Research Document - 2014/104

Optical, chemical, and biological oceanographic conditions on the Scotian Shelf and in the eastern Gulf of Maine in 2013

By C. Johnson, W. Li, E. Head, B. Casault, and J. Spry

Abstract

Ocean conditions were unusually warm and stratified on the Scotian Shelf in 2012, and the plankton response to the physical environment in 2012 set the initial conditions for the plankton in 2013. In particular, zooplankton biomass and the abundance of the two dominant herbivorous copepod species, Calanus finmarchicus and Pseudocalanus spp., were low, and there appeared to be a shift to a smaller-size phytoplankton community on the Scotian Shelf. In 2013, annual average temperature anomalies were still positive in the Maritimes Region, but less so than in 2012, and temperature and stratification anomalies were marked by strong sub-annual and mesoscale variability. Variability in the physical environment was reflected in nutrient and plankton conditions. Annual average deep-water and winter surface nitrate inventories were similar to average overall, but surface nitrate was higher than average. The magnitude of the spring bloom chlorophyll peak was above average on the Eastern Scotian Shelf (ESS) and below average on the Western Scotian Shelf (WSS) and in the eastern Gulf of Maine, but summer-fall blooms were above average in all areas. Zooplankton biomass and abundance were lower than average on the eastern transects, but anomalies were mixed on the Central Scotian Shelf, ESS and Bay of Fundy. The abundance of Pseudocalanus spp. was higher than average in the central and western part of the region. Although C. finmarchicus abundance was variable and lower than average overall, it was high on the WSS and in the eastern Gulf of Maine during the summer ecosystem trawl survey and higher than average at the Halifax-2 station and in Emerald Basin in autumn, suggesting a return to more typical abundances in the western part of the region at the end of 2013. Similar to the broader Scotian Shelf, the 2013 Bedford Basin annual average temperature was warmer than normal but not as warm as 2012. Small phytoplankton were more abundant than average in Bedford Basin and large phytoplankton less abundant. An initial evaluation of relationships among annual anomalies of physical variables, nitrate, spring bloom metrics and zooplankton at Halifax-2 from 1999 to 2013 identified bloom duration and enhanced upwelling as important correlates of zooplankton biomass and dominant copepod abundance at an annual scale. Continuous Plankton Recorder sampling showed that observations of phytoplankton bloom dynamics and abundance of C. finmarchicus at Halifax-2 in 2012 were representative of shelf-wide patterns.

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