Seasonal variation in physical and biological factors that influence sediment porosity on a temperate mudflat: Willapa Bay, Washington, USA

Abstract

A field study of a temperate mudflat in Willapa Bay, Washington was conducted to determine whether seasonal variation in physical forcings (e.g., temperature, rainfall, insolation) coupled with variation in key biological properties (e.g., seagrass and microphytobenthos abundance) would result in measurable changes in seabed porosity. On two mudflats and in a nearby secondary channel, replicate samples for surficial grain size, chlorophyll a content, and seagrass (Zostera japonica) shoot density were obtained at roughly 7–wk intervals for one year starting in November 2009. In addition, high-resolution profiles of porosity in the upper ∼9cm of the seabed were measured using an in situ resistivity profiler. Ancillary meteorological and river discharge data were obtained from nearby stations operated by Washington State University and the U. S. Geological Survey, respectively. Despite significant seasonal variation in seagrass abundance (0 to >3000 shoots m−2), a less dramatic, but still significant variation in chlorophyll a content, as well as seasonal changes in physical forcings, the near-surface porosity of the flats was a normally consolidated bed that remained steady throughout the year. In contrast, porosity of the channel often departed from a normally consolidated profile, with evidence for both erosion and deposition of several centimeters of sediment. These departures did not display a clear seasonal pattern (i.e., deposition in winter, erosion in summer), however, suggesting that the provision and removal of sediment to/from the channels was governed by processes (e.g., channel slumps, rainfall events) occurring on shorter time scales or that there was significant, unresolved small-scale variability.