* to be effective the size of the particles counted should be adjustable (a pin hole?) & be in 3 particle size ranges preferably: (1) < 0.25 mm, (2) 0.25 - 0.5 mm, and (3)0.5 - 1 mm.
the photographic equipment needed is not easily miniaturized
Do you need real-time numbers? As, mentioned previously, there are bench-top particle size analysers, but that would involve collecting samples. I suspect it would be hard to replicate for in situ measurements.Here's another approach that might work for you. Use a camera with a microscope lens of known power and an led light to take photographs in the water column. You would probably need several to get a representative sample. Then download the photos and use digital image processing to count particles and measure their sizes.