I am coming against some difficulties when trying to determine the mass of a object in the water. I cannot simply weigh the organism because it is too large.

I can determine the exact volume. I originally thought that it was neutrally buoyant and so I planned to determine the volume of the object, and then use the archimedes principle to show that the mass is equal to the mass of the water that the object displaces, which can be worked out through volume.

However, this turns out to be an incorrect assumption. The objects I am studying are actually very slightly negatively buoyant (I am not able to establish how -vely buoyant).

Is there some way of determining density without mass that I haven't managed to come up with yet? or would it be better to just add a coefficient for the negative buoyancy to my equation adapted from the archimedes principle??

I'm really going for accuracy, so not using a 'fudging factor' in my equation would be ideal...

Really hope this isn't a non-starter!!

Also, please be kind with the physics terminology, I am a marine biologist by trade, not a physicist!

Many thanks in advance

Jen