Spatial distribution and community structure of phytoplankton,ciliates and zooplankton coupled to environmental factors in the Sousse saltern (Sahel of Tunisia)

The composition and distribution of the main planktonic halophilic microorganisms were studied in 12 ponds of different salinity levels, ranging from 38 to 328, in the saltern of Sousse, Tunisia, in relation to environmental factors. Nutrient concentrations increased with decreasing salinity in the ponds. Phytoplankton, ciliate and zooplankton communities were very diverse and varied spatially in relation to salinity in the ponds and to nutrient availability. Phytoplankton were dominated by diatoms, followed by dinoflagellates, in primary ponds where salinities were <100, and by the Chlorophyceae and Cyanophyceae in ponds with higher salinity. Zooplankton were dominated by copepods in the ponds of lowest salinity and by the brine shrimp Artemia salina in the most saline ponds. Within the planktonic community as a whole, diatoms, Spirotrichaea and copepods (68.9%, 89.9% and 71.05%, respectively) dominated in the lower-salinity ponds whereas Chlorophyceae, heterotrichs and Artemia salina (49.19%, 50.4% and 90%, respectively) dominated in the ponds of higher salinity. Despite the considerable constraint imposed by salinity, other environmental factors such as temperature also play a role in regulating the planktonic communities.     

Biomass and trophic structure of the plankton community in subtropical and temperate waters of the northwestern Pacific Ocean

This study examined the biomass structure of autotrophic and heterotrophic plankton along a trophic gradient in the northwestern Pacific Ocean in an attempt to understand planktonic food web structure. Autotrophic biomass exceeded that of heterotrophic organisms in all sampling regions, but with lesser contribution to total planktonic biomass at stations of higher phytoplankton biomass, including the northern East China Sea, compared to the regions of lower phytoplankton biomass. The proportion of the biomass of heterotrophic bacteria, nanoflagellates (HNF), and dinoflagellates (HDF) relative to that of phytoplankton was all inversely related to phytoplankton biomass, but positive relationships were observed for both ciliates and mesozooplankton. Mesozooplankton biomass inclined greater than phytoplankton along the gradient of phytoplankton biomass, with biomass rise being most closely associated with ciliate and HDF biomass and, to a lesser degree, with large phytoplankton (>3?μm). Both bacteria and picophytoplankton were significantly and positively related to the biomass ratio of mesozooplankton to the sum of HDF and ciliates (i.e., proxy of mesozooplankton predation on protozoans), but no positive relationship was apparent either for HNF or for large phytoplankton. Such relationships may result from predation relief on lower food webs associated with mesozooplankton feeding on protistan plankton.     

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.     

The concept of ecological succession applied to phytoplankton over four consecutive years in five ponds featuring a salinity gradient

The distribution of phytoplankton abundance, biomass and species composition coupled with environmental factors and metazooplankton was studied relatively intensively and over a period of four consecutive years in five ponds featuring a gradient of increasing salinity from near to that of sea water to a nine-fold concentration from 2000 to 2003. The results indicate that the physical characteristics of the water (temperature and salinity) were quite similar over the years. Nutrients, which were concentrated in pond A1, decreased with increases in salt concentration. The composition of the phytoplankton community showed strong seasonality. Diatoms dominated in the first ponds A1, A16 and C2-1, followed by dinoflagellates. Chlorophyceae dominated the phytoplankton community in the hypersaline ponds M2 and TS. Cyanobacteriae were relatively abundant in ponds M2 and TS. The highest phytoplankton density and biomass were found in the ponds with the highest salinity due to the proliferation of Dunaliella salina (Chlorophyta: Volvocales). The inter-annual study of phytoplankton succession in the Sfax solar salterns showed slight differences among the years of study due to the stability of the environmental conditions. Phytoplankton communities were permanently primitive, stage 1 – structured as they failed to build complexity because of salt stress which operates for longer and above any other variables. This reduced frequency of disturbance to the existing course of regulation, allowed the community to “mature” from its “primitive” state, rather than experience frequent structural setbacks.     

Community structure and grazing of the nano-microzooplankton on the continental shelf of the Bay of Biscay

In order to investigate the parameters controlling the heterotrophic protists (nano-microzooplankton) on the continental shelf of the southern Bay of Biscay, plankton communities and their physico-chemical environment were studied 4 times in February, April, June and September–October 2004 at three stations in the euphotic zone in the Bay of Biscay. The abundance and carbon biomass of heterotrophic protists (ciliates, heterotrophic dinoflagellates and nanoflagellates) as well as all the others groups of plankton (picoplankton, nanophytoplankton, diatoms, autotrophic dinoflagellates, metazoan microzooplankton and mesozooplankton), the environmental parameters and the primary and bacteria production were evaluated at each sampling period. Microzooplankton grazing experiments were undertaken at the same time. Ciliates and heterotrophic dinoflagellates accounted for the main major component of nano- and microzooplankton communities in term of biomass. The total carbon biomass of heterotrophic protists was highest in spring and lowest at the end of summer. The development of heterotrophic protists started after a winter microphytoplankton bloom (principally large diatoms), the biomass was lower in June and was low in September (through inappropriate prey). The carbon requirement of microzooplankton ranged from 50 to more than 100% of daily primary, bacterial and nanoflagellate production. The heterotrophic protist community was predominantly constrained by bottom-up control in spring and at the end of summer via food availability and quality.     

Abundance, variability, and potential grazing impact of planktonic ciliates in the open subaratic Pacific Ocean

The abundance and variability of planktonic ciliates in the open subarctic Pacific were determined during four month-long cruises in 1987 and 1988. The ciliate community, numerically dominated by relatively small aloricate choreotrichs, was comparable in abundance to communities in a range of oceanic and neritic environments, including waters with much higher average chlorophyll concentrations. Integrated (0–80m) ciliate biomass was typically 100–200mgC m⁻², although 3- to 4-fold higher levels were observed on two occasions in spring. Ciliate community biomass, in general, was dominated by large (>20μm width) individuals, although in August 1988 the biomass of smaller cells was as great or greater. The estimated grazing impact of the ciliate community averaged 20% of the primary production. On one instance in May 1988, however, a large biomass of ciliates led to an estimated grazing impact equivalent to 55% of phytoplankton production. While ciliates may be major phytoplankton grazers during sporadic ciliate “blooms”, dino- and other heterotrophic flagellates, which make up the bulk of microheterotroph biomass, must normally be of equal or greater importance as herbivores in this ocean region.     

Seasonal variation in the community and size structure of nano- and microzooplankton in Gyeonggi Bay,Yellow Sea

To investigate the seasonal variation and community structure of nano- and microzooplankton in Gyeonggi Bay of the Yellow Sea, the abundance and carbon biomass of nano- and microzooplankton were evaluated at 10-day intervals from January 1997 to December 1999. Four major groups of nano- and microzooplankton communities were classified: heterotrophic ciliates, heterotrophic dinoflagellates (HDF), heterotrophic nanoflagellates (HNF), and copepod nauplii. The total carbon biomass of nano- and microzooplankton ranged from 10.2 to 168.8 μg C L⁻¹ and was highest during or after phytoplankton blooms. Nano- and microzooplankton communities were composed of heterotrophic ciliates (7.4–81.4%; average 41.7% of total biomass), HDF (0.1–70.3%; average 26.1% of total biomass), copepod nauplii (1.6–70.6%; average 20.7% of total biomass), and HNF (0.8–59.5%; average 11.5% of total biomass). The relative contribution of individual components in the nano- and microzooplankton communities appeared to differ by seasons. Ciliates accounted for the most major component of nano- and microzooplankton communities, except during summer and phytoplankton blooming seasons, whereas HDF were more dominant during the phytoplankton blooming seasons. The abundance and biomass of nano- and microzooplankton generally followed the seasonal dynamics of phytoplankton. The size and community distribution of nano- and microzooplankton was positively correlated with size-fractionated phytoplankton. The carbon requirement of microzooplankton ranged from 60 to 83% of daily primary production, and was relatively high when phytoplankton biomass was high. Therefore, our result suggests that the seasonal variation in the community and size composition of nano- and microzooplankton appears to be primarily governed by phytoplankton size and concentration as a food source, and their abundance may greatly affect trophic dynamics by controlling the seasonal abundance of phytoplankton.     

Feeding of Aurelia sp. (Scyphozoa) and links to the microbial food web

The impact of the scyphomedusa Aurelia sp. on planktonic assemblages was experimentally studied in enclosures incubated in situ in the sea lake of Mljet Island (Big Lake, Southern Adriatic), where jellyfish are present throughout the year. In situ feeding experiments using plankton at natural densities indicated a reduction in abundance for small calanoid and cyclopoid copepods, copepodites, nauplii and ciliates in the presence of Aurelia sp. In addition to direct predatory pressure, Aurelia sp. exerted an indirect cascading effect on autotrophic and heterotrophic microbial plankton. Phytoplankton biomass increases of up to 0.5 μg C·l^?1·h^?1 were mainly related to 19′‐hexanoyloxyfucoxanthin‐ and 19′‐butanoyloxyfucoxanthin‐containing phytoplankton. Bacterial production was about twice as high in the presence of Aurelia sp. and biomass was also consistently higher. It appears that the top‐down effect of predation along with material release by Aurelia sp. results in increases in microbial biomass and production.     

Release of dissolved amino acids by flagellates and ciliates grazing on bacteria

Release of amino acids was examined in the laboratory in the form of dissolved primary amine (DPA) by two marine planktonic protozoa (the oligotrichous ciliate, Strombidium sulcatum and the aplastidic flagellate Pseudobodo sp.) grazing on bacteria. DPA release rates were high (19–25 × 10⁻⁶ and 1.8–2.3 × 10⁻⁶ μmol DPA cell⁻¹ h⁻¹ for flagellates and ciliates, respectively) during the exponential phase, when the ingestion rates were maximum. Release rates were lower during the other growth phases. The release of DPA accounted for 10 % (flagellates) and 16 % (ciliates) of the total nitrogen ingested. Our data suggest that the release of DPA by protozoa could play an important role in supporting bacterial and consequently autotrophic pico- and nanoplankton growth, especially in oligotrophic waters, where the release of phytoplanktonic dissolved organic matter is low.     

Coastal Mediterranean plankton stimulation dynamics through a dust storm event: An experimental simulation

An enhancement of aeolian inputs to the ocean due to a future increase in aridity in certain parts of the world is predicted from global change. We conducted an experimental simulation to assess the biological response of NW Mediterranean coastal surface waters to an episodic dust addition. On the assumption that planktonic growth was limited by phosphorus, dust effects were compared to those induced by equivalent enrichments of phosphate. The experiment analyzed the dynamics of several parameters during one week: inorganic nutrients, total and fractioned chlorophyll a, bacterial abundance, phytoplankton species composition, abundance of autotrophic and heterotrophic flagellates, particulate organic carbon and particulate organic nitrogen. The maximum addition of dust (0.5 g dust L⁻¹) initiated an increase in bacterial abundance. After 48 h, bacterial numbers decreased due to a peak in heterotrophic flagellates and a significant growth of autotrophic organisms, mainly nanoflagellates but also diatoms, was observed. Conversely, lower inputs of dust (0.05 g dust L⁻¹) and phosphate enrichments (0.5 μmol PO₄³⁻ L⁻¹) only produced increases in phototrophic nanoflagellates. In our experiment, dust triggered bacterial growth, changed phytoplankton dynamics and affected the ratio of autotrophic to heterotrophic biomass, adding to the variability in the sources that affect system dynamics, energy and carbon budgets and ultimately higher trophic levels of the coastal marine food web.     

基于光合色素化学分类和流式细胞术结合的珠江口自养微微型浮游生物种群时空分布特征

为探究珠江口海域自养微微型浮游生物种群时空分布特征及其与环境之间的关系,于2013年5～11月,运用高液相色谱(HPLC)法和流式细胞术对珠江口海域表层水体中微微型浮游生物进行测定。流式细胞计数结果显示,珠江口海域自养微微型浮游生物由聚球藻(Synechococcus, Syn)和微微型真核生物(Picoeukaryotes,PEUK)组成。聚球藻始终占据总细胞丰度的主导地位。光合色素化学分类法(Chemotaxonomy,CHEMTAX)分析表明,自养微微型浮游生物群落结构具有明显的季节性变化,春季和夏季生物量以聚球藻为主,秋季生物量以青绿藻为主。CHEMTAX分析和流式细胞计数结果的相关性分析表明,在春季和夏季Syn细胞丰度与CHEMTAX生物量(即Syn贡献chla)之间呈现极显著正相关(P<0.01),PEUK细胞丰度与CHEMTAX生物量(即PEUK贡献chla)也存在显著正相关(P<0.05);然而,在秋季则无显著性相关关系(P>0.05)。冗余分析表明,温度和营养盐浓度是影响自养微微型浮游生物群落分布与组成的重要因素。另外,盐度、透明度、悬浮颗粒物对自养...     

Microzooplankton composition,biomass and grazing rates along the WOCE SR3 line between Tasmania and Antarctica

Microzooplankton species composition and grazing rates on phytoplankton were investigated along a transect between ∼46 and 67°S, and between 140 and 145°E. Experiments were conducted in summer between November 2nd and December 14th in 2001. The structure of the microbial food web changed considerably along the transect and was associated with marked differences in the physical and chemical environment encountered in the different water masses and frontal regions. On average microzooplankton grazing experiments indicated that 91%, 102%, and 157%, (see results) of the phytoplankton production would be grazed in the <200, <20 and <2 μm size fractions, respectively, indicating microzooplankton grazing was potentially constraining phytoplankton populations (<200 μm) along most of the transect. Small ciliates in general and especially oligotrich species declined in importance from the relatively warm, Southern Subtropical Front waters (6.8 μg C/L) to the colder waters of the southern branch of the Polar Front (S-PF), (∼0.5 μg C/L) before increasing again near the Antarctic landmass. Large changes in microzooplankton dominance were observed, with heterotrophic nanoflagellates (HNF), ciliates and larger dinoflagellates having significant biomass in different water masses. HNF were the dominant grazers when chlorophyll a was low in areas such as the Inter-Polar Frontal Zone (IPFZ), while in areas of elevated biomass such as the S-PF and Southern Antarctic Circumpolar Current (SACC), a mix of copepod nauplii and large heterotrophic and mixotrophic dinoflagellates tended to dominate the grazing community. In the S-PF and SACC water masses the tight coupling observed between the microzooplankton grazers and phytoplankton populations over most of the rest of the transect was relaxed. In these regions grazing was low on the >20 μm size fraction of chlorophyll a, which dominated the biomass, while smaller diatoms and nanoplankton in the <20 μm size fraction were still heavily grazed. The lack of grazing pressure on large phytoplankton contributes to this region's potential to export carbon with larger cells known to have higher sinking rates.     

Phytoplankton biomass and size fractions in surface waters of the Australian sector of the southern ocean

We collected surface water along the 142nd E meridian from Tasmania to Antarctica in December 1999. We measured temperature, salinity and total chlorophyll a; additionally, we collected suspended particle size fractions and used fluorometric analysis to determine the quantity of chlorophyll a in each of four cell size classes: picoplankton (<3 μm), two nanoplankton fractions (3–10 μm and 10–20 μm) and microplankton (> 20 μm). Changes in temperature and salinity show that we crossed 6 water masses separated by 5 fronts. We found low abundance (<0.2 mg m⁻³) of chlorophyll in all size classes, with the exception of higher values near the continent (0.2 to 0.4 mg m⁻³). Lowest chlorophyll values (<0.1 mg m⁻³) were found in the Polar Frontal Zone (51° to 54°S). Microplankton made up the largest portion of total chlorophyll throughout most of the region. We conclude that biomass of all phytoplankton fractions, especially pico-and nanoplankton, was constrained by limiting factors, most probably iron, throughout the region and that ecosystem dynamics within a zone are not circumpolar but are regionalized within sectors.     

Hydrodynamic control of phytoplankton in low salinity waters of the James River estuary,Virginia, U.S.A.

Autotrophic biomass and productivity as well as nutrient distributions and phytoplankton cell populations in the James River estuary, Virginia, were quantified both spatially and temporally over a 17-month period. Emphasis was placed on the very low salinity region of the estuary in order to gain information on the fate of freshwater phytoplankters. Differing amounts of freshwater plant biomass are advected into the estuary as living material, DOC or POC and the demonstrated variability of this input must play an important role in marine biogeochemical cycling.Late summer and fall maxima in both chlorophyll a and the photosynthetic production of particulate organic carbon in very low salinity regions were inversely correlated with river discharge.During periods of low river discharge greater than 50% of the chlorophyll a biomass measured at 0‰ disappeared within a narrow range of salinity (0–2‰). Cell enumeration data suggest that species introduced from the freshwater end-member tend to comprise the bulk of the biomass removed. Confounding factors, which may contribute to the regulation of both the abundance and species of phytoplankters mid-river, include the flocculation of colloidal material with phytoplankton cells, the presence of the turbidity maximum and the growth of endemic phytoplankton populations.An inverse relationship exists between the phytoplankton abundance in very low salinity waters and the abundance of biomass measured in the lower portion of the river (estuary). Thus, autotrophic production in the fresh and very low salinity areas may indirectly regulate the onset on the spring bloom in the estuary by controlling the amount of nutrients available.     

东海春季真光层分级叶绿素α分布特点的初步研究

于 1994年春季调查中 ,对东海海区真光层中的叶绿素 a含量进行了分级 ( 2 0～ 2 0 0μm,2～ 2 0 μm,<2 μm)测定。结果表明 ,在组成上 ,整个调查海域中 ,微微型和毫微型浮游植物是初级生产者中的主要组成者 ,分别占总叶绿素 a含量的 4 7%和 33% ,网采浮游植物平均占 2 0 %。在分布上 ,岸边及内陆架区 ( 50 m等深线以内 )毫微型浮游植物是初级生产的主要承担者 ,内外陆架交界区网采浮游植物占主要部分 ,外陆架区和大洋区微微型浮游植物占绝对优势。     

Plankton distribution associated with frontal zones in the vicinity of the Pribilof Islands

We studied the effect of four types of fronts, the coastal front, the middle front, the shelf partition front and the shelf break front on the quantitative distribution and the composition of plankton communities in the Pribilof area of the eastern Bering Sea shelf in late spring and summer of 1993 and 1994. The coastal fronts near St. Paul and St. George Islands and the coastal domains encircled by the fronts featured specific taxonomic composition of planktonic algae, high abundance and production of phytoplankton, as well as large numbers of heterotrophic nanoplankton. The coastal fronts also were characterized by high values of total mesozooplankton biomass, high concentrations of Calanus marshallae, as well as relatively high abundances of Parasagitta setosa and Euphausiacea compared to surrounding shelf waters. We hypothesize that wind-induced erosion of a weak thermocline in the inner part of the coastal front as well as transfrontal water exchange in subthermocline layers result in nutrient enrichment of the euphotic layer in the coastal fronts and coastal domains in summer time. This leads to prolonged high primary production and high phytoplankton biomass. In this paper a new type of front—the shelf partition front located 45–55 km to the north-east off St. Paul Island—is described, which is assumed to be formed by the flux of oceanic domain waters onto the shelf. This front features a high abundance of phytoplankton and a high level of primary production compared to the adjacent middle shelf. Near the southwestern periphery of the front a mesozooplankton peak occurred, composed of C. marshallae, with biomass in the subthermocline layer, reaching values typical for the shelf break front and the highest for the area. High abundance of phyto- and zooplankton as well as heterotrophic nanoplankton and elevated primary production were most often observed in the area adjacent to the shelf break front at its oceanic side. The phyto- and mesozooplankton peaks here were formed by oceanic community species. The summer levels of phytoplankton numbers, biomass and primary production in the shelf break frontal area were similar to those reported for the outer and middle shelf during the spring bloom and the coastal domains and coastal fronts in summer. In the environment with a narrow shelf to the south of St. George Island, the mesozooplankton peak was observed at the inner side of the shelf break front as close as 20 km from the island shore and was comprised of a “mixed” community of shelf and oceanic species. The biomass in the peak reached the highest values for the Pribilof area at 2.5 g mean wet weight m⁻³ in the 0–100 m layer. Details of the taxonomic composition and the numbers and production of phytoplankton hint at the similarity of processes that affect the phytoplankton summer community in the coastal domains of the islands, at the coastal fronts, and at the oceanic side of the shelf break front. The middle front was the only one that had no effect on plankton composition or its quantitative characteristics in June and July. Location of a variety of frontal productive areas within 100 km of the Pribilof Islands creates favorable foraging habitat for higher trophic level organisms, including sea birds and marine mammals, populating the islands.     

Changes in biomass structure and trophic status of the plankton communities in a highly dynamic ecosystem (Gulf of Venice,Northern Adriatic Sea)

The changes in the plankton biomass structure in relation to nutrient inputs were studied in the Gulf of Venice (Northern Adriatic Sea), an area characterized by a very marked trophic state variability. The investigation was carried out at two stations, in March, May and July 2005 and 2006, considering the whole water column. The size structure (from picoplankton to mesozooplankton) of both autotrophs and heterotrophs was analysed. Signals of diluted waters and nutrient inputs were more marked in 2005 than in 2006. In 2005, the total plankton biomass was almost double (87 ± 37 μg·C·l^?1) that in 2006 (44 ± 26 μg·C·l^?1). The variations were determined mainly by phytoplankton, with a 70% decrease, and a shift from a community dominated by microphytoplankton (49 ± 12%) in 2005 to one dominated by bacteria (43 ± 11%) in 2006 was observed. The relationship between the heterotrophic (H) and autotrophic (A) biomass indicated a rapid decline of the H/A ratio with increasing phytoplankton biomass. This study, although temporally limited, is consistent with the results reported for other marine environments and it seems to confirm the importance of nutrient inputs in structuring the biomass of plankton community.     

2012年楚科奇海及其邻近海域浮游植物现存量和初级生产力粒级结构研究

通过2012年夏季第五次北极科学考察期间在楚科奇海及其邻近海域现场调查所获得的数据分析研究了海域的粒度分级叶绿素a浓度和初级生产力。结果表明,叶绿素a浓度和初级生产力的高值均出现在楚科奇海陆架区,并且远高于深海区。去程时调查海域水层平均叶绿素a浓度的变化范围为0.32~15.66mg/m3,平均(2.77±3.96)mg/m3,高值区出现在南部邻近白令海峡海域、北部阿拉斯加巴罗近岸和冰缘区;初级生产力的范围为50.11~943.28mg/(m2d),高值出现在冰缘水华区。返程时水层平均叶绿素a浓度的变化范围为0.07~1.52mg/m3,平均(0.41±0.40)mg/m3,高值仍出现在陆架区,但比去程时低了一个数量级;初级生产力的分布范围为12.31~41.35mg/(m2d),高值出现在陆架区。浮游植物粒度分级测定结果表明,在生物量较低的深海区,叶绿素a浓度和初级生产力的粒级结构以微微型浮游生物(Pico级份)占优势(其贡献率分别为46.1%和56.9%),小型(Net级份)和微型(Nano级份)对总叶绿素a浓度的贡献差异极小,分别为26.6%和27.3%,对总初级生产力的贡献分别为23.8%和19.3%;而在生物量较高的水深小于200m的陆架区,Net级份叶绿素a浓度所占百分比最高,Pico级份次之,Nano级份最低,分别为59.8%、27.9%和12.3%,初级生产力的粒级结构中叶绿素a浓度所占百分比由高到低同样是Net、Pico和Nano,所占百分比分别为60.6%,32.2%和7.2%。     

Phytoplankton spring bloom of the Gironde plume waters in the Bay of Biscay: early phosphorus limitation and food-web consequences

During the spring 1995 (2–25 May), a cruise was carried on the RV Poseidon (Germany) on the continental shelf of the south Bay of Biscay. The objective was a comprehensive study of the planktonic food web within the Gironde plume waters. In these waters phosphate was present at very low concentrations (undetectable to < 0.1 μmol.L⁻¹), whereas nitrate, silicate and ammonium concentrations were much higher (several μmol·L⁻¹ for nitrate and silicate and 0.5 to 1.0 μmol·L⁻¹ for ammonium). The size distribution of the phytoplankton biomass (estimated from chlorophyll a measurements by high performance liquid chromatography) and primary production (measured by ¹⁴C in situ method) showed a great proportion of small (40 to 70 % < 3 μm) and active autotrophic cells (growth rates estimated from 0.4 to 0.8 d⁻¹ for the entire euphotic layer). Considering the very high values of NO₃-N:PO₄-P ratios and the high C:P and N:P ratios for the particulate organic matter, it is suggested that an early phosphorus depletion limits the spring bloom phytoplankton and particularly the new production (nitrate uptake coming from the Gironde waters).From these results and other simultaneous observations on the heterotrophic processes (such as grazing of microzooplankton), we can conclude that the planktonic food web would be close to a maintenance system as defined by Platt et al. The possible generalisation of these results for each spring is discussed with respect to the scarcity of previous and reliable phosphate data.     

台湾海峡小型浮游动物的摄食对夏季藻华演替的影响

于2004年8月1~6日对台湾海峡南部近岸的藻华过程进行了定点连续跟踪观测,用稀释法研究了浮游植物的生长率和小型浮游动物对浮游植物的摄食死亡率,同时运用高效液相色谱(HPLC)技术,分析了浮游植物不同光合色素类群的生长率和摄食死亡率.结果表明,观测期间处于藻华的消退期.8月1日时,浮游植物生物量(叶绿素a)和丰度分别为2.04μg/dm3和2.99×105个/dm3,主要优势种为尖刺伪菱形藻(Pseudo-nitzschia pungens)、冰河拟星杆藻(Asterionellopsis glacialis)和中肋骨条藻(Skeletonema costatum),8月6日时,浮游植物生物量和丰度分别减为0.37μg/dm3和1.54×104个/dm3;而蓝藻和甲藻的丰度和比例则呈现出逐渐增加的趋势,所占的比重分别从1日的0.04%和0.85%增加到6日的9.59%和41.97%.小型浮游动物主要由无壳纤毛虫、砂壳纤毛虫、红色中缢虫(Mesodinium rubrum)和异养甲藻等类群组成,总丰度于8月2日达到最大值,为3640个/dm3,之后逐渐减少,6日时,仅为436个/dm3.观测期间,小型浮游动物在群落组成上虽一直以无壳纤毛虫和异养甲藻为主,但在具体的类群结构上却表现出了一定的差异,30μm以下的无壳纤毛虫和异养甲藻总体呈下降的趋势,而红色中缢虫、砂壳纤毛虫和大于50μm的无壳纤毛虫总体呈增加的趋势.观测期间,浮游植物的生长率为0.40~0.91d-1,小型浮游动物的摄食率为0.26~1.34d-1,摄食率和生长率总体呈逐渐下降的趋势.结果还表明,小型浮游动物的摄食率与叶绿素a具有很好的相关性(R2=0.89),对各光合色素类群的现存量和初级生产力均具有较高的摄食压力(分别为37.97%~82.24%和70.71%~281.33%),是藻华消亡的重要原因之一;此外,小型浮游动物对甲藻和蓝藻的避食行为,可能是观测期间由“硅藻”水华向“硅藻-甲藻”水华转变的重要原因之一.