Relationships among vertical mixing, nitrate uptake, and phytoplankton growth during the spring bloom in the southeast Bering Sea middle shelf

Abstract

The temporal development of a spring diatom bloom in the southeast Bering Sea middle shelf for three consecutive ice-free years is analyzed. Physical and chemical criteria are used to divide the bloom period into prebloom, bloom, and postbloom stages. At this shelf depth, the shallowing of the mixed layer was most important in triggering bloom conditions by diminishing phytoplankton respirational losses. This occurred in late April to early May during a hiatus in wind mixing associated with low pressure systems. Sver drup's (1953, Journal du Conseil. Conseil International pour l'Exploration de la Mer, 18, 287–295) critical depth model accurately predicted changing prebloom growth conditions probably because it described the mean daily mixed layer light conditions to which the plant communities were exposed. The usefulness of this integrative model diminished as species composition changed since compensation depth light levels for later bloom species were not known. A nitrate advection diffusion model indicated that a cross pycnocline mixing rate of 2.1 m d −1 was associated with bloom conditions. Estimated convective vertical mixing rates in the upper water column were low during the period leading to nitrate exhaustion suggesting that cells remained at favorable light levels for extended periods. Since both net respirational loss and the supply of nutrients to the mixed layer were dependent on vertical mixing, bloom development could be described as a function of upper water stability. Discrepancies arose when rapid changes in mixing conditions decreased the correspondence between measured nitrate uptake rates and the prevailing surface layer buoyancy. Signals generated by periodic factors such as the M f (neap-spring) tide and aperiodic storm events were resolved during the spring bloom. Approximately 37% of new nitrogen productivity was due to wind mixing events that occurred after initial water column stabilization and prolonged high rates of nitrate uptake. This enhancement varied from 10 to 50% among years which suggests that large-scale meteorological factors determine May entrainment activity and influence the intensity and pattern of production on this shelf. The passage of low pressure systems temporarily pumped nutrient-rich outer shelf water ( 100m) into the middle shelf domain (<80m).