Wet winter-spring leads to periods of above normal river flow: stage set for poor dissolved oxygen conditions
Nutrient loading related to river flow
The Susquehanna River is the single largest source of nutrients (nitrogen and phosphorus) discharging into Chesapeake Bay. These nutrients have a significant effect on Bay health, in particular parameters such as dissolved oxygen and harmful algal blooms. Susquehanna River flows during the first five months of 2008 were characterized by large peak flows in January, February, and March. During these peaks, mean daily flow rates were significantly higher than the normal range. While Maryland experienced a very wet month of May (5th wettest on record), only a small fraction of the Susquehanna River watershed lies within Maryland and hence this rain does not appear to have had a significant effect on Susquehanna River flow rates.
Susquehanna River daily mean flow rates for January through June 2008. Flow data supplied by USGS and are still provisional.
Provisional Susquehanna River total nitrogen loads (fall-line) for the January to May 2008 period were the 6th highest since 1985. The high nitrogen loads were likely caused by the peak flows occurring between January and March. The high nitrogen load estimates are the reason anoxic volume is predicted to be relatively large this summer.
Susquehanna River total nitrogen loads (fall-line) for the period of January to May for each year since 1985. Provisional data supplied by USGS.
The forecasts do not account for unseasonable late spring or summer conditions, but represent the best available prediction based on past and present conditions.
This summer it is predicted that:
Anoxic condition (no dissolved oxygen) in the Bay's mainstem will be poor, with the average summer anoxic volume forecast to be 1.7 ± 1.27 km3. This year there is a large degree of uncertainty in the forecast due to limitations in the regression model at higher nutrient loads. Compared to the previous 23 summers, 2008 could have the 5th largest anoxic volume if this prediction holds true.
The average volume of hypoxic water (Dissolved Oxygen ≤2 mg·L‑1) in Chesapeake Bay this July is predicted to be 9.9 km3, with a 67% probability that the hypoxic volume will be between 7.7 and 12.3 km3. Compared to past years (1985 to 2007), this July is expected to have the sixth largest hypoxic volume. This forecast is based on a model that was developed to assess the impacts of changes in nitrogen loads on Chesapeake Bay hypoxia (Scavia et al 2006).
Low to moderate severity Microcystis blooms are forecast to occur in the tidal Potomac River this summer; 9% ± 2% of samples collected over the summer are predicted to have levels that are considered to be "bloom" concentrations (>10,000 cells·mL‑1). Typical sample collection during summer for the affected region is 72 samples. If this prediction holds true, this summer would be ranked as the 8th best (or 11th worst) among the last 18 years of monitoring.
Prorocentrum blooms will occur in the lower reaches of the Patuxent River in late June to early July, and within the Lower Choptank River in mid to late June. At this stage, no blooms are predicted to occur in the late summer and fall period in the forecast areas.