Redondo SA. 2022. Signals of an insidious pollutant: temporal, spatial, and biotic interplay of mercury in a terrestrial ecosystem. PhD Dissertation, Department of Biology, Stanford University.
Mercury (Hg) is a globally distributed trace metal element that is toxic in its various chemical forms. Hg concentrations have dramatically increased in the environment since the industrial revolution (circa ~1850) due to anthropogenic activities like mining and coal combustion. Climate change has exacerbated the pollutant problem by re-mobilizing Hg previously stored in glaciers, permafrost, soils, and forests. However, the fate and cycling of Hg in terrestrial ecosystems remains poorly studied. Understanding temporal deposition, spatial distribution, and biotic uptake of Hg all have important implications for environmental and human health. In this dissertation, I explore historic Hg deposition patterns in California using lake sediment cores collected at the foothills of the Santa Cruz Mountains. I also assess how vegetation, soil, and geology type influence the spatial heterogeneity of Hg concentrations across sixteen terrestrial habitats commonly found in the Bay Area of California. Lastly, I incorporate DNA metabarcoding and stable isotope techniques to investigate the transfer of Hg through soil arthropod communities. In chapter one, the sediment cores provided one of the first sub-annual records of sediment Hg encompassing over a century of trends. The combined record dates back at least 250 years, though likely up to ~1200 years. The record indicates an increase of 1.8-2.4x in Hg concentrations during this last century compared to the pre-industrial baseline. However, Hg deposition was also found to decrease near the surface of both records, which is consistent with declining consumption and production of Hg over the recent decades. There is also evidence suggesting that climatic conditions (e.g., drought, global cooling) affect the Hg deposition signals recorded in the sediment. In chapter two, I found that vegetation type and cover have a strong influence on Hg concentrations in terrestrial systems. Closed-canopy forests harbored the highest soil Hg concentrations, while the exposed grasslands and scrubland retained the least amount of Hg in their soil. These findings suggest foliar uptake combined with litterfall and throughfall as the primary mechanisms for inputting Hg into soil. I also found that some soil types can dictate soil Hg concentrations regardless of above-ground vegetation as seen at the boundaries between shifting soil types. This would suggest that inherent properties of the soil are still important for the retention of Hg. Furthermore, there is some evidence that heavily disturbed systems (e.g., tree farm) may not recover their soil Hg storage capacity for extended periods of time. In chapter three, I identified over 1,200 unique soil invertebrate taxa inhabiting the sixteen habitat types sampled. Stable isotope data suggested that relative trophic positioning of a composite community can provide similar insights from those obtained by assessing individual taxa alone. These findings indicate that higher trophic position is positively correlated with higher Hg concentrations. This implies that Hg biomagnification is indeed playing a role in the food web dynamics in several of these terrestrial communities. Our analyses corroborate the use of millipedes and collembolans as indicator species for understanding Hg dynamics in terrestrial food webs, but also supports the use of several coleopteran taxa as possible bioindicators. Some taxa were only found in low-Hg containing communities, indicating possible sensitivity. We recommend the continued refinement and use of these methods to expand their utility in assessing soil arthropod communities and their role in transferring pollutants through terrestrial food webs. This dissertation provides new insights into the cycling of Hg in terrestrial ecosystems. By expanding temporal and spatial records, this work enhances our understanding of where Hg is stored and how that might look in a changing climate. This work also develops a new approach for assessing Hg transfer in highly complex and abundant soil fauna. Together, this work has created new possible avenues of Hg research in terrestrial ecosystems that future researchers may expand upon. [link to publication]