Results for the
Upper Eastern Shore Basin

The nutrient limitation model was used to predict nutrient limitation for the ten stations in the Upper Eastern Shore. Results are summarized for the most recent three-year period (2001-2003) by season: winter (December-February), spring (March-May), summer (July-September) and fall (October-November). Managers can use these predictions to assess what management approach will be the most effective for controlling excess phytoplankton growth. Interpreting the results can be a little counter-intuitive, however. Remember that nitrogen limited means that phosphorus is in excess. Initially, it would seem that the best management strategy would be to reduce phosphorus inputs. However, it may actually be more cost effective to further reduce nitrogen inputs to increase the amount of ‘unbalance’ in the relative proportions of nutrients so that phytoplankton growth is even more limited. When used along with other information available from the water quality and watershed management programs, these predictions will allow managers to make more cost-effective management decisions.
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Northeast River (ET1.1) - On an annual basis, phytoplankton growth is nitrogen limited more than 25%, phosphorus limited about 5% and nutrient saturated (light limitation or no limitation) 70% of the time. Winter and spring growth is entirely nutrient saturated. Summer growth is nitrogen limited almost 60% of the time and phosphorus limited more than 10% of the time. Fall growth is nitrogen limited approximately 15% of the time and otherwise is nutrient saturated. Total nitrogen concentration is relatively fair and dissolved inorganic nitrogen concentration is relatively good and improving (decreasing); total and dissolved inorganic phosphorus concentrations are relatively good. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is high in winter and fall, indicating phosphorus limitation could occur. The ratio is low in summer, consistent with the strong nitrogen limitation in that season. Continued reductions in nitrogen concentrations would also further limit growth in the summer, while reductions in phosphorus may allow phosphorus limitation to occur in winter and fall.

C & D Canal (ET2.1) – On an annual basis, phytoplankton growth is phosphorus limited more than 5% of the time, but occurs only in the summer (20% of the summer samples); otherwise growth is nutrient saturated (light limited or no limitation). Total nitrogen concentration is relatively fair but dissolved inorganic nitrogen concentration is relatively poor; both are improving (decreasing). Total phosphorus concentration is relatively fair, but dissolved inorganic phosphorus concentration is relatively poor and degrading (increasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is decreasing. Both nitrogen and phosphorus concentrations will need to be reduced before phytoplankton growth is limited in winter, spring and fall; decreases in phosphorus will further enhance phosphorus limitation in the summer.

graph showing Northeast River et1.1graph showing C&D Canal et2.1

Bohemia River (ET2.2) – On an annual basis, phytoplankton growth is nitrogen limited more than 10% of the time and phosphorus limited 15% of the time. In the winter, growth is always nutrient saturated (light limited or no limitation). In spring, growth is nitrogen limited approximately 10% of the time otherwise is nutrient saturated. In the summer, growth is nitrogen limited more than 25% and phosphorus limited more than 25% of the time. Fall growth is phosphorus limited almost 30% of the time and is otherwise nutrient saturated. Total nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all relatively good; dissolved inorganic nitrogen concentration is fair. Total nitrogen and total phosphorus concentrations are improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is very high in winter, indicating phosphorus limitation could occur with continued reductions in phosphorus; continued reductions in nitrogen would further enhance nitrogen limitation in the summer.

Elk River (ET2.3) – On an annual basis, phytoplankton growth is phosphorus limited almost 10% of the time, but this occurs only in the summer (approximately 30% of the summer samples); otherwise growth is nutrient saturated (light limited or no limitation). Total nitrogen concentration is relatively fair and is improving (decreasing), but dissolved inorganic nitrogen concentration is relatively poor. Total phosphorus concentration is relatively good, but dissolved inorganic phosphorus concentration is relatively poor and degrading (increasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is decreasing; this ratio also suggests that further reductions in phosphorus concentration are needed to allow for phosphorus limitation, especially in the winter. Further reductions in nitrogen concentration are needed to allow for nitrogen limitation, especially in the spring, summer and fall.

graph showing Bohemia River et2.2graph showing Elk River et2.3

Sassafras River (ET3.1) – On an annual basis, phytoplankton growth is nitrogen limited more than 30% and phosphorus limited almost 10% of the time. In the winter, growth is always nutrient saturated (light limited or no limitation). In spring, growth is nitrogen limited more than 25% of the time and is otherwise nutrient saturated. In summer, growth is nitrogen limited more than 65% and phosphorus limited 10% of the time. In fall, growth is nitrogen limited more than 15% of the time and phosphorus limited 25% of the time. Total nitrogen, total phosphorus and dissolved inorganic nitrogen concentrations are all relatively fair and dissolved inorganic phosphorus concentration is relatively good. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is very high in winter, indicating that nitrogen is greatly in excess at this station, and phosphorus limitation could occur. In summer, this ratio is low, indicating nitrogen limitation could occur. Reductions in phosphorus would allow for phosphorus limitation in the winter and spring. Reductions in nitrogen would allow for increased nitrogen limitation, especially in the summer and fall.

Upper Chester River (ET4.1) - On an annual basis, phytoplankton growth is nutrient saturated (light limited or no limitation) more than 85% of the time. Growth is always nutrient saturated in the winter, spring and fall. In the summer, growth is nitrogen limited about 30% of the time and phosphorus limited almost 20% of the time. Total nitrogen, dissolved inorganic nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all relatively poor; total phosphorus concentration is improving (decreasing). The ratio of total nitrogen to total phosphorus and the ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus ratios are increasing. Dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is high in winter because dissolved inorganic nitrogen concentrations are very high, and phosphorus limitation could occur. Further reductions in phosphorus will be needed before spring phytoplankton growth will be phosphorus limited. Reductions in nitrogen will increase the occurrence of nitrogen limitation in the summer, but larger reductions will be needed to cause nitrogen limitation in the fall.

graph showing Sassafras River et3.1graph showing Upper Chester River et4.1

Lower Chester River (ET4.2) – On an annual basis, phytoplankton growth is nitrogen limited more than 35% of the time and phosphorus limited more than 30% of the time. In the winter, growth is nutrient saturated (light limited or no limitation) almost 90% of the time and is otherwise nitrogen limited. In the spring, growth is phosphorus limited more than 75% of the time and nitrogen limited 5% of the time. In the summer, growth is nitrogen limited approximately 70% of the time and phosphorus limited approximately 15% of the time. In the fall, growth is nitrogen limited more than 55% of the time and phosphorus limited almost 30% of the time. Total nitrogen concentration is relatively poor; total phosphorus, dissolved inorganic nitrogen and dissolved inorganic phosphorus concentrations are relatively fair. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is decreasing; this ratio is high in the spring, and is low in summer. Further reductions in phosphorus in the spring will further enhance phosphorus limitation. Reductions in nitrogen will further limit summer and fall phytoplankton growth, and would increase the occurrence of nitrogen limitation in the winter.

graph showing Lower Chester River et4.2graph showing Eastern Bay ee1.1

Eastern Bay (EE1.1) – On an annual basis, phytoplankton growth is nitrogen limited approximately 50% of the time and phosphorus limited more than 25% of the time. In the winter, growth is nitrogen limited 25% of the time and is otherwise nutrient saturated. In the spring, growth is phosphorus limited almost 70% of the time and is nitrogen limited 25% of the time. In the summer, growth is nitrogen limited almost 90% of the time and otherwise is phosphorus limited. In the fall, growth is nitrogen limited more than 60% of the time and phosphorus limited approximately 30% of the time. Total and dissolved inorganic nitrogen concentrations are relatively good and total nitrogen concentration is improving (decreasing). Total and dissolved inorganic phosphorus concentrations are relatively good. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is decreasing. This ratio is relatively low in the summer and fall, consistent with the strong nitrogen limitation at this station. Continued reductions in nitrogen will further limit summer and fall phytoplankton growth, and would increase the occurrence of nitrogen limitation in the winter. Reductions in phosphorus in spring will further enhance phosphorus limitation.

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

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