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Research Document - 2003/019

Biological and chemical oceanographic conditions on the Newfoundland Shelf during 2002

By Pepin, P., Maillet, G.L., Fraser, S., and Lane, D.

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. We focus on temporal and spatial series of the different biological, chemical, optical, and physical measures during 2002 but contrast those observations with previous information from earlier periods when data are available. The vertical attenuation coefficient increased nearly two-fold during the spring bloom in 2002 at the fixed coastal station compared to previous years, confining the penetration of incident solar radiation to shallower depths and the general reduction in the euphotic depth. The trend in optical conditions at Station 27 was not consistent with the general reduction in attenuation across different oceanographic sections and seasons in 2002, leading to deeper euphotic depths. The annual climatology for incident solar radiation suggested that recent solar radiation measurements collected during 2001-02, were among the highest recorded during the available time series for the months of April through to October. Water column stability and heating, inferred from stratification and integrated temperature, showed consistent trends between the seasonal occupations of Station 27 and AZMP sections. In general, stratification and integrated temperature in the upper 50m across the Newfoundland and Labrador Shelf were lower compared to earlier years. Time series of major nutrient inventories at Station 27 showed differences between years. The seasonal inventories of silicate and nitrate in the upper 50m were 2-3-fold higher along most oceanographic sections compared to earlier years (2000-01). Similar positive trends in the deep inventories were apparent for the southern Grand Banks, but smaller differences were observed along the other sections. The magnitude of the spring bloom has increased during the past 3 years peaking in 2002 and coincided with a similar trend in primary productivity at Station 27 during this period. Although the spring inventories of chlorophyll a (proxy of phytoplankton biomass) were higher in 2002 at Station 27 compared to earlier years, the pattern at the fixed station was not reflected in the offshore waters, where a negative trend was observed along the sections. Another prolonged maxima in mixed layer depth was observed at Station 27 during late winter-spring 2002 and may have contributed to the timing and short-duration (54 days) of the bloom. Both 2001 and 2002 had higher wind stress compared to the conditions observed during 2000. In 2001, this prolonged maxima coincided with a delay in the timing of the spring bloom and shortened it's duration by one-half compared to 2000. Major groups of phytoplankton were enumerated seasonally at the fixed station and along selected stations on oceanographic sections. The most notable difference observed between recent years was the increase in Diatom abundance during the spring bloom, and the presence of a fall bloom of coccolithophorids, not previously observed at Station 27.

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