On January 4, 2018, which coincided with a perigean spring high tide (Perigean tides occur when the moon is either new or full and closest to Earth), a single storm moved ice onto an area of salt marsh in Massachusetts from nearby creeks and bays, carrying the equivalent of more than 15 years’ worth of sediment onto the marshes. Also, prior to the event, there was a prolonged cold snap that lasted for more than a week when temperatures never climbed above 32 degrees.
In response to this event, researchers at Boston University are using high resolution aerial photography to investigate how important coastal ecosystems like salt marshes are responding to climate change. The study focused on that area of salt marsh in Massachusetts. Using high resolution aerial photography from Bluesky, researchers were able to examine the deposits in detail recording measurements of deposit’s distribution and total cover.
Bluesky recently completed a second survey of the site to map depressions in the marsh surface, called potholes, which demonstrate how the salt marsh is responding to rising sea level.
Salt marshes maintain elevation with rising sea level, in part by trapping mud and sand. The storm deposit was a consequence of prolonged ice formation followed by a strong nor-easter coinciding with a very high tide. While the temperatures were low for an extended period of time, ice formed in shallow water, and mud and sand froze to its base at low tide. The cold spell combined with the very low tides meant that the ice was especially thick and full of sediment when the storm moved in onto the marsh.
Salt marshes are among the most productive ecosystems providing a range of benefits including carbon sequestration, wildlife habitat and protection from storm surges and flooding. Coastal wetlands are progressively more at risk from the effects of climate change such as increasing carbon dioxide levels and rising temperatures. However, the primary impact is from rising sea level, which salt marshes are extremely vulnerable to due to their low, flat elevation.
Bluesky Geospatial undertook the original survey of the Parker River Wildlife Refuge in Newburyport, Massachusetts following a period of extended low temperatures, which caused prolonged ice formation, followed by strong winds and an extremely high tide. 300 exposures were captured using a large format, digital aerial photogrammetric camera which were processed to produce seamless coverage of the 130 km2 site at 10 centimeter resolution. The follow up survey was undertaken during early April 2021. The Bluesky aerial photomaps are shared with Boston University for analysis using the ArcMap Geographical Information System (GIS).
Professor Duncan M. FitzGerald of the Department of Earth and Environment at Boston University said, “We used the aerial photography to map the ice-rafted sediment that was deposited on the marsh surface. Some these ice rafts were the size of cars, whereas others coalesced into are that size of football fields.”
“Our major finding was that storm event such as this one deposits as much sediment on the marsh surface as 15 years of normal marsh deposition,” said Professor Zoe Hughes of the Department of Earth and Environment at Boston University.
“We had Bluesky fly a new mission this past spring, which we’re using to map detailed changes to the marsh surface as a consequence of accelerating sea level rise and marsh platform transition from high to low marsh,” said FitzGerald. “They provide really high resolution imagery that is required for this type of research.”
“Many scientists believe that salt marshes possess resilience to low level rates of sea level rise because they are able to boost plant productivity with increasing water depth, adding to organic accretion. Likewise increasing periods of inundation will cause greater mineral sedimentation,” commented Professor FitzGerald. “However, as the effects of climate change are predicted to include more extreme weather events, it is important that we understand how these events impact this important ecosystem.
“The Bluesky aerial photomaps allow us to observe and measure changes to the salt marsh and from this analysis we can get a better understanding of both short-term impact and the longer-term processes, which can then be used to help inform possible restoration methods,” he continued.
“Normally, aerial photo acquisition has to deal with the challenges of working around airspace, and having cooperating weather (low wind, absence of clouds),” Shaun Vincent, Sales and Mapping Specialist, Bluesky noted. “The photo acquisition presents additional challenges in that the tide has to be low, and the sun angle has to be low enough to prevent sun reflection form the water. So the available windows to get good photography are much smaller than a standard project.
Vincent really enjoy this project from an aesthetic perspective, because the infrared images around the shore can be quite beautiful. “I was a geology major in college so seeing the results of the scientific studies brings me back to the days of sedimentology class,” he said.
More than half of the world’s salt marshes exist at higher latitudes than Massachusetts and are also subject to freezing temperatures, low tides, and storm surges. Because of this ice rafted sedimentation may be a significant process, but not well reported on because many of the affected regions are isolated and not likely to be studied during the winter.
The Research Paper by Professor FitzGerald and Professor Hughes entitled Enhanced, Climate‐Driven Sedimentation on Salt Marshes was published in the journal Geophysical Research Letters.
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