The NBC initiated nutrient monitoring of the local urban rivers and the upper Bay (Providence and Seekonk Rivers) in 2005, and expanded the sampling locations in 2006. Nutrients can be a problem when they are in excess in the water column. An excessive amount of nutrients can lead to large phytoplankton blooms; when the phytoplankton die, this can cause the depletion of oxygen within a given body of water. Given a significant enough bloom with the right environmental conditions, low oxygen (hypoxia) or no oxygen (anoxia) can develop and persist. These low oxygen conditions impose a significant amount of stress on marine organisms, and can result in reduced reproductive capacity, smaller growth rates, and, in the worst case, mortality.
The NBC currently samples fifteen river stations (see map below) one to two times per month, depending on the station location. These sample locations were chosen to determine the amount of nutrient loading coming into the upper Bay from all river sources. This includes rivers that enter the upper Bay from Massachusetts as well as those that are located within Rhode Island.
NBC river nutrient samples are collected from just below the surface. All samples are collected and filtered at the sample station. A grab sample is taken, filtered through a 0.45 micrometer filter and put on ice. The NBC laboratory analyzes these freshwater nutrient samples for nitrate/nitrite, nitrite, total dissolved nitrogen, ammonia, orthophosphate, and silicate. All results are measurements of the dissolved (or soluble) phase. Grab samples for chlorophyll and TSS* (total suspended solids) are also taken at the same time as nutrient samples and analyzed by the NBC laboratory.
River Nutrient Results: What have we found to date?
These monitoring data have shown that the rivers are a large source of nutrients to the Bay. Data have shown that the river with the highest average nutrient loading is the Blackstone River, which enters the Seekonk River estuary. Rivers become a larger contributor to total nitrogen loading during wet weather when NBC data indicate that nutrient loading from the rivers increases by 88 to 152% over dry weather levels on average. This means that weather can have a significant effect on the level of nutrients entering the upper Bay and which sources have the most impact. The chart below shows total dissolved nitrogen (TDN) loading from several rivers and wastewater treatment facilities (WWTFs) along with the total rainfall for the sample date plus four days prior. This shows the dramatic difference in loading during dry and wet periods.
The NBC samples six bay stations (see map below) two times per month, depending on weather (inclement weather can prohibit access or may be a safety concern). Twice per month samples are taken at the surface (0.5-1 meter below the surface) and once per month samples are taken at the bottom (approximately 0.5-1 meter above the sediment). These water column nutrient measurements provide an important look at the amount of nutrients in the upper Bay from all sources, including river loading, surrounding WWTFs, atmospheric deposition, groundwater, runoff, leaky septic systems and nutrients from the middle and lower Bay area as well as from offshore.
As with the river nutrient samples, all bay samples are collected, filtered, and preserved at the sample station. A grab sample is taken, filtered through a 0.45 micrometer filter, a chemical preservative is added for ammonia, then all sample bottles are put on ice. The NBC laboratory analyzes both freshwater and saltwater nutrient samples for nitrate/nitrite, nitrite, total dissolved nitrogen, ammonia, orthophosphate, and silicate. All results are measurements of the dissolved (or soluble) phase. Grab samples for chlorophyll and TSS* (total suspended solids) are also taken at the same time as nutrient samples and analyzed by the NBC laboratory.
Bay Nutrient Results: What have we found to date?
According to EPA criteria as defined in their 2012 National Coastal Condition Report, 2014 summer average nutrient concentrations in two out of the six Bay nutrient stations fall into the “good” category, while the concentration remainder of the Bay stations falls into the “fair” category. (See map below)
* After a quality assurance/quality control review, the NBC found their TSS data was compromised. The NBC is currently evaluating their procedures. Past TSS data, historically found in the nutrients spreadsheets below, has been removed from the website. Once the issues are resolved, the NBC will resume TSS sampling and analysis and TSS data will be added to the nutrients spreadsheets.
2005 NBC River and Bay Nutrients Data
2006 NBC River and Bay Nutrients Data
2007 NBC River and Bay Nutrients Data
2008 NBC River and Bay Nutrients Data
2009 NBC River and Bay Nutrients Data
2010 NBC River and Bay Nutrients Data
2011 NBC River and Bay Nutrients Data