Species-specific patterns of distribution and abundance of the cryptic copepods Pseudocalanus moultoni and P. newmani on Georges Bank (NW Atlantic Ocean) during spring 1995–2012

Abstract Time-series analysis of zooplankton species diversity, distribution, and abundance are essential for ecosystem assessment and fisheries management on continental shelves. This study analyzed two morphologically cryptic species of the calanoid copepod Pseudocalanus, P. moultoni and P. newmani, in zooplankton samples collected during May–June 1995–2012 over Georges Bank, NW Atlantic Ocean. Samples were collected 1995–1999 by US Global Ocean Ecosystem Dynamics (US GLOBEC) and 2002–2012 by NOAA Northeast Fisheries Science Center Ecosystem Monitoring (EcoMon). The species were discriminated by real-time quantitative PCR (qPCR) based on DNA sequence variation of the mitochondrial cytochrome oxidase I (COI). Multivariate statistical analysis revealed significant positive correlation between the geometric mean abundance of P. moultoni and depth-averaged temperature at the collection locations; P. newmani abundances showed no relationship to temperature, suggesting different temperature niches and potential responses to environmental conditions. Interannual patterns of variation of the species-specific abundances of P. moultoni and P. newmani differed significantly from pooled Pseudocalanus spp. Nonmetric multidimensional (NMDS) and regression analyses confirmed significant interannual differences between P. moultoni and P. newmani geometric mean abundances during 1995–2012. This study demonstrates the need for discrimination of closely-related and cryptic zooplankton species to understand and predict impacts of environmental variation and climate change on marine ecosystems.