Phytoplankton

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.”


Phytoplankton convert the sun’s energy into biomass and are a key part of the food web. Phytoplankton can impact water clarity and compete with eelgrass and seaweeds for available light. Additionally, when large populations of phytoplankton die, their decomposition consumes the dissolved oxygen needed by fish and benthic invertebrates.
No increasing trends for phytoplankton.
National assessments note that less than 5 ug/L chlorophyll-a (chl a) is considered “good;” between 5 and 20 ug/L is considered “fair” and above 20 ug/L is considered “poor.”30, 31 For the years 2012 to 2015, monthly sampling results suggest that, much of the time, chl a levels in the Great Bay Estuary were within ranges regarded as “good” or “fair”, but that they sometimes exceeded 20 ug/L. As noted in Figure 5.1, changes since the last reporting period (2009–2011) vary, depending on the sampling station.

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

Figure 5.1 Reporting average concentrations by sampling station.

Figure 5.2 Chlorophyll-a concentrations at Adams Point. Box and whisker chart of data collected at low tide only. The horizontal line in each box is the median. Boxes encompass the middle 50% of the data points. Upper and lower vertical lines show the complete range of data values. Levels between the blue and the black line are considered “fair.” Levels above the black line are considered “poor.”

Figure 5.3 Chlorophyll-a concentrations at Great Bay. Box and whisker chart of low tide only. The horizontal line in each box is the median. Boxes encompass the middle 50% of the data points. Upper and lower vertical lines show the complete range of data values. Levels between the blue line and the black line are considered “fair.” Levels above the black line are considered “poor.”