Seasonal phytoplankton responses to N:Si:P enrichment ratio in the Bizerte Lagoon (southwestern Mediterranean)

This study investigated the role of N, P and Si enrichments on phytoplankton in the Bizerte Lagoon (southwestern Mediterranean Sea, Tunisia) during March, June, August, October and December 2004. Polycarbonate bottles were enriched with different nutrients according to four treatments N:Si:P ratios [+NSi/-P (40:40:1), +P/-NSi (20:20:2,5), +NP/-Si (16:0:1) and +Si/-NP (16:32:1)] and incubated in situ during six days. Chl a and carbon biomass of phytoplankton varied significantly during the course of months, with the highest levels recorded in summer (4-4.4 microg Chl a L(-1) or 1126-1721 microg C L(-1)). Dinoflagellates dominated the initial phytoplankton communities, except in August, when diatoms represented a high fraction of microalgae (48%). Enrichment experiments induced significant increases in Chl a and in the final phytoplankton carbon biomasses. In summer (June/August), Si was the main limiting element for phytoplankton. Diatoms strongly responded to +Si/-NP and +NSi/-P enrichments and dominated the final phytoplankton communities (52-61%) in both treatments. Si played the most important role in the growth and development of diatoms. The biomasses and growth rates of dinoflagellates were significantly stimulated by +P/-NSi and +NP/-Si enrichments. After 6 days, dinoflagellates represented more than 70% of the total phytoplankton biomass in samples subjected to these treatments. Moreover, the addition of +P/-NSi increased the biomasses of several dinoflagellates. This suggests that dinoflagellates were mostly controlled by P availability. Unlike diatoms and dinoflagellates, flagellates showed weak responses to nutrient treatments during only some months of the year. The results showed that phytoplankton dynamics in the lagoon were influenced by nutrients in different manners.     

An ecosystem model of the phytoplankton competition in the East China Sea, as based on field experiments

An ecological dynamic model for the simulation of two pelagic phytoplankton groups is developed in this article. Model parameters were adjusted and validated based on the light-limited field culture experiments and the mesocosm experiments in the East China Sea (ECS). The calculation comparisons from the proposed model, along with field experiment observations, show that the model simulate the datasets very well, qualitatively and quantitatively. The parameters’ sensitivity analysis indicates that the competition between the diatoms and dinoflagellates is most sensitive to the photosynthetic process, followed by the exudation process of the phytoplankton, while the autolysis and respiration processes of phytoplankton and the grazing and exudation processes of zooplankton can also influence this competition to some extent. The sensitive parameters include: the photosynthetic optimal specific rate; the optimal irradiance and optimal temperature for phytoplankton growth; and the half-saturation constant for limiting nutrients, etc. Results of the sensitivity analysis also indicate that light, temperature and limiting nutrients are the controlling environmental factors for the competition between the diatoms and dinoflagellates in the ECS. In order to explore the effects of light and nutrients on the phytoplankton competition, simulations were carried out with varying light and nutrient conditions. Model simulations suggest that the diatoms favor higher irradiance, lower DIN/PO₄–P ratios, higher SiO₄–Si/DIN ratios and higher nutrient concentrations, as compared to the dinoflagellates. These results support the speculation that the increase in the DIN/PO₄−P ratio and the decrease in the SiO₄–Si/DIN ratio in the ECS may be responsible for the composition change in the functional Harmful Algal Bloom (HAB) groups from the diatom to the dinoflagellate communities over the last two decades. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: L. Naselli-Flores     

Spatio-temporal variability of the winter phytoplankton distribution across the Catalan and North Balearic fronts (NW Mediterranean)

The qualitative and quantitative composition of the phytoplankton assemblages of the Catalano-Balearic Sea (NW Mediterranean) was examined in relationship with frontal boundaries and short-term meteorological forcing originated by an intense wind storm. Data were obtained during February 1990, before and after strong NW winds, from several transects across the Catalan (continental side) and North Balearic (Balearic Islands side) shelf/slope fronts. High chlorophyll concentrations and diatom dominance inshore of the Catalan Front were typical of the winter-spring phytoplankton maximum. However, offshore of the Catalan Front, diatoms were scarce and the phytoplankton assemblage was dominated by coccolithophorids. In spite of weak density stratification, a deep chlorophyll maximum (DCM) was well developed, before the storm, offshore and appeared to cause a pulse of new production, as suggested by increased oxygen concentrations in the vicinity of the DCM. The results presented here indicate the existence of a persistent association between the distribution of distinct phytoplankton assemblages and hydrographic characteristics of the studied area. Our results also show that, at least during an important part of the winter-spring bloom period of the Catalano-Balearic Sea, high phytoplankton biomass offshore of the Catalan Front is not associated with the diatom dominance typically found in coastal waters and other offshore areas of the NW Mediterranean. This finding has biogeochemical implications in relationship with vertical export fluxes of particulate matter.      

Temporal and spatial dynamics of phytoplankton diversity in the East China Sea near Jeju Island (Korea): A pyrosequencing-based study

The East China Sea (ECS) has long been considered an important monitoring site for oceanic ecosystem changes because many water currents and river discharges constantly influence this area. In this study, the community structure and diversity of phytoplankton in the northern part of the ECS adjacent to Jeju Island were explored using small subunit ribosomal RNA (SSU) pyrosequencing. We analysed samples collected from four stations from the surface and at 30-m and 50-m depths during April and September 2011. We observed spatial and temporal variations in the phytoplankton community. Among phytoplankton, diatoms and dinoflagellates constituted a major portion at all stations (60–90%). However, comparison of the April and September samples showed seasonal variation and shifts in the dispersion of diatom and dinoflagellates among stations. Among stations, diatoms dominated St. 1 and others were dominated by dinoflagellates. Furthermore, phylotypes of potentially toxin-producing genera such as Karlodinium, Heterocapsa, Gymnodinium, Gyrodinium, and Pseudo-nitzschia were dominant in this area.     

Influence of Nutrient Stress on the Relationships between PAM Measurements and Carbon Incorporation in Four Phytoplankton Species

Two methods of measuring primary production, modulated fluorimetry (PAM) and the traditional carbon incorporation method (¹³C), were compared in four phytoplankton species, two diatoms (Pseudo-nitzschia pungens and Asterionellopsis glacialis), and two dinoflagellates (Heterocapsa sp and Karenia mikimotoï), under N (nitrogen), P (phosphorus) and Si (silicon) limited semi-continuous culture. N and Si-limited cultures showed relatively high quantum efficiency of the PSII (F_v/F_m) values, confirming that F_v/F_m is not a good proxy for nutrient stress in balanced systems, whereas P limitation had a drastic effect on many physiological parameters. In all species, the physiological capacity of phytoplankton cells to acclimate to nutrient limitations led to changes in the cellular biochemical composition and the structure of the photosynthetic apparatus. The observed physiological responses were species and nutrient specific. The values of the chlorophyll-specific absorption cross section (a*) increased with nutrient limitation due to package effect, while the carbon/Chl a ratio was higher under N and P limitations. In diatoms, Si limitation did not affect photosynthesis confirming the uncoupling between Si and carbon metabolisms. In all four species and under all treatments, significant relationships were found between photosynthetic activities, ETR^Chl (electron transport rate) and P^Chl (carbon fixation rate) estimated using PAM measurements and ¹³C incorporation, showing that the fluorescence technique can reliably be used to estimate carbon fixation by phytoplankton. The relationship between ETR^Chl and P^Chl can be described by the shape and the slope of the curve (Φ_C.e). Linear relationships were found for dinoflagellates and P. pungens under all treatments. A decrease in Φ_C.e was observed under N and P limitation probably due to structural damage to the photosynthetic apparatus. A. glacialis showed a logarithmic relationship in N and P limited conditions, due to the alternative electron flow which takes place to optimise photosynthetic performances under high light and/or nutrient stress.     

Influence of terrestrial runoff on phytoplankton species richness-biomass relationships: A double stress hypothesis

The relationship between biomass and species richness (SR) was studied along productivity gradients in coastal phytoplankton assemblages under the influence of terrestrial runoff. In particular, the investigation focused on (a) whether phytoplankton blooms affect the shape of the relationship, (b) whether the relationship is taxon specific (between dinoflagellates and diatoms), and (c) the potential mechanisms regulating the shape of the relationship. Analysis was based on phytoplankton and physicochemical data from six coastal areas in the Aegean Sea, Eastern Mediterranean, covering the whole range from oligotrophy to eutrophication. A SR-biomass relationship was not observed for the more oligotrophic areas, whereas hump-shaped curves were found in areas yielding higher maximum biomass values, corresponding to bloom events occurring mainly during winter. Furthermore, the observed hump-shaped relationships were taxon specific since they were more pronounced in the case of diatoms than dinoflagellates. It is hypothesized that SR at the leftmost part of the hump-shape is nutrient regulated, whereas at the rightmost part is limited by extreme abiotic stress occurring during the formation of blooms in coastal ecosystems. The results of this study suggest that a double stress mechanism related to abiotic factors may be responsible for the hump-shaped SR-biomass relationships occurring in coastal phytoplankton communities at various levels of productivity.     

Phytoplankton and nutrients in the Helgoland region

During recent decades, phytoplankton stock on the one hand and inorganic nutrients (P and bound N) on the other have increased considerably in the southern North Sea, as demonstrated at a permanent station (since 1962) near the island Helgoland. This correlation between phytoplankton and inorganic P and N need not have anything to do with causality; exceptional algal blooms have been observed and reported in the literature since in the 19^th century. Furthermore, these increases (four-fold for phytoplankton and two-fold for nutrients) are in the same range as the fluctuations from year to year under different hydrographical conditions. A detailed investigation carried out in 1981 demonstrated the presence of a slowly growing phytoplankton population. Starting with a considerable stock of flagellates in spring, it reached a peak in cell numbers over a long reproduction period which contrasted with the normal duration of a spring bloom of diatoms. These processes were not related to a limited production by P or N. A considerable concentration of these nutrients was permanently available in the form of inorganic compounds. The total amount of nutrients surpassed by far the portion incorporated in the phytoplankton. This is a consequence of the fact that small organisms have a high metabolic rate. Therefore, the relation between stock and production (daily production ≈stock) is completely different from that known e.g. in agriculture. The nutrients exist during the vegetation period mainly in the form of dissolved organic matter that is accessible to plankton. The great dynamics of this system, including a phase shifting during the year between inorganic P, N, Si, and production, indicates the significance of permanent and fast remineralization. Calculations demonstrate that the natural nutrient content of seawater normally satisfies the demands of phytoplankton present in the North Sea area under study. Only in the more productive coastal region (salinity<30 associated with fresh water run-offs of low nutrient content — an unrealistic assumption in the German Bight) might some limitation be observed. For diatoms, silicate may represent a critical component, but a high dynamic force exists in the presence of small Si concentrations. Therefore, a lack of silicon must not represent any limitation; however, knowledge on the silicon system is insufficient up to now. Dedicated to Dr. Dr. h. c. Peter Kornmann on the occasion of his eightieth birthday.     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

大亚湾澳头水域浮游植物群落结构及周年数量动态

对1997年至1998年广东省大亚湾澳头水域的浮游植物群落进行调查和分析。结果发现浮游植物65属198种;硅藻在种类组成和数量上都比甲藻占有优势,存在春季和秋季高峰,主要优势类群依次是角毛藻、骨条藻、拟菱形藻等;甲藻只存在春季高峰,代表种类有裸甲藻、原甲藻等。主要优势种类的生长与调查水域的盐度没有明显关系,但全年水温的季节性变化对优势种类的消长影响显著。Simpson多样性指数、Shannon-Weaver多样性指数、均匀度的年平均值分别是0.611、2.107、0.557,多样性指数没有明显的季节变化规律和水平分布规律。       

Responses of Phyto- and Zooplankton Communities to Prymnesium polylepis (Prymnesiales) Bloom in the Baltic Sea

A large bloom of Prymnesium polylepis occurred in the Baltic Sea during the winter 2007 – spring 2008. Based on numerous reports of strong allelopathic effects on phytoplankton exerted by P. polylepis and its toxicity to grazers, we hypothesized that during this period negative correlations will be observed between P. polylepis and (1) main phytoplankton groups contributing to the spring bloom (i.e., diatoms and dinoflagellates), and (2) zooplankton growth and abundance. To test these hypotheses, we analyzed inter-annual variability in phytoplankton and zooplankton dynamics as well as growth indices (RNA∶DNA ratio) in dominant zooplankton in relation to the Prymnesium abundance and biomass. Contrary to the hypothesized relationships, no measurable negative responses to P. polylepis were observed for either the total phytoplankton stocks or the zooplankton community. The only negative response, possibly associated with P. polylepis occurrence, was significantly lower abundance of dinoflagellates both during and after the bloom in 2008. Moreover, contrary to the expected negative effects, there were significantly higher total phytoplankton abundance as well as significantly higher winter abundance and winter-spring RNA∶DNA ratio in dominant zooplankton species in 2008, indicating that P. polylepis bloom coincided with favourable feeding conditions for zooplankton. Thus, primary consumers, and consequently also zooplanktivores (e.g., larval fish and mysids), may benefit from haptophyte blooms, particularly in winter, when phytoplankton is scarce.     

Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.     

Decadal-scale changes of dinoflagellates and diatoms in the anomalous baltic sea spring bloom

The algal spring bloom in the Baltic Sea represents an anomaly from the winter-spring bloom patterns worldwide in terms of frequent and recurring dominance of dinoflagellates over diatoms. Analysis of approximately 3500 spring bloom samples from the Baltic Sea monitoring programs revealed (i) that within the major basins the proportion of dinoflagellates varied from 0.1 (Kattegat) to >0.8 (central Baltic Proper), and (ii) substantial shifts (e.g. from 0.2 to 0.6 in the Gulf of Finland) in the dinoflagellate proportion over four decades. During a recent decade (1995-2004) the proportion of dinoflagellates increased relative to diatoms mostly in the northernmost basins (Gulf of Bothnia, from 0.1 to 0.4) and in the Gulf of Finland, (0.4 to 0.6) which are typically ice-covered areas. We hypothesize that in coastal areas a specific sequence of seasonal events, involving wintertime mixing and resuspension of benthic cysts, followed by proliferation in stratified thin layers under melting ice, favors successful seeding and accumulation of dense dinoflagellate populations over diatoms. This head-start of dinoflagellates by the onset of the spring bloom is decisive for successful competition with the faster growing diatoms. Massive cyst formation and spreading of cyst beds fuel the expanding and ever larger dinoflagellate blooms in the relatively shallow coastal waters. Shifts in the dominant spring bloom algal groups can have significant effects on major elemental fluxes and functioning of the Baltic Sea ecosystem, but also in the vast shelves and estuaries at high latitudes, where ice-associated cold-water dinoflagellates successfully compete with diatoms.     

The effects of nutrients and their ratios on phytoplankton abundance in Junk Bay,Hong Kong

Eutrophication has been considered to be undoubtedly one of the key factors stimulating phytoplankton growth, since it involves the enrichment of a water mass with both inorganic and organic nutrients supporting plant growth. Nutrient enrichment as a result of anthropogenic activity occurs in estuaries and coastal waters as well as in lakes and freshwater impoundments, and blooms of phytoplankton are one of the effects of such an accelerated process of nutrient enrichment. This paper presents the results of a two-year survey of the nutrients and phytoplankton at 3 stations in Junk Bay, Hong Kong, carried out from 1997 to 1998. The relationships between nitrogen, phosphorus, and their ratio, with phytoplankton abundance have been studied. The results show that the highest nitrogen concentration was in Station 2 which is close to a sewage input, whereas the highest phosphorus concentration was in Station 1 which is close to a landfill area. The mean N:P ratios at the three stations were between 8 and 14. The diatoms were the dominant group during most of the year but it seems that diatoms were more sensitive than dinoflagellates and other algal groups to the increase in nutrients.     

Growth of Blue-Green Algae in the Saidenbach Reservoir and its Relationship to the Silicon Budget

The phytoplankton succession during the summer in the mesotrophic reservoir Saidenbach since 1975 may well be explained by the resource ratio hypothesis. Until 1980, only phosphorus controlled the phytoplankton growth, and diatoms prevailed, because an excesses of silicon existed. From 1981 to 1986, the ratio Si:P often was smaller than 90, a value, critical for the development of the diatom Fragilaria crotonensis. Its reduced growth caused an increased occurrence of blue-greens (mostly Aphanothece clathrata) immediately after the diatom mass development. During these years at first silicon limited phytoplankton growth in summer, later on the growth again was limited by phosphorus. Because of increased Si and P load since 1987 a simultaneous limitation of both nutrients occurs. This leads now to parallel mass developments of diatoms and blue-greens. In order to maintain the positive effect of diatoms (phosphorus transport into the sediment), it is to guarantee a sufficiently high Si:P ratio. If a reduction of P load isn't possible, Si remobilization from the sediment could be increased by artificial changes of the water level.     

THE ELEMENTAL COMPOSITION OF SOME MARINE PHYTOPLANKTON1

We analyzed the cellular content of C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Cd, and Mo in 15 marine eukaryotic phytoplankton species in culture representing the major marine phyla. All the organisms were grown under identical culture conditions, in a medium designed to allow rapid growth while minimizing precipitation of iron hydroxide. The cellular concentrations of all metals, phosphorus, and sulfur were determined by high‐resolution inductively coupled plasma mass spectrometry (HR‐ICPMS) and those of carbon and nitrogen by a carbon hydrogen nitrogen analyzer. Accuracy of the HR‐ICPMS method was validated by comparison with data obtained with ⁵⁵Fe radioactive tracer and by a planktonic reference material. The cellular quotas (normalized to P) of trace metals and major cations in the biomass varied by a factor of about 20 among species (except for Cd, which varied over two orders of magnitude) compared with factors of 5 to 10 for major nutrients. Green algae had generally higher C, N, Fe, Zn, and Cu quotas and lower S, K, Ca, Sr, Mn, Co, and Cd quotas than coccolithophores and diatoms. Co and Cd quotas were also lower in diatoms than in coccolithophores. Although trace element quotas are influenced by a variety of growth conditions, a comparison of our results with published data suggests that the measured compositions reflect chiefly the intrinsic (i.e. genetically encoded) trace element physiology of the individual species. Published field data on the composition of the planktonic biomass fall within the range of laboratory values and are generally close to the approximate extended Redfield formula given by the average stoichiometry of our model species (excluding the hard parts): While clearly this elemental stoichiometry varies between species and, potentially, in response to changes in the chemistry of seawater, it provides a basis for examining how phytoplankton influence the relative distributions of the ensemble of major and trace elements in the ocean.     

Recent phytoplankton productivity of the northern Bering Sea during early summer in 2007

Although the northern Bering Sea is one of the most productive regions in the northern North Pacific Ocean and currently considered a declining productivity region, no recent primary productivity measurements have been collected in this region. Phytoplankton productivity was measured in the northern Bering Sea in 2007 using a dual ¹³C–¹⁵N isotope tracer technique to quantify present rates of primary productivity and to assess changes under recent environmental conditions in this area. We found that large diatoms (mostly Fragilaria sp.) dominated the phytoplankton during the initial part of the cruise, whereas unidentified nano + pico phytoplankton largely dominated at the surface about 2 weeks later (at “revisited stations”). At the 1% light depth, diatoms and Phaeocystis sp. were the dominant species, whereas diatoms and unidentified nano + pico cells were dominant at the revisited sites. Based on nitrate and ammonium uptake rates, the estimated f-ratios (the ratio of nitrate uptake rate/nitrate + ammonium uptake rates of phytoplankton) were high (0.65–0.74), indicating that nitrate was an important nitrogen source supporting primary production in the northern Bering Sea during the cruise in 2007. Compared with previous studies performed several decades ago, we found significantly lower chlorophyll-a concentrations and carbon uptake rates of phytoplankton in the northern Bering Sea in 2007. This is consistent with recent studies that have shown lower rates of production in the Chukchi Sea and declines in benthic biomass and sediment oxygen uptake in the northern Bering Sea.     

黄海和东海营养盐分布及其对浮游植物的限制

根据1997～1999年黄海和东海4个季节的现场调查资料,分析探讨了黄海、东海的营养盐分布特征及其对浮游植物生长的限制状况．结果表明,在长江口以东及其东北部海域终年存在一个范围很大的营养盐高值区．分析表明,这些营养盐主要来自长江冲淡水的扩展及苏北沿岸流的输送．此外,还获得了1998年长江流域特大洪水期间,迄今被观测到的长江冲淡水中营养盐的最大扩展范围,计算并研究了黄海、东海上层水中Si／N／P比值,结果表明,黄东海上层水中Si／N比值较高,Si不会成为黄东海浮游植物生长的限制因子;但在南黄海南部尤其是西南部、东海近岸及长江口以东海域,N／P比值很高(>30),说明与一般海洋环境的情况不同,黄海、东海有很大一部分海域浮游植物的生长受磷限制,而不是受氮限制．     

Phytoplankton distribution in the Caspian Sea during March 2001

Phytoplankton abundance, biomass and species composition of the Caspian Sea were evaluated by using samples collected from the Iranian (southern Caspian Sea) and southern Kazakhstan (eastern Caspian Sea) surface waters in March 2001. A total of 45 taxa were found in the samples (20 diatoms, 17 dinoflagellates and 8 others). Abundance and biomass of diatoms were high at the eastern stations, while dinoflagellates were dominant in terms of both biomass and abundance in the southern region. Average abundance and biomass were 40 000 ± 35 000 cell l⁻¹ and 580 ± 690 μg l⁻¹. The mean biomass value found here for the Middle and southern Caspian Sea in March are difficult to compare with the past due to limited information, but seems higher than previously registered values. Higher chlorophyll values were also apparent from the SeaWIFS images in 2001 compared to those in 1998. This is suggested to be due to decreased grazing of phytoplankton by zooplankton which is voraciously preyed by the recent invasive ctenophore Mnemiopsis leidyi.     

Increased nutrient loads from the Changjiang (Yangtze) River have led to increased Harmful Algal Blooms

Based on observations collected during 15 cruises from 2002 to 2007 and on historical data, annual and seasonal variations in dissolved inorganic nutrients in the Changjiang (Yangtze) River estuary and the adjacent area were investigated. The importance of nutrients and nutrient fluxes to Harmful Algal Blooms (HABs) in the East China Sea (ECS) was also studied. The results showed an increasing trend in dissolved inorganic nitrogen (DIN) over the past fifty years. The changes in the PO₄-P concentration fluctuated from 1959 to 2000, but the level has been almost stable since then. By contrast, the SiO₃-Si concentrations decreased significantly over the past fifty years. Regarding seasonal variations, nutrients usually exhibited high values during autumn and winter, and the lowest values occurred in mid-summer. A strong positive relationship was observed between the DIN and PO₄-P concentrations and the frequency or scale of HABs, indicating that eutrophication played a crucial role in the occurrence of HABs. As for nutrient fluxes, DIN and PO₄-P fluxes have climbed sharply since 1980. In the ECS, the DIN increased sharply, but PO₄-P concentrations only changed slightly. The increased nutrient discharges, especially DIN, could also have caused the strong increase in the frequency and area of HABs. The areas of hypoxic bottom water off the Changjiang River estuary presented an increasing trend over the past fifty years, which may also be attributable to eutrophication. Changes in the nutrient composition were also investigated in the present study. The N/P ratio increased dramatically over the past fifty years, while the Si/N ratio decreased. The percentage of diatoms decreased from 99% to 73%, while dinoflagellates increased from less than 1% to over 25% during the past fifty years. Therefore, long-term changes in nutrients, especially excess DIN and increasing N/P, could be responsible for the shift in the phytoplankton community structure.     

Phytoplankton assemblages in the deep chlorophyll maximum layers off the Mediterranean coast of Israel

Phytoplankton assemblages in the deep chlorophyll maximum andnear-surface layers were compared at seven stations in the inshoreand offshore waters of the Mediterranean coast of Israel. Thestudy included the entire spectrum of taxonomic categories overa wide size range, comprising the nano/pico phytoplankton componentsdown to 1 µm and the larger phytoplankters consistingprimarily of diatoms and dino-flagellates. The coccolithophorids<20 µm and the monads constituted the most abundantcomponents of the phytoplankton at the deep chlorophyll maximum(DCM) and near surface layer. Certain individual species, mainlypennate diatoms and smaller dinoflagellates, seemed to adaptto the DCM to form a characteristic association.