Changes in structural and functional diversity of nematode communities during a spring phytoplankton bloom in the southern North Sea

The response of nematode communities to the sedimentation of a spring phytoplankton bloom in a sandy, well-oxygenated sediment at a single station (station 330) in the Southern North Sea was investigated monthly from early March to July 1999. Both structural (nematode density, diversity, vertical distribution and community composition) and functional (feeding type distributions, number of species within feeding groups) characteristics showed considerable changes shortly after the arrival of fresh organic material at the sediment surface. The general increase in numerical densities and diversity was related to changes within the groups of selective deposit-feeding and epistrate-feeding nematodes. It is hypothesised that sedimentation and subsequent remineralisation of fresh organic matter during the spring phytoplankton bloom result in an increase of suitable food items (both living and dead). This, combined with the availability of oxygen, creates conditions in which many nematode species can co-exist.     

Physical controls and interannual variability of the Labrador Sea spring phytoplankton bloom in distinct regions

We investigated the variability of the spring phytoplankton bloom in the Labrador Sea, dividing into distinct biogeographical zones, then analyzing the relationship between the bloom and physical forcings. The spring phytoplankton bloom in the north Labrador Sea varied in intensity by a factor of 4 and in timing of onset by 3 weeks over the 11-year record from SeaWiFS satellite ocean chlorophyll, 1998–2008. This north bloom (north of 60 °N and west of the Labrador shelves) is earliest and most intense, owing in part to the offshore-directed freshwater stratification from the West Greenland Current. On interannual timescales, significant correlations were found between the north bloom intensity and ocean processes, namely offshore advection, eddy activity and runoff from Greenland. In contrast, the central Labrador Sea is later and weaker, and only a correlation between the bloom timing and irradiance was found. As the subpolar gyre shifts in strength and shape, freshwater outflow from the Arctic and Greenland changes, we may expect further changes in the biological response as indicated by these relationships.     

Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider

We investigated the 2005 spring phytoplankton bloom in the Labrador Sea using Seaglider, an autonomous underwater vehicle equipped with hydrographic, bio-optical and oxygen sensors. The Labrador Sea blooms in distinct phases, two of which were observed by Seaglider: the north bloom and the central Labrador Sea bloom. The dominant north bloom and subsequent zooplankton growth are enabled by the advection of low-salinity water from West Greenland in the strong and eddy-rich separation of the boundary current. The glider observed high fluorescence and oxygen supersaturation within haline-stratified eddy-like features; higher fluorescence was observed at the edges than centers of the eddies. In the central Labrador Sea, the bloom occurred in thermally stratified water. Two regions with elevated subsurface chlorophyll were also observed: a 5 m thin-layer in the southwest Labrador Current, and in the Labrador shelf-break front. The thin layer observations were consistent with vertical shearing of an initially thicker chlorophyll patch. Observations at the front showed high fluorescence down to 100 m depth and aligned with the isopycnals defining the front. The high-resolution Seaglider sampling across the entire Labrador Sea provides first estimates of the scale dependence of coincident biological and physical variables.     

Behaviour of arsenic on the continental shelf off the Gironde estuary: Role of phytoplankton in vertical fluxes during spring bloom conditions

The arsenic (As) cycle in the marine environment is known to be sensitive to biological activity. Within the scope of the National Coastal Oceanography Program, we undertook a specific study of the behaviour of this element in a water column on the continental shelf of the Bay of Biscay off the Gironde estuary during two oceanographic cruises conducted in May 1994 and May 1995. Various chemical forms were measured: arsenate (As3), arsenate (As5), monomethylarsenic (MMA), dimethylarsenic (DMA) and dissolved total As after ultraviolet mineralization. The net flux of total As from the Gironde estuary was evaluated as well as its effect on the concentration in surface marine waters. The vertical profiles in the coastal zone during a period of active primary production confirmed a certain analogy between arsenates and phosphates as well as the formation of As3 and DMA. The importance of organoarsenic species not directly accessible to the formation of volatile hydrides is demonstrated. Their presence modifies assessments, reducing the deficit of dissolved total As in the euphotic layer to just 5%. For particles, the As content in phytoplankton was estimated at 6 μg·g⁻¹ compared to 20 to 30 μg·g⁻¹ in iron- and aluminium-rich terrigenous particles. These results do not invalidate the role of phytoplankton in the speciation of dissolved As but indicate that their involvement in vertical transfers was apparently not predominant in the zone under study.     

The spatial and temporal development of the spring phytoplankton bloom in the Celtic Sea,April 1979

The results are presented from three hydrogrpahic surveys in April 1979 of a 40 × 50 km region of the Celtic Sea, centred at 7°W and 51°N, using a towed undulating sensor system. In the $\text{5}\text{1}\text{2}$ days between Surveys 1 and 2, the seasonal thermocline was established, with surface to bottom temperature differences reaching 1.5°C, the average surface chlorophyll a level increased from ～ 1 to ～ 5.5 mg m^?3 due mainly to the growth of diatoms, and the surface nitrate concentration decreased from 6 to 1 μM. The third survey was carried out after a further two days and, although surface properties changed little, there was a general deepening of the mixed layer due to stronger winds, and a further increase in the standing stock of phytoplankton.By applying appropriate techniques of horizontal spatial averaging, which took into account possible advective effects, a quantitative comparison was made of the changes at two positions in the survey area which showed significant differences in the rate of development of the phytoplankton population. A simple model of phytoplankton growth was then developed, based on calculations of eddy diffusivity, on measurements of rates of photosynthetic carbon assimilation, and on observations of carbon to chlorophyll ratios, subsurface light attenuation and inorganic nutrient levels. In the absence of any data on zooplankton populations, the loss of phytoplankton by grazing was left as a free parameter.The model was only partly successful in reproducing the observed changes in chlorophyll concentrations during the first $\text{5}\text{1}\text{2}$ days and showed serious limitations for the subsequent 2 days. However, it emphasised several features of the dynamics of spring phytoplankton populations which require further experimental or observational investigation. These include more precise measurements of carbon to chlorophyll ratios and grazing pressure to which the model is sensitive over a rather narrow range (this has important implications in terms both of control by grazing and nutrient limitation), the potential significance of physiological photoadaptation by the plant cells in determining their vertical distribution, and the role of eddy diffusion across the developing thermocline in relation to the sinking of phytoplankton cells and the upward mixing of inorganic nutrients.     

The effects of horizontal advection on the spring bloom of phytoplankton in the central Southern Huanghai Sea

An algal bloom is defined as a relatively rapid increase in the biomass of phytoplankton in an aquatic system. During 30 March to 24 April 2007, a cruise was conducted in the central Southern Huanghai Sea to investigate the spring bloom processes. The spatial and temporal variations of phytoplankton are discussed based on the in-situ observations and simultaneous remote sensing data. The explosive algal blooming varied quickly in temporal and spatial scales, due to the highly patchy distribution. Data obtained at the 2 anchor stations (BM1 and BM2) were analyzed in the present study. Horizontal advection is speculated to be responsible for the abrupt decrease in the concentration of chlorophyll-a at stations BM1 and BM2. At station BM2, the intermediate high chlorophyll-a concentration, coinciding with the low temperature, was found to be advected from the inshore colder water mass located to the east of the site.     

Post-convection spreading phase in the Northwestern Mediterranean Sea

This is a study about the spreading of newly formed deep waters following open ocean deep convection in the Northwestern Mediterranean Sea. The main results are from the SOFARGOS large scale float experiment initiated in 1994–1995. During the SOFARGOS project, CTD stations and Lagrangian observations of ocean currents were carried out in the Gulf of Lion from December 1994 to July 1995. Hydrological observations confirmed that deep water formation occurred very early during winter 1994–1995 (late December, early January) in conjunction with atmospheric cooling, deep convection penetrating down to 2000 m in the so-called Medoc area. Numerous eddies (both anticyclonic and cyclonic) drifted away from the convection area and advected newly formed deep waters far away from the source region. In particular, compact anticyclones appeared to be the most coherent (long-lived) eddies and capable of transporting newly formed Western Mediterranean Deep Waters several hundreds of kilometers away from the convection area. Characterized by an inner core of about 5 km in radius, these eddies are submesoscale features in the outer domain and appear as key elements of the open ocean convection processes. During their long journeys, these eddies interacted with larger scale features such as the Northern Boundary Current, the North Balearic Front, topographic Rossby waves, and Sardinian eddies. These interactions influenced the long-term behavior of the eddies (mean drift, composition) and represented an important part of (1) the spreading phase following deep convection and (2) the large scale thermohaline circulation.     

Bacterioplankton and phytoplankton biomass and production during summer stratification in the northwestern Mediterranean Sea

We examined bacterioplankton biomass and heterotrophic production (BHP) during summer stratification in the northwestern Mediterranean in four successive stratification seasons (June–July of 1993–1996). Values of phytoplankton biomass and primary production were determined simultaneously so that the data sets for autotrophic and heterotrophic microbial plankton could be compared. Three standard stations were set along a transect from Barcelona to the channel between Mallorca and Menorca, representing coastally influenced shelf waters, frontal waters over the slope front, and open sea waters. Conversion factors from ³H-leucine incorporation to BHP were empirically determined and varied between 0.29 and 3.25 kg C mol^-1. Bacterial biomass values were among the lowest found in any marine environment. BHP values (between 0.02 and 2.5 μg C L^-1 d^-1) were larger than those of low nutrient low chlorophyll areas such as the Sargasso Sea and lower than those from high nutrient low chlorophyll areas such as the equatorial Pacific. Growth rates of bacterioplankton were highest at the slope front (0.20 d^-1) and lowest at the open sea station (0.04 d^-1). Phytoplankton growth rates were similar at the three stations (∼0.50 d^-1). Integrated values of bacterioplankton biomass, BHP and bacterial growth rates did not show significant differences among years, but differences between the three stations were clearly significant. Phytoplankton biomass, primary production, and phytoplankton growth rates did not show significant differences either with year or with station. As a consequence the bacterioplankton to phytoplankton biomass (BB/BPHY) and production (BHP/PP) ratios varied from the coastal to the open sea stations. The BB/BPHY ratio was 0.98 at the coast and ∼0.70 at the other two stations. These ratios are similar to those found in other oligotrophic marine environments. The BHP/PP ratio was 0.83 at the coast, 0.36 at the slope and 0.09 at the open sea station. The last value is also similar to values found in other oligotrophic marine environments. Vertical distribution of these ratios was also examined.The comparison of microbial parameters at the three stations indicates a different kind of relationship between bacterioplankton and phytoplankton in oligotrophic open sea waters and in coastal, nutrient-richer waters. According to such parameters and to the values of the BB/BPHY and BHP/PP ratios, open waters in the northwestern Mediterranean (despite their relatively short distance from the shore) were intermediate between the extremely oligotrophic waters of the eastern Mediterranean or the Sargasso Sea and the more productive waters of the equatorial Pacific.     

Arsenic,antimony and selenium speciation during a spring phytoplankton bloom in a closed experimental ecosystem

A study has been made of the effects of a spring diatom bloom on the levels and speciation of dissolved arsenic, antimony and selenium in the water enclosed in an experimental ecosystem moored in Loch Ewe (NW Scotland). Primary productivity resulted in severe depletion of phosphate and silicate in the bag, but had little effect on the levels and speciation of arsenic and antimony. Calculations based on phosphate depletion data strongly suggest that the field diatom population present during the experiment was capable of some degree of discrimination between phosphate and arsenate ions. Whilst biomethylation of arsenic was not observed in the upper region of the bag, where the phytoplankton population was at its greatest, the methylated form accounted for 64% of the dissolved arsenic at the base of the bag. In this region, however, the total dissolved arsenic levels were not higher than in the rest of the bag, suggesting microbial methylation of dissolved arsenic rather than the release of methylated arsenic from decaying phytoplankton.Total dissolved selenium and selenium(IV) showed some evidence of depletion during the development of the phytoplankton bloom, in support of previous observations of preferential selenite assimilation.     

Size-fractionated phytoplankton standing stock and primary production in the Bohai Sea during late spring

During June 1997 cruise by R/V Science No.l, observations on temporal and spatialvariations of the size-fractionated phytoplankton standing stock and primary production were carried out in the Bohai Sea. The size-fractionated chlorophyll a (Chl a) and primary production, photosynthet-ically available radiation (PAR), as well as the related physico-oceanographic and zooplanktonic parameters were measured at five time-series observation stations representing sub-areas of the sea. Results obtained show that there were the marked features of spatial zonation of Chl a and primary production in the Bohai Sea. The values in the Laizhou Bay, the Liaodong Gulf and the Bohai Gulf were high and showed close relation with tidal fluctuations, i.e. high Chl a concentration occurred during high tide in the Laizhou Bay, and during low tide in the Liaodong Gulf and the Bohai Gulf. In the strait and the central region of the Bohai Sea, the values were relatively low and no relationship with tidal fluctuation could be foun     

Spring phytoplankton bloom in the fronts of the East China Sea

Frontal areas between warm and saline waters of the Kuroshio currents and colder and diluted waters of the East China Sea (ECS) influenced by the Changjiang River were identified from the satellite thermal imagery and hydrological data obtained from the Coastal Ocean Process Experiment (COPEX) cruise during the period between March 1^st and 10^th, 1997. High chlorophyll concentrations appeared in the fronts of the East China Seas with the highest chlorophyll-a concentration in the southwestern area of Jeju Island (~2.9 mg/m³) and the eastern area of the Changjiang River Mouth (~2.8 mg/m³). Vertical structures of temperature, salinity and density were similar, showing the fronts between ECS and Kuroshio waters. The water column was well mixed in the shelf waters and was stratified around the fronts. It is inferred that the optimal condition for light utilization and nutrients induced both from the coastal and deep waters enhances the high phytoplankton productivity in the fronts of the ECS. In addition, the high chlorophyll-a in the fronts seems to have been associated with the water column stability as well.     

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.     

Deep strong currents and sediment transport in the Northwestern Mediterranean Sea

The Petit Rhône Canyon (Gulf of Lions) is incised by a narrow meandering thalweg. Two current meters were moored near the bottom, one in the channel and the other on the levee. None of the measured currents is associated with the sediment transport peculiar to deep-sea fan building. This transport is mainly influenced by relatively high speeds (1 h means up to 48 cm/s; 1 day means > 30 cm/s) measured in diverse directions.     

Particle fluxes dynamics in Blanes submarine canyon (Northwestern Mediterranean)

Within the framework of the multidisciplinary RECS project and with the aim of describing the particle flux transfer from the continental shelf to the deep basin, an array of five mooring lines equipped with a total of five pairs of PPS3/3 sequential-sampling sediment traps and RCM-7/8 current meters were deployed 30 m above the bottom from March 2003 to March 2004 inside and outside the Blanes Canyon. One mooring line was located in the upper canyon at 600 m depth, one in the canyon axis at 1700 m depth and other two close to the canyon walls at 900 m depth. A fifth mooring line was deployed in the continental open slope at 1500 m water depth.The highest near-bottom downward particle flux (14.50 g m⁻² d⁻¹) was recorded at the trap located in the upper canyon (M1), where continental inputs associated with the presence of the Tordera River are most relevant. On the other hand, the downward fluxes (4.35 g m⁻² d⁻¹) in the canyon axis (M2) were of the same order as those found in the western flank (M3) of the canyon. Both values were clearly higher than the value (1.95 g m⁻² d⁻¹) recorded at the eastern canyon wall (M4). The open slope (M5) mass flux (5.42 mg m⁻² d⁻¹) recorded by the sediment trap located outside the canyon system was three orders of magnitude lower than the other values registered by the inner canyon stations. The relevance of our data is that it explains how the transport pathway in the canyon occurs through its western flank, where a more active and persistent current toward the open ocean was recorded over the entire year of the experiment.Off-shelf sediment transport along the canyon axis showed clear differences during the period of the study, with some important events leading to strong intensifications of the current coupled with large transport of particle fluxes to the deepest parts of the canyon. Such events are primarily related to increases in river discharge and the occurrence of strong storms and cascading events during the winter.In summary, in this study it is shown that the dynamics of the water masses and the currents in the study area convert the sharp western flank of the Blanes Canyon in a more active region that favors erosion processes than the eastern flank, which has a smoother topography and where the absence of erosional conditions yields to steadier sedimentary processes.     

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.     

Kuroshio intrusion drives the Trichodesmium assemblage and shapes the phytoplankton community during spring in the East China Sea

Abs tractTo understand the influence of Kuroshio intrusion on the phytoplankton community, a field investigation was conducted in spring 2017 in the East China ...     

Early spring bloom phytoplankton-nutrient dynamics at the Celtic Sea Shelf Edge

Phytoplankton production was measured at the shelf edge region of the Celtic Sea in April/May 1994 at the beginning of the spring bloom. Size fractionated ¹⁴C uptake experiments showed that phytoplankton >2 μm dominated the bloom although, in the period immediately before the increase in phytoplankton biomass, picophytoplankton (<2 μm) was responsible for up to 42% of the production; in these late winter conditions, chlorophyll concentrations were generally <0.7 μg l^-1 and primary production was ca. 70 mmol C m^-2 d^-1. As the spring bloom developed, phytoplankton production rates of 120 mmol C m^-2 d^-1 were measured. Chlorophyll concentration increased to >2 μg l^-1 as a result of growth of larger phytoplankton, including diatoms, with large numbers of Nitzschia, Thalassionema and Chaetoceros dominating the assemblage. Picophytoplankton production declined as the spring bloom progressed. Nutrient concentrations were not depleted during the sampling period, and NO^-₃ concentrations were >6 μmol l^-1. Nutrient assimilation rates were measured at the same time as primary production was estimated. Before the development of any substantial phytoplankton biomass, the uptake rates for ammonium and nitrate were very similar, with f-ratios ranging from 0.5 to 0.6. Assimilation of ammonium remained relatively constant after the onset of stratification and bloom development, but nitrate uptake increased by a factor of 2 or more, resulting in f-ratios >0.8. There was significant phosphate uptake in the dark, which was generally ca. 50% of the rate in the light. The C : N : P assimilation ratios changed as the bloom developed; in the pre-bloom situation, when small phytoplankton cells dominated the assemblage, the C : N assimilation ratio was variable, with some stations having ratios less than (ca 2.5), and some higher than (ca. 9), the Redfield ratio. The most actively growing assemblages had N : P ratios close to the Redfield ratio, but the C : N ratios were consistently lower. New production was found to be closely correlated with the size of the species making up the phytoplankton assemblage, and high f ratios were measured when larger phytoplankton dominated the assemblage.     

2009 年晚春黄海南部浮游植物群落

在2009 年6 月对黄海南部及中部海域30 个站位进行综合调查, 对获得的131 个浮游植物样品用Uterm?hl 方法进行初步分析, 共鉴定浮游植物4 门51 属73 种(不包括未定名种), 其中硅藻32 属47种(不包括未定名种), 甲藻17 属24 种(不包括未定名种), 定鞭藻1 门1 种, 蓝藻1 门1 种, 硅藻在物种丰富度上占有优势。浮游植物的生态类型主要以温带近岸种为主, 优势物种为具齿原甲藻(Prorocentrum dentatum)、柔弱伪菱形藻(Pseudo-nitzschia delicatissima)、具槽帕拉藻(Paralia sulcata)等。调查区浮游植物细胞丰度介于0.089 × 10³～1 045.200 × 10³ 个/L, 平均为15.941 × 10³ 个/L, 甲藻的细胞丰度要高于硅藻。细胞丰度高值区位于调查区的南部海域, 以具齿原甲藻为主, 垂向上在10 m 层出现最大值, 随着深度的增加丰度降低。调查区的Shannon-winner 多样性指数和Pielou 均匀度指数的平面分布基本一致, 并且与细胞丰度的分布呈相嵌形式, 即在细胞丰度高的区域多样性指数较低。     

2009年晚春黄海南部浮游植物群落

在2009年6月对黄海南部及中部海域30个站位进行综合调查,对获得的131个浮游植物样品用Utermohl方法进行初步分析,共鉴定浮游植物4门51属73种（不包括未定名种）,其中硅藻32属47种（不包括未定名种）,甲藻17属24种（不包括未定名种）,定鞭藻1门1种,蓝藻1门1种,硅藻在物种丰富度上占有优势。浮游植物的生...