How often does dissolved oxygen (DO) in the estuary fall below 5 mg/L?
Datasondes, an automated water quality sensor or probe, in the bays and open waters located at the center of the Great Bay and in Portsmouth Harbor at the Coastal Marine Laboratory indicate dissolved oxygen levels well above 5 mg/L. Low dissolved oxygen events occur in all the tidal rivers. In August 2015–the most recent year we have data–most low dissolved oxygen events in the tidal rivers lasted between two and six hours.
The tidal portions of the major tributary rivers continue to experience many days when the minimum DO concentration value is below 5 mg/L. No long-term trends are notable at any stations, as exemplified by the data from the Salmon Falls River and Squamscott River datascondes (Figures 7.1 and 7.2). These datasondes were used in this long-term trend analysis because they had complete datasets going back as far as 2004, and because they represent different parts of the estuary.
It is important to note not only the number of low DO events but also the duration of those events because there are implications for organisms (such as small invertebrates in the sediment) that cannot move quickly to areas with higher DO levels. In 2015, the Lamprey and Squamscott Rivers had the highest number of low DO events, the majority of which took place in August and September. Figure 7.3 shows data taken every 15 minutes throughout August 2015 for the Squamscott River; this figure indicates that DO concentrations fell below 5 mg/L most days during the month, and that there was less than 5 mg/L for 12% of the month. These low DO events lasted anywhere from one to four hours.
In August 2015, 73% of the time Lamprey River DO levels were below 5 mg/L and stayed below the threshold for more than 24 hours on two occasions (Figure 7.4) with the second occasion lasting almost 168 hours (7 days). A 2005 study39 of the Lamprey River concluded that the datasonde readings were reflective of river conditions, but that density stratification—when salt water and fresh water stack in layers without mixing—was a significant factor in the low DO conditions in the Lamprey River.
In August 2015, the Oyster River experienced four low DO events, lasting between two and six hours each. The Salmon Falls River experienced two low DO events, each lasting approximately three hours. In the Cocheco River, data was only available for the month of September 2015. In that month, the datasonde indicates 12 low DO events, all lasting approximately two hours. More data and analysis is required to understand the relative importance of temperature, tidal stage, time of day, freshwater inputs, organic matter loading and nutrient loading as contributing factors to these low DO events.
Finally, this analysis does not include all DO data collected in the Great Bay Estuary. For information on other data, please see the 2017 Technical Support Document for Aquatic Life Use Support from NHDES.40