J.S. Page

The decline and fate of an iron-induced subarctic phytoplankton bloom

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the oceans interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

Seasonal and interannual variability in the distribution of surface nutrients and dissolved inorganic carbon in the northern North Pacific: Influence of El Niño

The seasonal and interannual changes in surface nutrients, dissolved inorganic carbon (DIC) and total alkalinity (TA) were recorded in the North Pacific (30–54°N) from 1995 to 2001. This study focuses on the region north of the subarctic boundary (∼40°N) where there was extensive monthly coverage of surface properties. The nutrient cycles showed large interannual variations in the eastern and western subarctic gyres. In the Alaska Gyre the seasonal depletion of nitrate (ΔNO3) increased from 8–14 µmol kg−1 in 1995–1999 to 21.5 µmol kg−1 in 2000. In the western subarctic the shifts were similar in amplitude but more frequent. The large ΔNO3 levels were associated with high silicate depletions, indicating enhanced diatom production. The seasonal DIC:NO3 drawdown ratios were elevated in the eastern and central subarctic due to calcification. In the western subarctic and the central Bering Sea calcification was significant only during 1997 and/or 1998, two El Niño years. Regional C/N stoichiometric molar ratios of 5.7 to 7.0 (>40°N) were determined based on the years with negligible or no calcification. The annual new production (NPa) based on ΔNO3 and these C/N ratios showed large interannual variations. NPa was usually higher in the western than in the eastern subarctic. However, values of 84 gC m−2yr−1 were found in the Alaska Gyre in 2000 which is similar to that in the most productive provinces of the northern North Pacific. There were also large increases in NPa around the Alaska Peninsula in 1997 and 1998. Finally, the net removal of carbon by the biological pump was estimated as 0.72 Gt C yr−1 in the North Pacific (>30°N).

Temporal and spatial distribution of dimethylsulfide in the subarctic northeast Pacific Ocean: a high-nutrient–low-chlorophyll region

Dimethylsulfide (DMS) was measured in the upper 400 m of the northeast Pacific from February 1996 to June 2001. The sampling took place along Line P (48°34.5’N, 125°30’W to 50°N, 145°W). DMS concentrations increased diurnally from a pre-dawn low to a mid-day maximum and from coastal waters to open ocean, in a high-nutrient—low-chlorophyll-a (HNLC) water mass. The mean surface DMS in winter was 2 nM, in spring 6 nM and in summer 10 nM. Our June surface DMS amounts were comparable with those obtained by the Pacific Marine Environmental Laboratory. A sea to air flux of DMS at Station P (50°N, 145°W) was high in the summer of 1997 and the early autumn of 1998 and 2000 and was significantly higher than at other Line P stations. The average flux along Line P was 27 µmol m−2 dm−1 by Wanninkhof’s formula while that by Liss and Merlivat was 16 µmol m−2 dm−1. DMS profiles showed a decreasing trend with depth, as did temperature and chlorophyll-a (Chl-a), but an increasing trend with salinity and nitrate. Average DMS concentrations at mixed-layer depth (DMSMLD) were low in winter at an average of 2.4 nM, moderate in spring at 8 nM and high in summer at 16 nM. For open ocean stations P20 and P26 DMSMLD was high, while Chl-aMLD was low for late spring and early summer during 1996–1998. That is the “summer paradox” phenomenon. The ratio of DMSMLD to Chl-aMLD was out of phase with the mixed-layer depth. Our data confirm the high DMS concentrations previously reported for this region and suggest that this is characteristic of the subarctic HNLC region.