Results for the Choptank Basin

The nutrient limitation model was used to predict nutrient limitation for the four stations in the Choptank 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). 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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Middle Choptank (ET5.1)
- Phytoplankton growth is entirely nutrient saturated (light limited or no limitation) at this station. This pattern is typical of turbid, nutrient enriched areas where nutrient limitation occurs primarily in the warmer/low riverflow periods (Fisher and Gustafson 2002). Total nitrogen, dissolved inorganic nitrogen and dissolved inorganic phosphorus concentrations are poor; dissolved organic nitrogen concentrations are degrading (increasing). Total phosphorus concentration is fair and improving (decreasing). Total nitrogen to total phosphorus ratio and dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio are also both increasing. Reductions in nitrogen concentrations in the summer and fall may allow for nitrogen limitation during that portion of the year, but reductions in phosphorus are necessary in the winter and spring to reduce phytoplankton growth when dissolved nitrogen concentration is at the annual high.

graph showing upper Choptank River et 5.1graph showing upper Choptank River et 5.2

Lower Choptank (ET5.2) - On an annual basis, phytoplankton growth is nitrogen limited about 40% of the time and phosphorus limited about 35% of the time. In the winter, phytoplankton growth is nutrient saturated (light limited or no limitation) 50% of the time, and nitrogen and phosphorus limited almost 40% and about 10% of the time, respectively. In the spring growth is phosphorus limited almost 65% of the time and nitrogen limited approximately 15% of the time. In the summer, growth is nitrogen limited almost 60% of the time and phosphorus limited 30% of the time. In the fall, phytoplankton growth at this location is nitrogen limited more than 65% of the time and phosphorus limited more than 15% of the time. This is typical of mesohaline areas, where river discharge dictates a seasonal pattern of limitation due to low light/temperatures in winter, high dissolved inorganic nitrogen/dissolved inorganic phosphorus in high river discharge in the spring, large fluxes of dissolved inorganic phosphorus fluxes from the sediments under anoxic conditions in the summer, and turnover of the water column in the fall (Fisher and Gustafson 2002). Ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus are high in winter and low in summer and fall. Total nitrogen and dissolved inorganic nitrogen concentrations are relatively poor and dissolved inorganic nitrogen concentration is degrading (increasing); total phosphorus and dissolved inorganic phosphorus concentrations are relatively fair and dissolved inorganic phosphorus concentration is degrading (increasing). This suggests that reductions in both nitrogen and phosphorus would reduce phytoplankton growth and removing even a little additional phosphorus in the spring may help to further limit phytoplankton growth in that key season.

Outer Choptank (EE2.1) - On an annual basis, phytoplankton growth is nitrogen limited 40% of the time and phosphorus limited more than 30% of the time. In winter, phytoplankton growth is nutrient saturated (light limited or no limitation) almost 90% of the time and is otherwise nitrogen limited. In the spring, phytoplankton growth is phosphorus limited approximately 70% of the time and nitrogen limited more than 15% of the time. In summer, growth is nitrogen limited 80% of the time and phosphorus limited 20% of the time. Fall growth is nitrogen limited 60% of the time and phosphorus limited 40% of the time. This is typical of mesohaline areas, where river discharge dictates a seasonal pattern of limitation due to low light/temperatures in winter, high dissolved inorganic nitrogen/dissolved inorganic phosphorus in high river discharge in the spring, large fluxes of dissolved inorganic phosphorus fluxes from the sediments under anoxic conditions in the summer, and turnover of the water column in the fall (Fisher and Gustafson 2002). Dissolved inorganic nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are relatively good total nitrogen concentrations are relatively fair; dissolved inorganic nitrogen and total phosphorus concentrations are also improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is also decreasing. This ratio is currently low in summer and fall, which leads to the large amount of nitrogen limitation that occurs in these seasons. Further reductions in phosphorus in the winter and spring will increase the occurrences of phosphorus limitation and further limit phytoplankton growth. Further reductions in nitrogen will also help bring the system into better balance and further limit phytoplankton growth in the summer and fall.

graph showing outer Choptank River ee2.1graph showing little Choptank River ee2.2

Little Choptank (EE2.2) - On an annual basis, phytoplankton growth is nitrogen limited about 45% of the time and phosphorus limited about 30% of the time. In winter, phytoplankton growth is nitrogen limited (about 20%) or nutrient saturated (light limited or no limitation, 70% of the time). In the spring, phytoplankton growth is phosphorus limited approximately 70% of the time and nitrogen limited more than 10% of the time. In summer, growth is nitrogen limited more than 80% of the time and phosphorus limited almost 20% of the time. In the fall, growth is nitrogen limited more than 65% of the time and phosphorus limited almost 35% of the time. This is typical of mesohaline areas, where river discharge dictates a seasonal pattern of limitation due to low light/temperatures in winter, high dissolved inorganic nitrogen/dissolved inorganic phosphorus in high river discharge in the spring, large fluxes of dissolved inorganic phosphorus fluxes from the sediments under anoxic conditions in the summer, and turnover of the water column in the fall (Fisher and Gustafson 2002). Total nitrogen, dissolved inorganic nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all good; total nitrogen, dissolved inorganic nitrogen and total phosphorus concentrations are improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is also decreasing. This ratio is low in summer and in summer and fall and moderately low in winter, which is leads to the large amount of nitrogen limitation that occurs in these seasons. Further reductions in phosphorus in the winter and spring will increase the occurrences of phosphorus limitation and further limit phytoplankton growth. Further reductions in nitrogen will also help bring the system into better balance and further limit phytoplankton growth in the summer and fall.

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

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