Nutrient pollution causes a long-term effect on Chesapeake Bay ecosystem

A team of scientists has determined that the growing worldwide problem of increased nutrient pollution, primarily nitrogen and phosphorous, on coastal waterways has altered the ecology of Chesapeake Bay as reported in the most recent issue of Marine Ecology Progress Series.

During the last 50 years, nutrient enrichment has reduced the size of sea grass beds and lowered dissolved oxygen concentrations, both contributing to the degradation of bottom habitats. Excess nutrients can cause large algae blooms which cloud the water. When the algae bloom dies it sinks to the bottom and decays through bacterial processes that rapidly deplete dissolved oxygen. Significant increases in the organic content of 200 year-old sediment suggest stimulation of algae blooms during pre-industrial times. Lead author Dr. W. Michael Kemp, of the University of Maryland Center for Environmental Science, said “By studying long-term effects of nutrient enrichment and detailed processes by which coastal ecosystems have been altered, we will be far better positioned to effect restoration of the estuary’s valuable resources.”

These trends have been made even worse by declines in oyster beds, caused by overfishing and disease. The oyster was considered one of the Bay’s dominant bottom feeders consuming vast amounts of algae. Extensive tidal marshes, which serve as effective nutrient buffers, are now being lost due to rising sea levels thus making the problems of nutrient loading even worse.

While the total fish production in the Bay is not believed to have declined, the type of fish have changed. Fish populations have shifted from species living near the seabed to those living in the upper waters. This shift may lead to food chain inefficiencies that favor production of bacteria and small invertebrates.

Even in some Chesapeake Bay tributaries such as the Potomac and Patuxent Rivers, where significant nutrient pollution reductions have occurred, complex ecological relationships have not been fully restored. For example, while algae blooms decreased in the Patuxent River with decreasing nutrients in the last fifteen years, the bottom water dissolved oxygen levels are still very low. Sea grass beds have not increased even with replanting.

The authors cited that an important component of the Chesapeake restoration effort is to revitalize its habitats. Kemp stated, “Even small increases in sea grass beds, oyster reefs, and tidal marshes may lead to substantial benefits toward recovery from nutrient enrichment due to their secondary effects. These habitats filter nutrients out of the system as well as trap and bind fine sediment particles. They support positive-feedback mechanisms that tend to accelerate Bay restoration once the reversal has been set in motion.”