Healthy beautiful salt marsh

Great Bay is a place where the ocean and rivers, land and water, and people and nature meet. It lies at the confluence of tidally driven salt water from the Gulf of Maine and fresh water from the Salmon Falls, Cocheco, Bellamy, Oyster, Lamprey, Squamscott, and Winnicut rivers. Before reaching the bay, seawater travels 15 miles inland—a geographic configuration makes Great Bay one of the nation’s most recessed estuaries. It is often referred to as New Hampshire’s “hidden coast.”

The Great Bay Estuary was formed by the melting of the glaciers more than 14,000 years ago. The rising ocean waters flooded the land and filled the river valleys that make up the bay today. Its habitats are extraordinarily diverse: eelgrass meadows, mudflats, salt marshes, oyster reefs, and rocky intertidal places that are home to hundreds of bird, fish and plant species, including 23 that are considered threatened or endangered. The rivers that flow into this Estuary drain a watershed that extends more than 1,000 square miles. The large quantities of water that move in and out of the estuary create some of the strongest tidal currents in North America. This tidal exchange affects water quality, habitats, and the distribution of species.

Because it has such a large tidal exchange and seasonal variation, the salinity in Great Bay can fluctuate quite a bit.  Salinity is measured in parts per thousand, or ppt, but what does that mean exactly?  It is simply a unit of measure that describes the concentration of something, typically in water or soil. A salinity of 1ppt is equivalent to one gram of salt (mostly sodium chloride) per 1000 ml of water.  Spring rains and snow melt bring a large influx of freshwater into the bay from 7 rivers and there are times when the salinity at the waterfront of the Great Bay Discovery Center drops from its more typical salinity of 25-30ppt to less than 12ppt seasonally.  During times of drought, like we are experiencing now, the salinity can be quite high, sometimes as high as ocean salinity which typically measures around 33-35ppt.  This is part of what makes Great Bay, and most estuaries, very unique and diverse.  Plants and animals need to be able to adapt to changing salinities in order to thrive in estuaries long term, so many estuaries have unique plants and animals that you may not see elsewhere.

Why is it important to monitor salinity changes in Great Bay and how do we do that?  “Our monitoring program provides baseline data to help track changes in the environment, identify new threats, and assess the effectiveness of management actions. We support and manage efforts to monitor water quality, habitats, and species throughout the Great Bay Estuary. With our partners, we work to ensure the data we collect is valuable at multiple scales and can be compared to other environmental datasets. We also participate in the NERRS System-Wide Monitoring Program (SWMP), a national effort that engages all Reserves in tracking weather, water quality, and habitat conditions with a standardized approach to data collection, analysis, and management. As a result, our data can be used locally and in regional and national comparisons of estuarine conditions.” (Chris Peter, Research Coordinator).  Just last week Chris was out in the field measuring porewater salinity and ground water levels at several sites around the Bay and salinity was much higher than normal while ground water levels were also abnormally low….a sign of a drought year for sure!  Tracking parameters regularly and monitoring baseline data can help the Reserve uncover potential impacts of climate change and highlight any other trends or problems that may not be apparent without regular monitoring.

Clearly seasonal fluctuations and climate change can have big impacts on salinity, but what about short term changes?  Large storms and flooding events such as the two, 100-year floods in 2006 and 2007, can have huge impacts on the salinity of Great Bay.  WMUR did a great recap of those two flooding events on the 10-year anniversary…..bottom line is that it is extremely unusual to have two, 100-year flooding events within 1 year!  What did these events do to the salinity of Great Bay?  If you go to the NERRS System-Wide Monitoring Program (SWMP) page you can learn a lot about how the Mother’s Day Flood affected Great Bay and you can use this tool to see many different parameters of the Bay.  Below is a graph of how the salinity changed following the flood.  You can make your own graph for any date you choose….

  1. Go to the website
  2. Click on the grasses logo for Great Bay
  3. Choose the station named Great Bay with data type water quality
  4. A small box will pop up on the map-click near the top of that box
  5. Next click on the blue box with the picture of a graph (when you hover over the box it will say ‘graph data’)
  6. Select the parameter you want to measure (such as salinity or turbidity) and the date range and click graph!
  7. For fun try this one….Great Bay NH Water Quality, turbidity for the parameter, Jan 2nd to Aug 19th 2020 for the date range. What can cause turbidity to spike?  What might a spike in turbidity mean for shellfish?