The Effects of Wetland Restoration on the Production and Bioaccumulation of Methylmercury in the Sacramento-San Joaquin Delta, California
By Darell G. Slotton 1,C, Shaun M. Ayers 1, Thomas H. Suchanek 2,3, Ronald D. Weyand 1, Anne M. Liston 1, Chance Asher 4, Douglas C. Nelson 4, and Brenda Johnson 2
1 Dept. of Environmental Science and Policy, University of California, One Shields Avenue, Davis, CA 95616 2 Dept. of Wildlife, Fish & Conservation Biology, University of California, One Shields Avenue, Davis, CA 95616
3 U.S. Fish and Wildlife Service, Div. of Environmental Contaminants, 2800 Cottage Way, Sacramento, CA 95825 4 Division of Microbiology, University of California, One Shields Avenue, Davis, CA 95616
C Corresponding Author: firstname.lastname@example.org
A collaborative, foundational project funded separately by the CALFED Bay-Delta Agency in a grant to the University of California at Davis, under CALFED contract 97-C05, July 1998 – March 2003
Submitted in collaboration with the multi-institution Directed Action research project:
Assessment of Ecological and Human Health Impacts of Mercury in the San Francisco Bay-Delta Wateshed
A CALFED Bay-Delta Program Project October 1999 – March 2003
DRAFT FINAL REPORT SUBJECT TO REVISION
UNPUBLISHED DATA – DO NOT CITE WITHOUT PERMISSION
September 25, 2002
UC DAVIS COLLABORATIVE DELTA STUDY: WETLAND RESTORATION EFFECTS ON MeHg PRODUCTION AND BIOACCUMULATION
Mercury (Hg) contamination and, particularly, the bioaccumulation of toxic methylmercury (MeHg) in food webs is one of the primary water quality issues in the San Francisco Bay-Delta watershed of California. This is the result, in large part, of the Gold Rush era legacy of extensive Hg use in Sierra Nevada gold mining, as well as the now-abandoned Hg mines in the California coast ranges that supplied this Hg. It is clear that both regions remain major sources of ongoing Hg contamination, both locally and downstream (Slotton et al. 1995, 1997, 1998, 1999, Suchanek et al. 1997, Foe and Croyle 1998, Domagalski 1998, Roth et al. 2000). During the past 150 years, significant amounts of Hg, coming from mining operations on both sides of the state, have been deposited in Bay-Delta sediments. The extensive Sacramento- San Joaquin Delta levee system that originated in the 1860’s effectively isolated and converted ("reclaimed") wetlands for the production of agricultural crops and other uses and, in so doing, dramatically altered the natural functioning of these wetlands. Many levees were likely constructed at locations which already contained significant Hg deposits, and some of these historic Hg-laden diked wetlands have long been isolated from normal tidal inundation.
In recent decades, substantial tracts of this levee-protected land have once again become flooded wetlands. This has occurred through the natural breaching of some of the Delta levees during storm/flooding events, as well as through the deliberate breaching of levees for the purpose of restoring wetland habitat. Plans are underway to similarly convert numerous additional tracts of leveed land to wetlands. Some previous, current, and planned future breaching projects include extensive manipulative restoration work, such as the current project at Prospect Island (US ACE & DWR 2001). Other breach sites are being allowed to return to vegetated wetland habitat along a more natural trajectory. The concern is that significant new wetland creation in the Delta may increase MeHg production and that this may result in increased MeHg bioaccumulation in aquatic organisms of the region, as well as their consumers, both wildlife and human.
Wetlands have been shown to be important sites of MeHg production (e.g. Zilloux et al. 1993; Hurley et al. 1995, Rudd et al. 1995, Cleckner et al. 1998). This has been linked to elevated rates of Hg methylation, primarily by sulfate-reducing groups of bacteria which typically occur near the oxic/anoxic interface in aquatic systems (Compeau and Bartha 1985; Gilmour et al. 1992). Highly organic and potentially sulfate-enriched wetlands can provide habitat for these microbes. MeHg production in wetlands has also been implicated as a major source of MeHg to adjacent and downstream environments (e.g. St. Louis et al. 1994, Krabbenhoft et al. 1999). In addition, the well known phenomenon of newly flooded and re- flooded terrestrial soils producing a surge in Hg methylation may be important in new restoration projects (Bodaly et al. 1984, 1997, Slotton 1991) as well as in existing flooded tracts which experience seasonal or inter-annual shifts in standing water level (Snodgrass et al. 2000).
A survey level, foundational study (the subject of this report) was conducted by UC Davis between 1998 and 2001 to begin to examine the potential effects of various flooded Delta tracts on the production and bioaccumulation of MeHg. Flooded tracts of varying age, ecological successional stage, and vegetation density were examined throughout the Delta region, together with adjacent aquatic habitats such as channels and sloughs. Sites weredistributed from tributary inflows to the estuarine entrapment zone where fresh and salt water mix. Sediment total Hg (THg), MeHg, MeHg:THg ratios, and relationships with organic carbon were examined at paired sites inside and outside various representative flooded wetland tracts. Relative Hg methylation potential was estimated in Hg-amended sediment slurry experiments. Concentrations of aqueous MeHg were assessed at a range of representative flooded wetland tracts in inflowing vs outflowing water during tidal cycles. Relative biological Hg exposure levels throughout the region and spatially among habitats were assessed with naturally occurring small fish and invertebrate indicator species, which were tested for THg, MeHg, individual variability in Hg bioaccumulation, and nitrogen and carbon stable isotopic ratios. The study was designed to provide an initial indication of prevailing Hg trends in the Delta, including relative MeHg bioaccumulation, and the potential role of wetland restoration sites on the Hg dynamics of the overall system.
Methylmercury (MeHg) production, export, and bioaccumulation were investigated at representative sites throughout the Sacramento-San Joaquin Delta in California, in relation to wetlands restoration efforts in the region. Sediment MeHg and MeHg:total mercury (THg) ratios were examined at paired sites inside and outside various flooded wetland tracts. Relative mercury (Hg) methylation potential was estimated in Hg-amended sediment slurry experiments. Concentrations of aqueous MeHg were assessed at a range of representative wetland tracts in inflowing vs outflowing water during tidal cycles. Relative biological Hg exposure levels throughout the region and spatially among habitats were assessed with naturally occurring small fish and invertebrate indicator species, which were tested for THg, MeHg, individual variability in Hg bioaccumulation, and nitrogen and carbon stable isotopic ratios.
Sediment MeHg concentrations and MeHg:THg ratios were found to be significantly greater in flooded tracts characterized by dense submergent or emergent aquatic vegetation, as compared to adjacent Delta channel, mudflat, or sandflat environments. Wetland sediments from vegetated flooded tracts exhibited 2-30 times greater potential to produce MeHg than aquatic sediments of adjacent channels and flats.
At these same locations, concentrations of aqueous MeHg and aqueous MeHg normalized to suspended solids were found to be substantially elevated in outflowing tidal water (off the tracts), relative to inflowing water. Consistent with the literature for other estuarine systems, all of these measures indicated that highly vegetated, flooded wetland sediments functioned as net producers and exporters of MeHg to the wider Delta.
However, biological findings indicated no discernible localized increase in biotic MeHg concentrations in flooded wetland tracts vs adjacent aquatic habitats. Vigorous tidal action may effectively mix MeHg from net methylating habitats into local areas, creating larger spatial patterns. Most surprising was the finding of notably lowest overall Hg bioaccumulation throughout a broad region of the south and central Delta that contained numerous wetland restoration sites identified as net methylating environments. This indicates that the linkages between sediment MeHg, aqueous MeHg, and ultimate bioaccumulation by aquatic organisms may be quite complex. The regions with most highly elevated biotic Hg identified in this work can all be characterized as being dominated by ongoing new inflows of Hg from upstream San Francisco Bay-Delta tributaries. Inputs of both elemental Hg from historic gold mining in the Sierra Nevada and abandoned mercury mine cinnabar in the Coast Ranges appear to be of importance. This suggests that upstream remediation efforts on either side of the watershed may be more regionally meaningful than previously anticipated. A secondary zone of relatively elevated Hg bioaccumulation occurred in the estuarine entrapment / salinity transition zone.