Seasonal and interannual variability in phytoplankton and nutrient dynamics along Line P in the NE subarctic Pacific

Abstract

We analysed mixed-layer seasonal and interannual variability in phytoplankton biomass and macronutrient (NO 3 and Si(OH) 4) concentrations from three decades of observations, and nitrogen uptake rates from the 1990s along Line P in the NE subarctic Pacific. Chlorophyll a concentrations near 0.35 mg m −3 were observed year-round along Line P except at the nearshore station (P4) where chlorophyll a concentrations in spring were on average 2.4 times the winter values. In contrast, the temporal variability in carbon-to-chlorophyll ratios at the two main end members of Line P (P4 and OSP) was high. Large seasonal and interannual variability in NO 3 and Si(OH) 4 concentration were observed along Line P. Highest upper mixed-layer (top 15 m) nutrient concentrations occurred on the continental shelf in late summer and early fall due to seasonal coastal upwelling. Beyond the shelf, maximum nutrient concentrations increased gradually offshore, and were highest in late winter and early spring due to mixing by winter storms. Interannual variations in upper mixed-layer nutrient concentrations beyond the shelf (>128°W) were correlated with E–W winds and the PDO since 1988 but were not correlated with either climate index between 1973 and 1981. Despite differences in nutrient concentration, nutrient utilization (ΔNO 3 and ΔSi(OH) 4) during the growing season were about 7.5 μM at all offshore stations. Variations in ΔNO 3 were correlated with those of ΔSi(OH) 4. The annual cycle of absolute NO 3 uptake (ρNO 3) and NH 4 uptake (ρNH 4) rates by phytoplankton in the upper mixed-layer showed a weak increasing trend from winter to spring/summer for the period 1992–1997. Rates were more variable at the nearshore station (P4). Rates of ρNO 3 were low along the entire line despite abundant NO 3 and low iron (Fe), at the offshore portion of Line P and sufficient Fe at the nearshore station (P4). As a result, new production contributed on average to only 32 ± 15% of the total nitrogen (N) uptake along Line P. NO 3 utilization in the NE subarctic Pacific is probably controlled by a combination of environmental variables, including Fe, light and ambient NH 4 levels. Elevated ambient NH 4 concentrations seem to decrease the rates of new production (and f-ratios) in surface waters of the oceanic subarctic NE Pacific. Contrary to expectation, phytoplankton biomass, nutrient utilization (ΔNO 3 and ΔSi(OH) 4), and nitrogen uptake (ρNO 3 + ρNH 4) varied relatively little along Line P, despite significant differences in the factors controlling phytoplankton composition assemblages and production. Future studies would benefit from including other variables, especially light limitation, to improve our understanding of the seasonal and interannual variability in phytoplankton biomass and nutrients in this region.