What is the distribution of microfibers and plastic debris in the water column of the Salish Sea and Inside Passage?

Methods

Summary

Water samples will be collected from the deck of my 40’ boat at depths of near surface, 4.5 meters, and 30 meters (~100 feet) at selected locations along the Inside Passage from southern Puget Sound through British Columbia and southeast Alaska as far north as Glacier Bay National Park.  The 4.5 meter depth was selected to because the only published data on environmental microfibers in marine waters used that depth and we wish to be consistent.  An effort will be made to include samples in the vicinity of sewage outfalls as these are suspected to be the source of the microfibers.  It is anticipated that between 200 and 300 total samples will be collected.

Samples will be collected using a hose attached to a diaphragm pump.  A conventional fishing downrigger with 10 pound weight attached will be used to place the mouth of the hose at the desired depth.  Using this setup, a measured quantity of water will be pumped through two sieves. The larger sieve will have a 5.6mm mesh and will be used to filter out seaweeds and any other large debris or biological material larger than what is defined as “microplastic” (< 5mm).  The smaller, 63um mesh filter will be used to collect the sample.  Sixty-three microns was selected to be consistent with previous published work on environmental microfibers.  Material collected by the 63um sieve will be washed into a sample bottle with fresh water and preserved by the addition HCl.  

In the laboratory, a stronger solution of HCl will be added to complete the digestion of biological material.  Once digested, the sample will vacuum filtered using 47mm diameter Millipore filters with a 5um mesh.  A USB microscope attached to a computer will be used to count and photograph samples.  The banner photograph for this project is an example of a sample processed in this manner.  After counting, infrared spectroscopy will be used to identify the chemical nature of the microfibers.

Challenges

Perhaps the biggest challenge is to quantify the sample variability.  To address this challenge, we plan on conducting a sample variability study in waters of central Puget Sound.  In this study we will take several replicates at each depth at several stations.  The data will be processed as above, except for the infrared, before heading on the journey north.  The extent of sample variability will dictate how extensive we will need to take replicates.  Given the time required to both collect and analyze samples, there will be a trade-off between number of stations that can be sampled and the number of replicates which we determine are required.

Pre Analysis Plan

Data will be analyzed statistically using the analysis of variance (ANOVA), analysis of means, and other appropriate statistical methods.  The null hypothesis will assume a uniform distribution of microfibers in both depth and geography.  Any statistically significant findings will indicate a non-random distribution which, in turn, will help identify the source of microfibers and whether there is any variable risk to marine life based on the likelihood of ingestion.