OpportunityTitle: Transformation and retention of nutrients in rivermouths of the Great Lakes
Description: Algal blooms and nearshore productivity are strongly influenced by the nutrient loads, the timing of nutrient delivery and nutrient form upon delivery. However, estimates of nutrient inputs are derived from models that use data from U.S. Geological Survey gages positioned outside the reach of direct lake influence. This leaves a gap between where loading estimates are made and the lakes themselves. This gap is occupied by rivermouths, which are zones of active mixing and nutrient transformation. Rivermouths seem to vary in their ability to alter nutrients and there is potential that restoration of rivermouth habitats could offer a mechanism to minimize impacts of excessive nutrient loads from upstream watersheds. For this reason, understanding the rivermouth effect on nutrients was recently identified as one of the areas in need of future research by a consortium of nearshore researchers (The Rivermouth Collaboratory) and is needed to accomplish objectives in the USGS GLRI Science Framework identified in both the Nearshore Health and Watershed themes. Rivermouths and other areas of the Great Lakes nearshore zone are the interface between the Lakes and the human communities that surround them. These areas provide abundant ecosystem services, including those related to provisioning services (food and water supply), cultural services (recreation and tourism) and regulating services (water quality regulation). Across much of the Great Lakes, changing land use practices, improved waste-water management programs and the invasion of dreissenid mussels have resulted in low-productivity open water environments and an increased reliance of the entire food web on nearshore processes. This is because nearshore habitats are often the direct recipients of terrestrial nutrient inputs (e.g., rivermouths) or are regularly exposed to terrestrial inputs (e.g., nearshore zones that are periodically flushed with river plume waters) that stimulate primary producers. Although nutrients stimulate primary production, the load, timing, ratio and form of nutrients delivered to the nearshore strongly influences the processes and thus services these nearshore areas provide. High concentrations of dissolved reactive phosphorus (DRP) appear to promote the proliferation of cyanobacterial species that provide poor food quality and thus poor support for many ecosystem services. Increases in DRP loading appear to have driven re-eutrophication over the past 15-20 years in Lake Erie and cyanobacterial abundance in nearshore habitats such as rivermouths and embayments is often controlled by the quantity of nitrogen (N), P and the N:P ratio. In the Great Lakes, nutrient loading estimates are made using data from river gages that are located outside the upstream extent of lake influence. However, some recent results have implied that rivermouths alter relationships between landscape nutrient sources (e.g., agriculture) and nutrients available to nearshore food webs. Natural rivermouths are often highly complex systems with associated wetland complexes, and these rivermouth wetlands can retain significant amounts of N and P. This rivermouth retention of nutrients appears to be minimized in heavily impacted systems such as the Maumee rivermouth. The potential for restoration activities within rivermouths and coastal wetlands to improve nutrient retention in rivermouths has not been fully explored (but see work by K. Kowalski on a Crane Creek restoration project). Further, even the transformation of nutrients from one form to another has the potential to influence the impacts of the nutrients on the nearshore zone. For example, toxic and non-toxic strains of cyanobacteria appear to be promoted differently by whether available N is in an organic or inorganic form.
GrantorContactEmailDescription: dsanta@usgs.gov
GrantorContactText: Desiree Santa &lt;br/&gt;Grant Specialist &lt;br/&gt;Phone 703-648-7382