U.S. GLOBEC: Interaction of zooplankton vertical migration with episodic mesoscale advective features: impacts on population retention and loss

Karen Wishner (URI) and Percy Donaghay (URI)

Episodic advective features may play a major role in the input and loss of zooplankton and larval fish populations on Georges Bank. However, the importance of these mesoscale events has not been quantified directly and may be strongly time- and feature-dependent. The impact is likely to vary with the type of feature and its location, season, and magnitude, all of which will affect the species, abundance, and developmental stages of zooplankton and fish larvae in the feature itself and on the Bank. This proposal seeks to determine the association of particular zooplankton species with selected episodic features of opportunity (such as Scotian Shelf streamers, slope or shelf water parcels, or warm core rings) and how the vertical migration of the zooplankton interacts with the flow field to affect the flux of zooplankton in these features. Features will be targetted using satellite imagery, and their water mass composition will be determined by both standard hydrographic techniques as well as the use of a SAFire spectral fluorometer to obtain characteristic fluorescent signatures. The latter technique provides an innovative way to identify water types bio-optically and can be used to follow mixing and identify water sources in regions of complex and ambiguous hydrography, such as the Georges Bank and Gulf of Maine region. During 3 two-week cruises spanning the winter- spring reproduction and stratification cycles (March to June, (Gifford cruises on the ship schedule), zooplankton will be sampled with vertically-stratified MOCNESS net tows day and night inside and outside of advective features of opportunity, repeatedly over a several-day time period. CTD casts and the spectral fluorometer on the MOCNESS will provide physical and bio-optical signatures of water masses. This will allow documentation of the degree of association of target zooplankton species with the water and bio-optical signature of the advective feature throughout the day / night cycle. Mixing or interleaving with adjacent waters, and changes over time, can potentially be detected independently for the hydrographic and bio-optical characteristics. The transport within the features will be documented by physical oceanographic components proposed by others, including analyses of acoustic Doppler current profiler (ADCP) records from shipboard, AVHRR satellite imagery, moorings, and drifters. Combining information on transport and zooplankton concentrations will provide estimates of the instantaneous zooplankton flux associated with these features. The MOCNESS tows taken outside of features for comparison can also serve as background or groundtruthing for work by other components proposing to use acoustics, the video plankton recorder, or pump samples. By looking at a variety of features across a seasonal gradient and at different locations, data will be obtained about a range of scenarios. Putting this information into the context of studies on vital rates and genetics, broad-scale distributions, physical oceanography, and modeling being proposed by others will provide an overall mechanistic understanding of the potential impact of episodic advective zooplankton input or loss on the target populations of plankton and fish on Georges Bank.