Results for the
Upper Western Shore Basin

The nutrient limitation models were used to predict resource limitation for the three stations in the Upper Western Shore Basin. 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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Bush River (WT1.1) - On an annual basis, phytoplankton growth is nitrogen limited 20% of the time and phosphorus limited 25% of the time. Winter growth is entirely nutrient saturated (light limited or no limitation). Spring growth is nitrogen limited and phosphorus limited more than 15% of the time each, and nutrient saturated the remainder of the time. Summer growth is nitrogen limited about 45% of the time and phosphorus limited almost 20% of the time. Fall growth is phosphorus limited 75% of the time and otherwise nutrient saturated. Relative status of total nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all good; dissolved inorganic nitrogen concentration is relatively fair and is improving (decreasing). Dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is decreasing; still, this ratio is high in winter, spring and fall, indicating that decreases in phosphorus will help limit phytoplankton growth. Further reductions in nitrogen concentration will enhance nitrogen limitation throughout the year.

graph showing Bush River wt1.1graph showing Gunpowder River wt2.1

Gunpowder River (WT2.1) - On an annual basis, phytoplankton growth is nitrogen limited more than 20% of the time and phosphorus limited 40% of the time. Winter growth is entirely nutrient saturated (light limited or no limitation). Spring growth is phosphorus limited almost 30% of the time and nitrogen limited almost 15% of the time. Summer growth is nitrogen limited about 35% of the time and phosphorus limited almost 45% of the time. Fall growth is nitrogen limited 25% of the time and phosphorus limited 75% of the time. Relative status of total nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all good; dissolved inorganic nitrogen concentration is relatively fair. Total nitrogen concentration is improving (decreasing). Dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio is decreasing; this ratio is high in fall. Reductions in both nitrogen and phosphorus have the potential to limit algal growth locally in the Gunpowder River.

graph showing Middle River wt3.1

Middle River (WT3.1) – On an annual basis, phytoplankton growth is nitrogen limited almost 25% of the time and phosphorus limited more than 35% of the time. Growth in the winter is nutrient saturated (light limited or no limitation). Growth in the spring is phosphorus limited 45% of the time and nitrogen limited 15% of the time. Growth in the summer is nitrogen limited almost 60% of the time and phosphorus limited almost 35% of the time. Growth in the fall is phosphorus limited almost 70% of the time and nitrogen limited more than 50% of the time. Total nitrogen, dissolved inorganic nitrogen, total phosphorus and dissolved inorganic phosphorus concentrations are all relatively good and total nitrogen and total phosphorus concentrations are improving (decreasing). The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus is high in winter and spring, indicating that continued reductions in phosphorus, especially in the spring, have the potential to better limit phytoplankton growth locally in the Middle River. Continued reductions in nitrogen in summer and fall will also likely help further limit phytoplankton growth in these seasons.

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

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