Native peoples on the west coast of North America have been using a natural form of alkalinity enhancement for thousands of years to boost their shellfish production. They have developed a special technique for breeding called clam gardening. The shellfish breeders construct walls in marine bays from debris and rocks along the low-water line. When surf water flows over this wall at high tide, silt, sand and gravel sediments are trapped behind it, settle to the bottom, and over time form a kind of terrace. Native clams then colonize the sediments on the terrace.
To encourage growth, the indigenous people have been scattering broken shells on the terraces and working them into the sediment for generations. The shells are composed of calcium carbonate, an acid-binding mineral. This raises the pH value of the pore waters, which especially benefits the acid-sensitive juvenile clams. Furthermore, the calcareous shell remains serve the young clams as a kind of storehouse for building material.
The results are impressive. Four times as many native butter clams (Saxidomos gigantea) and more than twice as many clams of the species Leukoma staminea grow on the terraces that have been spiked with clam shells than on a natural stretch of coastline. In addition, the shellfish grow faster, which is due not only to the addition of calcium carbonate, but also to other effects related to the terraced gardens.
Shellfish harvesters in other parts of North America are now also using alkalinity-enhancing methods to prevent production losses related to acidification. The amount of additional carbon dioxide that the clam gardens of the natives remove from the atmosphere, however, has not yet been measured.
fig. 7.3 > Native shellfish species grow faster and with greater densities in west-coast native clam gardens than on other stretches of coast without the artificial terracing and additional limestone input.