Experimental Study Of The Impacts Of Marine Plankton On Mass Transfer

The material exchange and mass transfer in the marine environment has been an important aspect of the ocean research. Zooplankton belongs to the family of marine organisms in small size but huge in quantity. It has very limited capability of motion, but such motion is an active stirring which will increase the local active mixing, especially in low Reynolds number flow. Some zooplankton can migrate hundreds meters vertically in group, the contribution is not clear but cannot be neglected in promoting mass transfer and substances diffusion. Due to the technical limitations’, there are few efforts on the role of marine organisms in mass transfer.The technology development for quantitative observation is the key for the study of mass transfer by zooplankton. In this study, we build a physical mass transfer model between saturated salt water and fresh water in lab. A brine shrimp larva is selected to be the model animals which can survival in both conditions. With the technology of holographic imaging, we are able to trace the larva movements specially and temporally.When illuminated by collimated and coherent light, the salinity fields change in time can be represented by the change of refraction index fields thus causes the change of interference fringe pattern. In this study, the digital holographic image recording system is constructed to capture the evolution of density field caused by the model animal. A calibration experiment is done to test the dissolution and diffusion of sugar particles in still water. We proved that (1) The shape evolution of the material exchange interface can be recorded and reproduced by interference images and (2) Salinity is proportional to the intensity of the image.The mixing efficiency in the salinity gradient field is quantified in this study to evaluate the contribution of a brine shrimp larva. Four results are obtained.(1) It is true that the organism has a positive effect on the mixed procedure when salinitygradient exist;(2) The organism behavior can be quantified which will be correlated with corresponding background salinity gradient field;(3) The motion details, such as the perturbation range of each individual animal can be obtained with Particle Image Velocimetry (PIV) to evaluate its contribution to the local mixing.(4) The biological salt wake is one of the main mechanisms to increase mixing. It is formed by the materials transformed by animal body directly, and will decay in space and in time. Because of its complex shape, the valid interface area of mater exchange increased correspondingly. We have paved the road to quantify the evolution of biological wake.We have proved that the existence of brine did increase the efficiency of the mixture;The major promotion mechanism is the expansion of the effective mass transfer area. Such exchange area variants with the size, shape and behavior of animal which can be quantified with the current imaging technology.The experimental study indicated that the bio mass transfer was the coupling between biological factors and physical factors. We are able to simultaneously obtaine and analyze both factors, thus pave the road to study the mixing and promoting mass transfer in ocean.