1. A method for density measurement of zooplankton in situ in an aqueous solution, the method comprising: acquiring at least one image of a volume of the aqueous solution; processing the at least one image and identifying particles in the at least one image; analyzing the identified particles based on a sharpness of each particle, and identifying zooplankton to be counted; wherein analyzing the sharpness of each particle comprises: tracing a boundary of each identified particle, calculating an intensity gradient for each pixel along the boundary of each particle from the at least one image based on a rate of change of intensity between neighboring pixels, calculating a mean intensity gradient of all the pixels along the boundary of each particle, and correcting the mean intensity gradient for a general contrast level in the at least one image.