Has Decreased Nitrogen Loading in Long Island Sound Improved Bottom Water Oxygen?

Sound Check is a new feature in which we ask Dr. James Ammerman, LISS’s science coordinator, about an indicator recently updated in the LISS  Status and Trends presentation. In our first installment, Dr. Ammerman looks at nitrogen loads into Long Island Sound. The 2015 data was added to the indicator in March, and shows a 2000-era target to reduce nitrogen is now more than 99 percent completed.

 

Q: Has Decreased Nitrogen Loading in Long Island Sound Improved Bottom Water Oxygen?

Dr. Ammerman: Nitrogen loading from sewage treatment plants to Long Island Sound has decreased by more than 42 million pounds per year since the early 1990s, greater than a 60 percent decline. This decrease is largely due to improvements in nitrogen removal by wastewater treatment plants in both New York and Connecticut and will meet the requirements of the Total Maximum Daily Load (TMDL) first implemented in 2000 by 2017.

One of the major results of excessive nitrogen loading to estuaries and coastal waters is bottom water hypoxia, or decreased oxygen concentrations (<3 mg dissolved oxygen per liter), which can imperil marine life, and which is measured from June to September by the Connecticut Department of Energy and Environmental Protection (CT DEEP) as part of the Long Island Sound Study Water Quality Monitoring Program. Hypoxia occurs because the plankton blooms fueled by the excess nitrogen are degraded by bacteria which use the oxygen in the process. Therefore, decreased nitrogen loading, as seen in Long Island Sound, should result in both a decreased area and intensity of hypoxia.

There is evidence that the area of hypoxia has declined in the last decade, to an area of 38 square miles in 2015, the second smallest since the start of monitoring in 1987. The one exception was 2012, an unusually warm year, when the hypoxic area was 289 square miles, the largest since 2003. In addition, the areas of severe hypoxia (<2 mg dissolved oxygen per liter) and anoxia (<1 mg dissolved oxygen per liter), either of which would likely be deadly to marine life, have also declined over the last decade. Severe hypoxia was zero in 2015, the first time severe hypoxia did not show up in the Sound since CT DEEP began sampling for that measure in 1991. Anoxia in Long Island Sound has been zero for the past three years.

So far, there is not a decrease in chlorophyll a concentrations in Western Long Island Sound, which would be expected as a result of the decrease in nitrogen loading.  However, the chlorophyll data is limited and may not tell a complete story. We are looking forward to this year’s monitoring data to determine whether these positive trends continue.

note: the links go to the indicator web pages on the LISS Status and Trends presentation.