Biological-physical interactions on Georges Bank : plankton transport and population dynamics of the ocean quahog, Arctica islandica

Abstract

Advective losses of bank water during winter because of strong wind forcing were hypothesized to be a significant factor limiting recruitment of Georges Bank cormnunities. This hypothesis was examined using biological-physical models of bank circulation with wind, tidal, and density driven circulation resembling winter conditions. Mode ls of stratification-driven flow over an idealized bank addressed effects of storms on the spring plankton bloom. An NPZ model and a copepod stage structure model were mode led as passive tracers. Results indicate that strong storms (13 m/s wind for 20 days) can cause marked replacement of bank water and loss of zooplankton and phytoplankton. These alterations in bank trophic structure may impair energy transfer from primary to secondary production and reduce recruitment of higher trophic levels. Georges Bank Arctica islandica abundance data indicates that adults appear primarily below 50 meters, with highest abundances on the South Flank. Age and size structures suggest that a large cohort, detected on the southeast flank in 1992 and 94 surveys, was spawned in 1986; no other comparable recruitment was seen. Larval transport was modeled using tidal forcing and winter wind data from 1974, 1978, and 1991. This work revealed that modeled transport driven by vector-averaged and realistic winds from the same periods differed. Circulation using realistic winds was highly variable; Ekman transport frequently overwhelmed tidal rectification and reversed the residual flow for several days. Transport and matrix mode ls of Arctica populations were compared with field data; correlation of models with NMFS Survey data was best for realistic wind simulations from 1974 and 1991. Projection matrix eigenvalues were most sensitive to changes in adult and larval survival and planktonic duration. Lower wind models identified the NE Peak region as having the highest reproductive value and sensitivity. This work indicates that winter wind forcing is a factor determining transport of plankton. Models suggest that interannual differences in Georges Bank transport depend partially on temporal wind variability. They indicate that the Northeast Peak may be a source region for larvae and that Arctica research should focus on adult survival and planktonic mortality and duration.