The Upper Limit of Barnacles as an Index of Sea-Level Change on the New England Coast during the past 100 Years

Dense colonies of the intertidal barnacle, Balanus balanoides Linné, occur at many places on rocks and pilings on the North Atlantic coast. The vertical range of the barnacle is from about mean low water to mean high water, and its upper limit is commonly sharply denned. The upper limit is controlled by the tolerance of the barnacle to continuous exposure to air. This tolerance, called the lethal limit, was found experimentally to be about 150 hours for first-year barnacles and about 185 hours for older barnacles. It is shown that the maximum emersion interval increases progressively from the base of the tidal zone to the top of the zone. The upper limit of the barnacle for any given year is determined by the lowest level at which the lethal limit falls, which commonly is intermediate between the levels of annual mean high water and annual high-water neap. To this level one must add a small wave factor to determine the barnacle limit for a given year at a given locality. A search was made for photographs showing barnacle-encrusted rocks in order to see if changes in the barnacle level reflected changes in sea level. Three localities in the vicinity of Boston were found. The fluctuations of the upper barnacle limit in reference to the level of the 1962 limit are plotted on a curve of annual mean high water from 1848 to 1961 compiled from tide-gauge records from Boston and New York. There is substantial agreement between fluctuations of the barnacle limit and the annual sea-level values. From this it follows that a series of photographs taken in different years can be used to determine an annual sea-level curve for the locality in the absence of tide-gauge data. The annual sea-level curve and the barnacle data also show that sea level at Boston in the mid-nineteenth century was at approximately the same level as in the mid-twentieth century, but that it was approximately 0.5 foot lower at the turn of the century.