Nutrient Limitation

The goal of the MD DNR Chesapeake Bay Monitoring Program, Nutrient Limitation Component, is to determine the specific factors, primarily nitrogen and phosphorus, that limit algal growth at various times in Chesapeake Bay and its tributaries. This information can be used both to target future nutrient reduction efforts, the keystone program to restore the Bay, and to interpret monitoring data used to track the restoration. The Nutrient Limitation component has been in place since August of 1990.

Like all plants, phytoplankton (microscopic algae) need light and nutrients (e.g. nitrogen, phosphorus, carbon) to grow. Light and nutrients are the ‘resources’ for phytoplankton growth. If light is not blocked by materials suspended in the water, phytoplankton will continue to grow as long as there are nutrients being added to the water. (Nutrients are added from both non-point and point sources, as well as regenerated from the Bay sediments under certain conditions such as anoxia). However, unless the nutrients are available in adequate amounts relative to each other (generally a ratio of nitrogen to phosphorus of 16:1), phytoplankton growth is ‘nutrient limited’ by one or the other nutrient. If both nutrients are added in enough excess (regardless of the relative proportion of them), phytoplankton will not be ‘limited’ even when they are growing as fast as they can, and the system is ‘nutrient saturated’.

How is nutrient limitation measured?

Water samples are collected from sample locations (see map below) in the Patuxent, Potomac, Choptank, and mainstem Chesapeake Bay. These water samples are tested using a bioassay. The results of these laboratory tests are used to determine if a sample is:

• Nitrogen limited (excess phosphorus),
• Phosphorus limited (excess nitrogen), or
• Nutrient Saturated (excess phosphorus and nitrogen or inadequate light).
  

Why is determining nutrient limitation important?

Determining what nutrient is limiting phytoplankton growth is important for many reasons. For example, these measurements have been made since August 1990, so we can compare the results over time to track changes due to management efforts or changes in watershed use and inputs. This type of comparison is included in the most recent annual report from the Nutrient Limitation Component (filename: 2002_level1_report.pdf, 11,982 kb).

In several areas, the Chesapeake Bay Monitoring Program has shown that nutrient concentrations in the Bay have been reduced but phytoplankton have not yet responded (e.g. Patuxent Estuary). The nutrient limitation data has shown why the expected response has not yet occurred and given managers confidence that we are still on the right path to restoring the Bay but have not yet reduced nutrients sufficient to limit algal growth.

Determining patterns of nutrient limitation can also be used to define future watershed management needs and priorities, and help determine what management strategies are most likely to have desired effects on controlling excess algal growth and associated negative impacts on the ecosystem.

In addition, the data from the Nutrient Limitation Component was used to develop a predictive model that uses routinely measured water quality components (total nitrogen, total phosphorus, dissolved inorganic nitrogen, dissolved inorganic phosphorus, salinity and water temperature) to estimate the nutrient limitation status for locations where bioassay samples are not collected. This model has been applied to determine annual patterns of nutrient limitation for all of the Maryland DNR Long-term Water and Habitat Quality Monitoring Program sample locations.

• Choptank Basin
• Lower Eastern Shore Basin
• Lower Western Shore Basin
• Patapsco-Back Basin
• Patuxent Basin
• Middle and Lower Potomac Basin
• Upper Eastern Shore Basin
• Upper Western Shore Basin

For more information, please contact Renee Karrh
at (410) 260-8628.

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