Research Document 2023/016
Optical, Chemical, and Biological Oceanographic Conditions on the Scotian Shelf and in the eastern Gulf of Maine during 2021
By Casault, B., Johnson, C., Devred, E., Head, E., and Beazley, L.
Ocean physical conditions in the Maritimes Region in 2021 were characterized by generally warm water with sea-surface temperatures approaching the record-high levels reached in 2012. Annual inventories of surface and deep nitrate and silicate were mainly above normal across the region with the exception of Prince-5 (P5). Annual inventories of surface and deep phosphate were also higher than in previous years although they remained near or below the long-term averages. Annual inventories of in situ chlorophyll-a over the 0–100 m layer were mainly near or below normal across the region in 2021, while annual mean surface concentrations of chlorophyll-a measured by satellite were mainly above normal. The spring phytoplankton bloom was mainly earlier than normal across the region, with variable duration, and consistently higher-than-normal amplitude and magnitude. Relatively intense fall bloom conditions were evident across the region. Phytoplankton composition at Halifax-2 (HL2) indicated near-normal abundance of diatoms and dinoflagellates in 2021 in contrast with recent years, while ciliate and flagellate abundances continued to be higher than normal. At P5, lower abundance of diatoms and higher abundance of ciliates and flagellates continued the long-term pattern. The abundance of C. finmarchicus was mainly lower than normal, particularly at HL2 and in the western area (Browns Bank and P5). Pseudocalanus spp. and non-copepod abundances were mainly higher than normal across the region, while total copepod abundance and mesozooplankton biomass were variable across the region. The abundances of Arctic Calanus and warm-shelf copepod species were mainly lower and mainly higher than normal across the region in 2021, respectively. Apart from Pseudocalanus spp. which were mainly more abundant than normal across the region in 2021, the copepod community indicated near- or lower-than-normal abundances of the small copepods O. similis, O. atlantica (HL2), and Microcalanus spp. and Paracalanus spp. (HL2 and P5).
Average surface and bottom temperatures in Bedford Basin were above normal in 2021, consistent with the general pattern observed across the broader Scotian Shelf. Surface nutrients were near or slightly below normal, while bottom nitrate, phosphate, and silicate were above normal in 2021, with bottom nitrate showing its highest values during the month of June since the start of the Bedford Basin time series in 1994. In contrast to 2020, no intrusions of shelf water into the deeper layers of Bedford Basin occurred during 2021. Dissolved oxygen concentrations revealed hypoxic conditions in the bottom 30–40 m of Bedford Basin, possibly as a result of the lack of replenishment through shelf-water intrusion.
The 2020 Continuous Plankton Recorder data indicated near-normal values for the phytoplankton colour index, a proxy for the phytoplankton biomass, and annual abundances of diatoms on the Eastern Scotian Shelf (ESS) and Western Scotian Shelf (WSS), while dinoflagellate annual abundance was above (ESS) or near (WSS) normal. Annual abundances of Calanus CI–IV were above normal in both regions, while those of C. finmarchicus CV–VI were above (ESS) or near normal (WSS). Annual abundances of C. glacialis CIV–VI were higher than normal in both regions, while annual abundances of C. hyperboreus CIII–VI, Para/Pseudocalanus, euphausiids and hyperiid amphipods were higher than normal on the ESS but normal on the WSS.
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