An annual harvest of clams provides overall environmental benefits of .

While some environmental benefits of clams can be easily documented, such as nutrient (for example, nitrogen) extraction associated with removing the product at harvest; others, such as denitrification, are not. The benefits in this calculator are based only on the clams harvested in a year, not on the entire standing crop of the farm (for example, seed, juveniles). This makes precise calculations of the economic value of these benefits difficult. The results presented here should be considered approximations of the benefits produced by clams. These benefits do not account for sales value of the clams harvested or costs (seed, gear, labor) associated with growing them.

The gills of clams, covered with tiny moving cilia, pump water through the clam, drawing it in the incurrent siphon. The gills capture phyto­plankton and remove oxygen from the water. The cleared water is then ejected from the excurrent siphon.

Your harvest of clams filtered gallons of seawater per day.

By the very act of feeding, shellfish filter phytoplankton (microscopic algae or plants) and nutrients out of the water, cleaning and clarifying the water. In fact, a single littleneck-sized clam can filter 4.5 gallons of water per day. Clearer water allows more sunlight to penetrate, which aids in the growth of important seagrasses and increases oxygen in the coastal environment. By removing phytoplankton and nutrients from the water, shellfish may also prevent harmful algal blooms.

The filtering rate of clams was determined in the lab using a fiber-optic colorimeter, which measures the turbidity of a phytoplankton solution. The turbidity of the water declined over time, as the clams removed the phytoplankton from the water. Using these data, the volume of seawater cleared of phytoplankton was calculated per day for the commercial harvest sizes.

Watch this time-lapse video to see how efficiently clams remove phytoplankton from seawater.

The role of clams in nitrogen cycling.
Adapted from Newell et al. 2005.

This year your harvest of clams removed pounds of nitrogen from the coastal environment.

Clams do not absorb nitrogen directly from their environment. Clams feed on naturally-occurring phytoplankton, which use dissolved inorganic nitrogen, available in the water, to grow. Thus, clams incorporate nitrogen from their food into their tissues and shells. When clams are harvested, the accumulated nitrogen is removed from the water. The nitrogen content of both clam tissues and shells was measured by drying, weighing, and grinding them into fine powders. Stable isotope mass spectrometry was then used to determine the proportion of nitrogen in the samples. From these data, total weight of nitrogen was calculated for each clam harvest size.

Shellfish also play an important role in the cycling of nutrients, including nitrogen. For example, bivalves release nitrogenous waste (urine) that can be used by phytoplankton as a source of nitrogen. In addition, some of the nitrogen filtered from the water by the clam is deposited to the sediment as feces and pseudofeces (rejected food particles). These biodeposits are decomposed by bacteria, which transform the nitrogen to a variety of other forms, including ammonia, nitrate, and nitrogen gas.

The role of clams in sequestering atmospheric carbon.

This year your harvest of clams sequestered, or stored, pounds of carbon.

Carbon dioxide (CO₂), a major greenhouse gas, dissolves in seawater and is incorporated by shell-producing marine organisms into calcium carbonate (CaCO₃). Shells of mollusks (clams, oysters) provide a long-term sink for atmospheric CO₂. In contrast, the carbon contained in most plant and animal tissues returns to CO₂ within a few years. The carbon content of both clam tissues and shells was measured by drying, weighing, and grinding them into fine powders. Stable isotope mass spectrometry was then used to determine the proportion of carbon in the samples. From these data, the total weight of carbon was calculated for each clam harvest size. The shells are about 12% carbon by weight; each littleneck clam represents about 2.8 grams of carbon in tissue and shell.

Bivalves not only sequester carbon in their shells and tissues, but also process it while they are growing. Just like other animals, they produce carbon dioxide as a waste product of respiration. In addition, the carbon deposited in the sediments as feces and pseudofeces (particulate organic carbon) is consumed by a variety of organisms, such as worms, brittle stars, and other deposit feeders. Some carbon will remain locked in the sediments as shell fragments, limestone, and dolomite.

Clam farming stimulates diversity and abundance of other marine plants and animals.

The culture gear (bags, cover netting) used by growers creates a favorable environment for a myriad of plants and animals, such as juvenile fish and crabs, by providing habitat, substrate, and protection. This is especially significant since shellfish aquaculture leases can only be located in areas that undergo a resource survey to ensure the site is devoid of seagrasses and other marine life. Once in production, a clam farm may have over 1000 bags planted per acre of lease bottom. The diversity and abundance of marine organisms stimulated through farming activities is one of the positive benefits that shellfish aquaculture provides to the coastal environment.

A pictorial guide of marine organisms commonly found in, on, and around clam culture bags on farms in the Big Bend area of Florida was developed by UF scientists in 2008. The guide features over 150 organisms and assists clam farmers in identifying them.

Visit What's in the Clam Bag?, a pictorial guide, to identify over 150 marine plants and animals that can be found on a clam farm.

The Florida Clam Farm Benefit Calculator allows clam growers to estimate the economic values associated with environmental benefits their farms provide to the coastal environment. The value of environmental services provided by clams is determined by considering how much it would cost to replace these services with human activities.

Value of Nitrogen Removal - Nitrogen removal could be provided through a wastewater treatment plant. For the Cedar Key area (Levy County), this would cost about $16.71 per pound of nitrogen removed (Burke 2009). This implies that each pound of nitrogen removed by clams saves $16.71 relative to obtaining the same water quality improvement through alternative means. Costs of providing the water treatment include land and labor costs, which vary by county in Florida. Consequently, the value of nitrogen removal will vary depending on where it is provided. Values are determined for 11 coastal counties where clam farming is occurring in Florida.

Value of Carbon Sequestration - Similarly, the carbon sequestration provided by clams could also be provided by converting existing pasture or agricultural land to forest. In the Cedar Key area, these activities would cost about $20.47 per ton (short) of carbon sequestered (Nielsen et al. 2014). This value includes the cost of planting forest as well as potential revenues lost from changing land uses. In areas with highly valued agricultural land uses, the cost of converting that land to forest will be higher. Consequently, in these areas, the value of sequestering carbon through clam production instead of alternative means will be greater. Of the clam producing counties in Florida, the highest replacement costs ($144.33 per ton) occur in Collier County, while the lowest ($0.86 per ton) occur in Franklin County, driven by county land values.

The Calculator was developed by the following University of Florida IFAS research and extension faculty: Leslie Sturmer, Cooperative Extension Service; Dr. Shirley Baker, School of Forest Resources and Conservation; Dr. Kelly Grogan and Dr. Sherry Larkin, Food and Resource Economics Department.

Graduate students who contributed to this project include Angelo (Jason) Spadaro and Jorge Avila.

Dacing Tree, Inc. developed the web-based calculator, which was inspired by the National Tree Benefit Calculator created by Casey Trees and Davey Tree Expert Company in association with the USDA Forest Service's Center for Urban Forest Research. The nitrogen and carbon cycles were illustrated by Anna Hinkelday. Photos were provided by the FDACS Bureau of Seafood and Aquaculture Marketing, University of Florida IFAS Communications, and Eric Zamora.

Funding for this project was obtained from the Florida Department of Agriculture and Consumer Services (FDACS) through the 2014-15 Florida Aquaculture Program (contract #00094300).

References:

Burke, S. 2009. Estimating Water Quality Benefits from Shellfish Harvesting; A Case Study in Oakland Bay, Wisconsin.
Technical Memorandum. Entrix, Inc.

Newell, R.I.E., T.R. Fisher, R.R. Holyoke and J.C. Cornwell. 2005. Influence of Eastern oysters on nitrogen and phosphorus regeneration in Chesapeake Bay, USA. IN: Dame, R. and S. Olenin (Eds), The Comparative Roles of Suspension Feeders in Ecosystems. NATO Science Series IV-Earth and Environmental Sciences, Vol 47: 93-120. Springer, Netherlands.

Nielsen, A.S.E, A.J. Plantinga and R.J. Alig. 2014. New Cost Estimates for Carbon Sequestration Through Afforestation in the United States. General Technical Report PNW-GTR-888. U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, 35 p.