DOI: 10.5061/dryad.nk98sf7s9
title: Microplastic changes the sinking and resuspension rates of marine mussel biodeposits

description: Microplastic (MP; &lt; 5mm) is ubiquitous in marine environments and
 is likely transported by biotic benthic-pelagic coupling. Mussels are key
 benthic-pelagic couplers, concentrating particles from the water column
 into dense and nutrient rich biodeposits. This study examined how MP
 affects benthic-pelagic coupling processes of mussels exposed to feeding
 regimes with and without MP by measuring four attributes of biodeposits:
 1) morphology, 2) quantity of algal and MP particles, 3) sinking rate, and
 4) resuspension velocity. We found interacting effects of particle
 treatment and biodeposit type on biodeposit morphology. Biodeposits from
 the algae treatment contained more algal cells on average than biodeposits
 from the MP treatment. Biodeposits from the MP treatment sank 34-37%
 slower and resuspended in 7-22% slower shear velocities than biodeposits
 from the algae treatment. Decreases in sinking and resuspension velocities
 of biodeposits containing MP may increase dispersal distances, thus
 decreasing in-bed nutrient input and increasing nutrient subsidies for
 other communities.
description: Pacific blue mussels (<i>Mytilus trossulus</i>; 35 ±
 2 mm) were collected from Argyle Lagoon (48.519401, -123.013180) on San
 Juan Island in Washington State, U.S.A. in August 2019. Byssal threads and
 epibionts were removed upon collection and mussels were acclimatized at
 11-13°C in flow-through seawater tables at Friday Harbor Laboratories
 (FHL), University of Washington. Two feeding treatments were tested, algae
 and MP + algae. The <b><i>algae</i></b> treatment
 used<i> Dunaliella </i>spp., grown in culture at FHL, in
 concentrations ranging 10,000 – 20,000 cells mL<sup>-1</sup>
 between trials (concentration was consistent within trials; concentration
 previously shown to not affect CR; Harris and Carrington 2019). The
 <b><i>m</i></b><b><i>icroplastic +
 algae</i> </b>(or MP) treatment was the same as the algae
 treatments, but with the addition of<i> </i>fluorescent violet
 polyethylene spheres 32-38 µm (Item # UVPMS-BV-1.00; Cosphereic; Harris
 and Carrington 2019). The spheres were soaked in Tween-20, a surfactant
 that reduces hydrophobicity and clumping, for 24 h prior to
 experimentation. Microplastic concentrations ranged from 0 – 675 particles
 mL<sup>-1</sup> (concentration previously shown to not affect
 CR; Harris and Carrington 2019). Mussels were placed in treatment
 containers (1 mussel per container with 1 L of aerated FSW) to feed for 1
 h. Water samples (1.5 mL) were taken from each container at 0, 30, and 60
 minutes to calculate mussel clearance rate. Particle concentrations were
 quantified with a flow cytometer (Guava C6, EMP Millipore, Hayward, CA)
 using a RedR vs side scatter plot where the two types of particles
 fluoresced at different intensities and granularities. 
 <b>Biodeposit classification and measurements</b>:
 Biodeposits from and associated mussel were collected and transferred to a
 200 mL beaker of FSW after experimental feeding treatments where the
 mussel continued to excrete biodeposits for an additional 24 h.
 Biodeposits were then selected from each mussel for one of three
 experimental measurements: particle quantification, sinking rate, or
 resuspension velocity. Selected biodeposits were photographed and measured
 for length and width using ImageJ and volume was calculated. All
 biodeposit classifications were based on morphology. 
 <b>Particle quantification</b>: Each biodeposit
 selected for particle quantification was homogenized with a pipette in a
 1.5 mL microcentrifuge tube with 0.5 mL FSW. Algal cells (live and whole)
 and MP particles were counted in each homogenate using a hemocytometer
 under a compound microscope.  <b>Sinking
 rate</b>: Sinking experiments were conducted in a 1 L graduated
 cylinder filled with FSW at 15°C. Biodeposits were placed a few
 centimeters below the water surface to avoid complications with surface
 tension. Biodeposits were initially allowed to sink 10 cm to reach
 terminal velocity, which was measured as the time to sink an additional 10
 cm.  <b>Resuspension velocity</b>: Mussel
 biodeposit resuspension velocity was measured in a flume (Rolling Hills
 Water Tunnel 2436; El Segundo, CA) filled with seawater held at 11-13°C
 and flow was manipulated by an external computer. Twenty-four biodeposits
 were placed 6 cm apart from each other in a 4 x 6 grid pattern at the
 bottom of the flume working section (40 cm x 40 cm x 2 m, width x height x
 length). Free stream velocity was ramped up to 3 cm
 s<sup>-1</sup> (shear velocity of 0.3 cm
 s<sup>-1</sup>) for 10 minutes and the
 biodeposits remaining were recorded. This procedure was repeated at
 progressively higher velocities, up to 64 cm
 s<sup>-1</sup> (shear velocity of 6.4 cm
 s<sup>-1</sup>) or until all biodeposits left the
 grid and were resuspended.