A Delicate Balance: Our Marshes and Sea Level
Sunny-day marsh flooding during full-moon tides is now more frequent.
Coastal marshes, tidal creeks, and bays are the cradles of life. They host a phenomenal diversity of wildlife, and their intrinsic beauty is a balm for the soul. They are the foundations of a complex web that is the building block for commercial and recreational fisheries, supports local economies, and sustains the coastal way of life.
Marshes and tidal creeks are critical nursery grounds for fish and shellfish. More than 75% of the commercially and recreationally important species like blue crabs, oysters, clams, striped bass, flounder, and many others rely on marshes during at least part of their life cycles. Marshes are the kidneys of our coastal waters, naturally improving the quality of water passing through them by trapping sediments and converting dissolved chemicals into harmless compounds. They are also crucial to coastal resiliency, offering protection against damage to property and infrastructure and reducing loss of life during storm surge and flooding – providing our coastal communities concrete and immediate benefits. Research from the insurance industry showed that marshes directly prevented more than $625 million in flood damages during Superstorm Sandy in 2012, further underscoring their value.
If you have either lived near the coast or visited it over many years, you have probably noticed the changes in water levels in our marshes. It used to be a really big deal when the meadows were flooded, because it didn’t happen very often. Times have changed. Now, they flood numerous times every month – even on calm, sunny days. What causes this, and what does it mean for our marshes and the wildlife that depend on them?
It is well-established fact: The sea level in our area is rising – and doing so too fast to sustain healthy marshes. The rate of sea-level rise is also accelerating dramatically. For the past 100 years or so, scientists directly measured this through local tide gauges that record how high the water level rises and falls each day. More recently, they have been using satellite technology as well. We are fortunate to have several gauges in the area that have relatively long records, so we can see how sea level has changed.
Averaged over long periods, sea-level measurements taken in both Atlantic City and Cape May show that sea level is rising at a rate of about 4.2 millimeters (or 0.16 inches) per year. This may not seem like much, but from 1911 to 2023 that is a change of more than 1½ feet. Due to a complex set of reasons – most importantly because southern New Jersey is also sinking – sea level has risen here at more than twice the global average.
Long-term averages only tell us part of the story, though, and downplay the increasing rate of sea-level rise. To really understand what is happening today to our marshes, we need to look more closely at the record. Over the past 15 years, the rate of sea-level rise measured right here on the 96th Street bridge into Stone Harbor is more than 6 millimeters (or a quarter-inch) per year.
Simply put, the water is rising higher each year and we see it now more dramatically because it’s flooding the marshes and covering over the grasses, making it impossible to ignore. It’s also more obvious because it’s also flooding bayside streets and stressing island infrastructure. It also brings water up higher on our beaches, accelerating beach erosion rates and increasing the frequency and need for beach replenishment projects.
So, what does this mean for our marshes?
The marshes that dominate the back bays are tidal marshes, meaning that the tides flood portions of the marshes twice each day. Other portions of the marshes are flooded only during the higher-than average tides that come with the full and new moons. Still other portions are flooded only by the tides that come with storms. Theirs is a life in a delicate balance: The tidal waters are life-giving and life-sustaining. But as seas rise, more of the marshes are being flooded more often and these same waters are now threatening the marshes’ well-being.
Marshes maintain their position relative to flooding tides by building up with sediment and organic matter. Mud washing into the marshes and marsh grass roots building up and trapping mud both help the marsh surface grow upward. As a result, the marsh surface will rise in relation to sea level – and the marsh can “keep up” with gradually rising seas. However, when sea level rises faster than the marsh surface can build, tidal marshes are drowned and replaced by mud flats or open water.
Salt-marsh grasses are very sensitive to water depth and flooding levels. They experience optimum growth over a very narrow depth range. If the water gets too deep or too shallow, or there are other stressors to healthy marsh grass growth, growth rates slow. When this happens, the marsh’s ability to maintain its level relative to sea level declines. Scientists working in our back bays in the Seven Mile Island Innovation Lab have directly measured the rate our marshes build up – which is only 4 millimeters (0.16 inches) per year. Yet the sea level is rising at a rate of more than 6 millimeters (1/4 inch) per year. We have reached the tipping point and the trained eye (and The Wetlands Institute’s detailed monitoring programs) can see this.
The projects of the Seven Mile Island Innovation Lab are working to help marshes right here add elevation more rapidly, and we are working as fast as we can. The negative effects of sea-level rise are no longer “if” or “sometime in the future.” They are here now. The work of the Innovation Lab has already restored more than 85 acres of marsh, and more work is planned for this fall. New Jersey is now a national leader in marsh restoration using tools developed right here.
Ultimately, the long-term fate of our coastal marshes depends on our collective efforts to slow the rate of sea-level rise primarily by reducing greenhouse gas emissions into our atmosphere. There are several ways to achieve this, and all of us have a role to play. Learn more about the Seven Mile Island Innovation Lab and our groundbreaking work at wetlandsinstitute.org/smiil-2. Also, learn more about actions you can take to curb greenhouse gas emissions.
