Too Much of a Good Thing
Fish kills illustrate harmful effects of excess nutrients on Bay ecosystem
Fish kills are often caused by the effects of excess nutrients entering the Bay through rivers and streams. Image credit: Adrian Jones / UMCES
November 2006 -- Nutrients are generally considered to be a good thing for humans because they are necessary for our health and strength. But just like anything else, too many nutrients can be too much of a good thing. In fact, excess nutrients are the main cause of the Bay's degraded water quality and aquatic habitat loss.
Nutrients occur naturally in air, water and soil. However, in addition to these natural sources, vehicle exhaust, treated sewage and runoff from urban, residential and agricultural areas all contribute nutrients to the Bay and its tributaries.
Together, these sources send too many nutrients to the water, causing serious problems:
- Excess phosphorus and nitrogen cause rapid growth of phytoplankton, causing algal blooms that reduce sunlight to bay grasses.
- Algae eventually sink and decompose, which depletes bottom waters of dissolved oxygen (DO). Without enough oxygen, fish and other underwater species die.
This summer, high nutrient levels were responsible for harmful algal blooms and low DO levels that led to fish kills in several Bay tributaries, including:
- in the Potomac River, where more than 8,000 fish died due to a combination of algal toxins and anoxic waters that sloshed into shallow areas due to a high wind event.
- in the Corsica River, where a similar fish kill occurred for the second year in a row. High nutrient levels to the river caused a harmful algal bloom to form. As the algae decomposed, DO levels dropped and 2,000 fish died. In comparison, 30,000-50,000 fish were killed after last year's bloom.
Bay scientists have found that reducing nutrient loads to the Bay in spring is critical to improving water quality conditions. The majority of nutrients are washed into the Bay in spring as snow melts and larger amounts of rain fall on the watershed. Planting cover crops in the fall and skipping spring fertilizer are two important ways people can reduce the amount of nutrients that enter the Bay.