Graduate Research Assistant - Andrea MacLennan

NOAA/Padilla Bay NERR Research Assistantship Recipient

Woody debris in salt marshes

Andrea MacLennan
M.S. Candidate - Geography
Western Washington University
Natural Resource Conservation and Management
Huxley College of the Environment
Bellingham, WA

An analysis of large woody debris in two Puget Sound salt marshes:
Elger Bay, Camano Island and Sullivan Minor, Padilla Bay

BACKGROUND
Beaches of the Pacific Northwest are commonly characterized by an abundance of drift logs. Drift logs, also called large woody debris, tend to accumulate in the upper portions of the intertidal zones of Puget Sound beaches. Accumulations of woody debris are dispersed throughout Puget Sound and their locations are influenced by a variety of controls including geomorphology, longshore currents, fetch, and sources of wood. While the influence of large woody debris has been studied extensively in fluvial environments, its function in the nearshore environment is not well understood. It is generally thought that small quantities of large woody debris can increase nearshore habitat quality; however, no significant research has been conducted to verify this hypothesis. Prior research conducted on the effects of log storage and rafting has shown that accumulations of woody debris can alter the ecosystem integrity of intertidal and shallow water areas. A greater understanding of how large woody debris functions in nearshore environments is needed in order to provide effective management.

There has been considerable discussion about the positive and negative influences of large woody deposits in salt marshes. Some think that anthropogenic wood is negatively affecting the marsh and should be removed. Others believe that while deposited wood is often anthropogenic in nature, it mimics a natural process, and removal of that wood deprives the marsh of important structural material.

Fig. 1. Aerial photo showing extent of woody debris accumulation in Elger Bay, WA.

This research project is designed to investigate the nature of the wood deposits at two sites in Washington State in order to determine whether these deposits are having an adverse effect on the ecology of the marsh. Two areas with significant wood accumulations were selected for this study. They are Elger Bay (Fig. 1), located on the southwest side of Camano Island, and Sullivan-Minor Marsh in Padilla Bay.

METHODS
In each marsh, logs were surveyed along quadrats to determine whether they were naturally recruited, cut logs, or from an unknown source. In addition, the size (diameter and length), relative elevation, and stage of decay for each log were determined. The mobility of the large wood deposits were measured by taking the GPS location of individual logs for summer and winter field seasons and after storm events in conjunction with high tides (Fig. 2). Shoreward extent of the wood deposits was delineated.

The effects of woody debris deposits on ecological processes were determined by measuring percent vegetation cover over the water and species composition near the log. Water quality parameters that were monitored include dissolved oxygen, pH, and salinity. Additional measures of redox-potential were taken to determined wood decay effects on the sediments.

Beach seines were conducted in May 2003 and May 2004 in order to determine whether these habitats were used by juvenile salmon.

Fig. 2. Field data collection for GPS location and delineation of large woody debris accumulations.

PRELIMINARY FINDINGS
Andrea is currently is the process of writing her thesis results. However, she has determined some trends in the data that can be reported here.

Quadrat sampling at the Sullivan-Minor field site showed that of the 97 logs sampled, 63.9% were anthropogenic in origin, 32% were biogenic, and 4.1% were of unknown origin. Thirty-two percent of all logs were acting as substrate for vegetation, mainly for the Atriplex patula (saltbush) and Salicornia virginica (pickleweed). Mobile logs were consistently observed along the storm berm (seaward end of the marsh) and throughout the middle of the marsh where tidal channels and standing water are typically found. Static logs were most frequently observed adjacent to the upland edge or cross-dike at the southern-most edge of the marsh.

The woody debris does appear to provide some positive function, mainly as a substrate for upland vegetation that could not survive the conditions found in the lower marsh. This was observed around the perimeter of the marsh where the logs are less mobile.

According to Andrea's findings, large woody debris does act as an agent of disturbance, where the mobile wood smothers emergent marsh vegetation. In each of the study sites, the percent large woody debris cover was negatively correlated with percent cover of the marsh vegetation.

PROJECT SIGNIFICANCE
Given the enormous value of the world's coastal ecosystem services and functions it is vital to identify and avoid harmful anthropogenic disturbances to salt marshes. This study will contribute to the understanding of the effects of large woody debris accumulations in salt marsh habitats and help in the identification of sites with restoration potential.

Andrea is currently finishing her Masters degree at Western Washington University (Fig. 3). Her graduate professor is  Dr. Thomas Terich of Western Washington University.  Andrea is a recipient of a Padilla Bay NERR Research Assistantship.  For more information about her research, contact the Reserve