Nutrient Cycling in Aquatic Ecosystems

Science Center Objects

Nitrogen and phosphorus are plant essential nutrients that are currently in excess in many aquatic ecosystems due to runoff from urban and agricultural areas. In high amounts, these nutrients are detrimental to aquatic ecosystem health, because elevated nutrients promote excessive growth or “blooms” of algae and other nuisance species. Many species that cause blooms can produce toxins which are harmful to humans and other animals. As these blooms die, decomposition of algal and plant tissues removes oxygen from the water column, creating low oxygen or “hypoxic zones” which lead to mortality in other aquatic organisms. Harmful algal blooms commonly occur in shallow areas of the Great Lakes, and a large hypoxic zone occurs every summer in the Gulf of Mexico.

USGS researchers and partners are measuring the nutrient retention potential in aquatic ecosystems to try to reduce the amount of nutrients in these systems and to decrease the possibility for harmful algal blooms and hypoxic zones. Nitrogen and phosphorus can be removed or transformed by microbial activity and buried in sediments. On-going research at UMESC is quantifying the removal and transformation of nitrogen and phosphorus in riverbed and floodplain sediments of the Upper Mississippi River and its tributaries, in tributary streams and rivermouths of the Great Lakes, and in coastal environments in the Great Lakes.

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USGS scientists measuring sand deposition from a summer flood on the Upper Mississippi River near La Crosse, Wisconsin

USGS scientists measuring sand deposition from a summer flood on the Upper Mississippi River near La Crosse, Wisconsin

(Credit: Lynn Bartsch, USGS - Upper Midwest Environmental Sciences Center. Public domain.)

Nutrient retention on the Upper Mississippi River Floodplain

Principal Investigator – Lynn Bartsch, Rebecca Kreiling

Rivers have a natural capacity to improve water quality when they are connected to their natural floodplains and are not overloaded with sediment and nutrient runoff. Where rivers have been disconnected from their historical floodplains and channelized to eliminate backwater areas, increased flow and nutrient loads have contributed to local and downstream problems of excessive nutrient enrichment. For example, high nutrient loads from the Upper Mississippi River have contributed to a large hypoxic zone in the Gulf of Mexico each summer. Allowing rivers to flood their historical floodplains may reduce the nutrient loads by trapping sediment and nutrients on the floodplain. Understanding how the reconnection of rivers with their floodplains affects nutrient cycling is important for managers trying to improve water quality.

 

 

 

 

 

 

 

 

 

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USGS scientist measuring sediment pH in a sample taken from a tributary of the Maumee River in Ohio

USGS scientist measuring sediment pH in a sample taken from a tributary of the Maumee River in Ohio

(Credit: Patrik Perner. Public domain.)

Nutrient cycling in agricultural watersheds of the Great Lakes

Principal Investigator – Rebecca Kreiling, Lynn Bartsch

Nutrients lost from agricultural areas in watersheds of the Great Lakes cause harmful algal blooms and hypoxia in some areas of the Great Lakes. Substantial efforts are being made in these watersheds to reduce the amount of nutrients entering the streams and rivers; however, additional work is needed to further reduce nutrient loads to meet international water quality standards. Limited research has been done to understand how nutrients are transformed, processed, and potentially removed in the river systems. This information is important to managers because the retention of nutrients in these river systems may be lowering the amount of nutrients entering the Great Lakes.