Guest essay by Eric Worrall
A new study claims global warming will increase biological production and bio-accumulation of methylmercury, a hideously toxic organic mercury compound. But the attempt by the study authors to attach their conclusions to future climate projections seems questionable.
The abstract of the study;
Terrestrial discharges mediate trophic shifts and enhance methylmercury accumulation in estuarine biota
Sofi Jonsson, Agneta Andersson, Mats B. Nilsson, Ulf Skyllberg, Erik Lundberg, Jeffra K. Schaefer, Staffan Åkerblom and Erik Björn
The input of mercury (Hg) to ecosystems is estimated to have increased two- to fivefold during the industrial era, and Hg accumulates in aquatic biota as neurotoxic methylmercury (MeHg). Escalating anthropogenic land use and climate change are expected to alter the input rates of terrestrial natural organic matter (NOM) and nutrients to aquatic ecosystems. For example, climate change has been projected to induce 10 to 50% runoff increases for large coastal regions globally. A major knowledge gap is the potential effects on MeHg exposure to biota following these ecosystem changes. We monitored the fate of five enriched Hg isotope tracers added to mesocosm scale estuarine model ecosystems subjected to varying loading rates of nutrients and terrestrial NOM. We demonstrate that increased terrestrial NOM input to the pelagic zone can enhance the MeHg bioaccumulation factor in zooplankton by a factor of 2 to 7 by inducing a shift in the pelagic food web from autotrophic to heterotrophic. The terrestrial NOM input also enhanced the retention of MeHg in the water column by up to a factor of 2, resulting in further increased MeHg exposure to pelagic biota. Using mercury mass balance calculations, we predict that MeHg concentration in zooplankton can increase by a factor of 3 to 6 in coastal areas following scenarios with 15 to 30% increased terrestrial runoff. The results demonstrate the importance of incorporating the impact of climate-induced changes in food web structure on MeHg bioaccumulation in future biogeochemical cycling models and risk assessments of Hg.
To their credit, the study authors appear to have performed actual physical experiments, they didn’t just rely on models, like far too many climate studies. But their conclusion references climate models – it is the model based assumptions behind the conclusion which I am questioning.
Delving into the study, the main issue appears to be nutrients released by land use changes and increased coastal rainfall will change the population balance of water based microorganisms. The new population balance (fewer microscopic photosynthetic plants, more fungi and bacteria consuming dissolved organic matter from runoff) and increased nutrient availability encourages increased microorganism conversion of inorganic mercury compounds into methyl-mercury.
… The observed differences in MeHg accumulation in these studies were proposed to be caused by differences in the chemical speciation of MeHg. Because of the relatively small increase (20%) in total DOC and the large fraction (>75%) of terrestrial NOM in the mesocosm water phase, the chemical speciation of MeHg was not affected by the applied treatment schemes in our study (table S2). Our results instead emphasize a shift to a heterotrophic pelagic food web as a major cause behind MeHg biomagnification in pelagic biota in response to moderate, and environmentally realistic, enhanced terrestrial NOM loadings to coastal marine environments. …
Read more: Same link as above
My first concern is the prediction of higher runoff. As WUWT recently discussed, global warming cannot cause significantly increased total rainfall, because this would violate the second law of thermodynamics – there is no additional energy available to power that increased rainfall. At worst any global warming will cause rainfall patterns to shift around.
The study itself admits that the prediction of increased runoff, if it occurs, will not apply everywhere. Any contamination problems which occur will be localised.
… The magnitude of the increase in DOC [dissolved organic carbon] concentration for the TM treatment is also relevant for climate change–induced runoff scenarios for large coastal regions worldwide (fig. S4) (7). However, it should be noted that the interpretations of data recently compiled by the Intergovernmental Panel on Climate Change (IPCC) (7) project a highly variable response in runoff with altered climate, with either increases or decreases, for different regions globally. …
Read more: Same link as above
Given that increased runoff, if it occurs at all, will only occur in some regions, there is a simple solution. In regions where nutrient rich runoff poisons the local fish, the solution is to apply existing waterway management regimes, to find ways to minimise organic matter and nutrients entering river systems.
This seems to be terrific land use study, the authors appear to have genuinely contributed to knowledge about how waterway contamination contributes to marine mercury pollution. Given their rigorous experimental effort, it seems a shame the authors seemed to find it necessary to spice up their study with references to very uncertain regional climate projections.
Update (EW) – fixed a typo in the first paragraph