Eelgrass

How many acres of eelgrass are currently present in the Great Bay Estuary and how has it changed over time?

The Great Bay Estuary, which includes seven tidal tributary rivers, the Piscataqua River and Portsmouth Harbor had 1,625 acres of eelgrass in 2016, which is 54% of the PREP goal of 2,900 acres. In Great Bay proper, there were 1,490 acres of eelgrass, which is a 31% reduction from 1981, the first year that data was collected. Over time, eelgrass habitat indicates a diminishing ability to recover from periodic disturbances, such as stress from extreme storms.


The long leaves of eelgrass (Zostera marina) slow the flow of water, encouraging suspended materials to settle, thereby promoting water clarity. Eelgrass roots stabilize sediments and both the roots and leaves take up nutrients from sediments and the water. Eelgrass provides habitat for fish and shellfish, and it produces significant amounts of organic matter for the larger food web.
Increase eelgrass distribution to 2,900 acres and restore connectivity of eelgrass beds throughout the Great Bay Estuary by 2020.
In 2016, there were 1,625 acres of eelgrass in the Great Bay Estuary. Figure 8.1 shows a statistically significant decreasing trend in eelgrass acreage since 1996 when the data became available for the entire estuary. The year 1996 also represents the highest amount of eelgrass on record for the Great Bay Estuary; this must be considered when evaluating the trend. Figure 8.2 compares 2016 eelgrass coverage with the acreage of eelgrass in 1996.

For Great Bay only, in contrast, data exists going back to 1981 (Figure 8.3). In 2016, there were 1,490 acres of eelgrass in Great Bay. The trend is not statistically significant; however, there is broad scientific consensus that eelgrass in the Great Bay shows a consistent pattern of being less and less able to rebound from episodic stresses. Current levels of eelgrass in the Great Bay are 31% reduced from 1981 levels. Connectivity of the remaining eelgrass habitat in the Great Bay Esutary is critical for habitat health and expansion. See figure 8.2 for 2016 eelgrass distribution.

In Portsmouth Harbor (Figure 8.4), there were 87.4 acres of eelgrass in 2016. The entire time series (1996-2016) shows a statistically significant decreasing trend. On a positive note, the number of acres in 2016 was higher than the previous 8 years.

The causes of eelgrass decline in the Great Bay continue to be the subject of great interest. Worldwide, the main causes of temperate (between the tropics and the polar regions) seagrass loss are nutrient loading, sediment deposition, sea-level rise, high temperature, introduced species, biological disturbance (e.g., from crabs and geese), and wasting disease.41 Toxic contaminants such as herbicides that are used on land can also stress eelgrass.42 All of these causes are plausible in the Great Bay Estuary and many magnify each other to stress eelgrass and make habitats less resilient. Proactive actions to increase resilience for eelgrass habitat are critical as climate science predicts an increase of stressful events, such as extreme storms with increased rains and higher winds. Since the 1930’s there have been three 100-year storms recorded by measurements of the river discharge at the Lamprey River – two of those storms occurred in 2006 and 2007, the third was in 1987. Increased rainfall during these events causes a large quantity of water flow to enter the estuary delivering increased sediments and nutrients as well as resuspending sediments throughout the water column. Since eelgrass relies on clear water to grow these events are important to note.

Research and discussions continue to focus on the type of recovery Great Bay Estuary can expect for eelgrass. In some cases, recovery requires only a decrease in the stressors that caused the problem. In other cases, conditions for recovery have to be better than conditions before the habitat loss began to occur.43 Figure 8.3 shows that eelgrass recovered after the wasting disease event of 1988-1989. After a drop in 2002-2003, eelgrass rebounded, but not quite to previous levels. Another three year downturn during 2006-2008 was followed by a weaker recovery.

Figure 8.1 Eelgrass cover in the Great Bay Estuary.

Figure 8.2 Map of eelgrass cover for 1996 and 2016. Map based on 2016 data from Kappa Mapping, Inc. and 1996 data provided by the UNH Jackson Estuarine Laboratory. To be counted as present, eelgrass must cover at least 10% of a given area. Therefore, this map does not distinguish between areas with dense versus sparse cover. With negligible exceptions, the 2016 areas also existed in 1996; the darker shade of green therefore represents areas that have been lost since 1996.

Figure 8.3 Eelgrass cover in the Great Bay only (not entire Great Bay Estuary). Missing data for years 1982-1985. Years 1988 and 1989 show very low values due to eelgrass wasting disease event. These data, however, are still included in linear regression calculations.

Figure 8.4 Eelgrass cover in Portsmouth Harbor. Linear regression showing a statistically significant trend.