Does predation control the diapausing stock of Calanus finmarchicus in the Gulf of Maine?

The variability of zooplankton populations is controlled by external and internal forcing, with the former being principally large-scale changes in circulation, and the latter being driven by in situ growth, competition, and predation. Assessing the relative importance of these forcings is challenging and requires analyses of multifaceted observational data. As part of the U.S. GLOBEC Georges Bank program, a series of cruises were conducted in fall 1997, 1998, and 1999 to survey diapausing populations of Calanus finmarchicus and their predators in Wilkinson, Jordan, and Georges Basins of the Gulf of Maine. Station and underway sampling were conducted using net (1 m(2) MOCNESS) and bioacoustic (BIOMAPER-II) systems, respectively, to acquire vertically stratified data for zooplankton biomass, taxonomic, size, and life-stage composition, together with associated environmental data. The results show that the autumn diapausing C. finmarchicus abundance was much lower in 1998 than in 1997 or 1999, even though the overall zooplankton biomass levels were comparable between the three years. The size frequency distribution of the diapausing individuals had a bi-modal pattern in 1997 and 1999, but a single mode in 1998, indicating the demise of an early cohort of the diapausing stock. The relative biomass and computed energy demand of potential invertebrate predators (euphausiids, decapods, medusae, and siphonophores) was found to be higher in 1998 and could account for the missing C. finmarchicus cohort. Evidence collected from this study supports the hypothesis that local predation has the potential to control the diapausing stock of C. finmarchicus in the Gulf of Maine.

Automated summary

The variability of zooplankton populations is influenced by external and internal forcing. External forcing, such as large-scale changes in circulation, and internal forcing, including in situ growth, competition, and predation, control the variability. This study analyzed observational data to assess these forcings in the Gulf of Maine. The results showed that the abundance of diapausing C. finmarchicus was lower in 1998 compared to 1997 and 1999, despite comparable overall zooplankton biomass levels. The study also found that potential invertebrate predators had a higher biomass in 1998, potentially accounting for the missing C. finmarchicus cohort. This supports the hypothesis that local predation can control the diapausing stock of C. finmarchicus.