Dynamics of surficial trace assemblages in the deep sea

Abstract

We present a steady-state model addressing factors that determine surfical trace concentration (fraction of sea floor covered by tracks and trails) relative to the areal density and composition of mobile epibenthic megafauna. Trace concentration is posited to be a direct function of the mean production rate of new traces and their mean residence time. Production rate, in turn, is governed by the density of trace makers, their average width and rate of movement, and the fraction of available (i.e. untracked) space on the sea floor. Trace-maker density is not solely a function of epibenthic megafaunal density, but is also influenced by the microscale roughness of the sediment surface; as roughness increases, a higher proportion of mobile epibenthos (although deforming the sediment surface) do not leave recognizable traces. Trace residence time is controlled by the combination of three destructuve mechanisms; diffusive sediment mixing by infauna, retracking by epifauna, and physical reworking. Explicit predictions of the model are supported by new survey and time-series data from two deep-sea sites that differ radically in epifaunal abundance, Santa Catalina Basin (1300 m, east Pacific) and the HEBBLE area (4800 m, west Atlantic). In the former, due to high sediment roughness, a large proportion of the abundant epibenthic megafauna do not produce recognizable traces; thus trace production rate is low and trace destruction is dominated by retracking. This combination results in a low mean trace concentration (1% of sea floor surface area) relative to the density of epibenthic megafauna (16.5 m −2). In the latter region, a much smoother sediment surface results in trace production by most epifaunal taxa. The extremely abundant and active infauna at this site, however, produce high sediment mixing rates and thus low trace residence times and concentrations (5% of sea floor surface area). Application of a steady-state model to data from the HEBBLE region is validated by solving for the unsteady case and showing that approach to equilibrium trace concentration essentially (95%) complete only 10 days after a benthic storm erases all traces. Previous studies of deep-sea lebensspuren and megafauna prove consistent with the model.