A. Areas of Research Emphasis and Funding in the OSU Aquatic Biogeochemistry Group. Our primary areas of research interest in the OSU Aquatic Biogeochemistry Lab include the cycling of carbon and associated elements (e.g., nitrogen, phosphorus, etc.) within and across watershed, river, estuarine and marine environments. The essential goals of our research in these and other systems are to 1) gain a better understanding of the production, transport and degradation of organic matter (OM relation to the overall C, N and P cycles in terrestrial, freshwater and marine environments, 2) investigate the dynamics of, and interactions between, carbon and nutrient cycling that occur at the interface (i.e., rivers and estuaries) between land and aquatic systems, and 3) evaluate the roles of biotic and abiotic transformations of organic and inorganic C and associated elements in these ecosystems.
Our current research focuses on the sources and ages of carbon and OM in aquatic systems and its utilization and cycling by heterotrophic bacteria. This is presently a topic of significant interest in the earth sciences because it relates to the interactions and exchanges of carbon between the continents (including fresh waters), oceans, and atmosphere. In particular, the loss of carbon and OM from land via the hydrologic cycle, and its fates (e.g., by respiration and recycling, utilization by aquatic food webs, export to the oceans, etc.) in rivers, estuaries and coastal waters is one of the most poorly constrained terms in the global carbon cycle.
We utilize a broad array of state-of-the-art microbial and biogeochemical tools in our research. These include Accelerator Mass Spectrometry (AMS), Stable Isotope Ratio Mass Spectrometry (IRMS), Gas Chromatography-Mass Spectrometry (GC-MS), Electrospray Fourier Transform Inductively Coupled Mass Spectrometry (ES-FTICR-MS), and various novel microbial ecological and photochemical techniques.
Our research projects have received continuous funding over the past two decades from a variety of National Science Foundation programs, as well as from the U.S. Department of Energy and state and regional funding agencies.
B. Ongoing and Future Research Directions. A primary research area in our group is the role of allochthonous OM and nutrients in supporting net ecosystem production (NEP) and degradation processes. In the vast majority of freshwater, estuarine and even coastal aquatic systems, NEP is negative, meaning that in order for the high levels of heterotrophy in them to be sustained, they must import significant amounts of allochthonous OM. The majority of this net heterotrophy is determined by the respiration of allochthonous OM by heterotrophic bacteria, while smaller amounts are driven by higher organisms. What we are beginning to realize is that the forms of allochthonous OM supporting aquatic systems are typically not tightly coupled in time to them – in fact, the terrestrial OM that dominates aquatic systems is generally highly aged (hundreds to thousands of years), indicating that it is stored in terrestrial or sedimentary systems for extensive periods of time before being exported to aquatic environments. Thus, we are finding that the allochthonous OM supporting aquatic systems appears to be separated in time from these systems, often by great periods of time (up to millennia). These new findings have the potential to fundamentally change our conceptual models and paradigms about how aquatic ecosystems function, including both lower microbial as well as grazer-based components of higher food webs, and possibly even fisheries. This work is currently being expanded from small and moderate-sized rivers to large rivers, including the Ohio River and other major tributaries of the Mississippi,the Great Lakes, and engineered systems such as dammed waterways and reservoirs.
Another important aspect of our present and future research involves bringing new and innovative state-of-the-art geochemical and microbiological techniques to bear on ecological and biogeochemical problems. In this regard, our group has used natural abundance isotopic (especially radiocarbon, 14C), stable isotopes (13C, 15N and Deuterium), and detailed organic biomarker approaches for tracing the sources and fates of organic and inorganic materials and energy in ecosystems. It is at the interface between state-of-the-art geochemical approaches and ecosystems ecology that we find our group’s future work being most motivated. Our collective background and experience with both ecological and geochemical principles and approaches including the following applications and approaches:
1) Interactions between watersheds and aquatic systems, and support of aquatic ecosystems by terrestrial materials.This has been a growing area of research emphasis and activity for our group over the years, and one which now dominates our research in rivers and estuaries. Recently we have moved from small and moderate-sized systems to the Mississippi watershed, where preliminary findings indicate that the forms and ages of organic materials supporting the aquatic communities there are a) different from those of the smaller temperate systems, and from large tropic river systems such as the Amazon, and b) different among Mississippi River basin sub-watersheds. We are particularly eager to explore components of the upper food web (planktonic grazers, fish, etc.) to see if the old forms of OM that have been found to support these communities in the Hudson also support large river systems (we predict that they do).
2) Relationships between the microbial loop and grazer-based food webs in aquatic systems. The traditional conceptual models of microbial and grazer-based food webs are that the two are largely independent of one another. The microbial loop is generally seen as a biotically mediated recycling pathway, where OM (particularly DOM) is respired and remineralized to its inorganic forms. Recycled nutrients such as N and P are then available to support phytoplankton and aquatic vegetation that can then support the tropho-dynamics of higher grazer-based food webs, mostly through feeding on POM components (both living and non-living). Our use of natural isotopic and organic biomarker (e.g., lipid) components of living and non-living OM have provided a completely independent geochemical assessment of the sources of OM supporting both microbial and higher food webs – and the findings are not always consistent with more traditional approaches. For example, bacteria in river systems have been found to rely on DOM having a diversity of sources and ages, and cladoceran and copepod grazers have been found to rely to a very significant extent on highly aged non-living POM for their nutrition in rivers such as the Hudson. Our group is planning to expand these types of studies to assess whether such findings are system-specific, or general across different aquatic systems (e.g., rivers vs. estuaries vs. lakes). The outcome of these studies may require a reassessment of not only lower food webs, but of higher food web components such as fisheries as well. We are eager to explore whether or not the microbial and grazer food webs overlap, and the extent of that overlap, across different aquatic systems (e.g., small vs. large rivers, lotic vs. limnic, natural vs. engineered), and the factors controlling the degree of overlap of the two food webs among different aquatic systems.
3) Impacts of environmental change on aquatic ecosystems and biogeochemical cycles. A key aspect of our group’s present work is assessing the role of changing land use on river and estuarine ecosystems. The effect of landscape-level features of watersheds such as the proportion of agricultural, forested and urbanized land use on aquatic biogeochemical cycles are poorly understood. Our current work is uncovering some important differences in the sources and cycling of organic materials and water quality depending on the land-use practices in the associated river and estuary watersheds. The dramatic (in many cases, nearly total) reduction in POM food sources in many temperate aquatic systems due to damming is predicted to cause dramatic shifts in aquatic food webs, yet there are few controlled studies to rely on for assessing this.
Our group will continue to explore the evolving linkages between watersheds and aquatic ecosystems. In our river-estuary studies, we would like to begin incorporating damming, dam removal (a growing practice as dams age) and invasive species’ impacts into our measurements and conceptual models of aquatic community energetics and biogeochemical cycles. For example, in the Hudson River and other systems the invasion of zebra mussels has both reduced phytoplankton chlorophyll to background levels (i.e., there is no longer a spring bloom in the river), as well as total particle and POM loads. These profound impacts, when extrapolated to other systems such as Lake Erie where similar invasions have taken place, might be predicted to exert fundamental changes on both food webs and biogeochemical features such as the timing and geographical extent of hypoxia, all the way to the diversity, distribution and recruitment of fish species.
4) Continued studies of continent-coastal ocean connections. Our current work in marine systems focuses on coastal environments, including the roles of both small and large river systems on coastal biogeochemical cycles. We continue to explore how allochthonous and authochthonous organic and inorganic materials regulate the metabolic balance of coastal systems, and how the balance between allochthonous and autochthonous materials regulates whether coastal systems are driven to net autotrophy or net heterotrophy. Coastal regions are to date one of the major ecosystem types whose carbon budgets are still largely unconstrained and have not yet been incorporated into either terrestrial or oceanic carbon budgets (and thus they are a key omission for closing the global carbon cycle). We intend to continue pursuing this work into the future because of its importance to regional, continent-scale, and global carbon cycling.