DISTRIBUTION OF ARCHAIAS ANGULATUS ON THE INNER WEST FLORIDA SHELF: SUBSTRATA AND WATER CHEMISTRY VERSUS TEMPERATURE

Archaias angulatus, a large symbiont-bearing foraminifer (Order Miliolida) that produces a Mg-calcite shell, is common throughout the Caribbean and warm western Atlantic region. This species lives abundantly in seagrass beds along the Springs Coast of northwest Florida, and in Florida Bay to the southwest. In contrast, live specimens are seldom found in the seagrass beds along the central-west coast of Florida, where barrier islands dominated by quartz sand occur. Our working hypothesis is that substratum and water chemistry are more important than temperature in explaining differences in abundance of A. angulatus associated with the seagrass meadows along the west Florida coastline and shelf. Differences in mean winter and summer temperatures between the Springs Coast and the west-central coast are negligible (17-30 °C), while Florida Bay temperatures are consistently warmer (21-30 °C). Water chemistry measurements were taken diurnally over 48-hour periods at four sites in January and May 2015. Salinity, temperature and pH were measured in situ, and sealed bottles of seawater were transported to the laboratory for analysis of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA). Salinity was lowest (~24) at the Springs Coast site and alkalinity was highest (>2900 μmol/kg-sw both samplings). At a seagrass site between the central-west coast and Springs Coast sites, where live A. angulatus were observed in situ, salinity was as low as 26 (TA~2700 μmol/kg-sw) in winter, and as high as 32 in spring (TA ~2400 μmol/kg-sw). At the central-west coast site, salinity was near normal marine in winter (~33) and spring (~35), while TA was ~2400 μmol/kg-sw in winter and ~2500 μmol/kg-sw in spring. Salinity at the Florida Bay site was ~36 both seasons, while TA was ~2600 μmol/kg-sw in winter and ~2200 μmol/kg-sw in spring. The broad, shallow Springs Coast shelf is underlain by Eocene-Oligocene limestone that host the aquifer and springs that give the region its informal name. The chemical composition of the limestone bedrock underlying the Springs Coast and shelf, as well as the extremely gradual slope, provide suitable habitat for A. angulatus despite the more northern location.