Zooplankton in a Thermohaline Staircase: Do fine-scale temperature and salinity gradients affect the distribution and abundance of zooplankton in the tropical N. Atlantic?


Peter H. Wiebe, Biology
Nancy Copley, Biology
Raymond W. Schmitt, Physical Oceanography
Grant Funded 2010
This Ocean Life Institute (OLI) project will enable the analysis of previously collected samples from a unique stratification regime in the thermocline of the tropical N. Atlantic. The site is within a large area of “thermohaline staircases” where temperature and salinity profiles display a remarkably regular staircase structure, with thin interfaces where temperature and salinity change rapidly alternating with well-mixed layers, 10-30 m thick, with uniform properties. These staircases are a permanent feature of the region, having been observed since the late 1960’s. They are maintained by salt finger convection in the high gradient interfaces and are characterized by enhanced vertical mixing. During a cruise for the Census of Marine Zooplankton (CMarZ) in 2006, a MOCNESS ext/javascript" src="http://www.whoi.edu/cms/tinymce3/jscripts/tiny_mce/themes/advanced/langs/en.js"> zooplankton tow was taken in a salt finger staircase such that each of the eight nets was collected on the horizontal ‘tread’ or mixed vertical ‘riser’ of each step, for 4 steps. Preliminary results from the “Staircase Tow” (MOCNESS-1 Tow 13) suggest some kind of species stratification within the staircase. The staircase structure might be an area of unique biology, but this has not been previously studied. We here propose to analyze this tow and other samples taken at similar depth ranges in nearby non-step areas to see how they differ. This data set presents a unique opportunity to examine zooplankton associations with strong and persistent structure in the temperature and salinity stratification. The results will provide a strong basis for a proposal to NSF to return to the area and conduct a coupled physical-biological study of the role that the staircase structures play in the ecology of tropical N. Atlantic.