Shallow Mixed Layer Depth Research Articles

Distribution of Mycosporine-Like Amino Acids Along a Surface Water Meridional Transect of the Atlantic

The composition and abundance of mycosporine-like amino acids (MAAs) were investigated in the surface waters along a 13,000-km meridional transect (52° N to 45° S) in the Atlantic Ocean (Atlantic Meridional Transect programme: Cruise AMT 18: 4/10/2008-10/11/2008). MAAs were ubiquitous along the transect, although the composition of the MAAs was variable. Highest concentrations were in the far south (below 40° S; MAA >1 μg L(-1)) and in north subtropical equatorial region (NER: 0-25° N; MAA up to 0.8 μg L(-1)). Highest MAA relative to chlorophyll-a occurred in the NER (MAA/chl-a ratio between 2 and 5). MAA/chl-a significantly correlated with the preceding month's mean daily UV dose and with UV-B/UV-A. In the far south, high MAA concentrations coincided with high phytoplankton biomass, high nutrients and a deep mixed layer associated with the austral spring. Here, the phytoplankton community was dominated by micro- and nano-eukaryotes. At the NER, the high MAA/chl-a coincided with low nutrient concentrations, a shallow mixed layer depth (20-70 m) and to a lesser extent to a shallow nitracline (40-90 m). Here, the phytoplankton consisted primarily of picophytoplankton (0-0.2 μm), dominated by the pico-cyanobacteria Synechococcus sp. and Prochlorococcus sp. and by the nitrogen fixing filamentous cyanobacterium Trichodesmium. The low nitrate concentrations (<0.1 μmol L(-1)) at the NER suggest that nitrogen fixation was required for MAA production. Specific MAAs could not easily be assigned to particular groups of phytoplankton and we could not rule out the possibility that MAAs were associated with symbiotic cyanobacteria contained within heterotrophic dinoflagellates or diatoms.

Spatial distribution of pCO2, ΔO2/Ar and dimethylsulfide (DMS) in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica

We report the first simultaneous measurements of surface water pCO2, biological oxygen saturation (ΔO2/Ar) and dimethylsulfide (DMS) concentrations in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica. Across our survey region, we observed large spatial variability in surface water gas concentrations, tied to strong gradients in hydrography and phytoplankton biomass. Variability in sea surface temperature and salinity was attributed to the interacting effects of surface ocean circulation and heat fluxes, sea ice melt and the upwelling of relatively warm, saline and nutrient-rich modified circumpolar deep water. Phytoplankton biomass ranged from <1μgL−1 to ∼40μgL−1 chlorophyll a (Chl a), with the highest values observed in regions of shallow mixed layer depths. Phytoplankton assemblages were dominated by the colonial haptophyte Phaeocystis antarctica at most sampling stations, with lesser abundances of diatoms found throughout the polynya and sea ice zone. Cryptophytes were abundant at a few stations along the continental shelf break. Across the open polynya waters and sea ice zone, ΔO2/Ar ranged from ∼−40% to 40% (mean 8.6%), pCO2 ranged from 80 to 530μatm (mean 250μatm) and DMS concentrations varied from <1nM to ∼350nM. Strong gradients in gas concentrations were observed over short (i.e. <10km) spatial scales. The distribution of pCO2 and ΔO2/Ar across our survey transects reflected the balance between deep water entrainment and net community production, with a significant imprint of air–sea exchange. Chl a concentrations were significantly correlated to both pCO2 and ΔO2/Ar, and the slope of the ΔO2/Ar vs. pCO2 relationship was consistent with photosynthetic stoichiometry (∼1.25molO2 evolved per mol DIC removed). DMS and Chl a concentrations were also correlated in surface waters, but the spatial distribution of DMS was often uncoupled from pCO2 and ΔO2/Ar, likely due to complex microbial cycling processes. Sea surface temperature and salinity were related to surface gas concentrations through their effects on mixed layer depth and as tracers of upwelling. During the time of our cruise, the Amundsen Sea acted as an overall net sink for CO2 (mean sea–air flux=−15.9mmolm−2d−1) and a DMS source (mean sea–air flux=23.1μmolm−2d−1). Sea–air CO2 and DMS fluxes were more than 2-fold higher in open polynya waters relative to the overall cruise means. Simple calculations suggest that the Amundsen Sea contributes ∼5% of total Southern Ocean CO2 fluxes, and ∼1% of DMS fluxes.

The Association of Antarctic Krill Euphausia superba with the Under-Ice Habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.

Interannual variability of the Tropical Indian Ocean mixed layer depth

In the present study, interannual fluctuations of the mixed layer depth (MLD) in the tropical Indian Ocean are investigated from a long-term (1960–2007) eddy permitting numerical simulation and a new observational dataset built from hydrographic in situ data including Argo data (1969–2008). Both datasets show similar interannual variability patterns in relation with known climate modes and reasonable phase agreement in key regions. Due to the scarcity of the observational dataset, we then largely rely on the model to describe the interannual MLD variations in more detail. MLD interannual variability is two to four times smaller than the seasonal cycle. A large fraction of MLD interannual variations is linked to large-scale climate modes, with the exception of coastal and subtropical regions where interannual signature of small-scale structures dominates. The Indian Ocean Dipole is responsible for most variations in the 10°N–10°S band, with positive phases being associated with a shallow MLD in the equatorial and south-eastern Indian Ocean and a deepening in the south-central Indian Ocean. The El Nino signature is rather weak, with moderate MLD shoaling in autumn in the eastern Arabian Sea. Stronger than usual monsoon jets are only associated with a very modest MLD deepening in the southern Arabian Sea in summer. Finally, positive Indian Ocean Subtropical Dipoles are associated with a MLD deepening between 15 and 30°S. Buoyancy fluxes generally appear to dominate MLD interannual variations except for IOD-induced signals in the south-central Indian Ocean in autumn, where wind stirring and Ekman pumping dominate.

Large-scale impacts of submesoscale dynamics on phytoplankton: Local and remote effects

The sensitivity of an idealized bio-physical model of seasonally varying subtropical and subpolar gyres to increased horizontal resolution is presented. Switching from mesoscale-resolving (1/9°) to submesoscale-resolving (1/54°) allows the emergence of a denser and more energetic vortex population sustained by submesoscale physics. The experiments display a global decrease in phytoplankton abundance of ∼10–20% as the resolution is refined. This result contrasts with previous studies, which suggested that eddy-driven vertical fluxes of nutrients locally boost phytoplankton growth at mid-latitudes in the open ocean. The explanation is that the long-term (50years) modification of the large-scale, or basin-scale, circulation and distribution of nutrients by submesoscale processes (remote effects), not taken into account by previous studies because of their much shorter time integration, balance the local, small-scale effects. More precisely, dynamical remote effects (involving only the physics) affect the location of the boundary between the two gyres as well as the large-scale mixed-layer depth (MLD) and thermocline depth. Biological remote effects result from the dynamical remote effects that strongly modify the physical–biological interactions at all scales, including at large scales. These biological remote effects involve changes of the nutricline depth. In the mid-latitude subpolar gyre, phytoplankton decrease in abundance at higher resolution is due to the shallower MLD and nutricline, which promote zooplankton grazing; in the subtropical gyre, it is due to deeper MLD and nutricline, which diminishes regenerated production. In addition, remote effects modulate the dynamical supplies of nutrients to the euphotic layer through a combination of changes in mean advection, eddy advection and vertical mixing.

The COPAS’08 expedition to the Patagonian Shelf: Physical and environmental conditions during the 2008 coccolithophore bloom

Here we present observations of the hydrography of the Patagonian Shelf, shelf break and offshore waters, with reference to the environmental conditions present during the period of peak coccolithophore abundance. Analysis of a hydrographic dataset collected in December 2008 (austral spring/summer), as part of the Coccolithophores of the Patagonian Shelf (COPAS) research cruise, identified 5 distinct surface water masses in the region between 37°S and 55°S. These water masses, identified through salinity gradients, displayed varying mixed layer depths, macronutrient inventories and chlorophyll- a fluorescence. Subantarctic Shelf Water (SSW), located to the north of the Falkland Islands and extending north along the shelf break, was also host to a large coccolithophore bloom. The similarities between the distribution of calcite, as seen in remote sensing data, and SSW indicate that the coccolithophore bloom encountered conditions conducive to bloom development within this water mass. Analysis of chemical and environmental data also collected during the COPAS cruise revealed that many of the commonly cited conditions for coccolithophore bloom development were present within SSW (e.g. low N:P ratio, high N:Si ratio, shallow mixed layer depth). In the other water masses present on the Patagonian Shelf greater variability in these same parameters may explain the more diffuse concentration of calcite, and the smaller size of possible coccolithophore blooms. The distribution of SSW is strongly influenced by the latitudinal variation in shelf break frontal width, which varies from 20 to 200 km, and consequently strong hydrographic controls underlie the position of the coccolithophore bloom during austral summer.

Mixed Layer Depth and its Variability in the Eastern Equatorial Indian Ocean as Revealed by Observations and Model Simulations

Mixed layer depth (MLD) variability in the Eastern Equatorial Indian Ocean (EEIO) from a hindcast run of an Ocean General Circulation Model (OGCM) forced by daily winds and radiative fluxes from NCEP-NCAR reanalysis from 2004 to 2006 is investigated. Model MLD compares well with the ∼20,000 observations from Argo floats and a TRITON buoy (1.5°S and 90°E) in the Indian Ocean. Tests with a one-dimensional upper ocean model were conducted to assess the impact on the MLD simulations that would result from the lack of the diurnal cycle in the forcing applied to the OGCM. The error was of the order of ∼12 m. MLD at the TRITON buoy location shows a bimodal pattern with deep MLD during May–June and December–January. MLD pattern during fall 2006 was significantly different from the climatology and was rather shallow during December–January both in the model and observation. An examination of mixed layer heat and salt budget suggested salinity freshening caused by the advective and vertical diffusive mixing to be the cause of shallow MLD.

Primary productivity off the Antarctic coast from 30°–80°E; BROKE-West survey, 2006

Abstract Primary productivity was measured in the Indian Sector of the Southern Ocean (30° to 80°E) as part of a multi-disciplinary study during austral summer; Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West Survey, 2006). Gross integrated (0–150 m) productivity rates within the marginal ice zone (MIZ) were significantly higher than within the open ocean, with averages of 2110.2±1347.1 and 595.0±283.0 mg C m−2 d−1, respectively. In the MIZ, high productivity was associated with shallow mixed-layer depths and increased Pmax up to 5.158 mg C (mg chl a)−1 h−1. High Si:N drawdown ratios in the open ocean (4.1±1.5) compared to the MIZ (2.2±0.79) also suggested that iron limitation was important for the control of productivity. This was supported by higher Fv/Fm ratios in the MIZ (0.50±0.11 above 40 m) compared to the open ocean (0.36±0.08). As well, in the open ocean there were regions of elevated productivity associated with the seasonal pycnocline where iron availability was possibly increased. High silicate drawdown in the north-eastern section of the BROKE-West survey area suggested significant diatom growth and was linked to the presence of the southern Antarctic Circumpolar Current front (sACCF). However, low assimilation numbers (12.8–23.2 mg C mg chl a−1 d−1) and Fv/Fm ratios indicated that cells were senescent with initial growth occurring earlier in the season. In the western section of the survey area within the MIZ, high NO3 drawdown but relatively low silicate drawdown were associated with a Phaeocystis bloom. NO3 concentrations were strongly negatively correlated with column-integrated productivity and chlorophyll biomass which was expected given the requirement for this nutrient by all phytoplankton groups. Regardless, concentrations of both NO3 and silicate were above limiting levels within the entire BROKE-West survey area (N>15.7 μM, Si>18.3 μM) supporting the high-nutrient low-chlorophyll status of the Southern Ocean.

Distribution and abundance of marine microbes in the Southern Ocean between 30 and 80°E

Our study, as part of the Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West) survey, emphasised the vital role of sea-ice retreat and upwelling in controlling the distribution, abundance and composition of marine microbial communities in the seasonal ice zone (SIZ). Autofluorescence or stains were used to detect the abundance of nanophytoplankton, heterotrophic nanoflagellates (HNF), virus like particles (VLP) and bacteria by flow cytometry. Correlations among microbial concentrations were determined and cluster analysis was performed to group sites of similar microbial composition and abundance. Distance to sea ice was the primary determinant of nanophytoplankton abundance and nanophytoplankton contributed up to 84% of the phytoplankton carbon biomass where melting sea ice caused shallow summer mixed layer depths. To the north, nanophytoplankton abundance was generally low except adjacent to the Southern Boundary (SB). HNF and bacterial abundance was positively correlated with the abundance of nanophytoplankton. Cluster analysis identified 5 groups of sites over the BROKE-West survey area. Clusters 1-4 grouped sites of different successional maturity of the microbial community along the continuum between bloom formation and senescence. Maturity increased with distance from the sea ice and, in areas of upwelling, with time since the development of phytoplankton blooms. Sites in cluster 5 occurred at the northernmost extreme of the survey area and were typical of communities in high nutrient low chlorophyll (HNLC) waters of the permanent open-ocean zone (POOZ) where phytoplankton growth was matched by mortality and decomposition. Synoptic-scale studies in Antarctic waters are rare but provide vital information about the control of microbial productivity, abundance and distribution in the Southern Ocean. Our study, covering over 40% of the SIZ off East Antarctica, enhances our understanding of the synoptic-scale factors that determine the structure and function of the microbial loop.

Low and high frequency Madden–Julian oscillations in austral summer: interannual variations

The Madden-Julian oscillation (MJO) is the main component of intraseasonal variability of the tropical convection, with clear climatic impacts at an almost-global scale. Based on satellite observations, it is shown that there are two types of austral-summer MJO events (broadly defined as 30-120 days convective variability with eastward propagation of about 5 m/s). Equatorial MJO events have a period of 30-50 days and tend to be symmetric about the equator, whereas MJO events centered near 8°S tend to have a longer period of 55-100 days. The lower-frequency variability is associated with a strong upper-ocean response, having a clear signature in both sea surface temperature and its diurnal cycle. These two MJO types have different interannual variations, and are modulated by the Indian Ocean Dipole (IOD). Following a negative IOD event, the lower-frequency southern MJO variability increases, while the higher-frequency equatorial MJO strongly diminishes. We propose two possible explanations for this change in properties of the MJO. One possibility is that changes in the background atmospheric circulation after an IOD favour the development of the low-frequency MJO. The other possibility is that the shallower thermocline ridge and mixed layer depth, by enhancing SST intraseasonal variability and thus ocean-atmosphere coupling in the southwest Indian Ocean (the breeding ground of southern MJO onset), favour the lower-frequency southern MJO variability.

Intra-Annual Studies of Mixed Layer Depth in the Arabian Sea Using A 3 ½ Layer Indian Ocean Model

Mixed layer is the upper layer of the ocean, where significant physical, chemical and biological activities take place. Knowledge of mixed layer depth variability is important in the studies of air-sea interaction, acoustic propagation, heat transport and fisheries. The Arabian Sea experiences extremes in atmospheric forcing that lead to intra-annual and in- ter-annual variability. Since the climatic conditions over the Arabian Sea are highly variable, the mixed layer depth also changes seasonally. Hence the intra-annual variability of mixed layer depth in the Arabian Sea has been examined using 3 � layer Indian Ocean model. The model is integrated for 8 years (1993 - 2000) with annually varying monthly averaged winds derived from ERS-2 scatterometer. In addition to this data, inter annually varying monthly averaged satellite esti- mates of precipitation from Global Precipitation Climatology were also incorporated in the model. Model results show a steady deepening of mixed layer depth in the central Arabian Sea during southwest monsoon period. The model shows its capability to predict the shallow mixed layer depths caused by coastal upwelling off Somalia during southwest monsoon period and is able to capture the Lakshadweep high/low as it predicted successfully the deeper/shallow mixed layer ob- served during these periods.

Mixed layer depth in the Aegean, Marmara, Black and Azov Seas: Part I: General features

Climatological fields of mixed layer depth (MLD) are presented over the Aegean, Marmara, Black and Azov Seas. Monthly fields of MLD are formed by historical individual temperature and salinity profiles from combination of various data sets with additional quality control procedures applied. Various definitions that are based solely on temperature ( T) or those that include the impact of salinity ( S) are applied to investigate the robustness in the pattern and values of the MLDs. Interpolation of the MLD fields to a 0.25° × 0.25° regular grid over the region is accomplished using a combination of median filter and ordinary kriging. Strong seasonal variability is noted in all regions. Given a density-based MLD criterion that includes both T and S, deep mixed layers (> 200 m) are noted in the Aegean Sea, especially eastern part of the region during winter while MLDs are generally much shallower (< 60 m) in the Black Sea. A criterion based on curvature method, which determines MLD according to first maximum of curvature of T may fail in representing deep MLDs in the Aegean Sea when the water column is well mixed. MLD fields obtained from all definitions are found to be strongly correlated to each other over the seasonal cycle, confirming the strong seasonal cycle. While the curvature method gives shallow MLDs only during winter, it has relatively large skill in comparison to the density-based MLD criterion. In general, MLD fields suffer from lack of input T and S profiles in the Marmara and Azov Seas, thus they may not be representative. Monthly MLD fields presented in this paper are available for various applications, such as mixed layer studies, ocean biology, ocean modeling and acoustic propagation.

Contrasting size evolution in marine and freshwater diatoms.

Diatoms are key players in the global carbon cycle and most aquatic ecosystems. Their cell sizes impact carbon sequestration and energy transfer to higher trophic levels. We report fundamental differences in size distributions of marine and freshwater diatoms, with marine diatoms significantly larger than freshwater species. An evolutionary game theoretical model with empirical allometries of growth and nutrient uptake shows that these differences can be explained by nitrogen versus phosphorus limitation, nutrient fluctuations and mixed layer depth differences. Constant and pulsed phosphorus supply select for small sizes, as does constant nitrogen supply. In contrast, intermediate frequency nitrogen pulses common in the ocean select for large sizes or the evolutionarily stable coexistence of large and small sizes. Size-dependent sinking interacts with mixed layer depth (MLD) to further modulate optimal sizes, with smaller sizes selected for by strong sinking and shallow MLD. In freshwaters, widespread phosphorus limitation, together with strong sinking and shallow MLD produce size distributions with smaller range, means and upper values, compared with the ocean. Shifting patterns of nutrient limitation and mixing may alter diatom size distributions, affecting global carbon cycle and the structure and functioning of aquatic ecosystems.

On the heat budget of the Arabian Sea

This study analyzes the heat budget of the Arabian Sea using satellite-derived sea-surface temperature (SST) from 1985 to 1995 along with other data sets. For a better understanding of air–sea interaction, canonical average monthly fields representing the spatial and temporal structure of the various components of the heat balance of the Arabian Sea are constructed from up to 30 years of monthly atmospheric and oceanic data. The SST over the Arabian Sea is not uniform and continually evolves with time. Cooling occurs over most of the basin during November through January and May through July, with the greatest cooling in June and July. Warming occurs over most of the basin during the remainder of the year, with the greatest warming occurring in March and September. Results indicate that the sign of the net heat flux is strongly dependent on the location and month. The effects of net heat flux and penetrative solar radiation strongly influence the change in SST during February and are less important during August and September. Horizontal advection acts to cool the sea surface during the northeast monsoon months. During the southwest monsoon horizontal advection of surface waters warms the SST over approximately the southern half of the basin, while the advection of upwelled water from the Somalia and Oman coasts substantially cools the northern basin. The central Arabian Sea during the southwest monsoon is the only area where the change in SST is balanced by the entrainment and turbulent diffusion at the base of the mixed layer. Agreement between the temporal change in the satellite-derived SST and the change calculated from the conservation of heat equation is surprisingly good given the errors in the measured variables and the bulk formula parameters. Throughout the year, monthly results over half of the basin agree within 3°. Considering that the SST changes between 8° and 12° over the year, this means that our results explain from 62% to 75% of the change in SST over 56% of the Arabian Sea. Two major processes contribute to the discrepancy in the change in SST calculated according to the heat budget equation and the change in SST derived from satellite observations. The first is the effect of the horizontal advection term. The position of the major eddies and currents during the southwest monsoon greatly affects the change in SST due to the large gradient in temperature between the cold upwelled waters along the Somali coast to the warm waters in the interior of the basin. The second major process is the thermocline effect. In areas of shallow mixed-layer depth, high insolation and wind speeds of either less than 3 m/s or greater than 15 m/s, the bulk formulae parameterization of the surface heat fluxes is inappropriate.

Primary production within the sea-ice zone west of the Antarctic Peninsula: I—Sea ice, summer mixed layer, and irradiance

In shelf waters of the western Antarctic Peninsula (wAP), with abundant macro- and micronutrients, water-column stability has been suggested as the main factor controlling primary production; freshwater input from sea-ice melting stabilizes the upper water column by forming a shallow summer mixed layer. Retreating sea ice in the spring and summer thus defines the area of influence, the sea-ice zone (SIZ) and the marginal ice zone (MIZ). A 12-year time series (1995–2006) was analyzed to address two main questions: (1) what are the spatial and temporal patterns in primary production; and (2) to what extent and in what ways is primary production related to sea-ice dynamics. Data were collected on cruises performed during January of each year, at the height of the growth season, within the region bounded by 641S and 641W to the north and 681S and 661W to the south. Average daily integrated primary production varied by an order of magnitude, from � 250 to � 1100 mg C m � 2 d � 1 , with an average cruise primary production of 745 mg C m � 2 d � 1 . A strong onshore–offshore gradient was evident along the shelf with higher production observed inshore. Inter-annual regional production varied by a factor of 7: maximum rates were measured in 2006 (1788 mg C m � 2 d � 1 ) and minimum in 1999 (248 mg C m � 2 d � 1 ). The results support the hypothesis that primary production in the wAP shelf is related to sea-ice dynamics. To first order, shallower summer mixed-layer depths in the shelf correlated with late sea retreat and primary production. Principal component analysis showed that high primary production in January was associated with enhanced shelf production toward the coast and in the south, explaining 63% of the variability in space and time. This first mode captured the inter-annual variability in regional production. Temporal variability in primary production (time series of anomalies defined for each location) showed spatial dependence: higher primary production correlated with shallow mixedlayer depths only at mid-shelf; in coastal and offshore waters, primary production correlated with deeper mixed layers. Thus, coastal primary production can show a non-linear relationship with summer mixed layers. Under conditions of large biomass (420 mg chl a m � 3 ) and shallow mixed-layer depth (e.g., 5 m) phytoplankton production becomes light limited. This limitation is reduced with a deepening of the summer mixed layer (e.g., 20 m). Dominance of diatoms and the ability to adapt and photosynthesize at higher light levels characterized the large phytoplankton blooms. No significant regional trend in primary production was detected within the 12-year series. We conclude that the regional average primary production on the wAP shelf is associated with shallow summer mixed layers in conjunction with late sea-ice retreat. An opposite relationship is observed for the highest production rates in coastal waters, associated with large biomass, where a deepening of the summer mixed layer relieves light limitation.

From climate regime shifts to lower-trophic level phenology: Synthesis of recent progress in retrospective studies of the western North Pacific

Pelagic ecosystems of the western North Pacific have experienced dramatic changes in recent decades. Prior retrospective analysis of several regions has shown that ecosystem or population changes have occurred coincident with or shortly following significant changes in ocean climate and atmospheric forcing. Here, we summarize these changes and attempt to identify specific mechanisms responsible for the changes in three regions in the western North Pacific: the Oyashio (OY), the subtropical water (ST), and the southern Japan/East Sea (JES). A cooling condition after the climate regime shift of 1976–1977 deepened the winter mixed layer depth (MLD) in both the OY and ST, but influenced lower-trophic level productivity differently between these regions. The deep MLD reduced winter phytoplankton and zooplankton biomass in the subarctic OY, presumably due to a decrease in wintertime light availability. Concurrently, it increased spring plankton biomass in the ST, presumably due to the replenishment of springtime nutrients. When a warming condition became prevalent after the 1988–1989 regime shift, the shallow winter MLD increased the winter plankton biomass in the OY, but decreased the spring plankton biomass in the ST through the same mechanism controlling light and nutrient availability. In the JES, with a complex water column structure consisting of the surface warm current and dense subsurface cold water, the hydrographic conditions and ecosystem responses could not be explained by the mechanisms for the OY and ST. We also detected cooling and warming phase shifts induced a phenological change in the OY. A model hindcast for the OY estimated an average 5-day (max. 20-day) delay in the timing of the primary production peak during the cool phase after the mid-1970s. This timing became earlier after the mid-1990s, several years after the 1988–1989 regime shift, reaching the level before the mid-1970s. The observed increase and decrease in winter and spring phytoplankton biomass, respectively, during the warm phase in the 1990s, indicate a moderate but prolonged productive season, which differs from the conventional seasonal variation pattern of the OY with an extensive but short spring bloom. This alternate phytoplankton seasonality coupled with the direct influence of a warm temperature anomaly might dramatically alter the zooplankton community structure in the OY after the 1990s, as suggested by the marked increase in the occurrence of warm-water copepods in spring. Based on these findings, we warn that retrospective analyses based on annual averages and lacking careful consideration of seasonal variation may result in erroneous findings of causes and consequences.

The impact of bio-optical heating on the properties of the upper ocean: A sensitivity study using a 3-D circulation model for the Labrador Sea

Abstract The impact of bio-optical heating on the properties of the upper Labrador Sea water was investigated by considering changes in light attenuation in water associated with the seasonal change of chlorophyll distribution. The time- and depth-dependent attenuation coefficients were obtained from remotely sensed SeaWiFS ocean-colour data. Sea-surface temperature (SST) and mixed-layer depth (MLD) were computed from a three-dimensional ocean circulation model. The model was integrated from 1999 to 2003 with 6-hourly atmospheric forcing. The changes in SST and MLD attributable to bio-optical heating were determined by comparing the model results using the observed attenuation coefficients (chlorophyll) to those using a weak and constant attenuation (clear water). The model results show that bio-optical heating is controlled mainly by chlorophyll concentration and MLD. The increase in SST is around 1 °C in most parts of the Labrador Sea and the shelves, and up to 2.7 °C in areas of shallow MLD and high chlorophyll concentrations (the Grand Banks and Northeastern Newfoundland Shelf). The increase is much higher than that found in previous studies, which was typically a fraction of a degree. Bio-optical heating also can enhance the stratification of the upper ocean and reduce the MLD by 20–50%.

Chlorophyll a variation in the Kuroshio Extension revealed with a mixed‐layer tracking float: implication on the long‐term change of Pacific saury (Cololabis saira)

AbstractVariation of chlorophyll a from March 2004 to July 2005 in the formation region of Subtropical Mode Water in the Kuroshio Extension was observed with a mixed‐layer tracking profiling float parking at around 40 m depth. Chlorophyll a concentration in the mixed‐layer is seasonally high from winter to early spring (January–April) even in the deep mixed layer; whereas during winter it tends to be lower for the corresponding deeper mixed layer. The chlorophyll a integrated over the winter mixed‐layer depth (MLD) increases with MLD for MLD &lt;200 m and for MLD &gt; 250 m the integrated chlorophyll a almost disappears, probably because of the large MLD exceeds in the critical depth with light limitation. These results suggest that the wintertime MLD influences the recruitment of Pacific saury (Cololabis saira) in the Kuroshio Extension because the wintertime food environment for larvae and juvenile of the Pacific saury could be better in the shallow wintertime MLD period of the 1950s and 1990s and worse in the deep MLD period of the 1970s and early 1980s.

Variability of mixed layer depth in Kuroshio/Oyashio Extension region: 2005–2006

In December 2005, the winter monsoon was abnormally strong around Japan. Therefore, it was expected that the cooling effect on the upper layer of the ocean would be very strong. However, the mixed layer depth (MLD) in the Kuroshio/Oyashio Extension region was not as deep as those observed during the last four winters. We investigated the reason for the shallow MLD by using a one‐dimensional turbulent closure model and the temperature and salinity data obtained using Argo floats. We found that summer heating plays an important role in the determination of the maximum MLD. In 2005, the heat storage rate was very high from July to October since summer heating was significantly stronger than those during the other summers. Consequently, a large vertical density gradient existed at the bottom of the mixed layer and prevented the deepening of the mixed layer.

Multi-nutrient, multi-group model of present and future oceanic phytoplankton communities

Abstract. Phytoplankton community composition profoundly affects patterns of nutrient cycling and the dynamics of marine food webs; therefore predicting present and future phytoplankton community structure is crucial to understand how ocean ecosystems respond to physical forcing and nutrient limitations. We develop a mechanistic model of phytoplankton communities that includes multiple taxonomic groups (diatoms, coccolithophores and prasinophytes), nutrients (nitrate, ammonium, phosphate, silicate and iron), light, and a generalist zooplankton grazer. Each taxonomic group was parameterized based on an extensive literature survey. We test the model at two contrasting sites in the modern ocean, the North Atlantic (North Atlantic Bloom Experiment, NABE) and subarctic North Pacific (ocean station Papa, OSP). The model successfully predicts general patterns of community composition and succession at both sites: In the North Atlantic, the model predicts a spring diatom bloom, followed by coccolithophore and prasinophyte blooms later in the season. In the North Pacific, the model reproduces the low chlorophyll community dominated by prasinophytes and coccolithophores, with low total biomass variability and high nutrient concentrations throughout the year. Sensitivity analysis revealed that the identity of the most sensitive parameters and the range of acceptable parameters differed between the two sites. We then use the model to predict community reorganization under different global change scenarios: a later onset and extended duration of stratification, with shallower mixed layer depths due to increased greenhouse gas concentrations; increase in deep water nitrogen; decrease in deep water phosphorus and increase or decrease in iron concentration. To estimate uncertainty in our predictions, we used a Monte Carlo sampling of the parameter space where future scenarios were run using parameter combinations that produced acceptable modern day outcomes and the robustness of the predictions was determined. Change in the onset and duration of stratification altered the timing and the magnitude of the spring diatom bloom in the North Atlantic and increased total phytoplankton and zooplankton biomass in the North Pacific. Changes in nutrient concentrations in some cases changed dominance patterns of major groups, as well as total chlorophyll and zooplankton biomass. Based on these scenarios, our model suggests that global environmental change will inevitably alter phytoplankton community structure and potentially impact global biogeochemical cycles.