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.
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.