Research Document 2019/055

Optical, Chemical, and Biological Oceanographic Conditions on the Newfoundland and Labrador Shelf during 2016-2017

By G. Maillet, D. Bélanger, G. Doyle, A. Robar, S. Fraser, J. Higdon, D. Ramsay, P. Pepin

Abstract

The overall pattern of variation among the nutrients and lower trophic levels surveyed in this report (phytoplankton biomass and zooplankton abundance) highlighted the relationship between biogeochemical conditions (nitrate standing stock) and primary (phytoplankton biomass) and secondary (zooplankton abundance and biomass) production.

Optical and chlorophyll a indices indicate reduction in phytoplankton biomass and delayed timing of the production cycle at the high frequency sampling station (S27) in 2016 – 2017. The replenishment of key macronutrients in the deep (> 50 m) strata has undergone further reduction in recent years. The absence of an autumn bloom may be related to changes in renewal processes. The deep inventories of macronutrients across the standard oceanographic sections have transitioned from mostly positive to mainly negative anomalies in the last decade. Phytoplankton biomass have declined overall in line with the trends in macronutrient inventories. Increased abundance of zooplankton observed throughout the northwest Atlantic may also contribute to higher grazing pressure and reduction of phytoplankton standing stocks.

The general trend of increasing zooplankton abundance observed over the past nineteen years continued during 2016 – 2017 with abundance reaching historical highs from southern Labrador to the southern Grand Banks. In contrast, the biomass of zooplankton remained below the climatology in all oceanographic sections and was at a record low on the southern Labrador shelf and on the Grand Banks. The abundance of large copepods (Calanus finmarchicus, Calanus hyperboreus), which has been declining for 3-5 years throughout much of the region, remained low on the Newfoundland and Labrador (NL) Shelf but showed signs of recovery on the Grand Banks. The abundance of small copepods (Pseudocalanus spp., Oithona similis, Microcalanus spp., Oncea spp.) and other non-copepod organisms continued to increase throughout the region. The production cycle of early copepodite stages of keystone copepods species (Calanus finmarchicus and Pseudocalanus spp.) was delayed by ~1 month in 2016 and 2017 compared to climatology at S27, possibly because of the delayed spring bloom and general cooling and freshening of the system in that area. More research is needed to understand the mechanisms behind the observed shift in zooplankton community structure and their potential impacts at higher trophic levels.

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