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Research Document - 2002/052

Biological and chemical oceanographic conditions on the Newfoundland shelf during 2001 with comparisons with earlier observations

By P. Pepin and G.L. Maillet

Abstract

We review the information concerning the seasonal and interannual variations in the concentrations of chlorophyll a, major nutrients, rates of primary production, as well as the abundance of major taxa of phytoplankton and zooplankton measured from Station 27 and standard oceanographic transects on the Newfoundland Shelf. We focus on temporal and spatial series of the different biological, chemical, optical, and physical measures during 2001 but contrast those observations with previous information from earlier periods when data are available. Variations in optical conditions including attenuation and euphotic depth were comparable to the previous year. The magnitude of the spring bloom was comparable between 2001 and 2000, but the timing and duration of the bloom occurred earlier in 2000 and persisted longer by nearly two-fold compared to 2001. Time series of major nutrient pools at Station 27 showed differences between years. Silicate and nitrate pools in the upper mixed layer showed expected seasonal trends during 2000-01. Depletion of nitrate was more prominent in 2000 during the production cycle in contrast to 2001. Integrated deep nutrient pools were nearly two-fold higher in 2000 compared to 2001. The magnitude, timing, and duration of stratification remained similar during 2000 and 2001, although the rate of onset in stratification appears too have been somewhat higher in 2001 compared to 2000. A prolonged maxima in mixed layer depth was observed during late Winter-Spring 2001 and may have contributed to the delay in the formation of the Spring bloom. Major groups of phytoplankton were enumerated seasonally at the fixed station and along standard AZMP transects. The most notable difference observed between recent years was the widespread reduction in Flagellates and Diatoms in coastal and Shelf areas during 2001. The numerical abundance of major zooplankton groups at Station 27 was generally similar to that previously observed in 1999 and 2000, although densities were generally lower in the fall of 2001. The most notable exception was found in the abundance of Calanus finmarchicus in the winter of 2001, when a strong cohort of young stages from the previous fall dominated. There was evidence of a delay in the peak production of Calanus finmarchicus stage I copepodites by approximately 30 days in contrast to the two previous years. The density of large calanoid nauplii also showed a similar delay and peak concentrations were approximately 50% of that observed in the previous year although overall levels were comparable to those observed in 1999. Overall, the zooplankton community was similar in composition to that observed in 1999 and 2000 although the relative abundance of pelagic gastropods remained high for an extended period in contrast to the two previous years. Overall zooplankton abundance on the Newfoundland Shelf was similar to previously observed densities throughout the spring and summer periods, with the exception of the coastal areas along the Bonavista Bay transect where densities of all major taxa were lower during the summer observation period. In contrast, densities of all major taxa along the coast of Labrador appeared to be elevated during the July Research Vessel survey.

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