Saline Water Mass Research Articles

Sources of Salinity Variation in a Coastal Lagoon in a Karst Landscape

Bahia de la Ascension (BA) is a shallow, mangrove-fringed coastal bay connected to the Caribbean through two inlets, outlined by the Mesoamerican Barrier Reef System. This work represents an initial investigation of the relative contribution of hydrometeorological and hydrodynamic forcing on salinity variation in this lagoon. Our objective is to assess the sensitivity of the salinity in BA to fluctuations in freshwater inflow and coastal oceanography. Two field trips were undertaken during rainy and dry seasons in 2007. Surface salinity was mapped across the system and CTD deployments carried out within BA and in the sea end-member to characterize temperature, conductivity, and water level. Also, cross-sectional CTD profiles were implemented to examine vertical stratification. The water balance indicated that 16 % of rainfall over the drainage basin (DB) becomes groundwater discharge plus surface runoff into BA during dry season, while 68 % of the precipitation input to the DB is supplied through groundwater–surface runoff to the bay during rainfalls. This combined inflow showed larger fluctuations than direct rainfall and, thus, has a greater potential to alter the seasonal salinity variations within BA. The tidal signal is selectively attenuated within BA, as diurnal frequencies are more readily filtered out than semidiurnal frequencies. Mesohaline conditions in the southwest bay are associated with freshwater sources, while saline water masses in the inlet are influenced by prevalent SE winds in the region and tidal phase, establishing a strong horizontal SW-NE estuarine salinity gradient.

Oceanographic variability in the South Pacific Convergence Zone region over the last 210 years from multi‐site coral Sr/Ca records

AbstractIn the South Pacific Convergence Zone (SPCZ), the variability in a sub‐seasonally resolved microatoll Porites colony Sr/Ca record from Tonga and a previously published high‐resolution record from Fiji are strongly influenced by sea surface temperature (SST) over the calibration period from 1981 to 2004 (R2 = 0.67–0.68). However, the Sr/Ca‐derived SST correlation to instrumental SST decreases back in time. The lower frequency secular trend (~1°C) and decadal‐scale (~2–3°C) modes in Sr/Ca‐derived SST are almost two times larger than that observed in instrumental SST. The coral Sr/Ca records suggest that local effects on SST generate larger amplitude variability than gridded SST products indicate. Reconstructed δ18O of seawater (δ18Osw) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64–0.67) but not local precipitation (r = −0.10 to −0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on δ18Osw in this region. The Sr/Ca records indicate SST warming over the last 100 years and appears to be related to the expansion of the western Pacific warm pool (WPWP) including an increasing rate of expansion in the last ~20 years. The reconstructed δ18Osw over the last 100 years also shows surface water freshening across the SPCZ. The warming and freshening of the surface ocean in our study area suggests that the SPCZ has been shifting (expanding) southeast, possibly related to the southward shift and intensification of the South Pacific gyre over the last 50 years in response to strengthened westerly winds.

Simulated Last Glacial Maximum ∆14Catm and the Deep Glacial Ocean Carbon Reservoir

∆14Catm has been estimated as 420 ± 80‰ (IntCal09) during the Last Glacial Maximum (LGM) compared to preindustrial times (0‰), but mechanisms explaining this difference are not yet resolved. ∆14Catm is a function of both cosmogenic production in the high atmosphere and of carbon cycling and partitioning in the Earth system. 10Be-based reconstructions show a contribution of the cosmogenic production term of only 200 ± 200‰ in the LGM. The remaining 220‰ have thus to be explained by changes in the carbon cycle. Recently, Bouttes et al. (2010, 2011) proposed to explain most of the difference in pCO2atm and δ13C between glacial and interglacial times as a result of brine-induced ocean stratification in the Southern Ocean. This mechanism involves the formation of very saline water masses that contribute to high carbon storage in the deep ocean. During glacial times, the sinking of brines is enhanced and more carbon is stored in the deep ocean, lowering pCO2atm. Moreover, the sinking of brines induces increased stratification in the Southern Ocean, which keeps the deep ocean well isolated from the surface. Such an isolated ocean reservoir would be characterized by a low ∆14C signature. Evidence of such 14C-depleted deep waters during the LGM has recently been found in the Southern Ocean (Skinner et al. 2010). The degassing of this carbon with low ∆14C would then reduce ∆14Catm throughout the deglaciation. We have further developed the CLIMBER-2 model to include a cosmogenic production of 14C as well as an interactive atmospheric 14C reservoir. We investigate the role of both the sinking of brine and cosmogenic production, alongside iron fertilization mechanisms, to explain changes in ∆14Catm during the last deglaciation. In our simulations, not only is the sinking of brine mechanism consistent with past ∆14C data, but it also explains most of the differences in pCO2atm and ∆14Catm between the LGM and preindustrial times. Finally, this study represents the first time to our knowledge that a model experiment explains glacial-interglacial differences in pCO2atm, δ13C, and ∆14C together with a coherent LGM climate. DOI: 10.2458/azu_js_rc.55.16295

Simulated Last Glacial Maximum Δ14Catm and the Deep Glacial Ocean Carbon Reservoir

Δ14Catm has been estimated as 420 ± 80% (IntCal09) during the Last Glacial Maximum (LGM) compared to preindustrial times (0%), but mechanisms explaining this difference are not yet resolved. Δ14Catm is a function of both cosmogenic production in the high atmosphere and of carbon cycling and partitioning in the Earth system. 10Be-based reconstructions show a contribution of the cosmogenic production term of only 200 ± 200% in the LGM. The remaining 220% have thus to be explained by changes in the carbon cycle. Recently, Bouttes et al. (2010, 2011) proposed to explain most of the difference in pCO2atm and Δ13C between glacial and interglacial times as a result of brine-induced ocean stratification in the Southern Ocean. This mechanism involves the formation of very saline water masses that contribute to high carbon storage in the deep ocean. During glacial times, the sinking of brines is enhanced and more carbon is stored in the deep ocean, lowering pCO2atm. Moreover, the sinking of brines induces increased stratification in the Southern Ocean, which keeps the deep ocean well isolated from the surface. Such an isolated ocean reservoir would be characterized by a low Δ14C signature. Evidence of such 14C-depleted deep waters during the LGM has recently been found in the Southern Ocean (Skinner et al. 2010). The degassing of this carbon with low Δ14C would then reduce Δ14Catm throughout the deglaciation. We have further developed the CLIMBER-2 model to include a cosmogenic production of 14C as well as an interactive atmospheric 14C reservoir. We investigate the role of both the sinking of brine and cosmogenic production, alongside iron fertilization mechanisms, to explain changes in Δ14Catm during the last deglaciation. In our simulations, not only is the sinking of brine mechanism consistent with past Δ14C data, but it also explains most of the differences in pCO2atm and Δ14Catm between the LGM and preindustrial times. Finally, this study represents the first time to our knowledge that a model experiment explains glacial-interglacial differences in pCO2atm, Δ13C, and Δ14C together with a coherent LGM climate.

Evolution of the northeast Labrador Sea during the last interglaciation

Boreal summer insolation during the last interglaciation (LIG) generally warmed the subpolar to polar Northern Hemisphere more than during the early Holocene, yet regional climate variations between the two periods remain. We investigate northeast Labrador Sea subsurface temperature and hydrography across terminations (T) I and II and during the LIG to assess the impact of two different magnitudes of boreal summer insolation increase on the northeast Labrador Sea. We use Mg/Ca ratios in Neogloboquadrina pachyderma (sinistral) as a proxy of calcification temperature to document changes in subsurface temperatures over Eirik Drift. Our corresponding record of δ18O of seawater documents changes in water mass salinity. Mg/Ca calcification temperatures peak early in the Holocene coincident with peak boreal summer insolation. In contrast, LIG temperatures are relatively constant through the interglaciation, and are no warmer than peak Holocene temperatures. During the first half of the LIG, δ18O of seawater remains depleted, likely from southern Greenland Ice Sheet retreat and enhanced Arctic freshwater and sea‐ice export to the Labrador Sea. The consequent stratification of the Labrador Sea and attendant suppressed convection explains delayed deep‐ocean ventilation and a cooler subsurface in the northeast Labrador Sea during the LIG.

Numerical Analysis of the Influence of Saltwater Intrusion on Qingcaosha Reservoir

A 2D hydrodynamics and salinity numerical model for the Changjiang River Estuary was set up by using Delft3D-Flow. The calculation results agree well with the real measured data, which indicates the model can be used to analyze the salinity in the Changjiang River Estuary. The calculated results of saltwater intrusion in the dry season of dry year shows that the main influence style of the salinity on the water intake of the Qingcaosha reservoir is the saltwater spilling over it from the North Branch during the neap tide and the saltwater intrusion from the open sea through the South Branch during the spring tide. The saltwater spilling over from the North Branch forms a saline water mass near the Xinjian sluice during the late period of a spring tide, which takes 5 days to reach the water intake of the Qingcaosha Reservoir.

Oscillation responses to tropical Cyclone Gonu in northern Arabian Sea from a moored observing system

In June 2007, tropical Cyclone Gonu passed over an ocean observing system consisting of a deep autonomous mooring system in the northern Arabian Sea and a shallow cabled mooring system in the Sea of Oman. Gonu was the largest cyclone known to have occurred in the Arabian Sea and to strike the Arabian Peninsula. The instruments on the moorings continuously recorded water velocities, temperature, conductivity, pressure, dissolved oxygen (DO) and turbidity at multiple depths and at hourly intervals during the storm.Near-inertial oscillations at all moorings from thermocline to seafloor are coincident with the arrival of Gonu. Sub-inertial oscillations with periods of 2–10 day are recorded at the post-storm relaxation stage of Gonu, primarily in the thermocline of the deep array and at the onshore regions of the shallow array. These oscillations consist of warm, saline water masses, likely originating from the Persian Gulf. Prominent 12.7-day sub-inertial waves, measured at a station ∼300km offshore, are bottom-intensified and have characteristics of baroclinic topographically trapped waves. Theoretical results from a topographically trapped wave model are in a good agreement with the observed 12.7-day waves at Murray Ridge. The wavelength of the 12.7-day waves is about 590km calculated from the dispersion relationship. Further analysis suggests that a resonant standing wave is responsible for trapping the 12.7-day wave energy inside the Sea of Oman basin. The observational results reported here are the first measurements of deepwater responses to a tropical cyclone in the Sea of Oman/Arabian Sea. Our study demonstrates the utility of sustained monitoring for studying the impact of extreme weather events on the ocean.

諫早湾内の塩淡成層構造に及ぼす南北排水門からの排水の影響評価

The flow processes of drain from the north and south gates of the Isahaya reclamation dike and its contribution on salinity stratification in Isahaya Bay have been investigated by using a 3-D flow simulation model. The low salinity water mass by the drain is seen in the inner area of Isahaya Bay. It spreads thinly to the central area of Isahaya Bay and flows out to the Ariake Sea along the Shimabara Peninsula in about one tidal cycle. In a flood period, the freshwater from the drain gates decreased the average salinity in Isahaya Bay by about 0.6 PSU. On the other hand, the inflow of low salinity water from the Ariake Sea caused the decrease of about 8 PSU. The inflow of low salinity water from the Ariake Sea plays a dominant role in the salinity stratification in Isahaya Bay.

Genesis of unusual lithologies associated with the Late Middle Devonian Taghanic biocrisis in the type Taghanic succession of New York State and Pennsylvania

The Middle Devonian (late middle Givetian) Tully Formation in New York State is the type regional expression of the global Taghanic Biocrisis. It is marked by anomalous development of massive to rhythmically bedded, micritic limestone deposits within a vastly thicker Acadian foreland basin clastic succession. Lower-medial Tully carbonates record the incursion of tropical Old World Realm (OWR) taxa (Tully Fauna) and the outage of the long-standing, higher diversity, cooler-water, endemic Eastern Americas Realm (EAR) biota (Hamilton Fauna). Succeeding Tully strata record a return of the Hamilton Fauna followed by its demise due, both to replacement by the lower diversity, cosmopolitan Genesee Fauna and by the overspread of widespread anoxic bottom conditions coupled with the onset of drastically increased sediment input following Tully Group deposition.Massive, micritic limestones, yielding modest diversity Tully Fauna assemblages are particularly characteristic of the lower-medial Tully succession on the western New York Tully platform; these are coeval with nearly barren ribbon limestone and siltstone deposits in the adjacent New Berlin (east-central New York) and central Pennsylvania basins. These depocenters trapped terrigenous sediment and helped to create sediment-starved conditions on the platform. Anomalous Tully carbonate deposition is believed to be coincident with strong super estuarine water mass stratification in offshore Tully shelf and basin settings where a warmer, denser, more saline water mass flowed northward (shoreward) across the study area beneath a counter flow of oxygenated, river-influenced surface water. Dysoxic conditions would have developed below the pycnocline, favoring the Tully Fauna incursion, precipitation of carbonate mud, and possibly promoting the formation of ooidal chamosite.At least five thin (1–30cm-thick) mappable beds of black, ooidal, chamosite, are observed in Taghanic shelf margin, slope, and basin deposits. These beds are typically intensely bioturbated, variably rich in associated siderite, contain corroded low-diversity fossil assemblages, and are regionally associated with condensed lag deposits above unconformities. Work by others suggests that chamosite formation occurs under minimally reducing, post-oxic, interstitial conditions which are followed, in turn, by more strongly reducing conditions which allow for variable siderite and pyrite formation. Terrestrial pedogenic alteration of silicates by evolving land plant communities under warm, humid conditions led to an increased supply of kaolinite as well as Iron oxides and hydroxides that served as precursors to chamosite formation. In the New Berlin Basin, the transgressive, top-Hamilton-base-Taghanic succession is marked by a succession of diastems associated with basin deepening and upward-increasing chamosite occurrence in dysoxic facies. Occurrence of ooidal chamosite in upper medial Tully platform deposits (Smyrna Bed) may reflect water mass stratification and extreme sediment-starvation at the shelf margin. Eastward spectral transformation of sparsely fossiliferous Smyrna ooidal chamosite in the western New Berlin Basin into richly fossiliferous, non-chamositic, equivalent neritic deposits east of that basin, suggests that the counter-flow of oxygen and nutrient-rich, lower salinity water had blocked the denser, restricted water layer from advancing over the eastern Tully clastic shelf closer to shore.

Chemical tracers, sterol biomarkers and satellite imagery in the study of a river plume ecosystem in the Yellow Sea

An attempt has been made using the molecular signatures of anthropogenic persistent organic chemicals such as DDTs and PCBs and sterol biomarkers in suspended particulate matter (SPM) to verify certain biogeochemical postulations, especially the origin of low salinity water mass and SPM in a river plume ecosystem in the Yellow Sea. This investigation was also assisted by satellite imagery using SeaWiFS and concurrent measurement of salinity. The target chemicals were measured in SPM, sampled near Yangtze River mouth to stations that were close to Jeju Island, Korea. ΣDDTs and ΣPCBs in SPM were at ultra trace levels (parts-per-quadrillion) ranging from 3.3 to 39 and from 20 to 68 pg L −1, respectively; while Σsterols occurred from 3200 to 32,000 pg L −1 range. The spatial distribution and molecular compositions were studied. This study illustrated the effectiveness of unconventional molecular tracers in identifying water masses and movement of riverine plumes to regional seas in the transport of sediments and contaminants. Additionally, it highlighted the importance of long range-longitudinal transport of contaminants (transboundary pollution) in regional seas and their impact on the ecosystem.

The Wind Response Analysis of Low Salinity Water Mass in Isahaya Bay

The mechanism of formation and the wind response of low salinity water mass in Isahaya Bay have been investigated by using the 3-D flow simulation model incorporating time-varying wind stress on the water surface. The northerly wind and the southerly wind are prevailing in Isahaya Bay and are repeated at the period of several days. The analytical results show that the change of the prevailing wind direction induces the appearance and disappearance of low salinity water mass. When the southerly wind blows, the low salinity water mass near the water surface is swept out Isahaya Bay and the salinity stratification is gone with the wind. On the other hand, the inflow of low salinity water mass to Isahaya Bay is accelerated by the northerly wind and the salinity stratification is formed.

SUBTROPICAL WATER MASSES IN HALMAHERA WATERS

Research of water masses by using conductivity temperature depth (CTD), are conducted in the eastern path of the Indonesia Throughflow (ITF) in the Halmahera, Seram and Banda seas during March-April 2007 under the Expedition of Widya Nusantara (EWIN). The objective of this research is to see maximum salinity spread of South Pacific Subtropical Water (SPSW) water masses enter the eastern Indonesia Waters. The temperature and salinity profiles show the presence of the presence of SPSW have been very much confined to the Halmahera Sea only. Little of this water masses have been detected in the eastern Seram Sea, but none in Banda Sea. Early data of Arlindo Mixing (ARMIX) experiment in southeast monsoon 1993 indicated that this water masses SPSW may entered the southern most part of the Moluccas Sea. Type of South Pacific Subtropical Water (SPSW) water masses appears in the Halmahera Sea at an average depth of 200m and the dominant flows between Halmahera and Obi Islands (Moluccas Sea continues to the east). Type of South Pacific Intermediate Water (SPIW) water mass appeared on average Halmahera Sea at a depth of 750m and the dominant flows between Halmahera and Obi Islands (Moluccas Sea continues to the east). Type of North Pacific Subtropical Water (NPSW) water masses at an average depth of ~ 150m found in the northern part of Halmahera, the dominant flow to the Celebes Sea, Makassar Strait, Flores Sea and partly flows into Lombok Straits. Type of minimum salinity water mass of North Pacific Intermediate Water (NPIW) obtained at an average depth of ~ 400m dominant flow towards the Celebes Sea, Makassar Strait and Flores Sea.Keywords: maximum salinity, SPSW, Halmahera, Seram, and Banda Seas

SUBTROPICAL WATER MASSES IN HALMAHERA WATERS

<p>Research of water masses by using conductivity temperature depth (CTD), are conducted in the eastern path of the Indonesia Throughflow (ITF) in the Halmahera, Seram and Banda seas during March-April 2007 under the Expedition of Widya Nusantara (EWIN). The objective of this research is to see maximum salinity spread of South Pacific Subtropical Water (SPSW) water masses enter the eastern Indonesia Waters. The temperature and salinity profiles show the presence of the presence of SPSW have been very much confined to the Halmahera Sea only. Little of this water masses have been detected in the eastern Seram Sea, but none in Banda Sea. Early data of Arlindo Mixing (ARMIX) experiment in southeast monsoon 1993 indicated that this water masses SPSW may entered the southern most part of the Moluccas Sea. Type of South Pacific Subtropical Water (SPSW) water masses appears in the Halmahera Sea at an average depth of 200m and the dominant flows between Halmahera and Obi Islands (Moluccas Sea continues to the east). Type of South Pacific Intermediate Water (SPIW) water mass appeared on average Halmahera Sea at a depth of 750m and the dominant flows between Halmahera and Obi Islands (Moluccas Sea continues to the east). Type of North Pacific Subtropical Water (NPSW) water masses at an average depth of ~ 150m found in the northern part of Halmahera, the dominant flow to the Celebes Sea, Makassar Strait, Flores Sea and partly flows into Lombok Straits. Type of minimum salinity water mass of North Pacific Intermediate Water (NPIW) obtained at an average depth of ~ 400m dominant flow towards the Celebes Sea, Makassar Strait and Flores Sea.</p><p>Keywords: maximum salinity, SPSW, Halmahera, Seram, and Banda Seas</p>

Amphipod abundance in sediment trap samples at the long-term observatory HAUSGARTEN (Fram Strait, ∼79°N/4°E). Variability in species community patterns

Since 2000, sediment traps have been deployed at the HAUSGARTEN (a long-term observatory established by the Alfred-Wegener-Institute for Polar and Marine Research in 1999) in the Fram Strait, (west of Spitsbergen at a water depth of 2,500 m, located in the confluence zone of the warm saline Atlantic water and water masses of polar origin) in order to investigate seasonal and inter-annual fluctuation of particle flux and the various contribution of zooplankton swimmers. Amongst these swimmers, amphipods are regularly observed occurring in a recurrent pattern and they dominate the biomass. Thus, we present data on amphipods regarding their seasonal and regional distribution pattern throughout the period 2000–2007. The most frequently observed amphipod species are Themisto libellula, T. abyssorum and T. compressa. While Themisto libellula is considered a true Arctic species associated to polar water masses, the boreal-Atlantic species Themisto abyssorum is imported into the Arctic by Atlantic water. A third species, Themisto compressa, occurred in 2004 and has been continually observed in the samples since then. The latter species has its main distribution in the warm regions of the North Atlantic and its occurrence in the Fram Strait points to the increased influence of warm Atlantic water masses. During 2000–2007, the amphipod composition in the samples has changed in favor of T. abyssorum and T. compressa. These shifts could suggest a northward movement of Atlantic species in the seasonally ice-covered area, a region of the ocean anticipated to react very sensitively to global warming.

2008년 하계 울릉분지에서 관측된 물리·화학적 외압에 대한 플랑크톤 군집의 반응

In Summer 2008, a multidisciplinary survey was conducted onboard R/V Haeyang 2000 to understand plankton response to the three distinct physico-chemical settings that developed in the Ulleung Basin of the East Sea. Baseline settings of hydrographic conditions included the presence of the thin (<20 m) Tsushima Surface Water (TSW) on top of the Tsushima Middle Water (TMW). It extends from the Korea Strait to <TEX>$37^{\circ}N$</TEX> along the <TEX>$130^{\circ}E$</TEX> and then turns offshore and encompasses the relatively saline (T><TEX>$26^{\circ}C$</TEX>, S>33.7) Ulleung Warm Eddy surface water centered at <TEX>$36.5^{\circ}N$</TEX> and <TEX>$131^{\circ}E$</TEX>. A relatively colder and saline water mass appeared off the southeastern coast of Korea. It was accompanied by higher nutrient and chlorophyll-a concentrations, suggesting a coastal upwelling. Most of the offshore surface waters support low phytoplankton biomass (0.3 mg chl-a <TEX>$m^{-3}$</TEX>). A much denser phytoplankton biomass (1-2.3 mg <TEX>$m^{-3}$</TEX>) accumulated at the subsurface layer between 20-50 m depth. The subsurface chlorophyll-a maximum (SCM) layer was closely related to the nutricline, suggesting an active growth of phytoplankton at depth. The SCM developed at shallow depth (20-30 m) near the coast and deepened offshore (50-60 m). A fucoxanthin/zeaxanthin ratio was high in coastal waters while it was low in offshore waters, which indicated that diatoms dominate coastal waters while cyanobacteria dominate offshore waters. The community structure and biomass of phytoplanktonare closely related to nitrogen availability. Zooplankton biomass was higher in the coastal region than in the offshore region while species richness showed an opposite trend. Zooplankton community structure retained a coastal/offshore contrast. These suggest that summer hydrography is a stable structure, lasting long enough to allow a hydrography-specific plankton community to evolve.

North–Central Aegean Sea surface and intermediate water masses and their role in triggering the Eastern Mediterranean Transient

Interrannual variability in the surface and intermediate water masses of the North–Central Aegean Sea is being investigated through data collected by the Hellenic Navy Hydrographic Service (HNHS) during the Eastern Mediterranean Transient (EMT) post-peak period. Data suggest that following the last major deep-water formation episode in the Aegean Sea (1993), Black Sea Water (BSW) is confined to the northernmost part of the basin, while saline Levantine Surface Water (LSW) becomes the dominant Central Aegean surface water mass, intruding below the BSW layer even north of Lemnos Island. Furthermore, highly saline intermediate masses transfer additional salt to the North–Central Aegean basin. After the early 90s, BSW gradually reclaims most of the Central Aegean while the salinity of both LSW and intermediate masses is reduced. The enhanced presence of these highly saline water masses in the early 90s, presumably signifies the intrusion of high salinity water of Levantine origin into the North–Central Aegean following the massive outflow of dense Aegean water into the Eastern Mediterranean. This salinity intrusion was one of the preconditioning factors that enhanced dense water formation in the Aegean during the last major deep-water formation phase of the EMT, affecting the characteristics of the deep-water masses produced.

Nutrient characteristics of the water masses and their seasonal variability in the eastern equatorial Indian Ocean

Nutrient characteristics of four water masses in the light of their thermohaline properties are examined in the eastern Equatorial Indian Ocean during winter, spring and summer monsoon. The presence of low salinity water mass with “Surface enrichments” of inorganic nutrients was observed relative to 20 m in the mixed layer. Lowest oxygen levels of 19 μM at 3°N in the euphotic zone indicate mixing of low oxygen high salinity Arabian Sea waters with the equatorial Indian Ocean. The seasonal variability of nutrients was regulated by seasonally varying physical processes like thermocline elevation, meridional and zonal transport, the equatorial undercurrent and biological processes of uptake and remineralisation. Circulation of Arabian Sea high salinity waters with nitrate deficit could also be seen from low N/P ratio with a minimum of 8.9 in spring and a maximum of 13.6 in winter. This large deviation from Redfield N/P ratio indicates the presence of denitrified high salinity waters with a seasonal nitrate deficit ranging from −4.85 to 1.52 in the Eastern Equatorial Indian Ocean.

Development and growth of ontogenetically migrating copepods during the spring phytoplankton bloom in the Oyashio region

We have evaluated development and growth of the ontogenetically migrating copepods in the northwestern subarctic Pacific by frequent sampling and by molting rate experiments during the spring phytoplankton bloom. Since different water masses frequently moved into the surface layers at our sampling station, the copepod abundance and species composition fluctuated greatly. Early copepodite stages of Eucalanus bungii and Neocalanus plumchrus appeared abundantly when a warmer and more saline water mass was present. Despite the fluctuating temporal pattern, the population structure revealed that each copepod species had a different life history strategy during the bloom. Eucalanus bungii were in the stages of gonad maturation, spawning and naupliar development when chlorophyll a concentrations were high. The phytoplankton bloom was utilized for development and lipid accumulation by late copepodites of Neocalanus cristatus and Neocalanus flemingeri and for development of nauplii and early copepodites of N. plumchrus. Molting experiments showed that carbon weight of ‘molters’ in the incubations was greater than that of ‘non-molters’, indicating that primarily animals with heavier body weight are those that molt into the next stage. Mean stage durations estimated by the molting rate method were on the order of 9.7 (C3) to 16.6 days (C4) for N. flemingeri, 13.9 (C3) to 29.1 days (C4) for N. plumchrus, and 12.2 days (C2) for E. bungii. Large fluctuations were observed for stage duration estimates, suggesting different development histories (i.e. ages-within-stage distributions) among the replicate incubations. From these results, we discuss development and growth of the ontogenetically migrating copepods during strongly fluctuating hydrographic conditions in the Oyashio.

Plankton trophodynamics at the subtropical convergence, Southern Ocean

Stable isotope signatures (δ 13 C, δ 15 N) in zooplankton tissues and particulate organic matter (POM) were determined to assess regional differences in the trophodynamics of zooplankton communities between 38 and 43°S, where the cool nutrient-rich subantarctic waters of the Southern Ocean meet the warm nutrient-poor subtropical waters of the southwest Indian Ocean at the subtropical convergence (STC). Significantly enriched values of δ 15 N were noted in populations of all major zooplankton groups inhabiting the warm and saline water mass north of the STC (maximum surface temperature 21°C), including the euphausiids, salps, amphipods, copepods, ostracods, pyrosomes, pteropods and chaetognaths, compared with those in the cool, less saline southern water mass (minimum surface temperature 11°C). Similar patterns of δ 15 N in POM collected throughout the region suggest that the large changes in zooplankton δ 15 N values across the frontal region are driven by variations in the phytoplankton communities. The differing trophodynamics in communities north and south of the STC provide compelling evidence of distinct bottom-up effects on planktonic food webs which have important implications in the determination of trophic positions and motility of plankton and higher consumers using δ 15 N signatures. Although expected, similar latitudinal variations in δ 13 C signatures were not found.

Short-term temperature and salinity variations in the Tsushima Strait in 2004: Behavior of the surface low-salinity water in the strait

Temporal variations in temperature and salinity observed in 2004 were investigated on a short time scale in the Tsushima Strait. The data were obtained by long-term in situ measurements at Mitsushima and Futaoi Island using an instrument equipped with a piston-type wiper to avoid biofouling. In addition, the temperature and salinity values of the surface layer obtained by a commercial ferryboat between Hakata and Busan were used to investigate their spatiotemporal variations. Temperature and salinity variations with a time scale of several days had a negative correlation in the summer. This evidence suggests that a warm and less saline water mass, which is considered to be mainly the Changjiang Diluted Water (CDW), flowed intermittently through the Tsushima Strait in summer. In late July 2004, a large low-salinity water mass was detected in the Tsushima Strait. At that time, the freshwater transport through the Tsushima Strait transiently reached about 12 × 104 m3s−1, which is estimated from observed acoustic Doppler current profiler (ADCP) data along a ferryboat line and inferred salinity profiles. This estimated value is more than double the maximum of the climatological monthly mean of the Changjiang discharge. Furthermore, salinity and surface current data obtained by high frequency ocean radar (HF radar) indicate that water properties at Mitsushima may occasionally represent part of the water flowing through the western channel via a countercurrent, although Mitsushima is geographically located in the eastern channel.