Looking for microplastics in lake sediment: where does it go?

Looking for microplastics in lake sediment: where does it go?

The problem.

Microplastics in the freshwater environment originate from many materials and processes, many of which started out as bigger items that have begun to disintegrate from items such as shopping bags, single use food and beverage containers, synthetic clothing, and other products. Defined as plastic fragments smaller than 5 mm, microplastics are problematic because their size renders them accessible to a wide range of organisms that ingest them. Since the environmental effects include absorption, bioaccumulation, and potential physical and toxicological harm, a more comprehensive understanding of microplastics in freshwater ecosystems is necessary.In order to explore the presence and accumulation of microplastics, students and staff at Leech Lake Tribal College measured microplastics in three lakes (Irvine, Bemidji, and Andrusia) which are located in the headwater lakes of the Mississippi River watershed, which includes parts of the Leech Lake Band of Ojibwe (LLBO) Reservation

A bit more about the sites...

The Mississippi River is a first order stream meandering through wetlands, forest and wild rice beds as it approaches Bemidji – the first city on the Mississippi. At Bemidji, the municipal waste water treatment plant (WWTP) is the first major discharge of wastewater into Lake Irving near the stream channel into Lake Bemidji. Water then flows through the LLBO Reservation, where water quality and quantity are viewed as “necessary to continue practicing the inherent traditional, cultural and spiritual life ways of the people” (LLBO, 2018).


As no known reports of microplastics in the headwater river systems exist, our research took us out onto three lakes to collect sediment samples with a Van Veen grab style sediment scoop using methods outlines by Hidalgo-Ruz et al. (2012).

How did we do it?

Our first samples were collected on Lake Andrusia, followed by Irving and Bemidji, while the lakes were still covered in ice. After drilling into the ice with an auger, we dropped the sediment scoop along transects that ran perpendicular to the shoreline and put the samples in deionized glass mason jars to be brought back for analysis. We processed the samples utilizing density separation with a saturated salt solution followed by vigorous agitation to separate the microplastics from organic or lithic material.
We then vacuumed-filtered the supernatant through Wattman 0.5 μ glass microfiber filters. The filter papers were finally placed in a petri dish, labeled and covered to allow them to dry. We analyzed the filters with a 3x microscope, with any suspect microplastics transferred to slides for further examination under higher power microscopy. The suspected microplastics were documented, photographed, and closely examined by both students and advisors.
What did we find?

Results from the first field season found the presence of microplastics at all 21 sites sampled. Under higher power microscopy, microfibers are easily distinguished from natural materials or filamentous algae  because they will not have cellular structures and usually exhibit a homogenous color and thickness throughout the sample. Initial results show a low microplastic content in the first headwater lake (Irving, n=26), with a marked increase in microplastics in the second lake (Bemidji, n=38). This is perhaps not surprising as the second lake is subject to heavy shoreline development, has a high recreational use, and is subject to WWTP effluent. The third lake (Andrusia, n=33) is within the LLBO Reservation, and has limited shoreline development and no industrial or municipal activity.
Sediments from this lake contained fewer microfibers than the upstream lake, but more than expected giving the low anthropogenic activity.

What does it mean? 

The results from this pilot project suggest that there is transport of microplastics into LLBO lakes from upstream. Because microplastics have the potential to both physically harm organisms as well as leach harmful chemicals, microplastic transport may be of concern to the aquatic traditional food systems of the Ojibwe people in the Mississippi headwaters area. Further analysis is needed to determine the magnitude of microplastic concentration as they move through the watershed.