How have phytoplankton concentrations changed over time?
Chlorophyll-a concentrations—an accepted proxy for phytoplankton biomass—show no statistically significant trends at the eight stations sampled in the Great Bay Estuary. The chlorophyll-a (chl a) levels recorded in the Great Bay Estuary are often within ranges considered “good” or “fair” in the peer-reviewed literature. Periodically, however, chl a levels increase to levels considered “poor.”
All of the data reported below were collected at low tide, when daily concentrations of chl a tend to be highest. None of the eight stations sampled on a monthly basis show a statistically significant trend (Figure 5.1). At Adams Point (Figure 5.2), between 2012 and 2015, median chl a levels ranged from 2.9 to 4.0 ug/L and maximum values ranged from 5.7 to 25.2 ug/L. At the Great Bay station (Figure 5.3), between 2012 and 2015, median levels ranged from 2.9 to 8.3 ug/L and maximum values ranged from 8.4 to 22.1 ug/L.
The Chapman’s Landing station indicated the highest levels of chl a. Since 2012, median levels ranged from 4.8 to 6.9 ug/L and maximum levels ranged from 18.3 to 71.7 ug/L. At the Lamprey River station, median levels ranged from 1.4 to 4.6 ug/L and maximum levels ranged from 2.1 to 21.0 ug/L. At the Upper Piscataqua River Station, median levels ranged from 2.1 to 3.2 ug/L with maximum levels from 4.1 to 24.5 ug/L. Note that 2012 was the only year that levels rose above 20 ug/L for this station. chl a levels at the remaining three stations (Squamscott River, Oyster River and Coastal Marine Laboratory) did not exceed 12 ug/L between 2012 and 2015.
Other parts of the Great Bay Estuary—in addition to the eight stations reported here—also show counts in excess of 20 ug/L. For example, Little Bay registered 25.2 ug/L in 2014 and the Cocheco River indicated a maximum of 28.9 ug/L in 2015.32