Introduction

The distribution of the Northern Rock Barnacle (Semibalanus balanoides) in the intertidal zone is affected by several different factors.  This study was conducted in Nahant, MA at Northeastern University’s Marine Science Center.  It was originally hypothesized that the densest masses of S. balanoides was found on vertical rock faces.  The study analyzed what variables affected the density of the barnacle masses and the average size of the barnacles in a given area.  The six different variables that were measured included: slope, direction, wave exposure, location in the intertidal zone, temperature, and the surface of the substrate.  Both graphical and regression analysis were performed for each variable against the percent cover and average barnacle size separately.  Analysis of the results revealed that in areas of high was exposure, both the barnacle density and average size increased.  It was also found that substrates with rough surfaces had significantly denser masses of barnacles.  Because the samples were chosen at random, some of the variables had skewed results and more data needs to be collected in order to make definite conclusions on the other variables.

            Throughout the rocky intertidal zone, the distribution of a given species is limited by a number of different factors.  The Northern rock barnacle (Semibalanus balanoides) can be found throughout the rocky intertidal zone and its distribution and abundance varies throughout a given area.  What factors determine the distribution of barnacles is dependant on a number of things.  In a closely related species, Balanus balanoides, a study found that the barnacles will settle in the intertidal zone primarily between the months of April and May (4).  The settlement occurs just weeks following the spring phytoplankton bloom.  The plankton larvae live for several weeks suspended in the water column and they depend heavily on the presence of high phytoplankton concentrations to feed upon.  S. balanoides larvae prefer to settle on cleared substrate without the presence of detritus and diatoms (5).  This indicates that they prefer to settle on a specific substrate. 

            The distribution and abundance of S. balanoides is dependent on several different physical factors as well.  Large barnacles are more susceptible to desiccation than smaller ones (6).  Thus, in the higher intertidal zone, primarily smaller barnacles are found.  In regards to the slope of the substrate, one study found that S. balanoides died at all significantly sloped surfaces that were higher than the mean tide level (7).  It also found that the barnacles were limited in growth and they approached the subtropics coast because of desiccation in the high summer temperatures.  Furthermore, the physiological condition of the larvae has also been found to affect the vertical distribution of the barnacles (8).  Finally, the amount of sun and shade can affect the distribution of the barnacles and how they compete with other species (9).  The study conducted in this paper examined the distribution and abundance of S. balanoides in the rocky intertidal in Nahant, MA.  The slope, direction, location in the intertidal, wave exposure, temperature, and surface of the substrate were measured at random.  These variable were then compared with the density and average size of the barnacles to assess the preferred habitat and optimal growth conditions of the barnacles.