Frank A. Whitney

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.

Evidence of change in the winter mixed layer in the Northeast Pacific Ocean

Sea-surface temperatures in the Northeast Pacific Ocean show a warming trend, and salinities show a declining trend, in data collected over the last 60 years. These changes combine to reduce the density of the surface layer over a large area of the Northeast Pacific. The declining surface density changes the energetic requirements for the formation of a surface mixed layer, and observations at Ocean Station Papa indicate that mid-winter mixed layer depths are showing a marked decline. The reduction in the depth of penetration of the winter-time mixed layer should reduce the nutrients entrained into the upper ocean each winter. Observations suggest that near surface nutrient levels are declining at Papa but remain well above levels that might inhibit productivity. However, at present the productivity of large phytoplankton appears to be limited by iron supply which is thought to be mainly from the atmosphere. A shallower mixed layer depth could increase the concentration of iron in this layer. The increase in iron would increase the utilization of nitrate, mainly by diatoms, and new production and the f ratio would increase.

Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean

Anticyclonic mesoscale eddies form near shore each winter in the Gulf of Alaska. One site near the Queen Charlotte Islands is shown to produce eddies that transport from 3000 to 6000 km3 of coastal water up to 1000 km westward. Eddies carry shelf nutrients either into the high nutrient, low chlorophyll waters of the NE Pacific, or in a more southerly direction into seasonally nitrate depleted waters. A large eddy sampled in summer 1998 was found to have elevated particulate levels on its perimeter. Nitrate supplied to the euphotic zone by this eddy during its natal summer is estimated to be three times greater than the usual seasonal nutrient transport in the Gulf of Alaska.

Ocean deoxygenation: Past, present, and future

To a first order, the oxygen content of the ocean interior is determined by the influx of the gas across the air-sea surface (i.e., ventilation) and consumption due primarily to microbial respiration. As these two competing processes vary in space and time, so does the concentration of oxygen in the ocean interior. Although oxygen concentrations on continental margins are declining in many regions due to increased anthropogenic nutrient loadings [e.g., Rabalais et al., 2002], oxygen also appears to be declining in both the central North Pacific Ocean and the tropical oceans worldwide [Emerson et al., 2004; Whitney et al., 2007; Keeling et al., 2010] (see Figure 1). It is unclear whether the loss throughout the basins in the open ocean is a long-term, nonperiodic (secular) trend related to climate change, the result of natural cyclical processes, or a combination of both (Figure 2). If related to climate change, a number of important factors may be involved, including decreased solubility of oxygen as waters warm, decreased ventilation at high latitudes associated with increased ocean stratification, and changes in respiration in the ocean interior.

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).

Impact of the 1997–1998 El Niño and 1999 La Niña on nutrient supply in the Gulf of Alaska

Nutrient surveys of the Gulf of Alaska, from 1997 through 1999, show that coastal waters of British Columbia and southern Alaska experienced nitrate depletion each spring and summer. Through the 1997–1998 El Nino, waters with less than 1 μM NO3 covered 250,000 km2 area greater than 1999. Silicate levels as low as 0.2 μM were observed in coastal waters, suggesting that diatom growth may have been nutrient limited both in 1998 and 1999. Detailed sampling off the southern coast of British Columbia revealed that 1998 nitrate levels were only half the average of that during the 1970s winter, were depleted 1 month earlier in spring and remained low throughout the summer. Satellite images show that, compared to 1997 and 1999, chlorophyll levels were much lower in the spring of 1998 throughout the coastal waters of the Gulf of Alaska. Conditions changed dramatically during the 1999 La Nina, with ocean-mixed layer depths increasing by 20 m in winter and 40 m in spring when compared to that during 1997–1998 El Nino. Winter nutrient levels increased and summer upwelling returned. Over the past several decades, a trend towards greater stratification of coastal waters appears to be affecting the supply of nutrients to the mixed layer. The effects of stratification were especially obvious during the 1998 El Nino.

Anomalous winter winds decrease 2014 transition zone productivity in the NE Pacific

Wind-driven transport from the North Pacific in winter provides nutrients to a highly productive region in the transition zone between the subarctic and subtropics. This region supports many species of fish, marine mammals, and seabirds. In winter 2013/2014, anomalous winds from the south weakened nutrient transport in the eastern North Pacific, resulting in substantial decreases in phytoplankton biomass. By January 2014, waters were warmer than usual by 3.5°C at the center of an affected area covering ~1.5 × 106 km2. South of this area, winter chlorophyll levels dropped to the lowest levels seen since the Sea-viewing Wide Field-of-view Sensor satellite began taking measurements in 1997. It is anticipated that impacts will be felt in some fisheries and among migrating predators this coming year.

Sponge reefs in the Queen Charlotte Basin, Canada: controls on distribution, growth and development

Sponge reefs in the Queen Charlotte Basin exist at 165–240 m depth within tidally influenced shelf troughs subject to near bottom current velocities of 25–50 cm s−1 where nutrient supply from coastal runoff is augmented by wind-induced upwelling of nutrient rich water from the adjacent continental slope. Large reef mounds to 21 m in elevation affect tidally driven bottom currents by deflecting water flows through extensive reef complexes that are up to 300 km2 in area. Three hexactinellid species construct reefs by building a siliceous skeletal framework through several frame-building processes. These sponge reefs exist in waters with 90 to 150 µM dissolved oxygen, a temperature range of 5.9 to 7.3°C and salinity of 33.2 to 33.9 ‰. Relatively high nutrient levels occur at the reef sites, including silica, which in bottom waters are typically >40 µM and may be up to 80 µM. A high dissolved silica level is potentially an important control on occurrence of these and other dense siliceous sponge populations. The sponge reefs are mainly confined to seafloor areas where exposed iceberg plough marks are common. Sediment accumulation rates are negligible on the relict, glacial surface where the reefs grow, and trapping of flocculated suspended particulate matter by hexactinosidan or framework skeleton hexactinellid sponges accounts for a large proportion of the reef matrix. Suspended sediment concentration is reduced within the nepheloid layer over reef sites suggesting efficient particle trapping by the sponges. The reef matrix sediments are enriched in organic carbon, nitrogen and carbonate, relative to surrounding and underlying sediments. The sponges baffle and trap suspended sediments from water masses, which in one trough have a residence time of approximately 6 days, ensuring a close association of the sponges with the bottom waters. The location of the reef complexes at the heads of canyons provide a means of regionally funnelling particulate material that sponges can trap to enrich their environment with organic carbon and biogenic silica. Like deepsea coral reefs, the sponge reefs are a remote and poorly known ecosystem that can present logistical challenges and survey costs. Also like deep-sea coral reefs, many of the hexactinosidan sponge reefs have been damaged or destroyed by the groundfish trawl fishery.

Seasonal changes in the distribution of dissolved organic nitrogen in coastal and open-ocean waters in the North East Pacific: sources and sinks

The distribution of dissolved organic nitrogen (DON) and nitrate were determined seasonally (winter, spring and summer) during three years along line P, i.e. an E-W transect from the coast of British Columbia, Canada, to Station P (50degreesN, 145degreesW) in the subarctic North East Pacific Ocean. In conjunction, DON measurements were made in the Straits of Juan de Fuca and Georgia within an estuarine system connected to the NE Pacific Ocean. The distribution of DON at the surface showed higher values of 4-17 muM in the Straits relative to values of 4-10 muM encountered along line P, respectively. Along line P, the concentration of DON showed an inshore-offshore gradient at the surface with higher values near the coast. The equation for the conservation of DON showed that horizontal transport of DON (inshore-offshore) was much larger than vertical physical mixing. Horizontal advection of DON-rich waters from the coastal estuarine system to the NE Pacific Ocean was likely the cause of the inshore-offshore gradient in the concentration of DON. Although the concentration of DON was very variable in space and time, it increased from winter to summer, with an average build up of 4.3 muM in the Straits and 0.7 muM in the NE subarctic Pacific. This implied seasonal DON sources of 0.3 mmol N m(-2) d(-1) at Station P and 1.5 mmol N m(-2) d(-1) in the Straits, respectively. These seasonal DON accumulation rates corresponded to about 15-20% of the seasonal nitrate uptake and suggested that there was a small seasonal build up of labile DON at the surface. However, the long residence times of 180-1560 d indicated that the most of the DON pool in surface waters was refractory in two very different productivity regimes of the NE Pacific

Long-term variability of surface nutrient concentrations in the North Pacific

We present the spatial distributions and temporal changes of the long-term variability of surface nutrient concentrations in the North Pacific by using nutrient samples collected by volunteer ships and research vessels from 1961 to 2012. Nutrient samples are optimally interpolated onto 1° × 1° monthly grid boxes. When the Pacific Decadal Oscillation is in its positive phase, nutrient concentrations in the western North Pacific are significantly higher than the climatological means, and those in the eastern North Pacific are significantly lower. When the North Pacific Gyre Oscillation is in its positive phase, nutrient concentrations in the subarctic are significantly higher than the climatological means. The trends of phosphate and silicate averaged over the North Pacific are −0.012 ± 0.005 µmol l−1 decade−1 and −0.38 ± 0.13 µmol l−1 decade−1, whereas the nitrate trend is not significant (0.01 ± 0.13 µmol l−1 decade−1).