GISS Lunch Seminar
July 5, 2023
This event is virtual and is presented on-line for remote attendance only. Please consult with the event host/contact for connection info.

Speaker: Xuemei Zhai (MIT)
Topic: What makes Stellwagen Bank a productive coastal region?

Zooplankton diel migration is a common feature in oceans and fresh waters, and it is an important part of the "biological pump" which exports carbon deeper in the ocean.  Zooplankton stay in deep water during the day and in surface waters at night.  This behavior allows them to feed on phytoplankton under the cover of darkness.  Zooplankton start to ascend to shallow waters after sunset and reach the depths where phytoplankton are.  At dawn, the zooplankton that previously ascended start to descend to deeper waters to avoid being preyed on during the daytime.  The migration of zooplankton reduces the grazing time, yet it is advantageous for zooplankton to avoid their predators (Hays, 2003; Zaret and Suffern, 1976).  Historical data from net collections in other studies show that distributions of vertically migrating zooplankton off California exhibit greater patchiness in oceanic areas (Genin, et al., 1988, 1994; Genin, 2004; Carmo et al., 2017).

As a case study we choose a small coastal area Stellwagen Bank as a study site to investigate the effects of zooplankton diel migration and several other factors on the distribution of zooplankton, which are an important food source for fish and mammals. Stellwagen Bank is a shoal in the Gulf of Maine with water depth ranging from 20 m to 40 m.  We first use a one-dimensional (vertical) ocean model to assess the impact of zooplankton vertical migration rates in coastal water of various depths.  The model includes five biological variables: phytoplankton, zooplankton, nitrate, ammonium, and detritus.  Then it is extended to a 2-D cross-bank profile model to investigate the biological and physical processes that can explain biomass patterns across Stellwagen Bank. The results show that zooplankton diel migration significantly alters the vertical distributions and the integrated amounts of the biological variables. 

By migrating away from the surface light, zooplankton can partially offset the lack of food in the deep water by avoiding predation.  In shallow water, the zooplankton migrating to near the bottom still have food available but remain less vulnerable to visual predators at night, as a result their biomass remains fairly high. 
Our study found that under the effect of tidal currents and underwater high lands, zooplankton diel migration can contribute to the spatial distribution of zooplankton and biomass in the surface water (30 m). It also affects the net biomass of phytoplankton and zooplankton, and DIN concentration as well as primary and secondary production over the Bank.  Diel migration at a maximum swimming speed, combined with tidal upwelling and downwelling gives a zooplankton biomass nearly 13 times more abundant than in the surrounding waters.

This more readily available food supply is expected to result in higher fish densities, as well as of their predators. Not all zooplankton migrate big distances, of course, and even in a single population some go further than others.  This happens in the model also.  It is an efficient way to get detritus and carbon through the density jump at the base of the mixed layer.