Gradient analysis of diatom assemblages in a Puget Sound salt marsh: can such assemblages be used for quantitative paleoecological reconstructions?

Abstract

Taphonomy is important to coastal paleoecologists because processes acting on diatom thanocoenoses tend to work towards obscuring original ecological relationships between diatom assemblages and the environment. The purpose of this paper is to briefly describe diatom taphonomy and present a method for quantitative reconstruction of environmental parameters from salt marsh diatom assemblages. The main hypothesis for this study is that major environmental and taphonomic processes (e.g., tides) act in predictable ways to distribute living and dead diatoms along environmental gradients. To test this hypothesis, a modern transect was established across a large salt marsh in southwestern Puget Sound for the purpose of determining modern species/environment gradients and calibrating species assemblages to environmental variables. Canonical correspondence analysis (CCA) relates modern species assemblages to environmental gradients, and weighted averaging calibration is used to develop transfer functions for predicting environmental information. CCA showed that the effect of salinity and elevation on the species distributions is significant, indicating that environmental processes control the distribution of sedimentary diatoms across the salt marsh surface in predictable ways. Salinity was strongly correlated with CCA Axis 1 and elevation with Axis 2. The calibration results indicate that, although mixing of allochthonous and autochthonous diatoms does occur, salt marsh diatom assemblages reflect major environmental gradients in Puget Sound salt marshes and can be effectively used for quantitative reconstructions of former environmental conditions.