Good thinking! My guess would be not too much. One time we were at the beach and the water was filled with tiny pumice stones bobbing all over the place. Because they were small and light though, none were getting left behind as the water retreated. They just kept getting sucked back out to sea. Think the microplastics would be the same because they're also small and float. Maybe?My opinion (guess) is I'd just give them the shelly sand for their calcium. But the grains might not be big enough to act as good grit for their gizzards. Maybe the sand has little rocks in it that are bigger than sand grains???
Just had a crazy thought, but I wonder how much of that microplastics stuff is in the beach sand now. Would it be an issue?