Distribution and Controls Over Habitat and Food Web Structures and Processes in Great Lakes Estuaries

Science Center Objects

Rivermouth ecosystems, or freshwater estuaries, are the focus of human and wildlife interactions with the Great Lakes. They are highly valued as the region’s urban, industrial, shipping and recreational centers; and home to recreational harbors, wildlife viewing and production, beaches and urban riverfronts. Rivermouths are also both the mixing zones where nutrients from upstream watersheds are incorporated into the Great Lakes ecosystem and important sites for fish nursery and passage to upstream spawning grounds. These estuarine processes have been broadly altered through watershed land use, floodplain development, harbor channel dredging, wetland filling, urban stormwater, shoreline hardening, road and bridge construction, pier construction and the introduction of non-indigenous species. Despite the importance of rivermouth ecosystems to humans, fish and wildlife, very little is known about how these ecosystems function or how anthropogenic changes have altered those natural processes. Our lack of understanding severely limits our ability to manage or restore these ecosystems effectively and efficiently.

The overall goal of this project is to link biological, hydrological and societal controls over rivermouth structure and function within an improved scientific framework that will form the basis for restoration, management and future research. To maximize usefulness, this framework will link rivermouth services (e.g., fish production, water quality) to important ecosystem processes, such as: 1) hydrogeomorphic influences, 2) distribution of habitat, 3) relationships between rivermouth habitats and Great Lakes biota (in particular fish), 4) structure and controls of food webs within rivermouths and the effects of upstream watersheds and 5) unique ecological and chemical processes that propagate the influence of rivermouths to the Great Lakes. Furthermore, this framework will be widely distributed, accessible and responsive to resource managers and scientists working on other aspects of Great Lakes ecology.

Characterizing Rivermouth Structure

Our understanding of rivermouth ecosystems is limited by a lack of information on their physical and biological structure. Other than a few well-studied systems (e.g., the St. Louis River), little is known about food-web structure, nutrient dynamics or the hydrology of Great Lakes rivermouths. What is known suggests that rivermouth ecosystem structure is controlled by inputs delivered from upstream catchments via rivers and those delivered via lake processes (i.e., seiches and lake currents). These bi-directional inputs include water, energy, nutrients, sediments and biota, resulting in a dynamic transition zone between riverine and coastal processes.


The St. Joseph rivermouth near St. Joseph, Michigan

(Public domain.)

To initiate a better undestanding of these dynamics, USGS scientists will use data collected both remotely and on-the-ground to characterize the large-scale physical structure, hydrogeomorphology, habitat and food-web structure of rivermouth ecosystems. This will be done in three parts: 1) mapping and characterization of rivermouths using GIS, 2) an extensive survey of rivermouth food-web structure and water chemistry and 3) an intensive survey of rivermouth habitat and hydrogeomorphlogy. These studies will proceed on a parallel track and interact with development of research and management strategies through a regional partnership (see Rivermouth Collboratory below).

Synoptic Characterization of Great Lakes Rivermouths

While many spatial data exist for coastal ecosystems, these data have been developed independently by numerous public and private organizations with minimal communication. Proliferation of spatial data has resulted in extensive knowledge but has also resulted in data gaps and duplication of effort. While a single spatial database is unlikely to meet all needs, a more comprehensive level of coordination is needed to combine local and regional spatial datasets.

USGS will inventory and classify the full spectrum of Great Lakes rivermouth estuaries, using three controlling elements 1) river inputs, 2) local geomorphic character, and 3) lake influences. Scientists then will work with a regional partnership (Rivermouth Collaboratory) to develop a common framework to support science, management and restoration across Great Lakes estuaries. These objectives will be met by compiling data available from national, state, regional and provincial sources to produce a geodatabase that will be made available to researchers working in Great Lakes coastal ecosystems.

Intensive and Extensive Surveys

Research on rivermouth ecosystems has been predominantly focused on a few river systems. However, little research has placed these studies into the context of variation in rivermouths seen across the entire Great Lakes. Variation across the Great Lakes basin in land use, soils, discharge, seiche properties, coastal geomorphology and climate are likely to have strong effects on both physical and ecological structure and function in rivermouths. Understanding of these effects is needed for successful restoration and management.

To assess the importance of various controls over rivermouth ecosystems, USGS plans to sample longitudinally (river, rivermouth, and adjacent Great Lake) within each studied rivermouth ecosystem. Sampling will occur in two parts: An extensive survey that will span the entirety of the U.S. Great Lakes with 20-30 rivermouths, and an intensive survey that will focus on 3 rivermouths (see diagram below). Our extensive survey will assess the source (lake or riverine) of nutrients supporting rivermouth food webs across the range of existing lake, watershed and discharge conditions, using food-web biomarkers. USGS also will characterize quality and quantity of seston (suspended particles) in rivers, rivermouths and lake to estimate effects of watershed land cover and river-lake mixing on seston characteristics. Seston is a key resource for many river and lake consumers that may be linked to watershed characteristics. Biomarkers (δ13C, δ15N, fatty acids ) derived from ingestion of seston will be measured in seston and tissues of dominant consumers and predators and will link watershed characteristics to rivermouth food webs. Using our geospatial data, scientists will correlate extant land cover and land-cover change to material flux, food-web characteristics and ecological processes contributing to Great Lakes productivity and fisheries health.

At the intensive sites USGS will additionally characterize the hydrogeomorphology, the mixing between riverine and lake inputs and the distribution and usage of habitat by aquatic biota (e.g., fish, benthos).


Sampling locations within each rivermouth

(Public domain.)

The extent of mixing between riverine and lake inputs along longitudinal, lateral and vertical transects of the rivermouth will be determined using metrics reflecting long-term (sediment, wetland vegetation, hydroperiod) and short-term (mixing, flow velocity and direction) hydrogeomorphic data. Along with water-level analysis and wetland mapping, these data will be used to characterize habitat connectivity and availability to Great Lakes biota. As a high-quality resource, the distribution of and food sources for migratory larval fishes are of particular interest and thus will be sampled throughout the spring spawning period.

The Rivermouth Collaboratory

Virtually every aspect of Great Lakes rivermouths has been al­tered: watershed land use, shoreline integrity, hydrology and the biotic community. The ability of rivermouth ecosystems to provide services for humans and wildlife has been diminished and may be diminished further by anthropogenic changes at many scales (from bridge construction to climate change). To help prevent future losses and take steps toward effective restoration of these ecosystems, USGS will engage the Great Lakes coastal ecology community to develop a common science agenda that consolidates knowledge about rivermouth ecosystems and their underlying processes and provides a framework for current and future research in support of restoration. Creating and disseminating this framework will be a major goal of the Rivermouth Collaboratory, which is being facilitated and organized by the Great Lakes Commission.

A collaboratory is a working environment where regionally-dispersed scientists and other stakeholders commit to a common science partnership. The purpose is to encourage critical discussions of science approaches; broad sharing of ideas, knowledge and tools; inter-disciplinary, inter-agency working partnerships and an ongoing, iterative, adaptive science process at the regional scale.

The Collaboratory’s rivermouth science agenda will provide a critical link among science disciplines often historically isolated by a focus on only watershed, coastal, nearshore or deepwater issues. The Collaboratory will provide a forum for scientific, management and stakeholder communities to define the current state of knowledge, strengthen the foundation for future research and ensure that restoration goals are met for these heavily used and impacted ecosystems.

The short-term objectives of the Collaboratory include: 1) conducting a series of workshops and webinars that will assemble, and improve, current paradigms of rivermouth ecosystems and their restoration and sustainability needs and 2) applying that knowledge in the development of a common rivermouth science agenda and institutional framework to guide and support restoration and management of these vital ecosystems. Over the long term, the results from this project can be used to construct a decision-support tool that meets the needs identified by resource managers and policy-makers.

This project is a collaborative effort among several USGS Science Centers in the Great Lakes region, including the Great Lakes Science Center, the Upper Midwest Environmental Sciences Center, and the Wisconsin Water Science Center.

Principal Investigator: William Richardson