Strong wind in reeds above and below water

Reeds grow in water; the tall stems and leaves catch the sun and wind above, while the roots are in the mud below. This track is a mix of normal mics listening to wind in the reeds combined with a mono underwater mic - in sync and at the same spot - picking up the below surface sound. The very present bass is all from the underwater mic. I don't know what creates this, maybe its the movement of the whole reed bed, which is extensive, or the sound of waves pounding the beach 300meters away transmitted through the ground.