Subtropical Frontal Zone Research Articles

Multidecadal modulations of the low‐frequency climate variability in the wintertime North Pacific since 1950

AbstractThe North Pacific decadal variability (PDV) is known to manifest itself as two distinct spatial patterns. Observations since 1950 reveal that the wintertime PDV underwent notable modulations of their dominance in sea surface temperature (SST) variability and accompanying atmospheric variability. Until the 1980s, decadal SST variability was strongest along the subarctic frontal zone (SAFZ), the boundary between the warm Kuroshio and cool Oyashio waters. The SAFZ variability was highly correlated with decadal variability of the surface Aleutian Low but not simultaneously with tropical SST variability. Since the 1990s, however, this extratropical ocean‐atmosphere variability has lost its predominance, taken over by SST variability in the subtropical frontal zone. It accompanies subtropical anticyclone variability, exhibiting significant anticorrelation with tropical SST variability. These long‐term PDV modulations have remotely modulated temperature variability over Canada and Alaska. Similar PDV modulations are simulated in a centennial integration of a global climate model.

Migratory Movements of Pygmy Blue Whales (Balaenoptera musculus brevicauda) between Australia and Indonesia as Revealed by Satellite Telemetry

In Australian waters during the austral summer, pygmy blue whales (Balaenoptera musculus brevicauda) occur predictably in two distinct feeding areas off western and southern Australia. As with other blue whale subspecies, outside the austral summer their distribution and movements are poorly understood. In order to describe the migratory movements of these whales, we present the satellite telemetry derived movements of eleven individuals tagged off western Australia over two years. Whales were tracked from between 8 and 308 days covering an average distance of 3,009±892 km (mean ± se; range: 832 km–14,101 km) at a rate of 21.94±0.74 km per day (0.09 km–455.80 km/day). Whales were tagged during March and April and ultimately migrated northwards post tag deployment with the exception of a single animal which remained in the vicinity of the Perth Canyon/Naturaliste Plateau for its eight day tracking period. The tagged whales travelled relatively near to the Australian coastline (100.0±1.7 km) until reaching a prominent peninsula in the north-west of the state of Western Australia (North West Cape) after which they travelled offshore (238.0±13.9 km). Whales reached the northern terminus of their migration and potential breeding grounds in Indonesian waters by June. One satellite tag relayed intermittent information to describe aspects of the southern migration from Indonesia with the animal departing around September to arrive in the subtropical frontal zone, south of western Australia in December. Throughout their migratory range, these whales are exposed to impacts associated with industry, fishing and vessel traffic. These movements therefore provide a valuable tool to industry when assessing potential interactions with pygmy blue whales and should be considered by conservation managers and regulators when mitigating impacts of development. This is particularly relevant for this species as it continues to recover from past exploitation.

Stratosphere‐Troposphere Exchange Corresponding to a Deep Convection in the Warm Sector and Abnormal Subtropical Front Induced by a Cutoff Low over East Asia

AbstractWe studied a deep convection in the warm sector and abnormal subtropical front caused by a cutoff low over East Asia in June 2009 by using cloud profile radar data from Cloudsat, temperature of black body from FY2C, atmospheric compositions from Aura/MLS, meteorological data from the ECMWF and HYSPLIT4 trajectory model. The analysis shows that for the sake of the background of southern trough, there is a weaker rainfall from the inner fronts, while nearly 1000km‐scale deep convection occurs in the warm sector. At the nearly mature stage of the low, the subtropical front induced by the cutoff low is abnormally developing throughout the whole troposphere, which is due to the warm and humid air from southwest gathering along the subtropical front. After rapid eastward movement of the front into the rain belt of the warm sector, the strong ascending branch of thermal indirect secondary circulation across the frontal zone enhances deep convection at the colder part below the front. But it is difficult to sustain because the supplement of water vapor is blocked by the abnormal front. Furthermore, intense stratosphere‐troposphere exchange occurs in the abnormal subtropical front zone due to the convective injection into stratospheric intrusions. On the scatter plot of ozone and water vapor these are two special gathering areas with both high (low) concentrations of the two species.

The Dynamical Subtropical Front

[1] The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and sea surface height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore, we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and is characterized by strong SST and SSH gradients and no seasonal cycle. At the center of each basin it merges with the Sub-Antarctic Front. On the eastern side of basins, the STF surface water mass boundary coincides with a separate region of multiple SST fronts. We call this the Subtropical Frontal Zone (STFZ). The fronts in the STFZ have a large seasonal cycle and no SSH signature. Despite lying close to the same water mass boundary, the DSTF and STFZ are completely unrelated. We therefore suggest the term STF only be used when referring to the surface water mass boundary. When studying the strong currents on the western side of basins the term DSTF is more relevant and, similarly, the term STFZ better describes the region of enhanced SST gradients towards the east.

The effect of coastal processes on phytoplankton biomass and primary production within the near-shore Subtropical Frontal Zone

This study evaluated drivers of phytoplankton net primary production (NPP) rates and chlorophyll-a (chl-a) concentrations within the coastally oriented Subtropical Frontal Zone (STFZ) off the South Island of New Zealand. Time series measurements of hydrographic parameters, macronutrients, size fractionated NPP and chl-a were conducted on a bi-monthly basis from July 2009 to November 2010. This study found that nutrient limitation in these waters is controlled by the dual influx of silicate inputs from riverine sources in coastal neritic water (NW) and oceanic inputs of nitrate from the high nutrient, low chlorophyll (HNLC) region of the offshore Sub-Antarctic Surface Waters (SASW). Total chl-a concentrations and primary production rates were perennially higher in near-shore NW and modified Subtropical waters (STW) than in the SASW, with highest indicators of biological production observed in the Austral spring and summer seasons (October to March). These periods of peak production and biomass were dominated in both parameters by microphytoplankton (>20 μm) size fractions. The coupled dominance by these large phytoplankton and the near depletion of silicate in all characterised waters within the frontal system indicate the importance of silicic diatoms as drivers of bloom production. The influence of coastal waters on the STFZ system is most pronounced with the intrusion of neritic water beyond the shelf boundary during periods of surface water thermal stratification and riverine dilution through flooding events. These two events were notably observed during the Spring 2009 sampling cruise in December 2009 and in the flood event in May 2010.

Interaction of the subtropical front with topography around southern New Zealand

The subtropical front zone (STFZ) is a fundamental feature of the southwest Pacific Ocean. It separates waters in the subtropical gyres from subantarctic waters. In this study we examine a section of the global STFZ that passes around southern New Zealand, using hydrographic sections and remotely sensed sea surface temperature (SST) data collected during the austral autumns of 2007, 2008 and 2011. The STFZ was composed of a single southern subtropical front (S-STF) that marked the poleward edge of the STFZ, while the region north of the S-STF was characterised by a series of shallow fronts (<50m depth) and a freshwater-induced neritic front. The S-STF around southern New Zealand is also distinguished by moderate-to-weak density compensation, in contrast to the strong density compensation to the west and east of New Zealand. We find strong coupling between the surface and subsurface expressions of the S-STF and demonstrate the practicality of tracing the pathway of the front using its surface expression from remotely sensed images. The S-STF is steered by topography as it crosses the Macquarie Ridge close to 49.6°S and to the east of 166°E as it turns equatorward following the upper continental slope. This is contrary to some previous descriptions that placed the S-STF lying across the Snares Shelf close to 47.5°S. Topographic interactions between the S-STF and sharply defined bathymetry around southern New Zealand appear to strongly influence the position of the front in this region. Cross-frontal temperature and salinity gradients at the S-STF were also found to be intensified in regions of shallow bathymetry. Both results are attributed to topographic steering of the front by bathymetry at approximately 500m depth.

Ocean nomads: Distribution and movements of sperm whales in the North Pacific shown by whaling data and Discovery marks

AbstractWe investigated the distribution and movements of sperm whales (Physeter macrocephalus) in the North Pacific by analyzing whaling data and movement data of whales marked with Discovery marks. Prior studies suggested that there were discrete “stocks” of sperm whales, assuming that the intervals between historical areas of concentration indicated subpopulation boundaries. Our analyses clearly refute this assumption: whaling and marking data suggest no obvious divisions between separate demes or stocks within the North Pacific. Sperm whales appear to be nomadic and show widespread movements between areas of concentration, with documented movements of over 5,000 km, time spans between marking and recovery over 20 yr, and ranges that cover many thousand km2. Males appear to range more widely than females. Sperm whales likely travel in response to geographical and temporal variations in the abundance of medium‐ and large‐sized pelagic squids, their primary prey. Our analyses demonstrate that males and females concentrated seasonally in the Subtropical Frontal Zone (ca. 28ºN–34ºN) and the Subarctic Frontal Zone (ca. 40ºN–43ºN), and males also concentrated seasonally near the Aleutian Islands and along the Bering Sea shelf edge. It appears that the sperm whales targeted by the pelagic whalers range widely across this ocean basin.

Zonal variability in primary production and nitrogen uptake rates in the southwestern Indian Ocean and the Southern Ocean

Hydrographic parameters along with the primary and new production measurements were carried out during the austral summer, 2009, in the southwestern Indian Ocean and Indian sector of the Southern Ocean (SO). The production varies from 185 to >900mgCm−2d−1 in different zones of SO. The zonal variations in production accompany variations in SST, salinity and nutrients. Further, the new production (0.3 to 4.1mmolNm−2d−1) covaries with the overall production, while the uptake of reduced forms of nitrogen (both NH4 and urea) show opposite trends. In the NO3 limiting environment (north of subtropical convergence), NH4 uptake dominates the total regenerated production, whereas, urea uptake dominates the regenerated production under Si, light and micronutrient (e.g., Fe) limiting conditions (found between the subtropical convergence and Antarctica). On the basis of the C and N uptake data, the studied region can be divided into five zones (from the south to the north) viz., located between (i) the Antarctic continent and the polar front (Antarctic zone; ANZ), (ii) the polar and subantarctic fronts (SAF) (Polar frontal zone; PFZ), (iii) SAF and Agulhas Retroflection fronts (ARF) (South Subtropical front; SSTF), (iv) subtropical frontal zone (STFZ), and (v) ARF and the north subtropical front (Subtropical zone; STZ). Except at SSTF, regenerated production dominates in all the zones. From the south to the north, this could be due to different reasons e.g., light, grazing by zooplankton, supply of key micronutrients (probably Fe), Si-limitation, or NO3-limitation. In the absence of such limitations, the maximum possible f-ratio in SO could be as high as 0.78±0.12 and under such conditions the region could export most of the total production to the deep. Supply of micronutrients through the Agulhas return current and from the Crozet Island supports the higher chl a, C uptake and new production at the 48°E transect relative to the 57.5°E transect. The C:N assimilation ratio is found to be 5.64, marginally lower than the canonical Redfield ratio. This slight difference is likely due to the variation in the composition of phytoplankton and NO3-limitation in some zones. A comparison with earlier results shows that seasonal and spatial variations in f-ratios in these zones are much higher than its inter-annual variability.

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on &amp;lt;sup&amp;gt;234&amp;lt;/sup&amp;gt;Th: &amp;lt;sup&amp;gt;238&amp;lt;/sup&amp;gt;U disequilibria

Abstract. We evaluated the export fluxes of Particulate Organic Carbon (POC) in the Subtropical Frontal zone (STF) of the SW Pacific sector of the Southern Ocean. The site is characterized by enhanced primary productivity, which has been suggested to be stimulated through so-called natural iron fertilization processes where iron-depleted subantarctic water (SAW) mixes with mesotrophic, iron-replete subtropical water (STW). We adopted the small-volume 234Th method to achieve the highest possible spatial sampling resolution in austral late autumn-early winter, May–June, 2008. Inventories of chlorophyll-a, particulate 234Th and POC observed in the upper 100 m were all elevated in the mid-salinity water type (34.5 &lt; S &lt; 34.8), compared with low salinity waters (S &lt; 34.5) which were of SAW origin with high macronutrients and high (S &gt; 34.8) salinity waters which were of STW origin with low macronutrients. However, Steady-State 234Th fluxes were similar across the salinity gradient being, 25 ± 0.78 ((1.5 ± 0.047) × 103) in the mid-salinity, and 29 ± 0.53 ((1.8 ± 0.032) × 103) and 22 ± 1.1 Bq m−2 d−1 ((1.3 ± 0.066) × 103 dpm m−2 d–1) in the high and low salinity waters respectively. Bottle POC/Th ratios at the depth of 100 m were used to convert 234Th fluxes into POC export fluxes. The derived POC flux did not appear to be enhanced in mid-salinity waters where the primary productivity was inferred to be the highest at the time of sampling, with a flux of 11 ± 0.45 mmol C m−2 d−1, compared to 14 ± 0.39 mmol C m−2 d−1 in high salinity waters and 8.5 ± 0.66 mmol C m−2 d−1 in low salinity waters. This study thus implied that natural iron fertilization does not necessarily lead to an enhancement of POC export in STF regions.

Changes in abundance of the neon flying squid Ommastrephes bartramii in relation to climate change in the central North Pacific Ocean

Interannual changes in abundance of the autumn cohort of neon flying squid Ommastraphes bartramii during 1979 to 2006 were examined in relation to the large-scale climate shift in the central North Pacific Ocean. Catch per unit of effort (CPUE) of driftnet surveys, which is assumed to reflect the squid stock level, was very low during 1999 to 2002, which was an anom- alous climate period. Given that the autumn cohort uses productive areas near and north of the transition zone chlorophyll front (TZCF) as favorable nursery and feeding grounds, we examined 3 covariates — (1) the winter position of the TZCF in the neon flying squid nursery ground, (2) the summer position of the TZCF in the feeding ground and (3) fishing mortality — to estimate which covariates were significantly correlated with low CPUE time periods. Multiple linear regression analysis suggested that only the effect of winter position of the TZCF in the nursery ground best predicted the change in the CPUE. The anomalous climate conditions caused a significant reduc- tion in net primary production over an extended area within the subtropical frontal zone (STFZ), which is the main nursery ground for the autumn cohort. Hence, we assume that food availability for juvenile neon flying squid could be critical for the level of stock recruitment of this cohort. Fur- thermore, the sea level anomaly (SLA), which is considered to be a useful index of variability in the upper ocean structure, significantly correlated to the CPUE during spawning and nursery periods in the STFZ. Our study supports the hypothesis that the bottom-up effect of the climate shift can be manifested rapidly in neon flying squid, indicating that squid abundance can act as a productivity integrator and a real-time ecosystem indicator that respond to large-scale environ- ment changes.

On North Pacific circulation and associated marine debris concentration

Marine debris in the oceanic realm is an ecological concern, and many forms of marine debris negatively affect marine life. Previous observations and modeling results suggest that marine debris occurs in greater concentrations within specific regions in the North Pacific Ocean, such as the Subtropical Convergence Zone and eastern and western “Garbage Patches”. Here we review the major circulation patterns and oceanographic convergence zones in the North Pacific, and discuss logical mechanisms for regional marine debris concentration, transport, and retention. We also present examples of meso- and large-scale spatial variability in the North Pacific, and discuss their relationship to marine debris concentration. These include mesoscale features such as eddy fields in the Subtropical Frontal Zone and the Kuroshio Extension Recirculation Gyre, and interannual to decadal climate events such as El Niño and the Pacific Decadal Oscillation/North Pacific Gyre Oscillation.

Diving of Great Shearwaters (Puffinus gravis) in Cold and Warm Water Regions of the South Atlantic Ocean

BackgroundAmong the most widespread seabirds in the world, shearwaters of the genus Puffinus are also some of the deepest diving members of the Procellariiformes. Maximum diving depths are known for several Puffinus species, but dive depths or diving behaviour have never been recorded for great shearwaters (P. gravis), the largest member of this genus. This study reports the first high sampling rate (2 s) of depth and diving behaviour for Puffinus shearwaters.Methodology/Principal FindingsTime-depth recorders (TDRs) were deployed on two female great shearwaters nesting on Inaccessible Island in the South Atlantic Ocean, recording 10 consecutive days of diving activity. Remote sensing imagery and movement patterns of 8 males tracked by satellite telemetry over the same period were used to identify probable foraging areas used by TDR-equipped females. The deepest and longest dive was to 18.9 m and lasted 40 s, but most (>50%) dives were <2 m deep. Diving was most frequent near dawn and dusk, with <0.5% of dives occurring at night. The two individuals foraged in contrasting oceanographic conditions, one in cold (8 to 10°C) water of the Sub-Antarctic Front, likely 1000 km south of the breeding colony, and the other in warmer (10 to 16°C) water of the Sub-tropical Frontal Zone, at the same latitude as the colony, possibly on the Patagonian Shelf, 4000 km away. The cold water bird spent fewer days commuting, conducted four times as many dives as the warm water bird, dived deeper on average, and had a greater proportion of bottom time during dives.Conclusions/SignificanceGeneral patterns of diving activity were consistent with those of other shearwaters foraging in cold and warm water habitats. Great shearwaters are likely adapted to forage in a wide range of oceanographic conditions, foraging mostly with shallow dives but capable of deep diving.

Mesoscale eddies northeast of the Hawaiian archipelago from satellite altimeter observations

Enhanced mesoscale eddy activity northeast of the Hawaiian archipelago was investigated by using the high‐pass filtered satellite altimetric sea surface height (SSH) data of the past 16 years. The level of eddy kinetic energy (EKE) shows significant interannual variations, which appear well correlated to the large‐scale atmospheric wind forcing associated with the Pacific Decadal Oscillation index; the EKE levels in the area along the Subtropical Front zone in 1997–1998 and 2003–2005, for example, were three times as large as those in the low EKE years. Spectral analyses in the 3° latitude by 5° longitude subregions revealed the dominant mesoscale periods ranging from 90 days near 18°N to 180 days near 36°N. A 3‐layer quasi‐geostrophic model constrained by World Ocean Atlas 2005 climatologies was used to explore instability of the regional circulation. The analysis shows a geographical match between baroclinic instability strength and the observed RMS SSH variability and a temporal match between seasonally varying instability strength and the observed seasonal EKE modulation, and suggests that baroclinic instability by the vertically sheared, regional mean circulation provides the energy source for the enhanced eddy variability northeast of the Hawaiian archipelago.

Characterizing Thermohaline Intrusions in the North Pacific Subtropical Frontal Zone

Abstract A monthlong field survey in July 2007, focused on the North Pacific subtropical frontal zone (STFZ) near 30°N, 158°W, combined towed depth-cycling conductivity–temperature–depth (CTD) profiling with shipboard current observations. Measurements were used to investigate the distribution and structure of thermohaline intrusions. The study revealed that local extrema of vertical salinity profiles, often used as intrusion indicators, were only a subset of a wider class of distortions in thermohaline fields due to interleaving processes. A new method to investigate interleaving based on diapycnal spiciness curvature was used to describe an expanded class of laterally coherent intrusions. STFZ intrusions were characterized by their overall statistics and by a number of case studies. Thermohaline interleaving was particularly intense within 5 km of two partially compensated fronts, where intrusions with both positive and negative salinity anomalies were widespread. The vertical and cross-frontal scales of the intrusions were on the order of 10 m and 5 km, respectively. Though highly variable, the slopes of these features were typically intermediate between those of isopycnals and isohalines. Although the influence of double-diffusive processes sometime during the evolution of intrusions could not be excluded, the broad spectrum of the observed features suggests that any role of double diffusion was secondary.

Hydrographic and productivity characteristics along 45°E longitude in the southwestern Indian Ocean and Southern Ocean during austral summer 2004

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 389:97-116 (2009) - DOI: https://doi.org/10.3354/meps08126 Hydrographic and productivity characteristics along 45°E longitude in the southwestern Indian Ocean and Southern Ocean during austral summer 2004 P. Jasmine1, K. R. Muraleedharan1,5,*, N. V. Madhu1, C. R. Asha Devi1, R. Alagarsamy2, C. T. Achuthankutty1, Zeena Jayan3, V. N. Sanjeevan4, Satish Sahayak1 1National Institute of Oceanography, Regional Centre, Dr. Salim Ali Road, PB No. 1918, Kochi – 682 018, India 2National Institute of Oceanography, Dona-Paula, Goa – 403 004, India 3National Centre for Anarctic and Ocean Research, SADA, Goa, India 4Centre for Marine Living Resources and Ecology, Ministry of Earth Sciences, Kendriya Bhavan, PB No. 5415, Kochi, CSEZ PO, Kakkanad – 37, India 5Present address: National Institute of Oceanography, Regional Centre, Dr. Salim Ali Road, PB No. 1918, Kochi – 682 018, India *Email: muraleedharan@nio.org ABSTRACT: During the austral summer 2004, an intensive multidisciplinary survey was carried out in the Indian Ocean sector of the Southern Ocean to study the main hydrographic features and the associated productivity processes. This sector includes circumpolar zones and fronts with distinct hydrographic and trophic regimes, such as the Subtropical Zone (STZ), Subtropical Frontal Zone (STFZ), Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), North Subtropical Front (NSTF), Agulhas Retroflection Front (ARF), South Subtropical Front (SSTF), Subantarctic Front, Surface Polar Front (SPF), and Subsurface Polar Front. Seasonal variations in the solar irradiance and day length, stratification, lack of micronutrients like iron and increased grazing pressure are the major factors that influenced or constrained biological production in this region. Even though broad differences in these controlling factors exist in time and space between the zonal regions, the upper 1000 m of the water column of the main zones, STZ, STFZ, SAZ, PFZ, supported almost identical standing stocks of mesozooplankton, 0.43, 0.47, 0.45 and 0.49 ml m–3, respectively, during the austral summer. This unexpected similarity can be explained either through the functioning of the microbial loop within STZ, STFZ and SAZ and the multivorous food web ecology within the PFZ. Dominance of ciliates in the microzooplankton community may be one factor resulting in the maintenance of a high mesozooplankton standing stock in SAZ. In contrast to the zones, frontal regions showed wide differences in hydrography and biological characteristics. The SSTF and SPF were far more biologically productive than that of NSTF and ARF. KEY WORDS: Fronts · Zones · Microzooplankton · Mesozooplankton · Microbial loop · Southern Ocean Full text in pdf format PreviousNextCite this article as: Jasmine P, Muraleedharan KR, Madhu NV, Asha Devi CR and others (2009) Hydrographic and productivity characteristics along 45°E longitude in the southwestern Indian Ocean and Southern Ocean during austral summer 2004. Mar Ecol Prog Ser 389:97-116. https://doi.org/10.3354/meps08126 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 389. Online publication date: September 04, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Life history of the neon flying squid: effect of the oceanographic regime in the North Pacific Ocean

The North Pacific Ocean population of the neon flying squid Ommastrephes bartramii, which un- dertakes seasonal north-south migrations, consists of autumn and winter-spring spawning cohorts. We ex- amined life history differences between the 2 cohorts in relation to the oceanographic environment. The differ- ences could be explained by seasonal north-south movements of the following 2 oceanographic zones: (1) the optimum spawning zone defined by sea surface temperatures; and (2) the food-rich zone defined by the position of the transition zone chlorophyll front (TZCF). The 2 cohorts use the food-rich zone in different phases of their life cycles. The spawning grounds for the au- tumn cohort occur within the subtropical frontal zone (STFZ), characterized by enhanced productivity in win- ter due to its proximity to the TZCF, whereas the spawning grounds for the winter-spring cohort occur within the subtropical domain, which is less productive. As the TZCF shifts northward in spring, the autumn co- hort continues to occur in the productive area north of the TZCF, whereas the winter-spring cohort remains in the less productive area to the south until it migrates into productive waters north of the TZCF in the summer or autumn. Consequently, the autumn cohort grows faster than the winter-spring cohort during the first half of its life cycle, whereas the winter-spring cohort grows faster during the second half. This growth pat- tern may be responsible for differing migration pat- terns; males of the autumn cohort do not have to migrate given their early fast growth in the STFZ, whereas those of the winter-spring cohort must mi- grate to the food-rich subarctic frontal zone to compen- sate for their slow growth. These biological and eco- logical differences between the 2 cohorts suggest flexibility of their life history response to oceanographic environments. Hatchling of the neon flying squid Ommastrephes bartramii

Vertical and lateral variations in coccolithophore community structure across the subtropical frontal zone in the South Atlantic Ocean

In this study the coccolithophore compositions of 11 plankton depth stations along a N–S transect from the South Atlantic Subtropical Gyre to the Subantarctic Zone were examined qualitatively and quantitatively. The lateral and vertical distribution patterns of not only the most abundant taxa but also of the morphotypes of distinct species complexes, such as Calcidiscus leptoporus, Emiliania huxleyi, and Umbellosphaera tenuis were the focus. Geographic variation among morphotypes mirrors different ecological affinities of the members of a species complex. Multivariate statistics were used to infer the relationship between a set of known environmental data and species concentrations. The results of the detrended Canonical Correspondence Analysis (CCA) revealed the presence of distinct species assemblages. The Subtropical Gyre assemblage within the upper 50 m of the photic zone is mainly composed of Umbellosphaera irregularis, U. tenuis types III and IV, Discosphaera tubifera, Rhabdosphaera clavigera, S. pulchra and E. huxleyi var. corona, adapted to warm and oligotrophic conditions. In the deeper photic zone abundant Florisphaera profunda, Gephyrocapsa ericsonii and Oolithotus spp. are encountered, benefiting from higher nutrient concentrations in the vicinity of the nutricline. A well-defined Subtropical Frontal Zone (STFZ) association is clearly dominated by E. huxleyi types A and C throughout the upper 100 m of the water column. Secondary contributors in the upper photic zone are Syracosphaera spp. (mainly S. histrica, S. molischii), Michaelsarsia elegans, Ophiaster spp. and U. tenuis type II. This assemblage is associated with cooler, nutrient-rich waters. E. huxleyi type B is found deeper in the water column. Here it is accompanied by Algirosphaera robusta, G. muellerae, and S. anthos indicating a tolerance of lower light availability in environments with elevated productivity. C. leptoporus spp. leptoporus shows relatively high cell numbers in all sampled water levels throughout the STFZ. Interestingly, its coccoliths are often smaller 5 μm in lith diameter. The mean coccolithophore assemblages of a station were compared to the underlying surface sediment assemblages. For the most part, the distribution of the morphotypes is reflected in the sedimentary archive, thus proving their potential as paleoecological proxies.

TurtleWatch: a tool to aid in the bycatch reduction of loggerhead turtles Caretta caretta in the Hawaii-based pelagic longline fishery

Operational longline fishery characteristics, bycatch information, and loggerhead turtle satellite tracks were all used in conjunction with remotely sensed sea surface temperature data to identify the environmental area where the majority of loggerhead turtle bycatch occurred in the Hawaii-based longline fishery during 1994 to 2006. In the first quarter of each calendar year from 1994 to 2006, the majority of shallow longline sets and associated loggerhead turtle bycatch were above 28° N, which corresponds to the area near the North Pacific Subtropical Frontal Zone. Based on the thermal ranges of bycatch, sets and the satellite-tagged turtles, it was recommended that shal- low sets should only be deployed in waters south of the 18.5°C (~65.5°F) isotherm to decrease logger- head turtle bycatch. This recommendation formed the basis for the TurtleWatch tool, a map provid- ing up-to-date information about the thermal habitat of loggerhead sea turtles in the Pacific Ocean north of the Hawaiin Islands. TurtleWatch was released to fishers and managers in electronic and paper formats on December 26, 2006, to assist in decision making during the first quarter of 2007. Fishery information from 2007 was later compared with data for the years 2005 to 2006 to assess the response of the fishery to TurtleWatch. The observed fleet movement during the first quarter of 2007 was to the north of the 18.5°C (~65.5°F) isotherm (i.e. in the area recommended for avoidance by the TurtleWatch product) with increased effort and lower bycatch rates. We discuss possible reasons for this decrease in turtle bycatch north of the frontal zone together with future research directions which may lead to refinement of the TurtleWatch product.

The spatial structure of the oceanic spawning of jack mackerel ( Trachurus murphyi) off central Chile (1998–2001)

The jack mackerel population has a widespread oceanic spawning habitat off central Chile, extending more than one thousand nautical miles offshore. In this paper, the spatial structure of jack mackerel eggs density is analyzed on the basis of four surveys carried out in oceanic waters (32°S–39°S, 75°W–92°W), from 1998 to 2001. In each survey, a grid of plankton stations was sampled through vertical hauls with WP2 plankton nets by using several purse-seine fishing ships sampling simultaneously along the E–W transects. With the aim of finding the bulk of the egg distribution within the surveyed area, an exploratory analysis between jack mackerel egg densities, latitude, longitude, and sea surface temperature (SST) was carried out. The spatial structure of the egg distribution was studied using geostatistical techniques. The bulk of the jack mackerel spawning tends to occur offshore between 80°W and 92°W, is maximal at 35°S and associated to SST warmer than 15–16 °C. All of the variograms showed clear spatial autocorrelations without anisotropy, with the range fluctuating between 125 and 252 nautical miles. The range of variograms suggests that the spawning of jack mackerel is a large scale process, probably reflected in the adult behavior of the spawning by favoring a high dispersion of eggs and/or associated with sea surface temperature characterizing the subtropical frontal zone (16–18 °C) off central Chile.

Assessing modern deep-water ages in the New Zealand region using deep-water corals

The radiocarbon levels of 19 deep-sea corals from seamounts and plateaus in the New Zealand region were determined to assess their ability to record contemporaneous radiocarbon levels in surrounding (in situ) waters. Four species of subthermocline-dwelling corals from 366 to 1680 m water depth were collected between 1964 and 2000 using dredge hauls. Carbonate directly underlying living polyps was analyzed for radiocarbon content. Corals underlying subantarctic waters to the south of the South Island and subtropical waters to the north of New Zealand were collected from 426 to 1680 m water depth and had Δ 14C levels ranging from −11‰ to −146‰ (45–1200 14C years). These radiocarbon levels are in good agreement with the radiocarbon levels of the regional subthermocline waters, as determined for the ∑CO 2 in the waters by GEOSECS in 1974 and WOCE in 1992. The differences in Δ 14C levels in corals sampled in the 1960s and 1990s trace a pattern of changing bomb radiocarbon influence on the carbon in subthermocline waters. The subtropical frontal zone, an area known for significant eddies and mesoscale mixing, sits astride the Chatham Rise to the east of New Zealand, and radiocarbon profiles there are correspondingly more complex. Corals collected on the Chatham Rise were retrieved from 366 to1100 m and document Δ 14C levels ranging from −116‰ to +12‰ (945 14C years before present to significantly bomb-influenced). Corals sampled from depths shallower than ∼500 m tended to have higher and more consistent levels of bomb-carbon input, whereas corals sampled from depths deeper than ∼800 m show little or no bomb-carbon influence. Published measurements of the radiocarbon levels of the dissolved inorganic carbon from a transect taken in 1988 across the Chatham Rise indicate localized injection of surface waters containing bomb radiocarbon to depths as great as 600 m, which appears to be faithfully recorded in the corals. These results indicate that deep corals record even decadal-scale changes in the radiocarbon signal of the water in which they grew, making them excellent recorders of deep-water radiocarbon past and present.