From the surface to the seafloor: How giant larvaceans transport microplastics into the deep sea

Abstract

Plastic waste is a pervasive feature of marine environments, yet little is empirically known about the biological and physical processes that transport plastics through marine ecosystems. To address this need, we conducted in situ feeding studies of microplastic particles (10 to 600 μm in diameter) with the giant larvacean Bathochordaeus stygius. Larvaceans are abundant components of global zooplankton assemblages, regularly build mucus "houses" to filter particulate matter from the surrounding water, and later abandon these structures when clogged. By conducting in situ feeding experiments with remotely operated vehicles, we show that giant larvaceans are able to filter a range of microplastic particles from the water column, ingest, and then package microplastics into their fecal pellets. Microplastics also readily affix to their houses, which have been shown to sink quickly to the seafloor and deliver pulses of carbon to benthic ecosystems. Thus, giant larvaceans can contribute to the vertical flux of microplastics through the rapid sinking of fecal pellets and discarded houses. Larvaceans, and potentially other abundant pelagic filter feeders, may thus comprise a novel biological transport mechanism delivering microplastics from surface waters, through the water column, and to the seafloor. Our findings necessitate the development of tools and sampling methodologies to quantify concentrations and identify environmental microplastics throughout the water column.

Fig. 1. Experiments were conducted on board R/V Western Flyer using DeepPIV’s particle injector deployed from ROV Doc Ricketts.

We conducted in situ feeding studies of giant larvaceans using a modified dye injector pump developed with the DeepPIV hardware (20). A peristaltic injector pump emptied a bag filled with microplastic particles and dispensed particle-rich fluid through the tubing nearby an animal.

Fig. 2. During feeding experiments, microplastics were observed inside and attached to the inner house and inside the gut of giant larvaceans.

Microplastic particles in varying size ranges (from 10 to 600 μm) are represented by different colors (for example, red, yellow, green, and orange; see table S1 for specific particle sizes). Image corresponds to B. stygius specimen D5 from dive D870 (table S1). Scale bar, 2 cm. ih, inner house; bt, buccal tube; mp, microplastics; g, gut; tr, trunk; m, mouth; t, tail; r, ramp of inner house.

Fig. 3. Collected giant larvaceans were maintained in a cold room, and after 12 hours, animals and fecal pellets were imaged under a microscope to determine microplastic particle size.

(A) After the 12-hour period, the guts of intact individuals were completely evacuated. (B) Microplastic particles were incorporated into the fecal pellets, where each color (for example, red, orange, yellow, and green) corresponds to specific size classes of particles shown at lower (B) and higher (C) magnifications. (See table S1 for specific particle sizes). The fecal pellet in (C) can be seen in the upper left corner of (B). Images shown here correspond to B. stygius specimen D5 from dive D870 (table S1).