Results for the Patuxent Basin

The nutrient limitation models  were used to predict nutrient limitation for the twelve stations in the Patuxent Basin. Results for each station 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). Overall, the upper river is largely nutrient saturated and seasonal patterns in nutrient limitation are controlled by riverflow (Fisher and Gustafson 2002). The lower river is largely nitrogen limited, probably due to sewage inflows with low Dissolved inorganic nitrogen to dissolved inorganic phosphorus ratios and lower variability in seasonal river flows (Fisher and Gustafson 2002).

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
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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.

US Rt. 50 (TF1.0) – Phytoplankton growth at this station is nutrient saturated (light limited or no limitation) at all times. Total nitrogen concentration is relatively fair and improving (decreasing). Total phosphorus concentration is relatively poor but improving (decreasing). The ratio of total nitrogen to total phosphorus is increasing. Further reductions in nitrogen concentrations will be needed, especially in summer and fall, before nitrogen limitation can occur at this station. Significant reductions in phosphorus will be needed to allow phosphorus limitation to occur in this portion of the Patuxent, but any reductions in phosphorus are important to reduce the amount of phosphorus being exported to areas downstream.

graph showing the Patuxent at US Route 50 TF 1.0graph showing the Patuxent at Western Branch TF 1.2

Western Branch (TF1.2) – Phytoplankton growth at this station is nutrient saturated (light limited or no limitation) at all times. Total nitrogen and dissolved inorganic nitrogen concentrations are relatively good and both are improving (decreasing); total phosphorus and dissolved inorganic phosphorus concentration is relatively good and dissolved inorganic phosphorus is improving (decreasing). Further reductions in nitrogen concentrations will be needed, especially in summer and fall, before nitrogen limitation can occur at this station. Significant reductions in phosphorus will be needed to allow phosphorus limitation to occur in this portion of the Patuxent, but any reductions in phosphorus are important to reduce the amount of phosphorus being exported to areas downstream.

MD Rt. 4 (TF1.3) – Phytoplankton growth at this station is nutrient saturated (light limited or no limitation) at all times. Total and dissolved inorganic nitrogen concentrations are relatively poor but both are improving (decreasing). Total and dissolved inorganic phosphorus concentrations are relatively fair and both are improving (decreasing). The total nitrogen to total phosphorus ratio is decreasing. Further reductions in nitrogen concentrations will be needed, especially in summer and fall, before nitrogen limitation can occur at this station. Significant reductions in phosphorus will be needed to allow phosphorus limitation to occur in this portion of the Patuxent, but any reductions in phosphorus are important to reduce the amount of phosphorus being exported to areas downstream.

graph showing the Patuxent at Maryland Route 4 TF 1.3graph showing the Patuxent at Jackson Landing TF 1.4

Jackson Landing (TF1.4) – Phytoplankton growth at this station is nutrient saturated (light limited or no limitation) more than 90% of the time and nitrogen limited less than 10% of the time. Summer growth is nitrogen limited about 20% of the time, and fall growth is nitrogen limited more than 10% of the time. Total and dissolved inorganic nitrogen and dissolved inorganic phosphorus concentrations are relatively fair and all improving (decreasing). Total phosphorus concentration is relatively poor but improving (decreasing). The ratio dissolved inorganic nitrogen to dissolved inorganic phosphorus is decreasing. The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in the summer, indicating that phosphorus is in excess relative to nitrogen and nitrogen limitation is possible. Further reductions in nitrogen, particularly in the summer and fall, may increase the duration of nitrogen limitation. Significant reductions in phosphorus will be needed to allow phosphorus limitation to occur in this portion of the Patuxent, but any reductions in phosphorus are important to reduce the amount of phosphorus being exported to areas downstream.

Nottingham (TF1.5) – On an annual basis, phytoplankton growth is nutrient saturated (light limited or no limitation) 75% of the time and nitrogen limited 25% of the time. Growth in the summer is nitrogen limited more than 50% of the time. Growth in the fall is nitrogen limited approximately 40% of the time. Total nitrogen concentration is relatively fair and dissolved inorganic nitrogen concentration is relatively good; both are improving (decreasing). Total phosphorus concentration is relatively poor and dissolved inorganic phosphorus concentration is relatively fair; both are improving (decreasing). The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in the summer, indicating that phosphorus is in excess relative to nitrogen and nitrogen limitation should occur. Further reductions in nitrogen, particularly in the summer and fall, would increase the occurrences of nitrogen limitation. Much larger phosphorus reductions would be needed in winter and spring for phosphorus limitation to occur.

graph showing the Patuxent at Nottingham TF 1.5graph showing the Patuxent at Lower Marlboro TF 1.6

Lower Marlboro (TF1.6) – On an annual basis, phytoplankton growth is nutrient saturated (light limited or no limitation) 75% of the time and nitrogen limited 25% of the time. Growth in the summer is nitrogen limited 50% of the time. Growth in the fall is nitrogen limited almost 45% of the time. Total nitrogen concentration is relatively good, dissolved inorganic nitrogen concentration is relatively fair, and total and dissolved inorganic phosphorus concentrations are relatively poor, but all are improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is low in the summer, indicating that phosphorus is in excess relative to nitrogen and nitrogen limitation should occur. Further reductions in nitrogen, particularly in the summer and fall, would increase the occurrences of nitrogen limitation. Much larger phosphorus reductions would be needed in winter and spring for phosphorus limitation to occur.

Above Benedict (TF1.7) – On an annual basis, phytoplankton growth is nutrient saturated (light limited or no limitation) 65% of the time and nitrogen limited 35% of the time. Growth in spring is occasionally nitrogen limited (approximately 5% of the time). Growth in the summer is nitrogen limited 60% of the time. Growth in the fall is nitrogen limited almost 55% of the time. Total nitrogen concentration is relatively good, dissolved inorganic nitrogen concentration is relatively fair, and total and dissolved inorganic phosphorus concentrations are relatively poor, but all are improving (decreasing). The ratio of total nitrogen to total phosphorus is decreasing. The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in summer and fall, indicating that phosphorus is in excess relative to nitrogen and nitrogen limitation should occur. Further reductions in nitrogen, particularly in the spring and fall, would increase the occurrences of nitrogen limitation. Much larger phosphorus reductions would be needed in winter and spring for phosphorus limitation to occur.

graph showing the Patuxent Above Benedict TF 1.7graph showing the Patuxent Below Benedict RET 1.1

Below Benedict (RET1.1) – On an annual basis, phytoplankton growth is nitrogen limited 70% of the time and rarely phosphorus limited (less than 5% of the time). Winter growth is nitrogen limited almost 40% of the time. Spring growth is nitrogen limited almost 60% of the time and phosphorus limited less than 5% of the time. Summer growth is nitrogen limited 95% of the time, and fall growth is nitrogen limited more than 65% of the time. Total nitrogen concentration is relatively fair, dissolved inorganic nitrogen concentration is relatively good, and dissolved inorganic phosphorus concentrations are relatively poor; all are improving (decreasing); total phosphorus concentration is relatively poor. The ratio of total nitrogen to total phosphorus is decreasing. The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in spring and very low in summer and fall, indicating that phosphorus is in excess relative to nitrogen and consistent with the strong nitrogen limitation at this station. Further reductions in nitrogen have the potential for limiting phytoplankton growth in all seasons. Reductions in phosphorus, particularly in the winter and spring, will bring the system into better balance.

Jack Bay (LE1.1) – On an annual basis, phytoplankton growth is nitrogen limited more than 80% of the time and rarely phosphorus limited (less than 5% of the time). Winter growth is nitrogen limited 50% of the time. Spring growth is nitrogen limited almost 80% of the time and phosphorus limited almost 10% of the time. Summer and fall growth is nitrogen limited 95% and 90% of the time, respectively. Total and dissolved inorganic nitrogen concentrations are relatively good and total and dissolved inorganic phosphorus concentrations are relatively fair, and all are improving (decreasing). The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in spring and very low in the summer and fall, consistent with the strong nitrogen limitation at this station. Continued reductions in nitrogen will help increase the occurrences of nitrogen limitation in the winter and spring, and further suppress algal growth throughout the year. Reductions in phosphorus concentrations, particularly in the spring, would help bring the system into better balance and allow for phosphorus limitation of growth as well.

graph showing the Patuxent Jack Bay LE1.1graph showing the Patuxent St. Leonard LE1.2

St. Leonard (LE1.2) – On an annual basis, phytoplankton growth is nitrogen limited more than 85% of the time and phosphorus limited almost 10% of the time. Winter growth is nitrogen limited approximately 55% of the time and phosphorus limited about 5% of the time. Spring growth is nitrogen limited almost 85% of the time and phosphorus limited 10% of the time. Summer growth is nitrogen limited more than 95% of the time and otherwise is phosphorus limited. Fall growth is nitrogen limited 90% of the time and is otherwise phosphorus limited. Total nitrogen and total phosphorus concentrations are relatively good and improving (decreasing); dissolved inorganic nitrogen and dissolved inorganic phosphorus concentrations are relatively fair and improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is decreasing; this is low all year, especially in the summer and fall, consistent with the strong nitrogen limitation at this station. Continued reductions in nitrogen will help increase the occurrences of nitrogen limitation in the winter and spring, and further suppress algal growth throughout the year. Continued reductions in phosphorus concentrations, particularly in the winter and spring, would help bring the system into better balance and allow for increased phosphorus limitation of growth as well.

Above Pt. Patience (LE1.3) – On an annual basis, phytoplankton growth is nitrogen limited almost 85% of the time and phosphorus limited almost 10% of the time. Winter growth is nitrogen almost 65% of the time and otherwise nutrient saturated (light limited or no limitation). Spring growth is nitrogen limited 75% of the time and phosphorus limited 20% of the time. Summer growth is always nitrogen limited. Fall growth is nitrogen limited almost 85% of the time and is otherwise phosphorus limited. Total nitrogen, dissolved inorganic nitrogen, and dissolved inorganic phosphorus concentrations are all relatively good and improving (decreasing); total phosphorus concentration is relatively good. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is decreasing; this ratio is very low, especially in the summer and fall consistent with the strong nitrogen limitation at this station. Continued reductions in nitrogen will help increase the occurrences of nitrogen limitation in the winter and spring, and further suppress algal growth throughout the year. Continued reductions in phosphorus concentrations, particularly in the winter and spring, would help bring the system into better balance and allow for increased phosphorus limitation of growth as well.

graph showing the Patuxent Abvove Pt. Patience LE1.3graph showing the Patuxent Drum Point LE1.4

Drum Point (LE1.4) – On an annual basis, phytoplankton growth is nitrogen limited almost 65% of the time and phosphorus limited more than 20% of the time. Winter growth is nitrogen limited 45% of the time and phosphorus limited more than 5% of the time. Spring growth is phosphorus limited approximately 45% of the time and nitrogen limited approximately 35% of the time. Summer and fall growth is nitrogen limited almost 95% and more than 70% of the time, respectively. Total nitrogen and dissolved inorganic nitrogen concentrations are relatively good and improving (decreasing). Total phosphorus concentration is relatively good; dissolved inorganic phosphorus concentration is relatively fair and improving (decreasing). The ratio of total nitrogen to total phosphorus and the ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus are both decreasing. The dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is low in the summer and fall. Continued reductions in nitrogen will help increase the occurrences of nitrogen limitation in the winter and spring, and further suppress algal growth throughout the year. Reductions in phosphorus would increase phosphorus limitation in the spring.

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

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