Columbia River Estuary

ETM Dynamics and the Impact of Anthropogenic Change

In landscape ecology terms, the Columbia River Estuary Turbidity Maxima (CRETM) LMER studies focus on the “relay node” at the estuarine land-sea interface, the bottleneck of the ecosystem. This is where localized physical, geochemical and biological processes transform and redistribute organic constituents borne by the river through the estuarine food web and adjacent coastal waters. In river-dominated estuaries such as the Columbia, these processes pivot around the dynamics of the estuarine turbidity maximum (ETM), where non-linear tidal and residual circulation processes promote trapping and increased residence of inorganic and organic particulate matter. The CRETM-LMER studies were initiated in late spring 1990 and will continue through fall 1994. The nine-investigator team from the University of Washington, Oregon State University and Louisiana University Marine Consortium are investigating the comprehensive hypothesis that ETM processes promote and control microbial conversion of entrapped organic matter and facilitate maintenance of dense populations of primary consumers, which exploit the microbial loop for food. Nutrient cycling, particularly nitrogen transformations, is also believed to be concentrated in the ETM. CRETM-LMER researchers are further testing the hypothesis that organic matter generated through the watershed landscapes and transported into and down river continua is not necessarily exported directly and exclusively to coastal waters; rather, the estuarine food web captures a significant portion of these terrestrial organics when ETM are active.

Site Characteristics

As LTER and LMER sites go, the ecosystem toward which the CRETM studies are directed is a landscape of colossal scale—660,480 kilometers². The Columbia River and estuary “site” was selected because

1. It has a prominent ETM of established importance to the estuarine food web
2. The basic physical processes that drive the ETM are better understood here than in most estuaries
3. It is representative of numerous river estuaries throughout the world having ETM
4. Anthropogenic changes in river inflow and tidal prism are well-documented, allowing model prediction of the influence on ETM of these and potential future changes, and subsequent impacts to the land-margin ecosystem

Sampling Design

Because the ETM varies spatially and over a range of temporal scales, from intertidal to climatic regimes, the research team needed to identify dominant, representative scales. Circulation in the estuary is typified under three conditions which account for the bulk of ETM variability in the Columbia River estuary:

1. Low flow, neap tide
2. Low flow, spring tide
3. High flow, spring and neap tide

Collectively, these include most of the ETM trapping mechanisms found in shallow estuaries. The sampling design is based on simultaneous physical and biological measurements over tidal, tidal- monthly and seasonal time scales. Thus, the unifying concept of the CRETM-LMER project is the physical characteristics of the ETM that determine the efficiency of the food web’s interception of terrestrial and riverine organic matter. The latter are determined principally by the river flow and tidal amplitude.

Measurements indicative of ETM processes are:

1. Riverine and oceanic inputs of living and detrital carbon and inorganic material;
2. Settling velocities of particles, and turbulent and tidal fluxes of water, salt and sediment that determine the fluid mechanical trapping mechanisms creating the ETM
3. in situ production of organic matter by ETM phyto-plankton
4. Geochemical determination of the origins and fates of organic matter (particulate and dissolved) imported to the ETM and its food web
5. Biomasses, sizes and growth and ingestion rates of microorganisms (bacteria, protozoa)
6. Biomasses and ingestion rates of key epibenthic and epipelagic metazoan consumers (i.e., calanoid and harpacticoid copepcxis)
7. Ecology and behavior of these consumers relative to the ETM flow field

Approach and Methodology

A combination of state-of-the-science hydrodynamic and ecological models and analytical techniques is being applied. These will include determining primary production and dissolved carbon into the ETM and measuring tidal and turbulent water, salt, and sediment fluxes using two connectivity temperature density meters (CTDs), an acoustic Doppler current profiler, and a boundary layer frame equipped with fast-response current meters and optical backscatter sensors. Water characteristics and constituents will be sampled with a high volume pump system operating concurrently in time and space with the circulation and sedimentation measurements.

Geochemical markers (stable isotopes, cupric oxidation products and lipids) will be used to identify the sources of organic matter to the ETM and follow the conversion of organic carbon through chemical and microbial processes to primary consumers. Bacterial and protozoan biomass and bacterial activity and the zooplankton that graze upon this “microbial loop” will be measured at sequential stages in the detrital pathway. These processes will be integrated via a circulation/suspended sediment transport model with a process-oriented ecosystem model to provide predictions of the impact of watershed and estuarine ecosystem change on ETM processes.

For more information contact Charles A. Simenstad, Fisheries Research Institute, WH-1O; or David A. Jay, Geophysics Program, AK-SO, University of Washington, Seattle, WA 98195.