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Nitrogen vital for health of bay - February 14, 2013

When we think about excess nitrogen in our coastal bays, many of us think about N from chemistry and that is it. What many of us overlook is that these important nutrients have their own cycles, where they are transformed and transported in our local watershed. N2, NH3, NO2 — and NO3 — are all various forms in the nitrogen cycle; nitrogen gas, ammonia, nitrite and nitrate respectively.

Due to numerous forms of nitrogen coupled with a multitude of sources, both anthropogenic and natural, the topic of excess nutrients in our waterways is a complex and inter-related problem.

Just like water and carbon, nitrogen has its own cycle that is vital for a healthy ecosystem. In our marine ecosystem, nitrogen plays a major role in keeping our seagrasses and animals healthy, along with maintaining proper water and soil chemistry.

Nitrogen plays an especially important role in keeping our bays clean and healthy. Nitrogen is needed by seagrasses to grow and is needed by animals for protein, amino and nucleic acid production. While all living things need nitrogen for survival, there is a difference between the forms that each organism can use.

Most nitrogen is stored in the atmosphere as gas. However, this is unusable for most organisms. Bacteria in the soil, and some in legume plants such as peanuts, fix atmospheric nitrogen into ammonium or usable nitrates. Ammonium is then turned into nitrite and finally nitrate via several more bacterial processes and is finally in a form fit for use by plants.

Plants uptake nitrate because it is soluble and less toxic compared to ammonium. Animals then eat the plants or other organic matter containing the nitrate in order to receive the nitrogen they need. Eventually when plants and animals die, decomposers turn the used nitrate back into ammonium for the process to start over again.

The nitrogen cycle is extremely vital because it keeps levels of harmful forms of nitrogen, such as ammonium, down, while constantly creating forms such as nitrate that can be used by plants and animals.

The coastal bays have a serious problem with excess nitrogen in our systems and this has had a dramatic effect on our watershed’s health. By adding nitrogen through fertilizers, which then gets into the bays via runoff, and through groundwater contaminated by septic systems, we have incorporated so much excess nitrogen that we can see a change in water quality and biodiversity.

When nitrates and other forms of nitrogen are added to our bays in large quantities, it causes an imbalance in the nitrogen cycle. This imbalance has played a major role in the disappearance of seagrasses. When there is an overabundance of nitrates in the water column, this creates an opportunity for epiphyte growth — non-parasitic plants that grow on other plants — on seagrasses along with an explosion of macro algae, which can grow faster than the grasses and can exploit the higher levels of nitrate.

This creates a difficult environment for seagrasses, so they start to die off. Meanwhile, phytoplankton grow in large quantities because they can exploit the excess nitrogen the best, and eventually the entire system changes from depending on seagrasses for primary production to relying on macro algae and phytoplankton. This in turn reduces water clarity and eliminates habitat for fish and crustaceans we commonly associate with our area.

When phytoplankton bloom, especially during the warmer summer months, they use up a lot of dissolved oxygen (DO), which is needed by all aquatic wildlife to survive.

Additionally, the phytoplankton that bloom can sometimes be toxic. Blooms of dinoflagellates and certain species of brown algae can lead to large fish kills and even health risks for humans entering toxic waters or consuming shellfish from them.

The only way to relieve this problem is to decrease the amount of nitrates and ammonium being added to our bays and increase the rate of denitrification, the process of transforming nitrate back into unusable nitrogen gas.

Unfortunately denitrification is complicated and only done by certain bacteria, which means our best bet is to curb our use of nitrogen rich fertilizers, try to prevent groundwater contamination and protect marshes which help retain excess nitrogen among other benefits. Even if we reduced our input of nitrogen today, we would still see an increase in nitrogen levels in the bay for many years due to the large amount in the groundwater reservoirs, which are slow moving and can take decades to respond to our efforts.

By understanding the roles of the various forms that nitrogen takes during its cycle through our system, we can hopefully start to mitigate the damage that we are doing to our environment by taking specific and planned action.
 

Harrison Jackson is the coastal stewards coordinator for the Maryland Coastal Bays Program.


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