Salt marsh foraminifera from the Great Marshes, Massachusetts: environmental controls

Abstract

By using sites in the Great Marshes at Barnstable (Massachusetts, USA) this study examines the effects of a set of environmental parameters on the foraminiferal distribution. The studied parameters are: elevation above mean high water; salinity of the porewater; various sediment characteristics; vegetation; and food source. Relations between the environmental parameters and foraminiferal properties (frequencies, densities and diversities) are quantified with correlation coefficients. For the first time Siphotrochammina lobata and Balticammina pseudomacrescens are documented in the New England region. The following species show a significant correlation with one or more of the studied parameters and are designated as key-species: Haplophragmoides manilaensis, Jadammina macrescens, Balticammina pseudomacrescens, Miliammina fusca and Tiphotrocha comprimata . Based on cluster analysis and the presence, absence or dominance of the key-species characteristic associations are distinguished. The distribution of three associations is indicative of specific marsh environments: the marsh fringe, the middle marsh and the marsh edge. These three marsh units are separated by their own salinity regime, flooding and sediment characteristics. The marsh fringe is typified by the H. manilaensis Association and experiences freshwater input (seepage, surface runoff and rainwater) and only slight marine influence, resulting in low salinity values (2.5–20‰). The width of the marsh fringe is variable, dependent on the amount of seepage which in turn is controlled by the permeability of the basement and the peat. The J. macrescens Association characterizes the middle marsh where salinities are controlled by infiltration of sea- and rainwater and by evaporation. Salinity values are higher than 20‰, while temporarily salinity can reach extreme high values during periods without flooding and high evaporation rates (e.g., 44‰). The fully marine M. fusca Association occupies the daily flooded marsh edge where the salinities have the same values as Cape Cod bay water (ca. 28‰). Unlike many other salt marshes the distribution of foraminiferal assemblages in the Great Marshes does not show a vertical zonation with respect to mean high water. This shows that a worldwide applicable model for paleoenvironmental studies in salt marshes based on foraminifera is not feasible. Each salt marsh has its own characteristics. Regional factors such as climate play an important role in the salinity regime, while the local upland characteristics determine if seepage takes place. Thus each marsh has its own foraminiferal fingerprint showing the opportunistic behaviour of the salt marsh agglutinants. A surface study is an indispensable first step in assessing the value of foraminifera as paleo-ecological indicators.