Ulf Skyllberg

Photodegradation in lakes is a major sink for the toxin methylmercury (MeHg) in forest-wetland-lake ecosystems. Previous attempts to estimate annual rates of MeHg photodegradation in lakes have followed a “black-box approach”, where the process has been related to incident sunlight rather than photon absorption. Here we use experimental data from three contrasting dark and clear boreal lakes to develop the first apparent quantum yield (AQY) model for spectral MeHg photodegradation rates in lakes. The model was proven universal by its ability to predict experimental data from 22 lakes representing five global regions, covering wide ranges in dissolved organic carbon (DOC), specific UV absorbance at 254 nm, and pH (1.8–39.5 mg C L^–1, 1.7–5.7 L mg^–1 C m^–1 and 4.8–8.5, respectively). The AQY model manifests a dual role played by dissolved organic matter (DOM) as a sensitizer, by producing reactive transient species (RTS) upon photon absorbance, and as an inhibitor, by scavenging of RTS by antioxidants associated with aromatic structures in DOM. Using site-specific data on direct and diffuse solar irradiance, MeHg concentrations, and spectral light absorption properties in 1033 lakes, we estimate an annual MeHg photodegradation rate of 12.1 kg y^–1 in the total volume of Swedish lakes. This value corresponds to 24% of the estimated 51 kg of MeHg that annually is transported with runoff from soil into the same lakes. By the AQY model, we calculate the first regional estimates of MeHg photodegradation in lakes of boreal and temperate Europe, temperate North America, subtropical North America, and tropical South America, providing a basis for the establishment of regional MeHg cycling models.

Monitored seabird populations have declined by up to 70% worldwide since the 1950s. Yet, data on long-term seabird population dynamics prior to the anthropogenic era are largely unknown. This limits our ability to understand future population trajectories, particularly in the Southern Ocean, where seabirds are facing multiple environmental threats. Here, we use mercury (Hg) derived from seabird guano in peatland catchments as a tracer of colony population sizes on sub-Antarctic Bird Island (South Georgia). Peat Hg flux and isotope signature results show that the first sustained seabird colonies after deglaciation were established on the island between 6800 and 6100 years ago, predating evidence for colonization on other sub-Antarctic islands by more than 1,000 y. The four subsequent periods with large local seabird populations occurred during phases of less intense Southern Hemisphere westerly winds. Our study unveils significant and repeated millennial-scale shifts in seabird abundance in response to natural climate changes, implying that the present-day increase in westerly wind intensity may lead to further declines in seabird populations in the Southern Ocean.

Monitored seabird populations have declined by up to 70% worldwide since the 1950s, yet little is known about population sizes or long-term trends prior to anthropogenic impacts. This is particularly true for the Southern Ocean, where seabirds and many other species are currently facing multiple environmental threats. Here, we identify shifts in seabird populations over the last 8100 years by applying a mercury (Hg) flux and isotope signature technique as a novel tracer of population size via guano input into peatlands on sub-Antarctic Bird Island (South Georgia). Our results show that seabirds have colonized Bird Island between 6800 and 6100 years ago, which pre-dates evidence for colonization of other sub-Antarctic islands by more than 1000 years. There are five periods when seabird populations were at maxima during the mid- to late-Holocene, coeval with periods of lower Southern Hemisphere westerly wind intensity. Our study provides unprecedented insights into cyclicity in seabird population sizes as a consequence of regional to large-scale environmental change.

Forest harvesting can lead to mercury (Hg) mobilization from soils to aquatic habitats and promote the transformation of inorganic Hg to highly neurotoxic and bioaccumulative methyl-Hg (MeHg). Multiple past studies reveal broad variation of stream water MeHg and total Hg (THg) concentration responses to forest harvesting, which has confounded messaging to forest and resource managers. To advance beyond divergent and sometimes contradictory findings, we synthesized information for 23 previously studied catchments in North America and Fennoscandia and compiled a uniform set of soil, landscape, and harvesting properties to identify forest management, riparian, and hillslope factors that influence responses of stream water MeHg and THg concentrations. From this synthesis, we found catchments with high soil moisture and organic soil layers >100 cm to be at highest risk for disturbance-induced increases in MeHg formation after harvest but not necessarily affecting concentrations of MeHg in stream waters. Instead, the combination of MeHg formation in soils along with factors that affect mobilization with runoff to streams most influenced how forest harvest affects MeHg concentrations in stream waters.

Knowledge of migration patterns plays an essential role for understanding spatiotemporal distribution of birds. Here we used >15,000 sightings of 1,473 colour-ringed Common Cranes Grus grus to study migration patterns of birds breeding in five regions along a longitudinal gradient of Fennoscandia. Our results confirm that Fennoscandian cranes mainly use either a Western European flyway (W-flyway), to winter mainly in France or Spain, or a Central European flyway (C-flyway), to winter in Hungary or Israel. Finnish cranes showed the greatest variation in migration patterns in terms of distance and direction and only Finnish cranes were recovered in Africa. Many of the Finnish cranes, starting along C-flyway change to the W-flyway and winter in SW Europe or NW Africa. On the other hand, the Scandinavian cranes are rarely observed along the C-flyway. However, substantial numbers of cranes from NE Sweden cross the Baltic Sea and migrate via Finland but then follow the W-flyway from Germany and southwards, especially during autumn. These results can be used for conservation and management, e.g. to coordinate monitoring. They are also relevant for land use planning, e.g. to avoid windfarms at sites important for migrating soaring birds while crossing seas, so-called ‘thermal bottleneck sites’.

Peatland vegetation takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remains, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg redox transformation processes. Isotope mass balances suggest photoreduction of Hg^II is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that above the water table, dark abiotic oxidation decreases peat soil gaseous Hg⁰ concentrations. Below the water table, supersaturation of gaseous Hg is likely created more by direct photoreduction of rainfall rather than by reduction and release of Hg from the peat soil. Identification and quantification of these light-driven and dark redox processes advance our understanding of the fate of Hg in peatlands, including the potential for mobilization and methylation of Hg^II.

The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions.

The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0-2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)₂ complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)₂, to complexes with lower availability, Hg(PEN)₂, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2-6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

Peatland vegetation constantly takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic Methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down Methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remain, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations above the water table, and that supersaturated gaseous Hg below the water table was likely created more by photoreduction of rainfall rather than release from peat soil. These findings constrain the possibilities for mobilizing previously deposited Hg. Identification and quantification of these processes further advance our understanding of the fate of Hg in peatlands and their export of Methyl-Hg to downstream ecosystems.

Migratory connectivity is a metric of the co‐occurrence of migratory animals originating from different breeding sites, and like their spatial dispersion, can vary substantially during the annual cycle. Together, both these properties affect the optimal times and sites of population censusing. We tracked taiga bean geese (Anser fabalis fabalis) during 2014–2021 to study their migratory connectivity and nonbreeding movements and determine optimal periods to assess the size of their main flyway population. We also compared available census data with tracking data, to examine how well two existing censuses covered the population. Daily Mantel's correlation between breeding and nonbreeding sites lay between 0 and 0.5 during most of the nonbreeding season, implying birds from different breeding areas were not strongly separated at other times in the annual cycle. However, the connectivity was higher among birds from the westernmost breeding areas compared to the birds breeding elsewhere. Daily Minimum Convex Polygons showed tracked birds were highly aggregated at census times, confirming their utility. The number of tracked birds absent at count sites during the censuses however exceeded numbers double‐counted at several sites, indicating that censuses might have underestimated the true population size. Our results show that connectivity can vary in different times during the nonbreeding period, and should be studied throughout the annual cycle. Our results also confirm previous studies, which have found that estimates using marked individuals usually produce higher population size estimates than total counts. This should be considered when using total counts to assess population sizes in the future.