Seasonal and interannual variability of sea surface chlorophyll a concentration in the Japan/East Sea (JES)

Seasonal and interannual variability of chlorophyll a concentration in the Japan/East Sea (JES) was detected spatially by ocean color satellite remote sensing. Start timing of the spring bloom was different spatially. The spring bloom started at the subpolar front and southward of it in March, northward of subpolar front, along the Primorye coast and off Hokkaido in April and in the middle of the Japan Basin in May. The start of the spring bloom showed interannual variability that corresponded with the wind speed in the area. The spring bloom in 1998 and 2002 appeared about four weeks earlier than in 1997, 1999 and 2001, and it corresponded with weak winds that can lead to an early development of the thermocline. The bloom was late in 1999 and 2001 in the Japan Basin and along the Primorye coast, and in the southern area in 2000. It corresponded with stronger wind stress that delayed seasonal thermocline formation. The bloom along the Primorye coast appeared later in 1999, and it corresponded with stronger wind stress, and at the same time, it seemed to be related with the delay of melting of sea ice in Mamiya Strait. The fall bloom appeared from early October to early December, and it did not have a clear temporal transition. The area where chlorophyll a concentration exceeded 0.8 μg l⁻¹ was wider in the western area than in the eastern area every year. The magnitude of the fall bloom was different between years, but it did not show a correlation with average wind speed in fall. Those results indicated that the timing of the seasonal bloom in the JES is largely affected by the variability of global climate such as ENSO events.     

Satellite-derived measurements of spatial and temporal chlorophyll-a variability in Funka Bay, southwestern Hokkaido, Japan

Chlorophyll-a (chl-a) concentration has an important economic effect in coastal and marine environments on fisheries resources and marine aquaculture development. Monthly climatologies the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) derived chl-a from February 1998 to August 2004 around Funka Bay were used to investigate the spatial and temporal variability of chl-a concentrations. SeaWiFS-derived suspended sediment, MODIS derived sea surface temperature (SST), solar radiation and wind data were also analyzed. Results showed two distinct chlorophyll blooms in spring and autumn. Chl-a concentrations were relatively low (<0.3 mg m⁻³) in the bay during summer, with high concentrations occurring along the coast, particularly near Yakumo and Shiraoi. In spring, chl-a concentrations increased, and a large (>2 mg m⁻³) phytoplankton bloom occurred. The spatial and temporal patterns were further confirmed by empirical orthogonal function (EOF) analysis. About 83.94% of the variability could be explained by the first three modes. The first chl-a mode (77.93% of the total variance) explained the general seasonal cycle and quantified interannual variability in the bay. The spring condition was explained by the second mode (3.89% of the total variance), while the third mode (2.12% of the total variance) was associated with autumn condition. Local forcing such as the timing of intrusion of Oyashio water, wind condition and surface heating are the mechanisms that controlled the spatial and temporal variations of chlorophyll concentrations. Moreover, the variation of chlorophyll concentration along the coast seemed to be influenced by suspended sediment caused by resuspension or river discharge.     

Population dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific

We characterized the community composition of phytoplankton in the western subarctic Pacific from the pre-bloom to the decline phase of the spring bloom with special reference to decreases in the silicic acid concentration in surface waters as an index for diatom bloom development. Furthermore, responses of heterotrophic bacteria and viruses to the spring bloom were also concomitantly investigated. Under pre-bloom conditions when nutrients were abundant but the surface mixed layer depth was relatively deep, chlorophyll (Chl) a concentrations were consistently low and green algae (chlorophytes and prasinophytes), cryptophytes, and diatoms were predominant in the phytoplankton assemblages as estimated by algal pigment signatures. Together with the shallowing of the mixed layer depth and the decrease in silicic acid concentration, diatoms bloomed remarkably in the Oyashio region, though the magnitude of the bloom in the Kuroshio-Oyashio transition (hereafter Transition) region was relatively small. A total of 77 diatom species were identified, with the bloom-forming diatoms mainly consisting of Thalassiosira, Chaetoceros, and Fragilariopsis species. It has become evident that the carotenoid fucoxanthin can serve as a strong indicator of the diatom carbon biomass during the spring diatom bloom. Differences in the species richness of diatoms among stations generally enabled us to separate the Oyashio bloom stations from the Transition and the Oyashio pre-bloom stations. Relatively high values of the Shannon-Wiener index for the diatom species were also maintained during the Oyashio bloom, indicating that a wide variety of species then shared dominance. In the decline phase of the Oyashio bloom when surface nutrient concentrations decreased, senescent diatom cells increased, as inferred from the levels of chlorophyllide a. Although the cell density of heterotrophic bacteria changed little with the development of the diatom bloom, viral abundance increased toward the end of the bloom, suggesting an increased likelihood of mortality among organisms including diatoms resulting from viral infection. This is the first report on the microbial trophodynamics, including viruses, during the spring diatom bloom in the western subarctic Pacific.     

东亚边缘海区浮游植物春华的纬向与年际变化

Combined studies of latitudinal and interannual variations of annual phytoplankton bloom peak in East Asian marginal seas(17°–58°N, including the northern South China Sea(SCS), Kuroshio waters, the Sea of Japan and the Okhotsk Sea) are rarely. Based on satellite-retrieved ten-year(2003–2012) median timing of the annual Chlorophyll a concentration(Chl a) climax, here we report that this annual spring bloom peak generally delays from the SCS in January to the Okhotsk Sea in June at a rate of(21.20±2.86) km/d(decadal median±SD). Spring bloom is dominant feature of the phytoplankton annual cycle over these regions, except for the SCS which features winter bloom. The fluctuation of the annual peak timing is mainly within ±48 d departured from the decadal median peak date, therefore this period(the decadal median peak date ±48 d) is defined as annual spring bloom period. As sea surface temperature rises, earlier spring bloom peak timing but decreasing averaged Chl a biomass in the spring bloom period due to insufficient light is evident in the Okhotsk Sea from 2003 to 2012. For the rest of three study domains, there are no significant interannual variance trend of the peak timing and the averaged Chl a biomass. Furthermore this change of spring phytoplankton bloom timing and magnitude in the Okhotsk Sea challenges previous prediction that ocean warming would enhance algal productivity at high latitudes.     

Enhanced chlorophyll associated with island-induced cyclonic eddies in the eastern channel of the Tsushima Straits

The relationship between island-induced cyclonic eddies and chlorophyll a (chl-a) was investigated using field data and satellite images in the eastern channel of the Tsushima Straits. The maximum chl-a concentration around the leeward eddy of the Tsushima Islands was two or three times greater than that of outside the eddy. Two different mechanisms of chl-a enhancement associated with island-induced cyclonic eddies were found in the post-bloom periods. In summer, when nutrients were depleted in the surface layer, eddy pumping increased the nutrient supply in the euphotic zone, resulting in enhanced chl-a around the shallow thermocline near the eddy core. In late autumn, when the mixed layer deepened over the euphotic zone, the mixed layer depth became shallow due to the doming effect of the cyclonic eddy, therefore, the improved irradiance condition led to an increase in the chl-a concentration in the surface mixed layer. Nighttime satellite visible images showed a number of fishing vessels in the lee region of the Tsushima Islands, implying that the enhanced phytoplankton biomass may have resulted in good feeding conditions for fishes and squids in the Tsushima Straits.     

Environmental Factors Affecting the Occurrence and Production of the Spring Phytoplankton Bloom in Funka Bay, Japan

The concentration of nutrients was measured during the spring phytoplankton bloom in Funka Bay over a 5-year period (1988–92). During the winter mixing period, nutrient concentrations were similar in every year except in 1990 when a high concentration of silicate was observed. There was interannual variation in the onset of the bloom, presumably depending on the stability of the water column. The bloom developed in early March when the Oyashio water (OW), which has a lower density than the existing winter water, flowed into the bay and the pycnocline formed near the bottom of the euphotic zone. In this case, high chl a was found only in the euphotic zone and nutrient utilization was limited to this zone. In the year when the inflow of OW was not observed by April, the bloom took place at the end of March without strong stratification and high chl a was found in the whole water column, accompanied by a decrease in nutrients. Interannual differences were found not only at the beginning of the decrease, but also in the thickness of the layer which showed a decrease in nutrients. Primary production from the beginning to the end of the spring bloom was estimated from the nutrient budget before and after the spring bloom. The integrated production over the spring bloom period ranged from 25 to 73 g C m^-2, which accounts for 19–56% of the annual production in this bay. We found that the timing of the bloom was strongly dependent on the inflow of OW, but the amount of production was not clearly related to this timing.     

Factors controlling the timing of major spring bloom events in an UK south coast estuary

Factors controlling the timing of major (>10 mg chlorophyll a m⁻³) spring bloom events in the estuarine waters of the Solent, on the south coast of the UK, have been investigated. Winter to summer variations in chlorophyll a concentration together with relevant meteorological and hydrographical data have been analysed for 5 years (1988, 1992, 2001, 2002 and 2003). Mean water column irradiance is demonstrated to be the main factor controlling the timing of the first major spring bloom event, usually dominated by large chain-forming diatoms. When chlorophyll a concentration first exceeds 10 mg m⁻³ in spring (usually in May) the mean water column photosynthetic active radiation (PAR) averaged for one week prior to the sampling date was always >380 W h m⁻² d⁻¹. Prior to the main spring bloom event surface incident radiation and water turbidity combine to limit chlorophyll a concentration to levels <10 mg m⁻³. Chlorophyll a concentrations >10 mg m⁻³ do not occur in the Solent until almost the entire 10 m water column is within the euphotic zone (i.e. above 1% light level) and light extinction coefficient (k) is ca. ≤0.5 m⁻¹. Statistically, river flow explains the largest percentage of the variations in k and the delayed bloom in June 2002 is due to increased cloud cover and high levels of rainfall in May, which caused a reduction in surface incident irradiance and increased turbidity. Chlorophyll a peaks during these major bloom events generally occur on spring tides when increased mixing rates favour net growth of diatoms.     

Springtime coupling between chlorophyll a, sea ice and sea surface temperature in Disko Bay, West Greenland

Alterations in sea ice and primary production are expected to have cascading influences on the food web in high Arctic marine ecosystems. This study spanned four years and examined the spring phytoplankton production bloom in Disko Bay, West Greenland (69°N, 53°W) (using chlorophyll a concentrations as a proxy) under contrasting sea ice conditions in 2001 and 2003 (heavy sea ice) and 2002 and 2004 (light sea ice). Satellite-based observations of chlorophyll a, sea ice and sea surface temperature were used together with in situ depth profiles of chlorophyll a fluorescence collected at 24 sampling stations along the south coast of Disko Island (5-30 km offshore) in May 2003 and 2004. Chlorophyll a and sea surface temperatures were also obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS: EOS-Terra and AQUA satellites) between March 2001 and July 2004. Daily SMMR/SSMI sea ice data were obtained in the same years. An empirical regional algorithm was developed to calibrate ratios of remotely sensed measurements of water leaving radiance with in situ chlorophyll a fluorescence. The optimal integration depth was 0-4 m, explaining between 70% and 91% of the variance. The spatial development of the phytoplankton bloom showed that the southwestern corner of the study area had the earliest and the largest spring phytoplankton bloom. The eastern part of Disko Bay, influenced by meltwater outflow from the glaciers, shows no signs of an early phytoplankton bloom and followed the general pattern of an accelerated bloom soon after the disappearance of sea ice. In all four years the coupling between phytoplankton and sea ice was bounded by average open water between 50% and 80%, likely due to the combined availability of light and stable open water. The daily incremental growth in both mean chlorophyll a density (chlorophyll a per volume water, μg l⁻¹) and abundance (density of chlorophyll a extrapolated to ice free areas, tons) estimated by linear regression (chlorophyll a vs. day) between 1 April and 15 May was highest in 2002 and 2004 (light ice years) and lowest in 2001 and 2003 (heavy ice years). In years with late sea ice retreat the chlorophyll a attained only slightly lower densities than in years with early sea ice retreat. However, the abundance of chlorophyll a in light ice years was considerably larger than in heavy ice years, and there was an obvious effect of more open water for light-induced stimulation of primary production. This observation demonstrates the importance of estimating chlorophyll a abundance rather than density in sea ice covered areas. This study also presents the first regional calibration of MODIS chlorophyll a data for Arctic waters.     

Temporal variability in phytoplankton pigments,picoplankton and coccolithophores along a transect through the North Atlantic and tropical southwestern Pacific

Biogeochemical processes in the sea are triggered in various ways by chlorophyll-containing phytoplankton groups. While the variability of chlorophyll concentration at sea has been observed from satellites for several years, these groups are known only from cruises which are limited in space and time. The Geochemistry, Phytoplankton and Color of the Ocean programme (GeP&CO) was set up to describe and understand the variability of phytoplankton composition on large spatial scales under a multi-year sampling strategy. It was based on sea-surface sampling along the route of the merchant ship Contship London which travelled four times a year from Le Havre (France) to Nouméa (New Caledonia) via New York, Panama and Auckland. Observations included the measurement of photosynthetic pigments, counts of picoplanktonic cells by flow cytometry (Prochlorococcus, Synechococcus, and picoeucaryotes) and counting and identification of coccolithophores. The results confirmed that tropical areas have low seasonal variability and are characterized by relatively high divinyl-chlorophyll a and zeaxanthin concentration and that the variability is strongest at high latitudes where the phytoplankton biomass and population structure are found to have large seasonal cycles. Thus, the spring bloom in the North Atlantic and an austral winter bloom north of New Zealand are marked by chlorophyll concentrations which are often higher than 0.5 μg l⁻¹ and by high concentration of fucoxanthin (a pigment used as an indicator for diatoms), while summer populations are dominated by Prochlorococcus sp. and have low chlorophyll concentrations. Apart from this yearly bloom at temperate latitudes, fucoxanthin is scarce, except in the equatorial upwelling zone in the eastern Pacific Ocean, where it is found in moderate amounts. In this region, relatively high chlorophyll concentrations extend generally as far as 14°S and do not respond to the seasonal strengthening of the equatorial upwelling during the austral winter. Prochlorococcus, which is known to dominate in oligotrophic tropical seas and to disappear in cold conditions, in fact has its minimum during the spring bloom in the North Atlantic, rather than during the winter. Coccolithophores are ubiquitous, showing a succession of species in response to oceanic conditions and provinces. 19′Hexanoyloxyfucoxanthin, the pigment generally considered as an indicator of coccolithophores, is relatively abundant at all times and in all regions, but its abundance is generally not tightly correlated with that of coccolithophores. The regional differences revealed by these results are in overall agreement with Longhurst's division of the ocean into ecological provinces.     

Chlorophyll distribution in a temperate estuary: The Strait of Georgia and Juan de Fuca Strait

Data collected during 7 years of seasonal surveys are used to investigate the distribution of phytoplankton biomass within the estuarine waters of the Strait of Georgia and Juan de Fuca Strait. Variability of the chlorophyll distribution is examined in relation to the density stratification, light availability and nutrient concentration. In the Strait of Georgia, both the horizontal and vertical distribution of chlorophyll are found to be linked to the presence of a near-surface layer of increased density stratification. Despite important year-to-year variability, the seasonal cycle of chlorophyll in the Strait of Georgia is dominated every year by relatively large near-surface concentrations in the spring that are linked to the seasonal increase in solar radiation onto the stratified near-surface layer. In the vertical, a sub-surface peak is observed around 10 m depth, corresponding to the depth of maximum water column stability. Nutrients within the euphotic zone are in general abundant, with the exception of the Strait of Georgia in summer where phytoplankton growth is potentially limited by low nitrate concentration near the surface. The depth of the euphotic zone is estimated along the thalweg of the estuary from transmissometer profiles. It appears to vary relatively little within the estuary from a minimum of 20 m in spring, near the mouth of the Fraser River, to an autumnal maximum of about 30 m in the northern Strait of Georgia. Finally, the estimated self-shading contribution to light attenuation is shown to be generally significant (5–10%) in the surface waters of the Strait of Georgia, during spring and summer, reaching values as high as 35% during the spring bloom.     

Diel vertical distribution of planktonic ciliates within the surface layer of the NW Mediterranean (May 1995)

The composition and vertical distribution of planktonic ciliates within the surface layer was monitored over four diel cycles in May 95, during the JGOFS-France DYNAPROC cruise in the Ligurian Sea (NW Mediterranean). Ciliates were placed into size and trophic categories: micro- and nano-heterotrophic ciliates, mixotrophic ciliates, tintinnids and the autotrophic Mesodinium rubrum. Mixotrophic ciliates (micro and nano) represented an average of 46% of oligotrich abundance and 39% of oligotrich biomass; nano-ciliates (hetero and mixotrophic) were abundant, representing about 60 and 17% of oligotrich abundance and biomass, respectively. Tintinnid ciliates were a minor part of heterotrophic ciliates. The estimated contribution of mixotrophs to chlorophyll a concentration was modest, never exceeding 9% in discrete samples. Vertical profiles of ciliates showed that chlorophyll-containing ciliates (mixotrophs and autotrophs) were mainly concentrated and remained at the chlorophyll a maximum depth. In contrast, among heterotrophic ciliates, a portion of the population appeared to migrate from 20–30 m depth during the day to the surface at night or in the early morning. Correlation analyses of ciliate groups and phytoplankton pigments showed a strong relationship between nano-ciliates and zeaxanthin, and between chlorophyll-containing ciliates and chlorophyll a, as well as other pigments that were maximal at the chlorophyll a maximum depth. Total surface layer concentrations showed minima of ciliates during nightime/early morning hours.     

东海区叶绿素a和初级生产力季节变化特征

依据2008年春季(5月)、夏季(8月)、秋季(11月)和2009年冬季(2月)的现场调查结果,分析了东海区叶绿素a、初级生产力的平面分布、垂直分布和季节变化的特征,并探讨了其影响因素。结果表明,四个航次叶绿素a浓度分别为1.33、0.93、1.61和0.65 mg/m3,秋季春季夏季冬季。春季、夏季和秋季最大值均出现在0—10m水层,冬季最大值出现在底层。叶绿素a浓度远海年季变化较小,近岸区和垂直分布年季变化较大。四个航次初级生产力平均为375.03、414.37、245.45和102.60 mg/(m3 h),夏季秋季春季冬季。叶绿素a浓度和初级生产力水平均高于历史同期值。鱼外渔场的年平均初级生产力最大,海州湾渔场最小。通过分析叶绿素a和环境因子的相关性表明,叶绿素a与浮游植物显著正相关;春季和秋季的低温以及春季和夏季的低盐比较适合浮游植物的生长;活性磷酸盐可能是限制春季和秋季叶绿素a的重要因素。     

南海U形海疆线的生态环境分区特征

南海U形海疆国界线（简称南海U形线）是我国的南海国界线。该研究分析多源卫星遥感和GIS数据,系统研究南海U形海疆线水域的水深地形和环境生态要素,并重点分析2014年生态要素的季节变化,首次整体展现了南海U形线立体水深分布特征。根据海底地形的平缓、波峰、波谷和递增四大特征,将南海U形线分为东北、西北、东、西和南区5个区间。南海U形线总长大于4 000 km。西北区和南区的水深浅且变化平缓（<1 000 m）,西区水深呈波峰分布（平均2 303 m）,东区水深由南向北递增（>2 000 m）;东北区水深最深且呈波谷分布（平均3 535 m）。南海U形线的5个区间,西北区与北部湾盆地、西区与越东断裂、南区与曾母盆地、东区与南海海槽、东北区与马尼拉海沟地形构造相吻合。研究发现季风对南海U形线5个区间海洋环境季节性变化有明显影响:西北区和东北区海表温度温差大,呈冬季最低夏季最高,混合层深度冬季最深春季最浅,海表流场和海表盐度季节变化小,但西北区海表叶绿素a浓度冬季爆发,其余季节呈对数分布,而东北区冬季区内中部略有增长;西区、南区和东区海表温度盐度季节变化小,海表风场和混合层深度冬季最强春季最弱,但海表叶绿素a浓度西区季节变化小,南区区内中部冬季增长明显,东区区内南部冬季小幅增长。西北区和南区（浅地形区）呈相似的季节分布。研究阐明了5个区间具有各自明显的区域性海洋环境特征:西北区海表温度和海表叶绿素a浓度的季节变化最大、西区混合层深度季节变化最大、南区海表流场季节变化最大、东区海表盐度季节变化最大、东北区风场变化大但海表叶绿素a浓度季节变化小。研究显示,南海U形线上的台风路径时空分布南北差异大,东西不均。1945—2016年共604个台风跨过南海U形线,年均8个,路径集中在东北、西北、东3个区,112.3°E以东台风537个,112.3°E以西415个。南海U形线东北区的生态环境受台风"风泵效应"影响最大。1991—2000年为台风多发期,跨线台风年均达11个。研究提出的南海U形海疆线5区间分法,具有科学意义和实践指导作用。     

Storm-induced convective export of organic matter during spring in the northeast Atlantic Ocean

Observations during a spring phytoplankton bloom in the northeast Atlantic between March and May 1992 in the Biotrans region at 47°N, 20°W, are presented. During most of the observation period there was a positive heat flux into the ocean, winds were weak, and the mixed layer depth was shallow (<40 m). Phytoplankton growth conditions were favourable during this time. Phytoplankton biomass roughly doubled within the euphotic zone over the course of about 7 days during mid-April, and rapidly increased towards the end of the study until silicate was depleted. However, the stratification of the water column was transient, and the spring bloom development was repeatedly interrupted by gales. During two storms, in late March and late April, the mixed-layer depth increased to 250 and 175 m, respectively. After the storm events significant amounts of chlorophyll-a, particulate organic carbon and biogenic silica were found well below the euphotic zone. It is estimated that between 56% and 65% of the seasonal new production between winter and early May was exported from the euphotic zone by convective mixing, in particular, during the two storm events. Data from the NABE 47°N study during spring 1989 are re-evaluated. It is found that convective particle export was of importance during the early part of that bloom too, but negligible during the height of the bloom in May 1989. The overall impact of convective particle export during spring 1989 was equivalent to about 36% of new production. In view of these and previously published findings it is concluded that convective transport during spring is a significant process for the export of particulate matter from the euphotic zone in the temperate North Atlantic.     

Phaeopigment distribution during the 1990 spring bloom in the northeastern Atlantic

The temporal and depth distributions of phaeopigments were determined during a spring bloom in the northeastern Atlantic in 1990 using reverse phase high performance liquid chromatography. Phaeopigment concentrations were very low (<8% by mole relative to chlorophyll a) and two forms of phaeophorbide and two forms of phaeophytin were separated and quantified. Phaeophorbides were the dominant phaeopigments, accounting for 80% of the total phaeopigment molar concentration, and phaeopigments closely tracked the variations in chlorophyll a by increasing during the development phase of the bloom and decliningin the post bloom stage. Production of phaeopigments was found to occur predominantly at night, and these rates exceeded the rates of phaeopigment destruction during the day. Lower phaeopigment-chlorophyll a ratios were determined for the surface waters, suggesting that photodegradation was possibly the primary mechanism of phaeopigment disappearance in the euphotic zone. A comparison of phaeopigment data and phyto- and zooplankton structure indicates that greater phaeopigment production and grazing pressure occurred in the development phase when diatoms dominated, while grazing and phaeopigment production declined in the post bloom stage dominated by prymnesiophytes.     

Relationships of interannual variability in SST and phytoplankton blooms with giant jellyfish (Nemopilema nomurai) outbreaks in the Yellow Sea and East China Sea

Giant jellyfish (Nemopilema nomurai) outbreaks in relation to satellite sea surface temperature (SST) and chlorophyll-a concentrations (Chl-a) were investigated in the Yellow Sea and East China Sea (YECS) from 1998 to 2010. Temperature, eutrophication, and match–mismatch hypotheses were examined to explain long-term increases and recent reductions of N. nomurai outbreaks. We focused on the timing of SST reaching 15 °C, a critical temperature enabling polyps to induce strobilation and enabling released ephyra to grow. We analyzed the relationship of the timing with interannual variability of SST, Chl-a, and the timing of phytoplankton blooms. Different environmental characteristics among pre-jellyfish years (1998–2001), jellyfish years (2002–2007, 2009), and non-jellyfish years (2008, 2010) were assessed on this basis. The SST during late spring and early summer increased significantly from 1985 to 2007. This indicated that high SST is beneficial to the long-term increases in jellyfish outbreaks. SST was significantly lower in non-jellyfish years than in jellyfish years, suggesting that low SST might reduce the proliferation of N. nomurai. We identified three (winter, spring, and summer) major phytoplankton bloom regions and one summer decline region. Both Chl-a during non-blooming periods and the peak increased significantly from 1998 to 2010 in most of the YECS. This result indicates that eutrophication is beneficial to the long-term increases in jellyfish outbreaks. Timing of phytoplankton blooms varied interannually and spatially, and their match and mismatch to the timing of SST reaching 15 °C did not correspond to long-term increases in N. nomurai outbreaks and the recent absence.     

Nematode community dynamics over an annual production cycle in the central North Sea

Nematode species composition, trophic structure and body size distributions were followed over an annual production cycle in the central North Sea; to test responses to temporally changing food quality and quantity in the sediment. Changes in the phytoplankton concentration in the water column were quantitatively reflected in the concentration of chlorophyll a and breakdown products in the sediment, with higher concentrations in spring and autumn following blooms, and lower concentrations in summer and winter. The taxonomic and trophic structure of nematode communities differed significantly among stations over relatively short distances, potentially masking some of the temporal dynamics. Spatio-temporal differences in nematode species composition were linked to changes in the quality and quantity of organic material reaching the seabed, reflecting a species-specific response to the nutritional quality of sedimenting organic material and the biochemical changes in the sediment associated with its decomposition. The size distributions of selected nematode species indicated that most species bred continuously throughout the sampling period, although one species, the epigrowth feeder Spilophorella paradoxa, had periods of increased growth following the deposition of the spring phytoplankton bloom. There was no consistent temporal relationship between the trophic composition of nematode communities and spring chlorophyll a or carbon sedimentation, most likely a result of the trophic plasticity of most feeding types and the capacity of the community to use both freshly sedimented material as well as the subsequent breakdown products and refractory organic matter. Community metrics implied that there were small responses to the seasonal production cycle, but these belied strong responses of a few species with life histories that allowed them to track the availability of suitable food resources.     

Role of sea ice on satellite-observed chlorophyll-a concentration variations during spring bloom in the East/Japan sea

The relationship between the spring bloom along the Primorye coast and the sea ice of the Tatarskiy Strait in the northern region of the East/Japan Sea, a semi-enclosed marginal sea in the North Pacific, was investigated using the ten-year SeaWiFS chlorophyll-a concentration data and DMSP/SSMI sea ice concentration data from 1998 to 2007. Year-to-year variations in the chlorophyll-a concentrations in the spring were positively correlated with those of the sea ice concentrations in the Tatarskiy Strait in the previous winter with a correlation coefficient of 0.77. Abrupt increases in nutrients, essential for the spring bloom in the upper ocean during spring, were supplied from sea ice-melted waters. Time series of vertical distributions of the nutrients indicated that phosphate concentrations were extremely elevated in the upper ocean (less than 100 m) without any connection to high concentrations in the deep waters below. The water mass from sea ice provided preferable conditions for the spring bloom through changes in the vertical stratification structure of the water columns. Along-coast ratios of stability parameters between two neighboring months clearly showed the rapid progression of the generation of a shallow pycnocline due to fresh water originating from sea ice. This study addressed the importance of the physical environment for biogeochemical processes in semi-enclosed marginal seas affected by local sea ice.     

Phytoplankton characteristics and hydrological conditions in the western part of the Sea of Okhotsk in the spring of 1999 and 2000 based on expeditionary and satellite data

Using the data obtained in 1999–2000 during the spring bloom of phytoplankton (late May–early June), the variability of the pigment concentrations, the phytoplankton biomass and species compositions, and the hydrological conditions on the eastern shelf of Sakhalin Island was studied. The study resulted in revealing 135 microalgae species belonging to eight divisions. The most diversely presented were the Dinophyta dinoflagellates and Bacillariophyta diatoms (70 and 53 species, respectively). The concentration of chlorophyll a in the euphotic zone amounted, on average, to 3.8 mg/m³ in 1999 and 2.4 mg/m³ in 2000. It was shown that, in the northern and southern parts of the coastal zone, the concentration of chlorophyll a and the phytoplankton density in the spring were considerably different and depended on the hydrological conditions. In the north, their maximum values were found in the area of the depth break and were determined by the tidal mixing. The increased algae concentrations and temperature inversions at depths of 400–600 m confirm the downslope sliding of the near-bottom shelf waters. In the southern part, the high phytoplankton concentrations in the surface layer in 1999 confirmed by the monthly averaged estimates from the SeaWiFS satellite color scanner were caused by the abnormal northward propagation of the Soya Current waters and by intense tidal mixing.     

Estimation of standing stock of chlorophylla and primary production from remote-sensed ocean color in the Oyashio region, the Western subarctic Pacific, during the spring bloom in 1997

In order to examine the applicability of remotely-sensed ocean color for the estimation of phytoplankton biomass and primary production in the Oyashio region, the western subarctic Pacific, vertical distributions of chlorophylla concentration and primary production were observed in April and May 1997. Spring bloom was observed in both April and May, and the surface concentration of chlorophylla exceeded 40 mg m⁻³. The relationship between the standing stocks of chlorophylla within the layer from the sea surface to one optical depth (0–1/k layer) and the surface chlorophylla concentration is expressed as a Michaelis-Menten equation. The mean ratio of the standing stock of chlorophylla in the euphotic layer to that in the 0–1/k layer was 4.41, this ratio did not significantly differ from 4.61 which was obtained at homogeneous distribution of chlorophylla within the euphotic layer. These facts suggest that the distribution of chlorophylla could be assumed to be homogeneous in the euphotic layer during the spring bloom. Results of primary production measurements by simulatedin situ method were compared with those by an algorithm with two variables; chlorphylla and non-spectral PAR. Daily primary production in the euphotic layer estimated by the algorithm varied in a range of 38–274% of that estimated by incubation, although the primary productions by the algorithm agreed with those by the incubation at a half of stations. Primary production within the euphotic layer calculated using simply the surface data was the same as that estimated using vertical distribution of chlorophylla. These results show that the primary production in the euphotic layer may be estimated from the remote sensed measurements during the spring bloom in the Oyashio region.