ABUNDANCE OF ARCHAIAS ANGULATUS ON THE INNER WEST FLORIDA SHELF SUGGESTS THE INFLUENCE OF CARBONATE ALKALINITY OVER SALINITY

Archaias angulatus (Fichtel and Moll, 1798), 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 (4 adults per gram of sediment) where spring-fed rivers emerge from a limestone aquifer, and in Florida Bay to the southeast (25 adults/g) where the sediment is primarily biogenic carbonate. 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. Water chemistry measurements were taken diurnally over 1-2 day periods at four sites in winter, spring and autumn of 2015. Salinity and temperature 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). The highest TA was found in the Springs Coast (2766 μmol/kg-seawater, three-season average), along with the lowest salinities, which reveals a strong contribution from the nearby rivers, springs and seeps. A TA end-member regression analysis predicts, and sampling confirms, higher TA values nearer to the river mouth, highlighting the atypical relationship of alkalinity and salinity in this carbonate province. A daytime increase in the TA to DIC ratio was seen at all sites; however, the Springs Coast site (~5km from the Weeki Wachee river mouth) exhibits stronger tidal influences on TA and DIC than diurnal influences. The gradual relief off the Springs Coast, as well as clarity of the water column, provide ideal physical habitat, and the input from spring-fed sources enhances the water chemistry for calcifying organisms. Presence of A. angulatus in low salinity waters influenced by high alkalinity riverine discharge led to a new hypothesis that calcification in A. angulatus requires high carbonate alkalinity but not high (normal seawater) salinity.