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1.
In order to detect iron (Fe) stress in micro-sized (20–200 μm) diatoms in the Oyashio region, western subarctic Pacific during spring, immunological ferredoxin/flavodoxin assays were applied to samples collected from the surface layer in May 2005. Concomitantly, the community composition of the micro-sized phytoplankton and hydrographic conditions, including dissolved Fe and macronutrient concentrations, were also examined. Chlorophyll (Chl) a concentrations were <2 mg m−3 at all sampling stations, except at a station where the Chl a level was 9.0 mg m−3 and a micro-sized diatom bloom occurred. A high abundance of ferredoxin in micro-sized diatoms was detected only at a rather near-shore station where dissolved Fe and macronutrient concentrations were higher, indicating that the micro-sized diatoms did not suffer from iron deficiency. On the other hand, flavodoxin in micro-sized diatoms was often observed at the other stations, including the bloom station, where macronutrients were replete but dissolved Fe concentration was low (0.31 nM). A significant amount of chlorophyllide a, a degradation product of Chl a, was also observed at the bloom station, suggesting a decline of the diatom bloom. The micro-sized phytoplankton species at all the stations were mainly composed of the diatoms Thalassiosira, Chaetoceros, and Fragilariopsis spp. Our study indicates that micro-sized diatoms were stressed by Fe bioavailability during the spring season in the Oyashio region  相似文献   

2.
Egg production rates and/or hatching success in the copepods Acartia clausi, Calanus helgolandicus and Temora longicornis were negatively affected by a late spring (May–June 2003) phytoplankton bloom in the North Adriatic Sea, dominated mainly by the large diatom Cerataulina pelagica. Highest total concentrations of 3.3·104 cells·ml?1 were located in the vicinity of the Po River, which also corresponded to the area where the highest numbers of phaeophorbides were measured (0.779, 0.528 and 0.419 μg·l?1, respectively, compared to an average of the remaining stations of 0.183 ± 0.049 SD), suggesting some grazing on the bloom. Phytoplankton biomass in terms of carbon was dominated by diatoms, representing on average 42% of total phytoplankton carbon and more than 80% at several stations. Cerataulina pelagica, Cyclotella spp., Chaetoceros spp. and small unidentified centric diatoms dominated the diatom community numerically but C. pelagica was by far the dominant diatom in terms of carbon due to its large cell size. This species represented more than 60% of the diatom biomass at nine of the 14 stations sampled, and was absent only at one station, which was the most offshore station sampled during the cruise. Although polyunsaturated aldehydes (PUAs) were not detected, other oxylipins which are hydroxy and keto derivatives of eicosapentaenoic and docosahexaenoic acids that affect copepod reproduction were found in these samples. Hence, we can attribute the negative impact of diatoms not only to PUAs, as previously believed, but also to these compounds. This is the first direct evidence of the presence of oxylipins other than PUAs in marine blooms dominated by diatoms.  相似文献   

3.
The relationship between changes in lipid classes and phytoplankton composition and abundance in the northern Adriatic was studied during spring and summer 2008 at two stations with different nutrient levels, i.e. at the western mesotrophic and eastern oligotrophic areas. Changes in the phytoplankton community depended on temporal surface nutrient depletion and bottom accumulation; that is, microphytoplankton, mainly diatoms Pseudo-nitzschia sp., developed at nutrient richer surface layers of the mesotrophic area in spring and at deeper layers of the oligotrophic site in late summer. In other periods nanophytoplankton dominated. Dissolved organic carbon (DOC) and lipid content were comparable for the two stations, while particulate organic carbon (POC) was richer at the mesotrophic side. Total lipid concentrations varied in the range from 8.0 to 92.2 μg l−1 and from 16.9 to 76.9 μg l−1 in the dissolved and particulate fractions, respectively. DOC and POC contents were in the ranges from 0.77 to 1.58 mg l−1 and from 0.06 to 0.56 mg l−1, respectively. Lipid and organic carbon distribution did not follow phytoplankton progression, indicating decoupling between organic matter production and decomposition throughout the investigation period. The main sources of lipids were marine phytoplankton and bacteria. Low nutrient conditions caused increased biosynthesis of lipids. Also, increasing oligotrophy led to an increasing number of phytoplankton taxa. The synthesis and accumulation of glycolipids by the developed taxa were enhanced during nutrient exhaustion, contributing in late summer, on average, 20.2 and 22.0% at the mesotrophic and oligotrophic stations, respectively, in the particulate fraction. The distribution of bacterial lipids and lipid breakdown products implies that bacterial lipid degradation was significant in spring, while very probably lipid abiotic degradation took place during summer.  相似文献   

4.
The East Sea(Sea of Japan)is a marginal,semi-closed sea in the northwestern Pacific.The Ulleung Basin area,which is located near the subpolar front of the East Sea,is known to have high primary production and good fisheries in spring season.After episodic wind-driven events during the spring of 2017,horizontal and vertical profiles of physical chemical biological factors were investigated at 29 stations located in the Ulleung Basin area.In addition,growth responses of phytoplankton communities to nutrient additions were evaluated by bioassay experiments to understand the fluctuation of phytoplankton biomass.Because of strong northwestern wind,phytoplankton biomass was scattered and upwelling phenomenon might be suppressed in this season.The phytoplankton abundances in the coastal stations were significantly higher than offshore and island stations.In contrast,the nutrient and chlorophyll a(Chl a)concentrations and the phytoplankton biomass were quite low in all locations.Bacillariophyceae was dominated group(>75.1%for coastal,40.0%for offshore and 43.6%for island stations).In the algal bioassays,the phytoplankton production was stimulated by N availability.The in vivo Chl a values in the+N and+NP treatments were significantly higher than the values in the control and the+P treatments.Based on the field survey,the higher nutrients in coastal waters affected the growth of diatom assemblages,however,little prosperity of phytoplankton was observed in the offshore waters despite the injection of sufficient nutrients in bioassay experiments.The growth of phytoplankton depended on the initial cell density.All of results indicated that a dominant northwestern wind led to a limited nutrients condition at euphotic layers,and the low level of biomass supply from the coasts resulted in low primary production.Both supplying nutrients and introducing phytoplankton through the currents are critical to maintain the high productivity in the Ulleung Basin area of the East Sea.  相似文献   

5.
Phytoplankton production and physiology were investigated at six selected locations during a research cruise in early October 2007 in Tanzanian coastal waters. The dataset included photosynthesis– irradiance and active fluorescence parameters, phytoplankton absorption coefficients, and pigment concentrations. Primary production was estimated to vary over the range 0.79–1.89 g C m?2 d?1. Diagnostic pigments indicated that micro–nanophytoplankton comprised the communities at three stations and nano–picoplankton at the other three stations. At all stations, the populations maximised their photosynthesis in the upper water column under elevated irradiance and low nutrient conditions. Significant photosynthetic activity was also observed at depth under very low light where the communities increased their quantum yield of photochemistry and the proportion of accessory chlorophylls b and c and photosynthetic carotenoids.  相似文献   

6.
Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August–September). Environmental conditions at six study stations varied broadly from open-ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 μM NO3 and 0.16 to 1.5 μg Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and 14C-uptake experiments gave comparable ranges of community growth rates (0.53–1.05 d−1 and 0.44–1.17 d−1, to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. Microzooplankton grazing utilized 81% of community phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d−1 (two doublings per day) and depth-integrated growth varied from 0.2 to 0.8 d−1. Synechococcus (SYN) growth ranged from 0.5 to 1.1 d−1 at offshore stations and 0.6 to 0.7 d−1 at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN, reaching 1.3 d−1 offshore and decreasing to 0.8 d−1 at the most coastal station. Microzooplankton grazing impact averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon production for all stations, and 50% at high-nutrient, upwelling stations. Prokaryotes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important at the coastal stations. Even during intense monsoonal forcing in the Arabian Sea, picoeukaryotic algae appear to account for a large portion of primary production in the coastal upwelling regions, supporting an active community of protistan grazers and a high rate of carbon cycling in these areas.  相似文献   

7.
Sendai Bay is located on the Pacific coast of northern Japan and suffered serious damage following the 2011 off the Pacific coast of Tohoku earthquake and tsunami in March 2011. To assess the impact on the marine ecosystem, information was needed on the phytoplankton communities and their seasonal variation. However, such information was limited. Therefore, an intensive monitoring of the phytoplankton was carried out from March 2012 to April 2014. Seasonal variation of the phytoplankton community was similar at coastal and offshore stations. Total phytoplankton biomass, based on Chl a concentration, peaked in spring and then decreased to a minimum in summer, before gradually increasing during early winter and peaking again in the following spring. This seasonal pattern was consistent with previous studies conducted before the earthquake and tsunami. Also, size structure of the phytoplankton community and its four main groups was estimated from the size-fractioned samples of Chl a. Our results also showed that the spring bloom consisted of large diatoms, with their growth ceasing due to nitrogen depletion. The bloom was followed by a summer period where cyanobacteria and picoeukaryote became dominant, with high cell densities in spite of low nutrient concentrations. In addition, sporadic environmental changes, such as those following typhoons, were observed. These resulted in large increases/decreases in individual phytoplankton groups.  相似文献   

8.
Zooplankton biomass consisting of large and small-size copepods, copepod nauplii and tintinnids were investigated over a period of one year at two stations in Funka Bay, Japan. The food requirement of zooplankton was also estimated using the method of Ikeda and Motoda. Estimated total carbon requirement of zooplankton in the coastal and central parts of the bay was equivalent to 52 and 38% of the annual primary production, respectively. These corresponded to zooplankton production of 12–13 gC·m–2·yr–1.The total carbon requirement at each station increased to 63 and 74% of the primary production during summer compared with 26 and 3% in spring or 19 and 17% in winter. The microzooplankton (copepod nauplii and tintinnids) accounted for about half of the carbon requirement from April to November.Food requirements reached 161% at the coastal station and 194% at the central station of the daily organic carbon production during September. Zooplankton may also feed on carbon sources other than living phytoplankton. This could account for the observed decrease in particulate organic carbon in a water column.Contribution No. 202 from the Research Institute of North Pacific Fisheries, Faculty of Fisheries, Hokkaido University.  相似文献   

9.
Primary production, nutrient concentrations, phytoplankton biomass (incl. chlorophyll a) and water transparency (Secchi depth), are important indicators of eutrophication. Earlier basin-wide primary production estimates for the Baltic Sea, a shallow shelf sea, were based mainly on open-sea data, neglecting the fundamentally different conditions in the large river plumes, which might have substantially higher production. Mean values of the period 1993–1997 of nutrient concentrations (phosphate, nitrate, ammonium and silicate), phytoplankton biomass, chlorophyll a (chl a) concentration, turbidity and primary production were calculated in the plumes of the rivers Oder, Vistula and Daugava and Klaipeda Strait as well as the open waters of the Arkona Sea, Bornholm Sea, eastern Gotland Sea and the Gulf of Riga. In the plumes, these values, except for primary production, were significantly higher than in the open waters. N:P ratios in the plumes were >16 (with some exceptions in summer and autumn), indicating potential P-limitation of phytoplankton growth, whereas they were <16 in the open Baltic Proper, indicating potential N-limitation. On the basis of in situ phytoplankton primary production, phytoplankton biomass and nutrient concentrations, the large river plumes and the Gulf of Riga could be characterized as eutrophic and the outer parts of the coastal waters and the open sea as mesotrophic. Using salinity to define the border of the plumes, their mean extension was calculated by means of a circulation model. Taking into account the contribution of coastal waters, the primary production in the Baltic Proper and the Gulf of Riga was 42·6 and 4·3×106 t C yr−1, respectively. Hence, an annual phytoplankton primary production in the whole Baltic Sea was estimated at 62×106 t C yr−1. The separate consideration of the plumes had only a minor effect on the estimation of total primary production in comparison with an estimate based on open sea data only. There is evidence for a doubling of primary production in the last two decades. Moreover, a replacement of diatoms by dinoflagellates during the spring bloom was noticed in the open sea but not in the coastal waters. A scheme for trophic classification of the Baltic Sea, based on phytoplankton primary production and biomass, chl a and nutrient concentrations, is proposed.  相似文献   

10.
Seasonal variations in diversity and biomass of diatoms, tintinnids, and dinoflagellates and the contribution of microplankton and faecal material to the vertical flux of particulates were investigated at one time series station T (station 18) between 2002 and 2005 and at a grid of stations during November 2004 in the coastal and oceanic area off Concepción (36°S), Chile. The variations were analysed in relation to water column temperature, dissolved oxygen, nutrient concentration, offshore Ekman transport, and chlorophyll-a concentration. Abundance was estimated as cell numbers per litre and biomass in terms of biovolume and carbon units.A sharp decrease with depth was observed in the abundance of both phytoplankton and microzooplankton during the whole annual cycle; over 70% of their abundance was concentrated in the upper 10 m of the water column. Also, a clear seasonality in microplankton distribution was observed at station T, with maxima for diatoms, tintinnids, and dinoflagellates every summer (centred on January) from 2002 to 2005.On the grid of stations, the maximum integrated (0-50 m) micro-phytoplankton abundances (>1 × 109 cells m−2) occurred at the coastal stations, an area directly influenced by upwelling. A similar spatial distribution was observed for the integrated (0-200 m) faecal carbon (with values up to 632 mg C m−2). Tintinnids were distributed in all the first 300 miles from the coast and dinoflagellates were more abundant in oceanic waters.At station T, the average POC export production (below 50 m depth) was 16.6% (SD = 17%; range 2-67%; n = 16). The biological-mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepción depend upon key groups of phytoplankton (Thalassiosira spp., Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively.  相似文献   

11.
Elevated levels of phytoplankton were observed at the Northern California coastal upwelling ecosystem studied as part of the CoOP-WEST project during monthly summer surveys in 2000, 2001 and 2002. The high concentrations of chlorophyll were made up mostly of larger cells; the phytoplankton communities that resulted were dominated by centric diatoms. The highest chlorophyll a concentrations were observed when large diatoms or small colony-forming species dominated the assemblages. In contrast, when smaller nano-flagellates and picoplankton were dominant, total chlorophyll a concentrations were over four or five-fold lower than when diatoms were prevalent, illustrating the importance of large diatoms to total phytoplankton biomass. Each year, when chlorophyll concentrations were greatest, Chaetoceros species created a Chaetoceros-dominated system. A few other genera contributed to the upwelling diatom community, mostly the centric diatoms Asterionella, Thalassiosira, Rhizosolenia (including Guinardia and Proboscia), and the pennate Nitzschia. These diatoms have been described in other coastal upwelling ecosystems, and at this study site form a functional group that are apparently well adapted to the high-nutrient, turbulent conditions that are typical of these coastal regions.  相似文献   

12.
Living benthic foraminiferal faunas of six stations from the continental shelf of the Bay of Biscay have been investigated during three successive seasons (spring, summer and autumn 2002). For the three investigated stations, bottom water oxygen concentration, oxygen penetration into the sediment and sediment organic carbon contents are all relatively similar. Therefore, we think that the density and the composition of the foraminiferal faunas is mainly controlled by the quantity and quality of organic input resulting from a succession of phytoplankton bloom events, occurring from late February to early September. The earliest blooms are positioned at the shelf break, late spring and early summer blooms occur off Brittany, whereas in late summer and early autumn, only coastal blooms appear, often in the vicinity of river outlets. In spring, the benthic foraminiferal faunas of central (B, C and D) and outer (E) continental shelf stations are characterised by strong dominance in the first area and strong presence in the second area of Nonionella iridea. In fact, station E does not serve as a major depocenter for the remains of phytoplankton blooms. If station E is not considered, the densities of this taxon show a clear gradient from the shelf-break, where the species dominates the assemblages, to the coast, where it attains very low densities. We explain this gradient as a response to the presence, in early spring, of an important phytoplankton bloom, mainly composed of coccolithophorids, over the shelf break. This observation is supported by the maximum particles flux values at stations close to the shelf break (18.5 g m− 2 h− 1) and lower values in a station closer to the coast (6.8 g m− 2 h− 1). In summer, the faunal density is maximum at station A, relatively close to more varied phytoplancton blooms that occur off Brittany until early June. We suggest that the dominant species, Nonion fabum, Cassidulina carinata and Bolivina ex. gr. dilatata respond to phytodetritus input from these blooms. In autumn, the rich faunas of inner shelf station G are dominated by N. fabum, B. ex. gr. dilatata, Hyalinea balthica and Nonionella turgida. These taxa seem to be correlated with the presence of coastal blooms phenomena, in front of river outlets. They may be favoured by an organic input with a significant contribution of terrestrial, rather low quality organic matter.  相似文献   

13.
Temporal variations in water mass properties and the composition of phytoplankton pigments in the central part of Sagami Bay were investigated by monthly observations from June 2002 to May 2004. Eleven pigments were quantified using high-performance liquid chromatography (HPLC) from 100%, 20%, and 5% light depths relative to the surface; the class-specific composition of phytoplankton community was then obtained by CHEMTAX analysis. The study area was influenced by the Kuroshio water for most of the observation period. The mean contribution of diatoms in all samples was relatively low (29%), while that of flagellates, mainly chlorophytes or cryptophytes, was quite high (60%). The phytoplankton composition at the three depths was uniform throughout the observation period, indicating that the vertical structure of the phytoplankton community did not develop significantly over time. A distinct temporal pattern was observed: flagellates dominated during the summer of 2002 and the winters of 2002–2003 and 2003–2004, while diatoms dominated during the summer of 2003. This pattern was associated with water mass changes. The community in the summer of 2003 was influenced by coastal water. While no distinct spring bloom of phytoplankton was observed, a weak increase in chlorophyll a was observed during the spring of 2004. Ocean color satellite data showed that fluctuations in chlorophyll a concentrations at time scales much shorter than a month occurred during the spring of 2003 and that the elevations in chlorophyll a levels were not continuous. The fluctuations were probably associated with rapid flushing by the Kuroshio water, which has low chlorophyll a content.  相似文献   

14.
Whereas diatoms (class Bacillariophyceae) often dominate phytoplankton taxa in the Amazon estuary and shelf, their contribution to phytoplankton dynamics and impacts on regional biogeochemistry are poorly understood further offshore in the western tropical Atlantic Ocean (WTAO). Thus, relative contribution of diatoms to phytoplankton biomass and primary production rates and associated environmental conditions were quantified during three month-long cruises in January–February 2001, July–August 2001, and April–May 2003. The upper water column was sampled at 6 light depths (100%, 50%, 25%, 10%, 1% and 0.1% of surface irradiance) at 64 stations between 3° and 14°N latitude and 41° and 58°W longitude. Each station was categorized as ‘oceanic’ or ‘plumewater’, based on principal component analysis of eight physical, chemical and biological variables. All stations were within the North Brazil Current, and plumewater stations were characterized by shallower mixed layers with lower surface salinities and higher dissolved silicon (dSi) concentrations than oceanic stations. The major finding was a much greater role of diatoms in phytoplankton biomass and productivity at plumewater stations relative to oceanic stations. Mean depth-integrated bSi concentrations at the plumewater and oceanic stations were 14.2 and 3.7 mmol m−2, respectively. Mean depth-integrated SiP rates at the plumewater and oceanic stations were 0.17 and 0.02 mmol m−2 h−1, respectively. Based on ratios of SiP and PP rates, and typical Si:C ratios, diatoms contributed on average 29% of primary productivity at plumewater stations and only 3% of primary productivity at oceanic stations. In contrast, phytoplankton biomass (as chlorophyll a concentrations) and primary production (PP) rates (as 14C uptake rates) integrated over the euphotic zone were not significantly different at plumewater and oceanic stations. Chlorophyll a concentrations ranged from 8.5 to 42.4 mg m−2 and 4.0 to 38.0 mg m−2 and PP rates ranged from 2.2 to 11.2 mmol m−2 h−2 and 1.8 to 10.8 mmol m−2 h−2 at plumewater and oceanic stations, respectively. A conservative estimate of annual integrated SiP in offshore waters of Amazon plume between April and August is 0.59 Tmol Si, based on mean SiP rates in plumewaters and satellite-derived estimates of the area of the Amazon plume. In conclusion, river plumewaters dramatically alter the silicon dynamics of the WTAO, forming extensive diatom-dominated phytoplankton blooms that may contribute significantly to the global Si budget as well as contributing to energy and matter flow off of the continental shelf.  相似文献   

15.
Concentrations and sinking rates of particulate biogenic silica (BSi), chlorophyll a (chl a) and phaeopigments (phae) (< 3 μm, 3–10 μm, > 10 μm and total), as well as the abundances of the major phytoplankton species, were studied during September 1991 in the Eastern Laptev Sea and the lower Lena River (Siberian Arctic). The highest chl a concentrations were found in two major “new” production regimes of the study area: (1) a deep chl a maximum (5.8 mg chl a m−3) (formed by the diatom Chaetoceros socialis) at 30 m depth on the outer shelf of the northern Laptev Sea, and (2) in the Lena River, where the phytoplankton community was dominated by fresh water diatoms (1.5 to 4.5 mg chl a m−3). Elevated chl a concentrations were also found in the river plume phytoplankton community (dominated by brackish water diatoms), NE of the Lena delta. In the Laptev Sea, the low chl a (0.1 to 3 mg chl a m−3) and high phae concentrations (0.5 to 14 mg phae m−3) indicated that the phytoplankton community (dominated by picoplanktic algae and nanoflagellates) was already senescent and affected by grazing losses. Biogenic silica values were highest in the Lena River (4 to 17 μM) as compared to the low values found in the Laptev Sea (0.3 to 4 μM). The large chl a size fraction, phae and BSi in the Lena River samples revealed the highest measured sinking rates (1.4, 2.3, and 1.5 m d−1, respectively). The formation of a strong halocline, decreasing turbulence, and possible nutrient deficiency resulted in death, disintegration and rapid sedimentation of fresh water diatoms. This was accompanied by a decrease in the BSi concentration and growth of the picoplanktic size fraction (< 3 μm) in the estuarine mixing zone (Gulf of Buorkhaya). Only a minor part of BSi was bound to intact diatom cells (< 3%) in the surface layer, most of which being apparently associated with detrital particles. In the Lena River, approximately 12% of the total silica was bound to BSi fraction, yet elsewhere in the Laptev Sea and in the estuarine mixing zone the BSi:total silica ratio was ≤ 5%. Thus, the results reflected the successional stage of a late summer phytoplankton community, characterized by dominance of small autotrophs and patchy distribution of senescent diatoms no longer able to affect the relative high levels of dissolved silica supplied by the Lena River.  相似文献   

16.
The spatial and temporal dynamics of dissolved inorganic nitrogen, dissolved phosphate, dissolved silica and chlorophyll a were measured seasonally at eight stations in the Ria de Aveiro. Between December 2000 and September 2001, the seasonal succession of phytoplankton assemblages, inferred after the spatial and seasonal variation of silica and of chlorophyll a concentrations, showed that diatoms (μmol Si L−1) dominated from late autumn until early spring, while chlorophytes (μg Chl a L−1) bloomed during late spring and summer. The Si:N:P ratios and Si concentrations indicated no seasonal depletion in dissolved silica, as in other temperate systems, possibly because of abnormal precipitation and flood events prolonging the supply of dissolved Si to the system. The Si:N:P ratios suggested P limitation at the system level. Despite the relative proportions of available nutrients, the mean phosphorus concentration (5.3 μmol L−1) was above the reported half-saturation constants for P uptake by phytoplankton. Thus, in Ria de Aveiro, the seasonal succession of phytoplankton assemblages may also be dependent on the grazing capacity of the pelagic community through top-down regulation.  相似文献   

17.
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.  相似文献   

18.
To test the hypothesis that phytoplankton assemblages dominated by different taxa have distinct biogeochemical characteristics and cycles, the temporal and spatial variations in phytoplankton biomass and composition were studied within the Ross Sea polynya, where diatoms and the haptophyte Phaeocystis antarctica are thought to have spatially distinct distributions. Two cruises were completed, with the first conducted in spring, 1994, and the second in late spring–early summer, 1995/1996. Ice concentrations decreased substantially from spring to summer. Mixed layer depths for the region decreased markedly in early spring and were relatively invariant thereafter; the strength of the stratification varied both in time and space. Mixed layers were greater in spring in assemblages dominated by diatoms (as determined by HPLC pigment concentrations) than those dominated by Phaeocystis antarctica, whereas in summer no difference was observed. Nutrient concentrations were initially high and near winter values, but decreased throughout November and December. Nitrate : phosphate removal ratios varied widely, with ratios exceeding 20 in spring but decreasing below 14 in summer. N : P removal ratios at stations dominated by diatoms were less than the Redfield ratio in both spring and summer, and at those stations dominated by P. antarctica the N : P removal ratio was ca. 19 in both seasons. Chlorophyll and particulate matter concentrations increased as nutrients decreased. Spatial and temporal variations of phytoplankton pigments occurred, with 19′-hexanoylfucoxanthin, a pigment of P. antarctica, exceeding 3.9 μg l−1 during spring in the south-central polynya, and fucoxanthin, an accessory pigment of diatoms, found in concentrations >1 μg l−1 in the western Ross Sea. The distributions were not mutually exclusive, and concentrations of both pigments were greatest in spring. The early growth of P. antarctica appears to be related to earlier stratification and disappearance of ice from the south-central Ross Sea. Ratios of FUCO/CHL were relatively invariant, but substantial changes in the HEX/CHL and POC/CHL ratios were observed through time. A one-dimensional nitrogen budget for the spring–early summer period suggests that much of the surface production was partitioned into particles, most (53%) of which remained in the upper 200 m. The rest was partitioned into dissolved organic matter (14%), remineralized as ammonium (19%), or sank from the surface layer as particles (13%). The region may serve as a useful analog to other polar systems, and an understanding of the processes controlling assemblage composition, production, and biomass accumulation may provide insights into biogeochemical cycles of other Antarctic environments.  相似文献   

19.
The Bay of Concepcion (36°40′S; 73°02′W) is a semi-enclosed and shallow embayment in which biogeochemical processes are seasonally coupled to coastal upwelling during the austral spring and summer. The nutrient cycle in the bay is complex due to the combined effects of a pronounced O2 minimum layer and high nutrient concentrations both originating from subsurface equatorial water during coastal upwelling and a rapid rate of sediment nutrient recycling. The sediments are characterized by a high content of organic matter mainly due to the extremely high rates of phytoplankton production and deposition. During the upwelling period, a black flocculent layer frequently covers the sediment–water interface in the inner part of the bay where an extensive mat of Beggiatoa spp. develops. Three approaches are used to analyse the extent to which the benthic system recycles or retains nutrients at two stations, located at the centre (station C, St. C) and mouth (station B, St. B) of the bay for a 1-year period (March 1996–1997): (1) estimation of C and N remineralization rates based on SO42− reduction measurements, (2) calculation of C and N turnover rates using a diagenetic model applied to total organic carbon and total nitrogen vertical distributions and, (3) construction of C and N budgets from direct measurements of sedimentation (from a sediment trap) and estimates of the C and N burial rates. Depth-integrated SO42− reduction rates varied between 3.4 (winter) and 25.5 (summer) mmol m−2 d−1. Estimated C and N oxidation rates ranged between 7.9 and 87.8 mol C m−2 yr−1 and between 0.9 and 6.9 mol N m−2 yr−1, respectively. Each approach yielded minor differences in the C and N remineralization rates (and also minor differences between both studied stations), except when the kinetic model was applied to C and N distribution without including the presence of the flocculent layer. The rates of carbon oxidation and sulphate reduction were considerably higher than in other coastal sediments with similar depositional regime. The C and N burial rates were 2.23 and 0.21 (St. C) and 1.30 and 0.09 (St. B) mol m−2 yr−1, respectively. The C/N ratio of the buried fraction was ca. 10.6 at St. C and 14.4 at St. B. Because the observed differences in burial rates could not be ascribed to distinctive depositional (both stations have similar sediment accumulation rates) and oceanographic (similar O2 concentration and hydrography) conditions, differences may be due to in part spatial heterogeneity in the supply of organic matter. The degree of preservation of organic matter as plankton detritus and nitrogen accumulating bacterial biomass associated with Beggiatoa spp. at St. C may also be involved.  相似文献   

20.
Two microcosm experiments were carried out to simulate the effect of sporadic oil spills derived from tanker accidents on oceanic and coastal marine phytoplankton assemblages. Treatments were designed to reproduce the spill from the Prestige, which took place in Galician coastal waters (NW Iberia) in November 2002. Two different concentrations of the water soluble fraction of oil were used: low (8.6 ± 0.7 μg l−1 of chrysene equivalents) and high (23 ± 5 μg l−1 of chrysene equivalents l−1). Photosynthetic activity and chlorophyll a concentration decreased in both assemblages after 24–72 h of exposure to the two oil concentrations, even though the effect was more severe on the oceanic assemblage. These variables progressively recovered up to values close or higher than those in the controls, but the short-term negative effect of oil, which was generally stronger at the high concentration, also induced changes in the structure of the plankton community. While the biomass of nanoflagellates increased in both assemblages, oceanic picophytoplankton was drastically reduced by the addition of oil. Effects on diatoms were also observed, particularly in the coastal assemblage. The response of coastal diatoms to oil addition showed a clear dependence on size. Small diatoms (<20 μm) were apparently stimulated by oil, whereas diatoms >20 μm were only negatively affected by the high oil concentration. These differences, which could be partially due to indirect trophic interactions, might also be related to different sensitivity of species to PAHs. These results, in agreement with previous observations, additionally show that the negative effect of the water soluble fraction of oil on oceanic phytoplankton was stronger than on coastal phytoplankton.  相似文献   

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