CESN Main Page

Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

You can have future issues delivered to your email inbox on a quarterly basis. Sign up today!

October 2022

Table of Contents

How Blue Crabs Invaded Portugal in Under Three Years

Rising temperatures will likely facilitate their continued expansion

The Atlantic blue crab (Callinectes sapidus) is native to the western Atlantic but is now widely distributed in northwestern Europe and the Mediterranean. However, a viable population had not been detected in Portugal until 2016 when fishers began catching multiple specimens in southeastern estuaries. Soon afterwards, a newly launched citizen science campaign called New Marine Species of the Algarve tracked the establishment and westward expansion of this booming population. Researchers used observations made by citizen scientists during 2019 and 2020 along with museum collections and oceanographic data to gain insight into why this invasion was successful where previous introductions had failed.

In 2018 and again in 2019—a year before each of the two years when adults were reported—sea surface temperature records suggest that warm bands of water were transported along the entire length of southern Portugal and even northwards up the western coast. This oceanographic feature formed during the crabs’ reproductive period from summer to early fall, and it is a perfect mechanism for transporting larvae: Current speed and direction data suggest larvae could be transported from southeastern Portugal to the west coast in just 14 days.

Blue crabs are extremely fecund and can tolerate a wide range of water temperatures, and it is likely that estuaries along the west coast of Portugal will serve as stepping stones to support their continued expansion northward as temperatures continue to rise.  This study highlights the value of citizen science for the early detection and tracking of non-indigenous species over wide geographic areas that may otherwise go unobserved.

Source: Encarnação, J. et al. 2022. Coastal Countercurrents Increase Propagule Pressure of an Aquatic Invasive Species to an Area Where Previous Introductions Failed. Estuaries and Coasts. DOI: 10.1007/s12237-022-01092-8

Coconut Fibers for Oyster Reef Restoration

A biodegradable alternative to plastic

Restoring oyster reefs typically involves constructing artificial reefs using oyster shells (or cultch), which help live oysters colonize the surface. However, the most commonly used material to contain cultch is high-density polyethylene plastic, which can break down into microplastic particles with problematic ecological and health effects. As a result, there is an active interest in testing new construction techniques that use alternate materials.

Coconut fiber (or coir) is a renewable, biodegradable natural fiber with high lignin and low cellulose content, which offers strength and resistance to weathering. To evaluate the suitability of coir for oyster reef construction under low-wave-energy conditions, researchers created a network of subtidal oyster reefs in Florida’s St. Andrew Bay using two different designs—high-profile reefs consisting of cultch-filled woven bags and low-profile reefs made of woven mattresses assembled from two coir layers enclosing a thin layer of cultch.

After 5 years of monitoring, the team found coir to be suitable for creating oyster reef habitat. The natural fiber maintained its structural integrity and adequately contained cultch for an average of 9 months, enough time to recruit oysters and stabilize underlying cultch. The most successful strategy featured a perimeter wall of coir oyster bags surrounding a 22-centimeter-tall mound of loose cultch. This design continued to provide substrate for oyster settlement and recruitment for years after construction, despite continuing to decline in height over time. However, even reefs constructed in this way would not likely persist over the long-term with the current conditions in the bay, and a larger analysis of the interacting abiotic and biotic stressors driving reef degradation is still needed to better understand the concerning trend of adult oyster mortality observed in the region.

Source: Hatchell, B. et al. 2022. Use of Biodegradable Coir for Subtidal Oyster Habitat Restoration: Testing Two Reef Designs in Northwest Florida. Estuaries and Coasts. DOI: 10.1007/s12237-022-01094-6

Return to Top

Managing Delta Smelt in the San Francisco Estuary

Is X2 a good predictor of habitat quality?

Protecting the delta smelt (Hypomesus transpacificus), an endangered species endemic to the San Francisco Estuary, is a major focus of water management efforts in the region. Delta smelt tend to occur in waters with relatively low salinity, and current water management practices routinely use the location of a low-salinity zone proxy (known as the X2 isohaline) as an indicator of habitat quality for the protected species. When fresh water enters the estuary, X2 moves downstream and westward towards the Golden Gate Bridge. This effect is thought to increase the area of high-quality habitat for the fish. However, habitat is not one-dimensional, and X2 is not the only factor known to affect their survival and recruitment.

Regional experts from the Delta Smelt Scoping Team specified hypotheses about environmental variables that potentially drive the quality of delta smelt habitat. The researchers then evaluated a suite of 16 Bayesian models to identify which of those variables best predicted the presence of delta smelt on the basis of catch data collected during the Fall Midwater Trawl survey from 1980 through 2015. According to the most strongly supported model, occupancy was associated with salinity and, to a lesser degree, temperature. Other supported models indicated that competitor abundance, predation intensity, and water clarity were also associated with occupancy.

Although salinity and temperature predicted occupancy in the top model, the relationship between X2 and salinity is complex, and none of the models that explicitly included X2 were supported by the data. The authors conclude that management on the basis of salinity oversimplifies habitat quality and quantity, and focusing on X2 may be less effective than focusing on other metrics of delta smelt habitat. For example, it may be worthwhile to explore options for increasing habitat quality in the northern subregions where probabilities of occupancy were consistently high during autumn and spring.

Source: Hendrix, A.N. et al. 2022. Relations Between Abiotic and Biotic Environmental Variables and Occupancy of Delta Smelt (Hypomesus transpacificus) in Autumn. Estuaries and Coasts. DOI: 10.1007/s12237-022-01100-x

Return to Top

Including Estuaries in Greenhouse Gas Reduction Strategies

Increasing sediment availability can bolster carbon accumulation

Estuaries have historically been overlooked as major contributors to climate change mitigation due to their comparatively small area. However, estuarine habitats capture greater amounts of carbon per hectare than many upland terrestrial forests. The ability to sequester carbon depends on the vertical accretion of sediment and the horizontal migration of tidal marshes, which means that if rates of sea level rise (SLR) outpace the vertical accretion rate, the resilience of estuarine habitats will be diminished—or worse, soil destabilization could release stored carbon.

A team of researchers developed a tool to help predict the effects of SLR on soil carbon accumulation in sediments. They used a process-based model of soil accretion combined with a habitat classification model to generate habitat distribution maps of Puget Sound’s Nisqually River Delta over the next 100 years under a range of sediment input and SLR scenarios. Under present-day sediment levels, substantial changes to high salt marsh habitats are projected to occur beginning at 100 cm SLR, with much of their area converted to low (or transitional) salt marshes and mudflats. Increasing sediment availability increased projected carbon accumulation by reducing the amount of transitional salt marsh area that would convert to mudflats, but didn’t prevent losses of high marsh habitat. Allowing marsh migration by opening flow paths via causeway removal also bolstered carbon accumulation. According to their analysis, the economic value of carbon accumulation continued to rise over time even as total carbon accumulation was projected to plateau or decline—suggesting that the value of this ecosystem service can be resilient to climate change.

Applying this modeling process to sediment-starved systems may help identify SLR thresholds where tradeoffs emerge between maintaining present-day habitat distributions and carbon accumulation. Without targeted management that includes planning for SLR, estuarine habitats, and the carbon they contain, will continue to decline.

Source: Moritsch, M.M. et al. 2022. Can Coastal Habitats Rise to the Challenge? Resilience of Estuarine Habitats, Carbon Accumulation, and Economic Value to Sea‑Level Rise in a Puget Sound Estuary. Estuaries and Coasts. DOI: 10.1007/s12237-022-01087-5

Return to Top