When most people think of plastic pollution in the ocean, they may think of sea turtles swallowing plastic straws or fishing lines in seagull stomachs. While macroplastics make up a majority of ocean plastic pollution’s mass, the majority of individual particles are made up of microplastics, plastic particles smaller than 5mm in length. Typically, microplastics come from the breakdown of larger pieces of plastic, fibers that come off of synthetic clothes in the washer, and other sources such as industrial or cosmetic products. Increased microplastics can have significant effects on phytoplankton. Fonseca and Gaylarde, researchers at University of Oklahoma and Fluminense Federal University, find that the two are interconnected, and part of a positive feedback cycle.
Phytoplankton are responsible for around half of Earth’s productivity. Equally as importantly, they are the base of marine food webs. Basically productivity = lots of photosynthesis = phytoplankton growth = food supply for marine organisms, and throughout this process, carbon is taken up from the ocean / atmosphere.
Microplastics affect phytoplankton in a variety of ways, including toxic effects due to chemicals. At higher concentrations, photosynthetic efficiency is significantly decreased, up to 45% at 250mg/L.
Additionally, phytoplankton also acts as a bridge for carbon to be taken out of the atmosphere and into carbon sinks deep in the ocean.Typically, phytoplankton fix carbon through photosynthesis, and then sink as marine snow or are consumed by animals that also later sink to the ocean floor. However, microplastics can disrupt this mechanism too. They can attach to plankton and other organic matter, altering their size/density, which can slow sinking rates. This results in decreased carbon export to deeper waters, exacerbating the effects of global warming and climate change.
In the context of a global feedback loop involving climate change, microplastics reduce phytoplankton productivity and CO2 uptake. As a result, global warming causes more frequent weather events that increase microplastic fragmentation and dispersion, and as a result more microplastics are formed increasing the climate effects that increase microplastics. However, results are tentative and often derived from climate modeling. It’s unknown how accurate these are due to the limited data available on microplastic composition, concentration, and tracking.
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