Susceptibility of Atlantic Surfclams to Ocean Acidification

This project aims to understand how surfclams are affected by the changing chemistry of their habitat.

New England/Mid-Atlantic

Female scientist wearing an orange life vest and knit hat stands in shallow water beside a small boat with four Massachusetts Maritime Academy students, all wearing jackets and waders. Water and the sunset make up the background. — Massachusetts Maritime Academy students (right to left) Ben Tuttle, Shannen Allen, and Grace Calvert work with Molly Roberts (left) to sample seawater and surfclams to bring back to the lab. Credit: NOAA Fisheries / Emily Roberts

Atlantic surfclams support an important commercial fishery in the Northeast United States, with landings valued at nearly $30 million in 2019. They also provide environmental benefits, such as removing excess nutrients from the water, and are one of the largest bivalves on the East Coast. We characterize the seafloor habitats of Atlantic surfclams to understand their susceptibility to ocean acidification.

What is Ocean Acidification?

About a quarter of the human-produced carbon dioxide released into the atmosphere is absorbed by the ocean. This causes changes to water chemistry, such as lowering pH, which is called ocean acidification. We are studying how ocean acidification may affect commercial bivalve species, including Atlantic surfclams.

An off-white surfclam shell on a sandy beach, with the ocean in the background. — A surfclam shell on the beach on Cape Cod, Massachusetts. Credit: NOAA Fisheries/Molly Roberts

What We Do

Our activities include:

Characterizing the carbonate chemistry of the seawater and sediment where Atlantic surfclams live
Evaluating the growth of the northern and southern subspecies of Atlantic surfclam to better understand the role of the environment and genetics on the growth of surfclams
Clarifying the role of subspecies versus the environment on their growth
Using real world data to validate a model of surfclam growth and reproduction under future ocean acidification conditions

Validating the Model with Real World Data

Milford Lab scientists built a model that projects Atlantic surfclam growth based on water temperature and the partial pressure of carbon dioxide, a commonly used metric of ocean acidification. This model indicates that ocean acidification will negatively affect surfclam growth and reproduction by the year 2100, under the Intergovernmental Panel on Climate Change’s high carbon dioxide emission scenario.

Now we are studying surfclam populations in their natural habitat to see how well real-world observations match these model predictions. We will use data collected in the field to refine our growth model for surfclams, making it more accurate.

Sampling the Porewater

We sample surfclam seafloor habitats associated with surfclams monthly to find out whether their growth rate correlates with the chemistry of the sediment in which they burrow. We call the water between sediment grains porewater. Scientists currently know much less about changes to sediment porewater chemistry than they know about the overlying waters, yet surfclams spend most of their time burrowed in the seafloor.

One Surfclam, Two Subspecies

One student takes a water sample while standing in knee-deep water while two other students look on from a small boat labeled “MMA Research Vessel”. The sun is setting in the background and the students are wearing jackets and waders. — Three undergraduate students from Massachusetts Maritime Academy assist with taking water samples of surfclam habitat in Cape Cod, Massachusetts. Credit: NOAA Fisheries/Molly Roberts

Although the Atlantic surfclam fishery is managed as a single species, there are actually two genetically distinct subspecies native to Massachusetts waters. Spisula solidissima solidissima thrives in deeper water off the northern coast of Cape Cod. This subspecies grows larger and has a longer lifespan than the southern subspecies, Spisula solidissima similis. The smaller subspecies thrives in areas with riverine influence and is more tolerant of warm water and lower pH conditions.

The two subspecies may adapt to different sediment carbonate chemistry conditions in their respective habitats. We will use a transplant experiment to better understand the role of the environment and genetics on the growth of surfclams.

This information will help the commercial surfclam industry, recreational fishermen, and the developing surfclam aquaculture industry.

Student Involvement

Undergraduate students at Massachusetts Maritime Academy are providing field support for this project as they learn about shellfish biology and ocean acidification.

Study Methods

Study Locations

Map of Cape Cod showing two sites on the southern coast, Eel Pond (west) and Cockle Cove, Chatham (east), and two sites on the northern coast, Horseshoe Shoal, Barnstable Harbor (west) and Nobscusset Harbor (east). — Map of the four study sites: Barnstable Harbor, Falmouth, East Dennis, and Chatham, all towns on Cape Cod in Massachusetts. Credit: NOAA Fisheries/Molly Roberts

We are monitoring environmental conditions in surfclam habitats at five sites on Cape Cod, Massachusetts:

Barnstable Harbor
Falmouth
East Dennis
Chatham
Provincetown

We are also conducting transplant experiments at three of these sites:

Falmouth
East Dennis
Provincetown

Research Questions

Does temperature or carbonate chemistry of the seawater or sediment porewater limit the growth of surfclams?
Do differences between the subspecies affect the relationship between environmental conditions and growth?
How well do our lab-developed growth models predict surfclam growth rates in the field based on sediment carbonate chemistry of their habitat?

Habitat Characterization/Environmental Monitoring

We sample habitats and their associated surfclams monthly to find out whether their growth rate correlates with the sediment chemistry where they live. We are collecting seawater, sediment porewater, sediment, and surfclams at each of our study sites. Sediment porewater is collected from depths of: 2 cm, 5 cm, 15 cm, 20 cm. In each seawater sample, we measure:

Carbonate chemistry (pH, alkalinity, and dissolved inorganic carbon)
Temperature
Salinity
Food available for surfclams (Because surfclams filter feed, food available to them is organic and inorganic material suspended in the water.)

We also take sediment cores to measure grain size as well as organic and carbonate content. For each sediment sample, we measure:

Temperature
Redox potential
Percentage of organic matter
Percentage of carbonates

Transplant Experiment

A square of PVC pipe surrounds four green circles stuck into the sand in shallow water. — Surfclam enclosures for the transplant experiment, Provincetown, Massachusetts. Credit: NOAA Fisheries/Molly Roberts

In spring 2022, we began a transplant experiment to evaluate the growth of the northern subspecies in their native habitat. We also evaluated the habitat dominated by the southern subspecies. Our research will help define the role of the environment on growth compared with the role of genetic differences in populations and subspecies.

The transplant experiment will last for 9 to 12 months and include at least three sampling periods. We will keep surfclams in nylon mesh enclosures and study their growth during this time, with 4 to 6 enclosures per treatment.

During the three sampling periods, we will measure growth and monitor environmental conditions in each enclosure, including carbonate chemistry, temperature, and chlorophyll a, a measure of food availability. We will also conduct biodeposition experiments to measure feeding and metabolism.

wo side-by-side images of clam shells inside plastic buckets. The bucket on the left contains white fragments of surfclam shell, while the one on the right contains ground-up shell called hash. — Surfclam shell (left) is ground up to shell hash (right) that will be used in the transplant experiment. Credit: NOAA Fisheries/Emily Roberts

We will expose each subspecies to two different substrates: sediment from the site and sediment from the site mixed with crushed shell. Previous studies found that adding crushed shell can raise the pH of the surface sediment. Shell is made of calcium carbonate and buffers the sediment against changes in pH. This comparison will help us learn if adding crushed shell raises sediment pH and improves the growth of surfclams. If this strategy is effective, the developing surfclam aquaculture industry could use it to adapt to lower pH conditions in the area.

Toward the end of the experiment, we will use transcriptomics to look at differences in gene expression between surfclams kept in ambient sediment and those in sediment with crushed shell.

We will test whether the genes that are “turned on” and expressed by surfclams differ depending on sediment pH. We will determine if, in sediments with lower pH, more genes associated with greater physiological stress, metabolism, and calcification are “turned on” or upregulated. This result would indicate that there is a genetic component to acclimation to lower pH conditions.