Exploring Ecosystem Response After Nitrogen Is Reduced by 50 Percent: A Case Study

After a major fish kill in Narragansett Bay, Rhode Island, a plan was put in place to reduce nitrogen (N) inputs from wastewater treatment facilities located in the upper portions of the bay. This plan, enacted in 2004, called for a 50% reduction of N inputs from 1995/96 levels. These reductions focused on the summer nitrogen loading, as they strongly affect the hypoxic conditions of Upper Narragansett Bay. Before treatment plants were upgraded to achieve nitrogen reduction, they contributed approximately 78% of the annual average daily loading of total nitrogen to Upper Narragansett Bay. By 2013, the 50% reduction goal was met.

Simultaneously, the need for more data in Upper Narragansett Bay was recognized, and a fixed-site monitoring network expanded to accommodate. Prior to this point, most of the long-term data were collected in areas of Narragansett Bay which do not experience frequent hypoxia. We now have decade-long measurements from a bay-wide fixed-site network, and multiple assessments of water quality parameters (including, but not limited to, dissolved oxygen, chlorophyll, water clarity), habitat and community data (seagrasses, benthos, and salt marshes) with sampling locations in the most impacted regions of the bay.

Ecosystem response to the 50% reduction is being assessed. This presentation will report on results from multiple efforts. One effort found that the decrease of primary production has increased water clarity and that dissolved oxygen levels have improved but that freshwater stratification from wet summers continues to cause hypoxia in areas of the Bay. Another effort is the development of the State of the Watershed report by the Narragansett Bay Estuary Program, which compiles data and studies from partners (research and government scientists) who study Narragansett Bay. These two efforts will show a holistic view of the ecosystem and preliminary responses.

Nutrient reductions of this magnitude were a grand experiment on a shifting foundation. Evidence points to improving water quality, but weather (i.e. wet/dry years), and climate (i.e. long-term water temperature increases) may play a role and may explain mixed results of reduced fish biomass and the winter/spring bloom. Assessment of this experiment is ongoing, and will continue for years to come. Each piece of data or new information gives us opportunities to ask more questions and tease apart the effects of nutrient reductions from other factors, like weather and climate.