My Teaching
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Research
I have diverse research interests that contribute to vibrant areas of research within the fields of freshwater ecology, ecosystem ecology, aquatic microbial ecology, biogeochemistry, and restoration ecology. In addition, my research experience has encompassed a wide variety of aquatic habitats and ecosystems such as shallow groundwaters, mountain streams, lowland rivers, freshwater wetlands, tidal marshes, and large estuaries. Although I plan to continue my work on energy flow through aquatic ecosystems, my breadth of interests and expertise provides me with a sound foundation for a wide diversity of collaborative projects with students and faculty throughout the Department of Biology and interdisciplinary program in Environmental Studies. In addition, Worcester’s geographic location allows easy access to an amazing diversity of aquatic habitats and ecosystems that will facilitate comparative studies and provide undergraduate students with a wide-range of research opportunities under my guidance.
My broad research interests have focused on five important areas of freshwater ecology:
Variation in the metabolism of organic matter among aquatic ecosystems
Dissolved organic matter (DOM) influences many aspects of the ecology and biogeochemistry of aquatic ecosystems. DOM can regulate biotic processes such as bacterial productivity which in turn influences dissolved oxygen concentrations, food web structure, and microbially-mediated biogeochemical transformations. Stream ecosystems integrate DOM fluxes from adjacent terrestrial and groundwater environments and downstream fluxes of DOM are typically the largest component of stream organic matter budgets. I have found that the proportion of DOM susceptible to metabolism is highly variable among regional streams in the Northeast U.S. My research suggests that the role of streams and rivers in the retention and transport of organic matter appears highly variable and translation of findings from individual streams to regional and global scales remains a major challenge for freshwater ecologists.
Terrestrial-aquatic linkages in organic matter supply have been well documented in headwater streams draining forested catchments, and are becoming increasingly documented in large rivers and estuaries. Recent research on the metabolism of organic matter among habitats in the San Francisco Estuary's Delta has shown that DOM from the Sacramento and San Joaquin Rivers dominates the Delta's organic-matter mass balance and supports the majority of the Delta's ecosystem metabolism. However, my results also suggest that most of the DOM flux through the Delta is not metabolized. This finding contributes to an emerging general pattern of carbon cycling in large rivers and estuaries. Organic matter in large rivers may be much older and more recalcitrant to microbial metabolism than previously thought, but the ultimate fate of riverine DOM continues to be debated.
Factors that regulate the abundance and growth of bacteria in stream sediments
The important role of the microbial loop in supplying organic matter to planktonic food webs is well established in many aquatic ecosystems, but its role in stream food webs is less certain. In streams, most microbes are associated with organic particles or biofilms coating sediments. Estimates of bacterial abundance and growth in stream sediments are surprisingly uncommon and only rudimentary efforts have been made at uncovering broadly applicable controlling factors. My research has demonstrated that benthic algal production and terrestrial-derived organic matter can be important, yet spatially and temporally variable controlling factors on bacterial abundance and growth in surface biofilms in streams. My research also suggests that bacterial growth and activity in stream sediments is closely related to organic matter supply, but comparative studies are needed on a wide diversity of streams and rivers.
Controls on the flux of nutrients and carbon from terrestrial to aquatic ecosystems
Microbial processes that occur at the interface between terrestrial and aquatic ecosystems can influence both the amounts and forms of nitrogen and organic matter that move from upland terrestrial ecosystems to downstream aquatic ecosystems. However, experiments that examine the mechanisms that control the magnitude and fate of nutrient and organic matter flux remain limited. My research has shown that microbial metabolism at soil-stream interfaces can decrease the flux of DOM to downstream ecosystems. My research has also shown that variability in organic matter bioavailability may contribute to spatial and temporal differences in nitrogen export from forested catchments. These findings contribute to a growing body of research suggesting that land-water interfaces can be metabolic regulators of the downstream flux of nutrients.
Modes of energy flow into food webs among freshwater habitats and ecosystems
Ecologists have long recognized the potential importance of terrestrial-derived organic matter in supporting respiration and productivity within aquatic ecosystems. While terrestrial-aquatic linkages in organic matter supply and respiration have been documented in increasingly larger aquatic ecosystems, the corresponding strength of detrital pathways to higher trophic levels in planktonic food webs is much less certain.
I studied the source and bioavailability of organic matter supporting the planktonic food web in a diversity of aquatic habitats in San Francisco Estuary’s Sacramento-San Joaquin River Delta. My research on energy flow among the Delta’s aquatic habitats revealed that internal phytoplankton production was the major food source for the ecosystem’s planktonic food web. The relative importance of phytoplankton as a food source was surprising because phytoplankton is a small component of the ecosystem's organic-matter mass balance. In addition, my research suggests that planktonic food webs in large rivers and estuaries throughout the world may be especially sensitive to changes in the production and transport of algal-derived organic matter, regardless of its significance in the flux of organic matter through the ecosystem.
Role of environmental science in aquatic ecosystem restoration & conservation
My research on the sources and fate of organic matter among aquatic habitats in San Francisco Estuary's freshwater Delta provides important information for constructing ecological forecasts that must accompany large-scale restoration actions aimed at augmenting the flow of energy to higher trophic levels in the Delta's food web.
My research illustrates an important role for ecosystem science in support of restoration: identification of factors that limit sustainability of diverse biota and constrain forecasts of restoration actions. In low-productivity estuaries, a critical limiting factor is the supply rate of bioavailable organic matter to pelagic food webs. These findings provide critical information for predicting ecosystem responses to future manipulations of organic matter supply, both planned and unplanned. Planning and decision-making can be improved by such forecasts of ecosystem state, but predicting ecosystem change in response to multiple stressors is among the greatest challenges to ecology. Ecological forecasts and linking science with environmental-decision making have been recently highlighted in Science as an emerging imperative (Science 293: 657).
Forecasting stream ecosystem responses to a regional landscape disturbance: indirect ecological consequences of the removal of Eastern Hemlock from New England forests
Eastern hemlock dominates many New England forests and stream-side riparian areas, thus influencing the supply of detritus and availability of light to many regional streams. Regional declines of hemlock have been documented following the invasion of the woolly adelgid, an exotic forest pest. The continued expansion of the woolly adelgid range into New England and subsequent loss of eastern hemlock from New England forests will result in an array of ecological consequences. Terrestrial ecosystem responses are just now being documented, yet aquatic ecosystem responses to regional changes in riparian forest composition are less certain. The new research program aims to understand how the ecology and biogeochemistry of the region’s stream ecosystems will be altered as riparian forests change. I plan to examine changes in 1) stream hydrologic and thermal regimes, 2) stream light regimes and primary production, 3) quantity and quality of organic matter inputs, 4) microbial metabolism and downstream organic matter fluxes, and 5) macroinvertebrate biomass and functional feeding groups. The proposed research will help identify ecological effects of a regional landscape disturbance (the woolly adelgid invasion) on neighboring aquatic ecosystems and help scientists and managers forecast future environmental change.
Publications (most available as pdf)
Findlay, S.E. G., R. L. Sinsabaugh, W. V. Sobczak, and M. Hoostal. 2003. Metabolic and structural response of hyporheic microbial communities to variations in supply of dissolved organic matter. Limnol. Oceanogr., 48:1608–1617. (PDF)
Sobczak, W. V., J. E. Cloern, A. D. Jassby, and A. Mueller-Solger. 2002.
Bioavailability of organic matter in a highly disturbed estuary: The role of detrital and algal resources. Proceedings of the National Academy of Sciences USA 99: 8101-8105. PDF
Sobczak, W. V. and S. Findlay. 2002. Variation in bioavailability of dissolved organic carbon among stream hyporheic flowpaths. Ecology: 83: 3194-3209. PDF
Sobczak, W. V., S. Findlay, and S. Dye. 2002. Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach. Biogeochemistry 62: 309-327 PDF
Lovett, G. L., K. W. Weathers, and W. V. Sobczak. 2000. Nitrogen saturation and retention in forested watersheds of the Catskill Mountains, New York. Ecological Applications 10:73-84. PDF
Findlay, S. and W. V. Sobczak. 2000. Microbial communities in hyporheic sediments. IN: Streams and Ground Waters. Jones, J. & P. Mulholland (Eds.). Academic Press, New York.
Sobczak, W. V., L. O. Hedin, and M. J. Klug. 1998. Relationships between bacterial productivity and organic carbon at a soil-stream interface. Hydrobiologia 386: 45-53. PDF
Findlay, S., R. O. Hall, and W. V. Sobczak. 1998. Book Review: Methods in Stream Ecology (R. Hauer & G. Lamberti eds.). Limnology and Oceanography 43: 1021-1022.
Sobczak, W. V. 1996. Epilithic bacterial responses to variations in algal biomass and labile DOC during biofilm colonization. Journal of the North American Benthological Society 15:143-154. PDF
Findlay, S. and W. V. Sobczak. 1996. Variability in removal of dissolved organic
carbon in hyporheic sediments. Journal of the North American Benthological Society 15:35-41. PDF
Sobczak, W. V. and T. M. Burton. 1996. Epilithic bacterial and algal colonization among a stream run, riffle, and pool: a test of co-variation. Hydrobiologia 332:159-166. PDF
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