring a suite of trace metals in sediment cores from 24 lakes in three ‘lake districts’ of the UK (Cumbria, Glasgow and Norfolk) (Figure 3). One of our aims is to use the palaeotoxicity approach to see how differences in connectivity amongst lakes can influence the scale of contamination in lakes through time from which we can also reconstruct the likelihood of detrimental effects to aquatic biota. Furthermore, there is growing evidence that pollutants previously deposited from the atmosphere and stored in soils are now becoming remobilized as a result of climate-enhanced soil erosion. Metals and POPs are being transferred from soils to aquatic systems such that pollutant inputs to lakes may remain high despite emissions reductions. We hope the palaeotoxicity approach can be used to identify the scale of threat to aquatic biota as well as highlighting those chemicals most likely to be causing harm.
Figure 3: Easedale Tarn in the Lake District, UK. One of our Hydroscape study sites.
Neil Rose and Simon Turner (UCL)