RAPID/Collaborative Research: Estuarine dam removal as an ecosystem disturbance: Examining the impacts of seawater intrusion on functional stability of benthic N cycle communities

College Of William & Mary Virginia Institute Of Marine Science, Gloucester Point VA

Investigators

Abstract

Large dams (> 15m high) are a major man-made disturbance to watersheds altering the structure and function of river ecosystems. The Nakdong River is one of the watersheds heavily affected by the dam constructed at the mouth of the river to stop tidal seawater exchange. When it was built in 1987, the dam caused dramatic shifts in lower river ecosystems converting the area to a man-made freshwater reservoir. The alteration of river water flows resulted in seasonal harmful algal blooms and water quality degradation. Increasing concerns about these negative impacts along with associated social and economic demands have led to restoring the river ecosystem by removing the dam. The dam removal will be done in a controlled way through a series of events that open the dam's gates, thereby allowing seawater intrusion into upstream ecosystems. This provides a unique and immediate opportunity to study the impacts of seawater intrusion on a freshwater ecosystem. The series of planned dam openings can be effectively used as a large-scale, unprecedented, manipulative ecosystem experiment. This RAPID project takes advantage of this unique opportunity to study the sediment microbial communities responding to the programmed seawater intrusion in the river. The project will include training opportunities for undergraduate and graduate students, and results will disseminated via public lectures and a scientific network for river researchers. This project aims to study the functional stability of sediment nitrogen (N) cycling communities responding to the disturbance of seawater intrusion in the Nakdong River, where dam removal is in progress via a series openings of dam gates. Multi-omic approaches including metagenomic, metatranscriptomic and metaproteomic analyses will be conducted to evaluate both sensitivity and resilience of individual N metabolisms in a community, while the measurements of N cycling processes (fluxes and rates), including denitrification and dissimilatory nitrate reduction to ammonium, reveal a whole community metabolism responding to the seawater intrusion. This is a unique and transformative project to determine the functional stability of different metabolisms within the community exposed to a programmed seawater intrusion in a freshwater ecosystem. The combined multi-omics and biogeochemical measurements will provide novel knowledge of sensitivity and resilience in different systematic levels from genes to individual organisms to the community level. Dissimilatory N cycling metabolisms in sediment communities are selected as a model of microbial functions since benthic N cycling is critical to attenuate excess N loading, a cause of eutrophication. Given the importance of benthic N cycle, the impacts of seawater intrusion on sediment N cycling communities and their functions have been mainly examined with ex-situ experimental designs such as microcosms and mesocosms or field survey along salinity gradients of rivers and estuaries. This first project represents the first ecosystem-scale manipulation examining sediment N cycling communities responding to seawater intrusion. Two graduate students will be involved in the project and will be trained in multi-omic and biogeochemcial methods. The project will also include training activities for undergraduates at the two institutions, dissemination of results in public outreach lectures and via the Worldwide Hydrobiogeochemical Observatory Network for Dynamic River Systems (WHONDRS). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.