Barnacle colonization on Spartina alterniflora in Georgia salt marshes

The distribution of barnacles within a habitat is influenced by numerous biotic and abiotic factors that impact barnacle recruitment and/or settlement. In Georgia salt marshes, barnacles (specifically Chthamalus fragilis) have been observed colonizing leaves and stems of S. alterniflora, which is the dominant plant species in marshes along the Gulf and East Coast. The aims of this study were to: 1) identify spatial and short-term monthly patterns of barnacle colonization within individual S. alterniflora as well as across sites of varying marine influence during peak barnacle recruitment and plant growth and 2) determine the effect of barnacle settlement on S. alterniflora growth and productivity. Patterns of barnacle settlement and measurements of plant growth and productivity were assessed through a field survey and field experiment in three salt marshes of varying proximity to oceanic inputs. Two marsh sites were dominated by tall-form S. alterniflora and one was composed of short-form plants. Results suggested spatial and short-term monthly patterns in barnacle settlement both across sites and within individual plants. Specifically, there was significantly greater barnacle density on stems and leaves of plants in salt marshes with greater marine influence. Furthermore, there was a decrease in barnacle settlement on S. alterniflora leaves and increase in settlement on stems from June to August, suggesting short-term monthly patterns in the portion of the plant colonized. Within individual plants, there was a significantly greater density of large barnacles on the middle segments of the stem and greater barnacle colonization in the non-axils of the stems compared to leaf axils. A potential negative impact of barnacle settlement on S. alterniflora was observed at the site with the greatest barnacle density and marine influence, which was also the site dominated by short-form S. alterniflora. These plants showed significantly lower photosynthetic efficiency in August, but it is unclear whether this was related to barnacle settlement, edaphic conditions, or both. Information on the patterns of barnacle settlement and its impact on S. alterniflora will be vital to coastal managers in salt marsh conservation strategies.