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Study Examines Crab Impact on Salt Marsh Health

Written on: September 20th, 2019 in NERRResearchSt. Jones Reserve

A study within the National Estuarine Research Reserve System to determine the destructiveness of crabs to salt marshes found that, while crabs can be a problem in some areas, a far greater threat facing marshes is sea level rise.

Crabs play an important role in maintaining a balanced ecosystem in salt marshes, but too many crabs can upset that balance and wreak havoc on a marsh.

If you’ve walked along the river banks of a salt marsh at low tide and seen holes in the mud that make it appear like Swiss cheese, then you’ve seen the burrows that are a hallmark of the crab community. Those burrows are actually good. Among the benefits, they are a way for oxygen to get into the sediment and can also increase a marsh’s ability to drain water.

The crabs also help keep vegetation and the population of other marsh inhabitants in check, as well as serve as a food source for predators, like the Clapper Rail.

Too many crabs, however, can upset the balance of the ecosystem, and crab populations have been blamed for having a negative impact on salt marshes.

As the crabs feed they graze down the salt marsh grasses that hold the marsh soil in place.

To find out how much of a threat crabs are to marshes, the Elkhorn Slough National Estuarine Research Reserve in California collaborated with scientists from 14 other sites in 13 coastal states, including Delaware’s National Estuarine Research Reserve on a study looking at their impact.

The results of the study showed that, in most of the marshes, sea level rise was a bigger contributor to marsh degradation than the crabs.

Dr. Kerstin Wasson, research coordinator at the Elkhorn Slough Reserve in California, said their reserve was one of only four where the study showed crab burrow density having a significant negative relationship with marsh vegetation. Even so, Wasson said, the effect of elevation was stronger than that of crabs across the entire marsh.

“Our marshes are drowning faster than they’re getting attacked by crabs, which is why we’re investing in major marsh restoration initiatives to raise elevation,” she said.

At the St. Jones Reserve in Dover, Dr. Kari St. Laurent, research coordinator at the DNERR, said the crab community isn’t a problem here either.

“We’re not seeing it because our ecosystem is functioning as it should be,” she said.

Sea level rise, however, is an issue. Of the areas included in the study, 97 percent had unvegetated bare ground, she said.

“Low-lying areas in the marsh could be seeing more prolonged and intense inundation periods,” she said. “We’re more vulnerable to sea level rise.”

The study sampled salt marshes along transect lines – straight lines along which samples are taken at fixed, predetermined intervals – extending from uplands down to the edge of tidal creeks. In contrast, most recent studies showing crabs caused dramatic die-off of marsh vegetation occurred in areas where researchers had identified lots of crab burrows, which tend to be at lower elevations and near tidal creek banks.

Crabs collected in the trap over night.

Wasson said patterns in nature differ across scales. “What is true at one scale may not be true at another,” she said. “We found that crabs can cause a lot of harm at a local scale, in some parts of the marshes, but they don’t seem to be a main cause of marsh dieback at a national scale.”

Dr. Kenny Raposa of the Narragansett Bay Reserve in Rhode Island, another lead scientist in the study, said the crab population will increase in marshes as waters rise.

“The health of our marshes will be challenged by rising seas and increasing crab burrows at the same time.”

St. Laurent said various marsh restoration techniques, such as adding sediment to build them up so they don’t drown, and allowing for natural marsh migration are ways to ensure the continued health of marshes being degraded by sea level rise.

She said the study was also valuable for the Reserve because no invasive crabs were found in the study in the St. Jones. Red jointed fiddler crabs were documented along with sand fiddler crabs, as well as purple marsh crabs. Purple marsh crabs were responsible for salt marsh losses in New England, but St. Laurent said the native crab most likely is not a problem in the small numbers found here.

Each crab caught in a trap was measured and identified by species.

The results of the study — Pattern and scale: evaluating generalities in crab distributions and marsh dynamics from small plots to a national scale — were published in the journal Ecology.


Fractals and Flow

Written on: November 19th, 2018 in Education & OutreachGuest BlogSt. Jones Reserve

Outdoors, patterns abound. One need not be a mathematician in order to view and appreciate them.

Striking patterns against a vivid sky.

The widening whorls of seeds in a sunflower head are often used to illustrate the Fibonacci sequence of numbers (0, 1, 1, 2, 3, 5, 8….) in which each figure is the sum of the two preceding digits. Other Fibonacci examples include the curves of nautilus shells and the swirls of hurricanes.

Brackish water flowing gently through marsh mud.

On a larger scale, trees also exhibit mathematical beauty, and they, too, spring from Fibonacci and fractal patterns. That’s the fancy way of saying their branch arrangements are not entirely random but express an inner logic. Leonardo da Vinci was fascinated by tree branches and studied them — a good example for all of us to follow.

Even the leaves of plants have patterns. The scientific term phyllotaxis describes the positioning of leaves around a plant’s stem. Some ascend the stem helically, in stair-step fashion. Some alternate left to right.

Everything around us contains an inherent rhythm, an intrinsic flow, and sometimes-subtle waves of movement or energy. Water and wind and sunbeams and flowers are all “flow-ers” of one sort or another.

How many fractals, waves, and intriguing patterns can you spot when next visiting DNERR?Accompanying this blog post are a few images from the St. Jones Reserve that entice the eye with their pleasing and varied forms.

Wind blown waves in strands of marsh grass.


Text and photos by M.L. Christmas

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M.L. Christmas, MSM, is a freelance writer/editor living in the Dover area. She is a longtime member of Delaware Press Association and the National Federation of Press Women. Her approach to nature writing, she says, is part Henry David Thoreau and part Dave Barry.


Delaware’s Rainfall: Seriously Spikey! Just ask Dr. Kari St. Laurent

Written on: October 23rd, 2018 in ResearchSt. Jones Reserve

A bright, bristly weather-chart spied on the research-laboratory wall at the St. Jones Reserve, on National Estuaries Day 2016, cried out for revisiting. Because a pirate’s best friend just may be the weather, or possibly a good chart, an irresistible combination of the two caused this would-be pirate to return recently to the Reserve for a special interview with Dr. Kari St.Laurent.

This chart of rainfall data means business–and so does DNERR’s Research Scientist, Dr. Kari St.Laurent.

Kari joined DNERR in April 2016, and according to the announcement posted on this blog, she:

 “…holds a Ph.D. in oceanography from the University of Rhode Island’s Graduate School of Oceanography where she studied black carbon and persistent organic pollutants in the subtropical Atlantic Ocean. Before that, she received a B.S. in Environmental Science and a B.A. in Environmental Chemistry from Roger Williams University where she did research on oyster restoration, bay scallop diets, and estuarine hypoxia.”

Impressive! She has also been a contributor in her own right to DNERR’s blog. We now welcome the chance to hear from her some more. Why? Because apart from all of the aforementioned, meteorology is her superpower! As that earlier blog goes on to state:

“After completing her Ph.D., Kari worked as a postdoctoral researcher at the University of Maryland Center for Environmental Science on a project investigating climate variability and extreme change in Chesapeake Bay and its implications to environmental issues, such as submerged aquatic vegetation dieback events.”

Dr. Kari St.Laurent poses alongside this “snapshot” of her ongoing meteorological research.

Sounds intimidating, right? Well, that’s where the knowledgeable, but very approachable, Kari comes in — with a bright smile and a ready explanation that informs even the most weather-challenged audience member. She is accustomed to explaining scientific data in everyday terms, and she especially enjoys when she can correlate the weather to a visitor’s own, personal experience. This became readily apparent when she worked that very magic on this guest‑blogger…but we’ll get to that in a few moments. First things first:


MLC: OK, what’s with the spikey chart?

KStL: The chart came out of my post-doctoral work and my inquiries into weather patterns in the Chesapeake Bay Region, including Delaware. The chart is a printout of the rainfall data from Dover Air Force Base. It came about when we wanted to display the data at an Open House at the University of Maryland, two years ago: a “family fun” day.

 At that event, I would have visitors write, on sticky notes, the month and year they were born and then put them on the corresponding spot on the chart. People found their birthdays often correlated to, or were close to, a big weather event. The chart also helped people recall a weather event from when they were a child or from when they were an adult.

We had people who remembered the big storms of the 1930s — such as grandparents who recalled when they were 4 or 5 years old and the power went out for a week. They wrapped potatoes in aluminum foil and threw them into the fireplace so they could have a meal. That’s just one example of illustrating how weather affects our everyday lives.

 We have been using that same sticky-note method on this chart here at DNERR, for our own outreach events such as National Estuaries Day. We are trying to relate WEATHER and PEOPLE ‑‑ as opposed to simply stating the expected rate of climate change, which has seemingly less of a personal connection.

It was at that point that I covertly eyeballed the chart for my own birth year and noticed a particular spike. Yup, a significant weather event connected to me, too! I had heard some mention of it before, in family lore, but there it was in black and white–or should I say, in blue, yellow, red, and green.


MLC: The chart is certainly eye-catching, not to mention powerful! After all, it levitated me back to the St. Jones Reserve several weeks after the event at which I first spotted it.

KStL: The chart actually illustrates the total rain by month, from about 1920 to the first half of 2016. Instead of listing the rainfall day-by-day, it’s streamlined by using the greatest single-day precipitation event each month. Converting the display from 365 bars down to 12 bars per year makes it much more manageable.

Kari exclaims, “I love oceanographic acidification equations!” (That smiley face was only our first hint.)

You will see the months are color-coded by season. Blue spikes are winter months, green spikes represent the spring, summer is red, and fall is yellow. Do you notice anything about the patterns?


MLC [with a chuckle]: You are asking me a question? No problem. Let me take a closer look. Well…the most significant spikes, over the years, appear to be either red or yellow. There is only one relatively large green spike, and it was back in the 1940s.

KStL: Yes, the rainiest times of the year tend to be the summer and fall, though there are occasional exceptions. Do you notice anything else?


MLC: Uhhh…the biggest spike, located in the mid-1970s, is labeled simply “severe thunderstorm.” It’s a spike even bigger than all the hurricanes named on the chart; and no hurricane-names at all appear on the left half of the chart. Obviously that’s not because there were no hurricanes prior to the mid-1900s.

KStL: That’s correct. Named storms really only started in the 1950s. Before that, weather events were called “Hurricane 1,” “Hurricane 2,” etc. Some storms have been named in hindsight, like Ocean City’s “Great Storm of 1933,” “The Mother’s Day Storm” [of May 12, 2008], etc.

As for that big “severe thunderstorm” spike you see, I had to do some research in order to determine whether a specific rain event was associated with it. The daily values underlying these monthly totals can represent a trickle of precipitation over time or a single popup thunderstorm.

 This chart is just a snapshot at one location that matters to us, as residents of the Dover area. Everyone is impacted by the weather. The storms that people remember are because they were directly affected by that event, or because they saw on the news that it caused a lot of damage. For scientists, we monitor these things because they also can suddenly affect or disturb the ecosystem, particularly at a place like DNERR. Disturbances we find in the ecosystem could have come from a storm surge, for instance, because not everything [in an estuary] is salt tolerant.

Scientists are always aware of these effects and are always on the lookout for long-term changes. Weather is dynamic!


MLC: And so are you, Dr. Kari St.Laurent! Thank you for your time and for your thought-provoking insights!


Text and photos by M.L. Christmas

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M.L. Christmas, MSM, is a freelance writer/editor living in the Dover area. She is a longtime member of Delaware Press Association and the National Federation of Press Women.



We Monitor “What” in the Water?

Written on: August 15th, 2018 in Blackbird Creek ReserveEducation & OutreachResearch

An experience that is avoided by most college students became the beginning of a morning adventure… I was up before the sun; a rarity in itself. I had risen from the depths of my room to explore the waters of Blackbird Creek on a nekton trawl. The word nekton was absent from my vocabulary prior to this summer. Yet, within the first week of my internship I was gladly informed that nekton are any aquatic organisms that actively swim. Immediately intrigued by this foreign concept, I wanted to further my understanding and asked to join one of these so called “nekton trawls.”

I was later informed of an opportunity to assist Michael Mensinger and Drew Faulhaber on one of their monthly surveys. On August 6th, that fateful morning came and I was inquisitive as ever. Various questions flowed through my mind and I pondered: How does a trawl even work? What kind of data are we collecting? Why is this data important?” Although, all of my contemplations were soon contented when we set off and I was able to experience all of these things first hand.

Mike Mensinger and Drew Faulhaber measuring a striped bass.

The Blackbird Creek has been segmented into seven distinct zones based on salinity, then a randomized GPS point is selected within each zone.  The GPS point marks the place to begin the trawl. It is important to note that trawling is a method of fishing where a net is driven open by wooden boards, known as otter boards and runs along the bottom to collect various bottom-dwelling organisms. Setting up the trawl and driving the 22 foot motorized boat fell onto the two men I accompanied, while my job was to record start & end times and GPS coordinates. After reaching 300 meters in distance the trawl was lifted from the water and the captured organisms were placed into a fish processing bin. Once they were measured, and the data was recorded they would be returned to the water. Finally, before progressing into the next zone water quality and creek depth measurements were recorded.

Although this seems like a straightforward procedure, there is always room for mistakes to happen. During trawling in one of the zones, the net became caught and ripped from unknown debris residing at the bottom of the water way. The largest lesson to be learned from the entire situation is that scientists and people alike need to have the adaptability to overcome obstacles. Mr. Mensinger took this set back with a level head and having planned ahead was able to fix the net and continue the survey.

As we continued and eventually came to a close, it became evident that the high abundance of animals of this month were: white perch (Morone americana), hogchokers (Trinectes maculatus), channel catfish (Ictalurus punctatus), and blue crab (Callinectes sapidus). The data collected this month appears to be relatively consistent with Michael Mensinger’s  data collected last year, with the most abundant species being white perch, hogchokers, weakfish (Cynoscion regalis), and blue crabs (Mensinger, 2018).

A hogchoker caught during the trawling survey at Blackbird Creek on August 6th.

By comparing the data between the months and years a clearer picture of the area’s short-term and long-term biodiversity and biodiversity changes will become clearer, which helps scientists understand the effects of climate change and other forces on the nekton community. I am appreciative of Delaware’s National Estuarine Research Reserve for presenting me with a new perspective and the skills to match so that I may one day be able to improve society’s understanding of nature… and also for helping me get me up before the sun rose.

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Mensinger, M., 2018. Monitoring Nekton Biodiversity in Blackbird Creek. Report – submitted to the Delaware National Estuarine Research Reserve, Dover, Delaware.

Written by Jared Ryan, 2018 Environmental Education Intern.



DNERR’s Tree Swallows and Bluebirds Need YOU

Written on: July 11th, 2018 in Blackbird Creek ReserveEducation & OutreachSt. Jones ReserveVolunteers

DNERR’s Tree Swallows and Bluebirds Need YOU

Spring is here, and the St. Jones and Blackbird Creek Reserves have sprouted a number of mysterious, wooden boxes affixed to PVC poles. Where did these boxes come from? And why?

At the Blackbird Creek Reserve, a nest box is clearly marked as being monitored by DNERR. Photo: M.L. Christmas

With such questions in mind, this writer approached Christina Whiteman, DNERR’s Environmental Scientist and Stewardship Coordinator, and Kari St. Laurent, DNERR’s Research Coordinator. Here is what they said; and you, the readers of this blog, can volunteer to be in on the fun!

What is the purpose of the nest boxes? Are they for Wood Ducks? Or Bluebirds? Or just what? Christina responded, “They are for the use of cavity‑nesting birds, such as Eastern Bluebirds and/or Tree Swallows, for breeding and brooding purposes.”

Why the PVC posts? Aren’t they typically wood? “That is a strategy being tried for avoiding the incursion of predators, like snakes. Snakes cannot easily climb PVC, unlike when the traditional, wooden posts are used.” (One can certainly imagine why a snake and a nest box full of eggs or chicks is not a good mix.)

It’s “problem solved,” then, with PVC! “Not exactly. One ongoing problem we have is House Sparrows, a species that often takes over nest-box spaces and crowds out the other species. We are still trying to devise strategies for deterring them.”

 So, people are an important part of the equation? “Yes! Some human involvement is needed: for monitoring the nest boxes and documenting the avian activities.”

A line of nest boxes at the ready at Blackbird Creek Reserve, just a portion of those on the premises. Photo: M.L. Christmas

This writer did not need to inquire where this help was needed. She had been at the Blackbird Creek Reserve earlier in the week and had noticed a number of nest boxes throughout the complex, including a sweeping array in the Reserve’s “back 40”; and on the way to this interview, she also saw a line of nest boxes at the St. Jones Reserve.

So, what specific type of help is needed, and how is it accomplished? Christina showed this writer an amped-up, angle-y, oversized, dental-mirror-like thingy that is used for peering into the nest boxes without disrupting the inhabitants. Any Philadelphia-area folks familiar with Elfreth’s Alley would liken the implement to a “Busybody Mirror,” a Colonial-era device attributed to Benjamin Franklin. That specialized type of mirror, still being sold today, allows a homeowner on the second floor of a home to discreetly view who is calling at the front door. This is similar, but in reverse: allowing the nest-box checker, when standing at the “front door” of the nest box, to peer inside without disturbing the avian occupants.

Can anyone participate? “Yes, and the nest boxes are also an ongoing research-intern project. The training in the proper protocol is provided via the Cornell Lab of Ornithology’s NestWatch program. The database tracks the location of a nest box and which species is using it. We have been monitoring our boxes at DNERR for two years. Our interns and volunteers can become certified in nest-box monitoring, and they help monitor the boxes throughout the summer. We also have a partnership with Delaware State University [for monitoring the boxes at the DNERR], and Del State bands the Eastern Bluebirds that are in our boxes.”

How has that been going? Any discoveries? “They have been banding some of the birds to see who comes back or to determine if they are overwintering.

A peek inside a nest box to check on its precious occupants: Babies on board! Photo: Molly Williams, EPSCoP research intern from 2016.

We start checking the nest boxes, and scouting the arriving birds for color‑coded bands, in April. That was when, the other year, we learned of a female Bluebird, at the Blackbird Creek Reserve, laying white eggs. They were lacking the normal light-blue pigmentation. A small percentage of Bluebirds have white eggs, and the interns from Del State banded the chicks that hatched from those particular eggs, for the purposes of future monitoring.”

What else is needed, apart from human assistance — or should that be “assistants”? “We would like to get more Bluebirds at the St. Jones Reserve, though we might not have quite the right environment for them here, as we do at Blackbird Creek. Here at the St. Jones Reserve, the conditions are good for Purple Martins. We have found that the houses with the crescent-shaped openings work best here. We had been trying to attract Purple Martins, with mixed results in the past, but were not fully successful until last year.”

Bottom line for the readers of our blog? “We would like to get some volunteers involved with the monitoring process and with helping clean out DNERR’s nest boxes in the fall. The Cornell Lab of Ornithology’s NestWatch website contains a lot of helpful information,” Christina said.

Kari St. Laurent remarked, “One of the things I like best about the Cornell Lab’s nest-box program is that participants can be certified in monitoring and then take home the ‘recipe’ [for maintaining nest boxes in their own yards]. It is something that people of all ages can do. I know of a research paper from about two years ago that used NestWatch data about Tree Swallows. That is an example of how citizen-science projects, such as these, can be useful for others’ research.”

Christina then noted that anyone “can see the real-time data at the Cornell Lab of Ornithology’s NestWatch site: <https://nestwatch.org/>.”

Baby birds: What’s not to love? Photo: Kathleen Carey, conservation intern from 2016.

Kari continued, “Nest-box monitoring is ‘warm and fuzzy’ — but is also very important. As mentioned, we can fold this data into other research projects. We even studied the effect of emptying most of the boxes but leaving a few old nests in others. Would the birds have a preference, when selecting a box for the new season, between an empty box or using one containing a nest left over from a previous brood? Would the existence of the old nests help or not? It turned out the preference was really for where a box was located — adjoining the meadow at Blackbird Creek, as opposed to being closer to the woods — and not whether a nest was already in place inside a particular box. Of course, we always need more data before we make conclusions.”

This guest-blogger could not help but mentally make the correlation that, just like the real-estate-agent adage, even for Bluebirds the key to a property’s success is “location, location, location”!

And to further anthropomorphize: Who can look at these baby-bird photos provided by past DNERR interns and not think “Cootchie-cootchie-coo”? Adorable, yes, but the nestlings are also an important part of our ecosystem. Who knew citizen science could be so cute?

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Text by M.L. Christmas; photo credits as noted.

M.L. Christmas, MSM, is a freelance writer/editor living in the Dover area. She is a longtime member of Delaware Press Association and the National Federation of Press Women.