Nick Woods, Physical Oceanography


Development of a vertical current meter to measure vertical velocity associated with coupled physical-biological zooplankton aggregation mechanisms

Dense aggregations of zooplankton are an important part of the marine ecosystem and the primary source of food for whales and fish that reside off the coast of Cape Cod. The patchy nature of these aggregations may be associated with local ocean dynamics, specifically vertical motion in the water column.  Using COI support, I developed a current meter by modifying a profiling float to measure vertical water velocity.  

My design uses an APEX profiling float from the Autonomous Systems Laboratory fleet.  Profiling floats descend to a chosen depth, drift at that depth for a specified time and return to the surface with a record of their journey.  A typical profiling float records pressure and temperature, but does not provide information on vertical velocities which are on the order of mm/s. Most ADCP's are not able to measure such small velocities, which motivates the development of the vertical current meter. We attached custom-built vanes (similar to blades on a small fan, which were constructed in the machine shop at WHOI) to the sides of the profiling float that spin the entire apparatus when water moves vertically past them. I also built a small compass heading recorder (using a Parallax BASIC Stamp microprocessor and an OceanServer Digital Compass) that records the rotation rate of the float.  Using this information we can estimate the vertical velocities felt by the float.  

We have tested the system a number of times in tanks at WHOI, and have also done two short deployments off the coast of Cape Cod.  The next step is to improve the precision, reliability, and lifetime of the system to allow it to be deployed for several days to weeks.  The Coastal Ocean Institute made the development of the vertical current meter possible, and I am very grateful for their support.