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SHORE CRABS DEPRESS PLANT BIOMASS AND SOIL DENSITY ALONG TIDAL CREEK EDGES

https://doi.org/10.1002/ecs2.3703

UCSC’s Press Release on the study can be found here. Or read below…

A long-term study in Elkhorn Slough revealed the impact of superabundant crabs on salt marsh vegetation and the vulnerability of tidal creek banks to erosion

August 09, 2021

By Tim Stephens

Coastal marshes are vulnerable to erosion caused by rising seas, pounding waves, and tidal flows. In Elkhorn Slough, these vulnerabilities are made worse by superabundant crabs found at their highest densities along the estuary’s tidal creeks, according to a new study published August 8 in Ecosphere.

The striped shore crab (Pachygrapsus crassipes) is a small crab found all along the West Coast of North America, and it is extremely abundant in Elkhorn Slough. The study demonstrated the dual role of these crabs as both consumers of salt marsh vegetation and as ecosystem engineers.

“Their burrowing weakens the creekbank edges, so that whole chunks of marsh will sometimes calve off, and by lowering biomass they are reducing the ability of marsh plants to prevent erosion,” said lead author Kathryn Beheshti, who earned her Ph.D. in ecology and evolutionary biology at UC Santa Cruz in 2021 and is currently a California Sea Grant State Fellow at the Ocean Protection Council’s Climate Change Program.

Beheshti and her coauthors conducted a five-year field experiment to assess the effects of crabs on the vegetation and sediments along eroding creekbank edges. Using fencing and traps made of empty tennis-ball cans to exclude crabs from experimental enclosures, they found that reducing crab abundance led to increased growth of salt marsh vegetation and enhanced sediment density.

The researchers also found that the number of burrows did not change over the study period, even with researchers experimentally removing crabs. The unexpected persistence of the burrows highlights the value of long-term field experiments. The experiment was maintained for five years thanks in large part to the efforts of a team of over 50 UC Santa Cruz undergraduate students and high school interns.

“Field experiments that span multiple seasons and years are rare,” said coauthor Kerstin Wasson, research coordinator of the Elkhorn Slough National Estuarine Research Reserve and an adjunct professor at UC Santa Cruz. “This work demonstrates the value of long-term studies by showing that burrows, which weaken the stability of tidal creek banks, persist despite the near absence of the crabs that build them.”

Coauthor Brent Hughes, assistant professor at Sonoma State University, noted that the crabs were most abundant in spring and summer, when the pickleweed marshes are at peak production. “This synchrony suggests that the effect of crabs as consumers is more punctuated than their more chronic effect as engineers,” he said.

Elkhorn Slough is one of the largest estuaries in California, with the largest tract of tidal salt marsh in the state outside of San Francisco Bay. It has been highly altered by human activities, however, and erosion along the edges of the tidal creeks and main channel is steadily eating away at the marsh.

“It’s a big issue, because when the marsh erodes away along the tidal creeks it’s a permanent loss,” Beheshti said.

The impacts of crabs on marsh biomass and soil structure near tidal creek banks are likely to make the marsh less resilient to erosion and sea-level rise, presenting a unique challenge to managers. Restoring populations of crab predators, such as herons, racoons, and sea otters, may be one way to mitigate these negative effects.

“In this system, top-predator recovery is key,” said coauthor Brian Silliman, distinguished professor at Duke University.

This collaborative study brought together marsh ecologists from both the East and West Coasts who have led the field in exploring how animals affect the marshes they inhabit. Over the past few decades, the U.S. East Coast has been the epicenter of studies exploring top-down effects in salt marshes, and this study is one of the few to explore such effects in a West Coast salt marsh.

“Southeastern U.S. marshes appear to be a harbinger of what’s to come for marshes along the Pacific coast, with sea-level rise amplifying the effects of what would otherwise be considered an innocuous crab,” said coauthor Christine Angelini, associate professor at University of Florida.

The authors called for similar long-term studies to be conducted in other West Coast marsh systems to determine how widespread these crab effects are. “It’d be great for contextualizing our findings,” Beheshti said. “We’d like to know if Elkhorn Slough is the canary in the coal mine, signaling yet another pathway for accelerated marsh edge loss for one of California’s rarest coastal habitats.”

This work was supported in part by grants from the David H. Smith Conservation Research Fellowship, the Myers Ocean Trust, and Friends of Long Marine Laboratory.

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AN INVASIVE SPECIES IN DECLINE

Wasson, K., Fabian, R.A., Fork, S. et al. Biol Invasions (2020).

https://doi.org/10.1007/s10530-019-02172-w

This paper was a collaborative effort with current and former graduate students of lead PI, Kerstin Wasson, NSF REU interns, UCSC field interns, ESNERR research staff, and folks from the University of Virginia, University of Georgia, CDFW, CSU Monterey Bay, Oregon State, Hopkins Marine Station, and the Smithsonian Environmental Research Center.

Fig 8 pulled from the manuscript showing shell dissolution and crab damage to the snail shells. A: Live mudsnail with intact shell with low taphonomic index; B: live mudsnail with very dissolved shell with high taphonomic index and algal growth; both A and B were used for tethering experiment and show attachment with dental floss. C–F: crab damage on empty shells with typical peeled apertures and puncture wounds. G–J: illustrations by D. Jolette (see complete series in Figs. S8-9). G: idealized intact shell with taphonomic index of 0; H: highly dissolved shell with taphonomic index of 4; I: peeled aperture and puncture wound typical of crab predation; J: shell repair typical following crab predation
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PATTERN AND SCALE: EVALUATING GENERALITIES IN CRAB DISTRIBUTIONS AND MARSH DYNAMICS FROM SMALL PLOTS TO A NATIONAL SCALE

Are Crabs Killing the Nation’s Salt Marshes?


Watsonville, California – August 20, 2019- As they munch on plants and burrow through the soil, crabs are wreaking havoc on salt marshes around the country. Or are they? Turns out it depends on where you look, according to a recent study published in the journal Ecology. Conducted by the NOAA-sponsored National Estuarine Research Reserve System, this assessment explored how crabs are affecting the nation’s salt marshes.

California’s Elkhorn Slough Reservecollaborated with scientists from 14 other sites in 13 coastal states to conduct this study. They found crab burrows in just under half of the plots they sampled. In only four estuaries did researchers find evidence that crabs were having a negative effect on marsh vegetation. This indicates that it is unlikely that crabs pose a major threat to U.S. salt marshes at a broad scale.

“Elkhorn Slough was one of the four sites where crab burrow density showed a significant negative relationship with marsh vegetation,” said Kerstin Wasson, Research Coordinator at the Elkhorn Slough Reserve.  “Crabs burrow into the lower marsh and bank edge, giving it the look of swiss cheese—more holes than anything else. Even so, 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.”

There are many other types of crabs that live in estuaries, including those that support commercial fisheries. This study focused on the types of crabs that live in tidal creeks and salt marshes. At first glance, the study appears to contradict recent experiments that demonstrated that crabs can cause dramatic die-off of marsh vegetation. It turns out, however, that when it comes to crab research, location is everything. These experiments are labor intensive, involving the installation of many cages. As a result, researchers often position cages where they see lots of crab burrows, which tends to be at lower elevations and near tidal creek banks.

Crab burrows near bottom of marsh  in Elkhorn Slough, CA

This study systematically sampled salt marshes along transect lines that extended from the uplands down to the edge of tidal creeks. Transects are straight lines along which samples are taken at fixed, predetermined intervals to minimize bias. Few crabs were found in most places, but they were abundant near tidal creeks in some estuaries— the same sorts of places where past experiments have been conducted.

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

 
While crabs did not have a strong overall influence on salt marshes, elevation did. Lower parts of the marsh are submerged more often and tend to have poorer overall health. This study suggests that rising sea levels associated with climate change pose a bigger threat to the nation’s salt marshes than do crabs. However, since the study found that crabs are more abundant, and cause more harm, at low elevations, climate change will also impact the effects that crabs have on marshes.  

“We’ll be seeing more crabs in our marshes as waters rise,” says Dr. Kenny Raposa of the Narragansett Bay Reserve in Rhode Island, another lead scientist of the study. “The health of our marshes will be challenged by rising seas and increasing crab burrows at the same time.”  

 
Entire marsh transect at Elkhorn Slough seen from the top: crabs were only found at the bottom.

Restoration measures that help marshes stay higher, where they are less likely to be susceptible to rising seas or crab burrows, will be essential to protect the salt marshes that provide economic benefits to communities nationwide.   Raposa and Wasson are leading another collaborative study across eight Reserves that is examining the effectiveness of adding soil to raise marshes to help them survive in the future. 

“With scientists following consistent protocols that monitor coastal plants at 29 sites around the country, the Reserve system is providing high quality data and science that communities need to manage our changing coasts,” says Rebecca Roth, the executive director of the National Estuarine Research Reserve Association. “Reserves are places with scientists that can detect early signs of threats and then test strategies to make communities safer and more resilient.”

Kathryn Beheshti, a co-author on the recent paper, sampling crabs near the marsh edge at Elkhorn Slough, CA


About the National Estuarine Research Reserve System

The National Estuarine Research Reserve System (NERRS) is a national network of 29 sites dedicated to the effective science-based management of coastal and estuarine environments. Established by the Coastal Zone Management Act in 1972, each Reserve is a partnership between the National Oceanic and Atmospheric Administration (NOAA) and a U.S. state or protectorate, university, or nonprofit. NOAA provides funding and national guidance for each Reserve. The local partner provides a place and vision for the Reserve’s work and administers programs in support of Reserve goals.

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HESTER UPDATE – CRABS HAVE MOVED IN!

A cohort of newly recruited crabs have moved into the Hester marshes. When visiting the site you can see crab burrows of migrant adults (indicative by the size of the individuals and the burrow sizes) and in the small branching tidal creeks small burrows blanket the shallow creeks.

Now we see many crab burrows in the Hester tidal creeks.
& so we are beginning to set up our crab exclusion experiment to track the effect of crabs on virgin salt marsh
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HOW DO WE MEASURE % COVER?

Percent cover is a common measurement taken by marsh ecologists…

When collecting percent cover data we often use a gridded quadrat like the one pictured in panels A &B. At each intersection point we place a narrow rod and record all the cover types that the rod makes contact with at the canopy, sub-canopy and benthos.

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INTERN TAKE-OVER: OUR SUMMER IN THE SLOUGH

Mason and Annakate measuring restoration plot area during low tide.

Written by Annakate Clemons and Mason Emery

Figure above: Left to Right; Annakate Clemons, UCSC Undergraduate (2013-2017); Kat Beheshti, PhD Candidate; Mason Emery, UCSC Undergraduate (2013-2017)

BACKGROUND

As keystone predators, the Southern Sea Otter (Enhydra lutris) plays a critical role in maintaining the stability and health of the ecosystem in which they live.

A trophic cascade discovered by Dr. Brent Hughes (Hughes et al. 2013) showed that there was an indirect positive effect of otters on eelgrass bed health. Elkhorn Slough eelgrass (Zostera marina) beds have been slowly recovering from a 10 hectare loss of eelgrass habitat following the construction of Moss Landing Harbor.

A TYPICAL FIELD DAY

A good day on the slough, or any outdoor adventure for that matter, starts with protection from the elements—sunscreen, a hat, and multiple layers. Weather in the slough changes hour by hour. Many days start cold and foggy, with the fog casually burning off through the day, and then just as the sun graces you with its presence the wind picks up—making that extra layer you packed worth it. Pack a good lunch, sit back, observe and record. The greatest lesson we learned through planning these days is that timing is everything for kayaking in the slough, which we picked up on quickly through trial and error. We made the mistake of heading out early morning on the windiest of days and discovered that the usually mellow, lake-like water of the slough had become a rough and turbulent channel blanketed in whitecaps. As we struggled to stay in one place long enough to observe any otter, we watched as beached kayakers were rescued by boats and towed back in. We quickly called it quits, but as soon as we started battling our way back, my (AK Clemons) paddle snapped in half. I managed to power row myself back to the harbor after multiple attempts of trying to connect our kayaks. After that, we learned to check the wind forecast prior to going out in the field.

RESTORATION

Restoration efforts of eelgrass habitat led by Kat Beheshti, PhD Candidate at UC Santa Cruz, have been widely successful. We joined Kat’s research team in early 2018 and participated in her 2018 restoration project. This year’s goal is to enhance restoration success via plot size while concurrently monitoring sea otter usage within the proximity of our newly transplanted plots.  We are interested in integrating the trophic cascade that Dr. Hughes discovered in the existing beds of Elkhorn Slough and apply it to our restoration plots to see whether there is the capacity for otters to enhance restoration success.

OUR EXPERIENCE AS PROJECT LEADS

Our goal for this project is to assess sea otter activity in and around our 2018 restoration plots. Through observing otter activity in proximity to our plots, we hope to quantify the impact otters may have on restoration success. Otter activity that we are interested in includes: feeding, forage diving, mating, in transit, and whether otters present are mom and pup pairs. Proximity of the otter to the plot is recorded using a GPS. Our mode of transportation: kayaking. As project leads, we drafted a schedule with the goal of completing 40 hours of otter observations that span across all of our 2018 restoration plots. We organized a team of currently enrolled and recently graduated UCSC students to complete this task from June 2nd-16th.

THE NITTY GRITTY

Each field day includes four hours of otter observations. We arrived at the slough roughly two hours before high tide. That way we could let the rising tide carry us through the slough, easily reaching our furthest observation point by high tide. We could then turn back with the change of tide, allowing us to effortlessly glide back to the harbor, only occasionally being thwarted by the afternoon wind. Sampling in this way allowed us to record each area twice in a day, once on our way out and again on our way back in. This sampling method combined with the changing times of high tide from week to week allowed us to compile a record of otter behavior at various times of day—from early in the morning to early evening. We successfully designed a sampling protocol that would account for the temporal and spatial variability in sea otter usage in relation to our restored plots.

One of the most rewarding consequences of this work is our heightened understanding of the dynamics and structure of Elkhorn Slough. We witnessed massive algal production, learned where in the channel was the best to flow with the tide, expected the unexpected with the weather, and observed a variety of marine mammal activity throughout varying habitat types.

We’d like to thank Kat and the Wasson Lab for giving us this opportunity and for involving in their many slough projects!

Thank you for reading our blog post!

-Annakate Clemons & Mason Emery

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THE MARSH FOLLOWS

June 2018

TUNA STEAK ON A BED OF PICKLEWEED IN FRANCE!

Our fancy meal in Monaco!

While on her honeymoon in France, grad student Kat Beheshti’s husband was served tuna steak on a bed of steamed pickleweed!

Now the image she sent to the folks at ESNERR while abroad is being used on tours as a tool for public outreach demonstrating how, what may seem like a common marsh plant is actually cultivated and harvested for consumption in other parts of the world!