Remote sensing of phytoplankton groups in case 1 waters from global SeaWiFS imagery

Ocean color sensors enable a quasi-permanent monitoring of the chlorophyll a concentration (Chl a) in surface waters. This ubiquitous photosynthetic pigment cannot, however, be used to distinguish between phytoplankton species. Distinguishing phytoplankton groups from space is nevertheless necessary to better study some biochemical processes such as carbon fixation at the global scale, and is thus one of the major challenges of ocean color research. In situ data have shown that the water-leaving radiances (nLw), measured by ocean color sensors at different wavelengths in the visible spectrum, vary significantly for a given Chl a. This natural variability is due partly to differences in optical properties of phytoplankton species. Here, we derive relationships between nLw and phytoplankton species by using a large set of quantitative inventories of phytoplankton pigments collected during nine cruises from Le Havre (France) to Nouméa (New Caledonia) in the framework of the GeP&CO program. Coincident SeaWiFS nLw data between 412 and 555 nm are extracted and normalized to remove the effect of Chl a. These normalized spectra vary significantly with in situ pigment composition, so that four major phytoplankton groups, i.e., haptophytes, Prochlorococcus, Synechococcus-like cyanobacteria and diatoms, can be distinguished. This classification (PHYSAT) is applied to the global SeaWiFS dataset for year 2001, and global maps of phytoplankton groups are presented. Haptophytes and diatoms are found mostly in high latitudes and in eutrophic regions. Diatoms show a strong seasonal cycle with large-scale blooms during spring and summer. These results, obtained with only five channels in the visible spectrum, demonstrate that ocean color measurements can be used to discriminate between dominant phytoplankton groups provided that sufficient data are available to establish the necessary empirical relationships.     

Modeling the long-term variability of phytoplankton functional groups and primary productivity in the South China Sea

Primary productivity (PP) and phytoplankton structure play an important role in regulating oceanic carbon cycle. The unique seasonal circulation and upwelling pattern of the South China Sea (SCS) provide an ideal natural laboratory to study the response of nutrients and phytoplankton dynamics to climate variation. In this study, we used a three-dimensional (3D) physical–biogeochemical coupled model to simulate nutrients, phytoplankton biomass, PP, and functional groups in the SCS from 1958 to 2009. The modeled results showed that the annual mean carbon composition of small phytoplankton, diatoms, and coccolithophores was 33.7, 52.7, and 13.6 %, respectively. Diatoms showed a higher seasonal variability than small phytoplankton and coccolithophores. Diatoms were abundant during winter in most areas of the SCS except for the offshore of southeastern Vietnam, where diatom blooms occurred in both summer and winter. Higher values of small phytoplankton and coccolithophores occurred mostly in summer. Our modeled results indicated that the seasonal variability of PP was driven by the East Asian Monsoon. The northeast winter monsoon results in more nutrients in the offshore area of the northwestern Luzon Island and the Sunda Shelf, while the southwest summer monsoon drives coastal upwelling to bring sufficient nutrients to the offshore area of southeastern Vietnam. The modeled PP was correlated with El Niño/Southern Oscillation (ENSO) at the interannual scale. The positive phase of ENSO (El Niño conditions) corresponded to lower PP and the negative phase of ENSO (La Niña conditions) corresponded to higher PP.     

天鹅洲长江段浮游植物群落季节演替及环境因子研究

通过研究长江天鹅洲白鱀豚国家级自然保护区长江段浮游动植物群落组成及其与环境因子之间的关系,揭示长江干流浮游植物群落的结构特征,以期为该区域生态系统保护与研究工作提供基础资料和科学依据。2014—2015年按季节分四次在干流沿途设置的10个采样点开展了浮游植物调查,并同步开展水温、溶解氧、总磷等水质因子监测,并采用典范对应分析(canonical correspondence analysis,CCA)研究了不同季节浮游植物群落结构变化及其与环境因子的关系。结果表明:浮游植物种类以硅藻、绿藻和蓝藻为主;全年来看生物量组成主要是硅藻和隐藻,不过春秋季时蓝绿藻密度组成占优。CCA分析则进一步显示浮游植物群落组成四季变化明显,其中磷酸盐、溶解氧和氧化还原位等水质因子对春季浮游植物群落结构存在显著的影响,但是夏冬季的浮游植物群落变化则可能是水文情势等条件起到了重要作用。     

Phytoplankton community structure,as derived from pigment signatures,in the Kuroshio Extension and adjacent regions in winter and spring

The relationships between the spatiotemporal variation in phytoplankton community structure and environmental variables were investigated in the Kuroshio Extension (KE) region from winter to spring by analysing biomarker pigments. In winter, when the mixed layer was deep, phytoplankton communities were characterised by low biomass and a relatively high dominance of cryptophytes, followed by chlorophytes and pelagophytes. In spring, phytoplankton biomass generally increased with shoaling of the mixed layer. In April, when nitrate was not exhausted, chlorophytes became the most dominant group throughout the KE region, followed by cryptophytes. In May, in the south of the KE, phytoplankton biomass decreased with the depletion of nitrate and cyanobacteria dominated, whereas at the northern edge of the KE, phytoplankton biomass remained high. A predominance of diatoms occurred sporadically at the northern edge of the first ridge with a shallow mixed layer and an elevated nutricline. In contrast, the contribution of diatoms was low at the northern edge of the second ridge, despite high levels of nitrate and silicic acid, suggesting that factors other than macronutrient depletion limited diatom production. In general, the contribution of diatoms to the total phytoplankton biomass in the KE region was small in both winter (2.9%) and spring (16%). This study showed that the phytoplankton communities in the KE region during the spring bloom were generally composed of non-diatom phytoplankton groups, chlorophytes, cryptophytes, and prasinophytes. It is necessary to identify the roles of non-diatoms in grazing food chains to more accurately evaluate the KE as a nursery area for pelagic fish.     

Grazing impact of microzooplankton on phytoplankton in the Xiamen Bay using pigment-specific dilution technique

Phytoplankton group-specific growth and microzooplankton grazing were determined seasonally using the dilution technique with high-performance liquid chromatography （HPLC） in the Xiamen Bay, a subtropical bay in southeast China, between May 2003 and February 2004. The results showed that growth rates of phytoplankton ranged from 0.71 to 2.2 d^-1 with the highest value occurred in the inner bay in May. Mierozooplankton grazing rates ranged from 0.5 to 3.1 d^-1 with the highest value occurred in the inner bay in August. Microzooplankton grazing impact ranged from 39% to 95% on total phytoplankton Chl a biomass, and 65% to 181% on primary production. The growth and grazing rates of each phytoplankton group varied, the highest growth rate （up to 3.3 d^-1 ） was recorded for diatoms in August, while the maximum grazing rate （ up to 2.1 d ^-1 ） was recorded for chlorophytes in February in the inner bay. Among main phytoplankton groups, grazing pressure of microzooplankton ranged from 10% to 83% on Chl a biomass, and from 14% to 151% on primary production. The highest grazing pressure on biomass was observed for cryptophytes （83%） in August, while the maximum grazing pressure on primary production was observed for eyanobacteria （up to 151% ） in December in the inner bay. Net growth rates of larger phytoplanktons （diatoms and dinoflagellates） were higher than those of smaller groups （ prasinophytes, chlorophytes and cyanobacteria）. Relative preference index showed that microzooplankton grazed preferentially on prasinophytes and avoided to harvest diatoms in cold seasons （December and February）.     

Composition and interannual variability of phytoplankton in a coastal upwelling region (Lisbon Bay,Portugal)

From July 2001 to May 2005, at a fixed station located in Lisbon Bay (Cascais: 38° 41′ N, 09° 24′ W), surface seawater samples were collected on a weekly basis. We aimed to describe at different temporal scales, short-term to interannual, the phytoplankton community in relation to hydrographic conditions.Maxima of the main phytoplankton groups varied according to the seasonality of upwelling/downwelling cycles and nutrient availability and were associated with particular hydrological mesoscale structures highlighted by satellite images. Short succession cycles were identified dependent on coastal upwelling events. Intermittent and weak pulses allowed the coexistence of species from different succession stages and groups, although having consecutive maxima. The interannual differences observed in the phytoplankton community, in Lisbon Bay, varied according to both the duration and strength of the upwelling events and to precipitation and Tagus river flow regimes.Diatoms developed and were dominant, during spring–summer under prevailing upwelling conditions and silicon availability. Short upwelling pulses appeared to be unfavourable for diatoms maintenance. When upwelling weakened and SST increased due to onshore advection of warmer waters, coccolithophores dominated. This assemblage was the second most abundant during the study, in particular during the short transition period from upwelling (summer) to downwelling seasons (autumn) distributing in the largest range of hydrographical conditions between diatoms (maximum turbulence) during early spring and dinoflagellates (maximum stratification) during summer to further dominate during autumn and winter. Nitrites and nitrates seemed to favour greater developments of this group. Dinoflagellates peaked mainly during summer and were the less abundant through the four years due to the decrease of lasting convergence periods. Like coccolithophores, a preference for warmer waters emerged but this group seemed to have a narrow tolerance to turbulence and temperature changes.     

南流江河口区春季浮游植物群落结构组成与分布特征

通过2016年3月底现场航次11个站点的调查,应用反相高效液色谱（RP-HPLC）并结合二极管阵列检测器分析技术,分析了春季广西南流江河口区浮游植物光合色素组成,进而由CHEMTAX软件估算全粒级浮游植物的群落结构。结果表明:春季含量较高的浮游植物特征光合色素含量以叶绿素b最高,其次为岩黄藻素;浮游植物的优势类群为隐藻,其次为绿藻和硅藻,它们分别平均占据了浮游植物生物量的54.95%、23.36%和17.37%,其他藻类所占比例很低。南流江河口区浮游植物群落结构东西部入海分支有较大差异:东部分支营养盐较西部分支低,隐藻所占生物量比例最高,其次为绿藻和硅藻,浮游植物群结构与分布受营养盐因素影响较大;西部分支营养盐含量明显比东部分支高,绿藻和硅藻的所占比例有所提升,隐藻的生物量所占比则有所下降,浮游植物群落结构与分布受非营养盐因素的影响较大。南流江河口区浮游植物生物量和群落结构除了受营养盐影响外,还与浊度、盐度等密切相关,表明南流江浊度增加已明显影响着生态系统结构与功能,需要密切关注和进一步研究。     

Trophic cascades and future harmful algal blooms within ice-free Arctic Seas north of Bering Strait: A simulation analysis

Within larger ice-free regions of the western Arctic Seas, subject to ongoing trophic cascades induced by past overfishing, as well as to possible future eutrophication of the drainage basins of the Yukon and Mackenzie Rivers, prior very toxic harmful algal blooms (HABs) – first associated with ∼100 human deaths near Sitka, Alaska in 1799 – may soon expand. Blooms of calcareous coccolithophores in the Bering Sea during 1997–1998 were non-toxic harbingers of the subsequent increments of other non-siliceous phytoplankton. But, now saxitoxic dinoflagellates, e.g. Alexandrium tamarense, were instead found by us within the adjacent downstream Chukchi Sea during SBI cruises of 2002 and 2003. A previous complex, coupled biophysical model had been validated earlier by ship-board observations from the Chukchi/Beaufort Seas during the summer of 2002. With inclusion of phosphorus as another chemical state variable to modulate additional competition by recently observed nitrogen-fixers, we now explore here the possible consequences of altered composition of dominant phytoplankton functional groups [diatoms, microflagellates, prymnesiophyte Phaeocystis colonies, coccolithophores, diazotrophs, and dinoflagellates] in relation to increases of the toxic A. tamarense, responding to relaxation of grazing pressure by herbivores north of Bering Strait as part of a continuing trophic cascade. Model formulation was guided by validation observations obtained during 2002–2004 from: cruises of the SBI, CHINARE, and CASES programs; moored arrays in Bering Strait; other RUSALCA cruises around Wrangel Island; and SBI helicopter surveys of the shelf-break regions of the Arctic basin. Our year-long model scenarios during 2002–2003 indicate that post bloom silica-limitation of diatoms, after smaller simulated spring grazing losses, led to subsequent competitive advantages in summer for the coccolithophores, dinoflagellates, and diazotrophs. Immediate top-down control is exerted by imposed grazing pressures of the model’s herbivores and bottom-up control is also effected by light-, nitrate-, ammonium-, silicate-, and phosphate-modulated competition among the six functional groups of the simulated phytoplankton community. Similar to the history of the southern North Sea adjacent to the Rhine River, possible farming of northwestern Alaska and Canada, in conjunction with other human activities of ice retreat and overfishing, may lead to future exacerbations of poisonous phytoplankton. These potential killers include both toxic dinoflagellate and diazotroph HABs, deadly to terrestrial and marine mammals, as well as those of prymnesiophytes, some of which have already foamed beaches, while others have killed fishes of European waters.     

Phytoplankton dynamics associated with the monsoon in the Sulu Sea as revealed by pigment signature

Phytoplankton dynamics during the northeast monsoon was investigated in the Sulu Sea from algal pigment analysis. We visited the Sulu Sea in February 2000, a mid period of the northeast monsoon, and in November and December 2002, the beginning of the northeast monsoon. SeaWiFS images showed generally low concentrations of surface chlorophyll a (Chl a) during the southwest monsoon and higher concentrations with several peaks during the northeast monsoon. In the beginning of the northeast monsoon, subsurface chlorophyll maxima (SCM) occurred, where vertical variation in class-specific composition as estimated from pigment signatures was prominent. Prochlorococcus, cyanobacteria, prymnesiophytes and crysophytes were important groups above the SCM, and the contribution of cyanobacteria to Chl a became much lower at and below the SCM. Contributions of chlorophytes and prasinophytes to Chl a generally showed maxima near the SCM. This distribution was accompanied by vertical changes in the concentration of photoprotective pigments relative to photosynthetic accessory pigments. During the mid northeast monsoon, the upward supply of nutrients was probably enhanced at some stations due to vertical mixing, and as a consequence diatoms dominated in the upper 100 m water column of these stations, and other eukaryotic flagellates including prymnesiophytes, chrysophytes and cryptophytes were secondary major components of the community. The elevation of Chl a concentration and changes in phytoplankton community during the northeast monsoon likely influence the variation in biological production at higher trophic levels in the Sulu Sea.     

Seasonal patterns of growth and composition of phytoplankton in the lower Chesapeake Bay and vicinity

A twenty-three month study of lower Chesapeake Bay phytoplankton was made from February 1982 to December 1983. The major seasonal growth periods were dominated by a diatomaceous flora and a pico-nanoplankton complex composed mainly of cyanobacteria, chlorophytes and other cells <5μm in size. There was a pattern of multiple pulses throughout the year. However, the trend for maximum development occurred during the winter-spring and fall periods. The dominant diatoms were Skeletonema costatum, Leptocylindrus danicus, and Asterionella glacialis.Seasonal comparisons were made to earlier phytoplankton composition studies in the Bay. Over the past 60 years there has been a change in phytoplankton assemblages and concentrations in the Bay. Skeletonema costatum has remained a dominant species, but a more abundant and broader base of small sized, chainforming diatoms have become established. In addition, high concentrations of the pico-nanoplankton, chrysophyceans and cryptophyceans were abundant.     

中国东部海域浮游植物类群遥感反演研究

浮游植物类群遥感反演能够为全面认识浮游植物在海洋生态系统中的作用提供重要的数据资料.但由于复杂的水体光学特性,近海浮游植物类群遥感反演存在着巨大挑战.本研究以复杂光学二类水体—中国东部海域为研究区,通过使用3种建模方法,即波段组合法、基于奇异值分解的多元线性回归法、基于奇异值分解的XGBoost回归法,利用遥感反射率数...     

Phytoplankton community structures in shelf and oceanic waters off southeast Brazil (20°–25°S), as determined by pigment signatures

The influences of the hydrological features and environmental conditions in the phytoplankton community found in the Campos Basin area in the Atlantic Ocean (20° to 25°S; 42° to 38°W) were studied using HPLC/CHEMTAX pigment analysis. Samples were collected at 72 stations distributed along the 25–3000 m isobaths at two depths during two seasonal periods (rainy and dry). Seven taxonomic groups of phytoplankton were detected (diatoms, dinoflagellates, prasinophytes, cryptophytes, haptophytes, pelagophytes and cyanobacteria). Redundancy analysis showed that the spatial and temporal patterns observed in the distribution of the phytoplanktonic groups were primarily related to variations in the availability of light and nutrients. Nutrient variations were caused by South Atlantic Central Water seasonal intrusions over the continental shelf region. Cyanobacteria predominated in the rainy season, while diatoms, Haptophyceae and Prasinophyceae, were associated with higher nutrient availability in the dry season. In the inner shelf region, diatoms dominated and were associated with increased conditions of turbulence and nutrient availability. Haptophytes and prasinophytes were predominant on the outer shelf and shelf-break regions associated with high nutrient concentrations and availability of light. Prochlorococcus was related to oceanic waters (in both dry and rainy periods) or to low nutrient/strongly stratified shelf waters (rainy period). In contrast, Synechococcus was widely distributed in both the shelf and oceanic regions. Variation in the quality of light between coastal and oceanic waters was probably responsible for the distributions observed. Through HPLC/CHEMTAX pigment analysis we have developed a detailed picture of the influence of hydrological regime on the dynamics of the phytoplankton community in an under-studied shelf/ocean system in the tropical southern Atlantic Ocean.     

Size scaling patterns of species richness and carbon biomass for marine phytoplankton functional groups

Phytoplankton assemblages were clustered into associations according to functional taxonomic (diatoms, dinoflagellates and coccolithophores) and “ataxonomic” unimodal (nanoplankton, microplankton and macroplankton) size‐based criteria. Scaling relations of species richness‐cell size were performed in terms of histogram and log‐transformed data analyses for both taxonomic and ataxonomic groups. Frequency distribution histograms were fitted to a negative power function, which was strongly unimodal and right skewed and invariant across taxonomic and ataxonomic units. Regression analyses of the log‐transformed data were fitted to negative linear curves, which had common patterns and they were independent of taxonomic or ataxonomic affiliation. Species carbon biomass–cell size spectra produced by log transformation of the relevant data yielded positive slopes for both taxonomic and ataxonomic groups. In contrast, comparisons of the relative cell abundance, cell volume and carbon biomass levels showed large differences among these variables across taxonomic and ataxonomic groups. This work demonstrates that phytoplankton taxonomic and ataxonomic functional group relationships should be considered when developing future models of phytoplankton community structure.     

Phytoplankton-pigment signatures and their relationship to spring–summer stratification in the Gulf of Gabes

We investigated the phytoplankton dynamics (determined by CHEMTAX analysis of HPLC pigment data) and its relationships with nutrients and water column structure, during two oceanographic cruises in May–June and September 2006 in the Gulf of Gabes (south-eastern Mediterranean). The May–June cruise coincided with the beginning of the summer stratification, while a strong stratification occurred in September with a more than 30 m deepening of the thermocline, and a reduction of the euphotic depth. This strong stratification resulted in a shift in nitrogen sources from nitrates to ammonium as well as phosphate depletion (0.2 μM) and a decrease in silicate concentrations (<2 μM). With the exception of chlorophyll a, pigment concentrations were higher in September than in May–June samplings. The pico- and nanophytoplankton were the major contributors to phytoplankton total biomass, accounting for 90% and 87% of total chlorophyll a in May–June and September, respectively. Picoplankton persisted throughout the entire survey, occupying different depth layers. Chlorophytes were present at substantial amounts (average 23% of total chlorophyll a) during May–June; however, they declined in September (average 5%). Diatoms were overall poorly represented in this study (2% of total chlorophyll a), due probably to silicate shortage. Apparently, the nutrient availability, but also the water column stability seemed to be among the major factors determining phytoplankton dynamics. Indeed, cyanobacteria were prominent in surface samples during the period of strong stratification, whereas the relative contribution of chlorophytes decreased, probably due to low phosphate availability.     

Phytoplankton pigment and absorption characteristics along meridional transects in the Atlantic Ocean

Pigment patterns and associated absorption properties of phytoplankton were investigated in the euphotic zone along two meridional transects in the Atlantic Ocean, between the UK and the Falkland Islands, and between South Africa and the UK. Total chlorophyll a (TChla=MVChla+DVChla+chlorophyllide a) concentrations and the biomarker pigments for diatoms (fucoxanthin), nanoflagellates and cyanobacteria (zeaxanthin) appeared to have similar distribution patterns in the spring and in the autumn in the temperate NE Atlantic and the northern oligotrophic gyre. Divinyl chlorophyll a levels (prochlorophytes) were greater in spring at the deep chlorophyll maximum in the oligotrophic gyre, however. Marked seasonal differences were observed in the NW African upwelling region. TChla concentrations were twice as high in the upper mixed layer in the spring, with the community dominated by diatoms and prymnesiophytes (19′-hexanoyloxyfucoxanthin). A layered structure was prevalent in the autumn where cyanobacteria, diatoms and prymnesiophytes were located in the upper water column and diatoms and mixed nanoflagellates at the sub-surface maximum. In the South Atlantic, the Benguela upwelling ecosystem and the Brazil-Falklands Current Confluence Zone (BFCCZ) were the most productive regions with the TChla levels being twice as high in the Benguela. Diatoms dominated the Benguela system, while nanoflagellates were the most ubiquitous group in the BFCCZ. Pigment concentrations were greater along the eastern boundary of the southern oligotrophic gyre and distributed at shallower depths. Deep chlorophyll maxima were a feature of the western boundary oligotrophic waters, and cyanobacteria tended to dominate the upper water column along both transects with a mixed group of nanoflagellates at the chlorophyll maximum.Absorption coefficients were estimated from spectra reconstructed from pigment data. Although absorption was greater in the productive areas, the TChla-specific coefficients were higher in oligotrophic regions. In communities that were dominated by diatoms or nanoflagellates, pigment absorption was generally uniform with depth and attenuating irradiance, with TChla being the major absorbing pigment at 440 nm and photosynthetic carotenoids (PSC) at 490 nm. Absorption by chlorophyll c and photoprotective carotenoids (PPC) was much lower. Populations where cyanobacteria were prevalent were characterized by high PPC absorption, particularly at 490 nm, throughout most of the euphotic zone. The data suggested that the effect of pigments on the variability of phytoplankton absorption was due primarily to the variations in absorption by PPC.     

Phytoplankton chemotaxonomy in the Atlantic sector of the Southern Ocean during late summer 2009

A chemotaxonomic investigation of surface phytoplankton was undertaken on a research cruise to the Atlantic sector of the Southern Ocean during late austral summer 2009. Based on pigment signatures, several distinct regions emerged that were delineated by physical features. CHEMTAX analysis of high performance liquid chromatography (HPLC) pigment data indicated that diatoms generally dominated communities south of the Antarctic Polar Front (APF), particularly in regions of elevated biomass where chlorophyll-a (chl-a) was >1.5 µg l⁻¹ and diatoms comprised >80% of biomass. Pigment signatures representative of haptophytes-8, indicative of Phaeocystis antarctica, were dominant near the ice shelf. Chl-a concentrations were 0.2–0.6 µg l⁻¹ between the APF and the Subtropical Front (STF) and outputs suggested that chlorophytes, haptophytes-8 and haptophyte-6, in the form of coccolithophores, were the major constituents. Very low chl-a levels (<0.2 µg l⁻¹) were observed north of the STF and the prokaryotes Synechococcus spp. and Prochlorococcus spp. were the dominant groups in these oligotrophic waters.     

球石藻及其生态功能

球石藻(Coccolithophore)是一类在全球海洋中广泛分布的海洋微型浮游植物,它们在海洋浮游植物功能群落中是一类极其重要的钙化生物类群,也是海洋中生源无机碳的重要来源,并且在海洋的碳循环过程中起到重要的作用。球石藻由于快速增殖而发生水华的过程中能够释放大量的具有挥发性的二甲基硫(DMS)和丙烯酸(acrylic acid),它们是影响气候变化,特别是引起区域性环境效应(温室效应)的关键性物质。本文针对球石藻的生物学特征及其生态重要性作了简要阐述。     

Environmental drivers of phytoplankton crops and taxonomic composition in northeastern Antarctic Peninsula adjacent sea area

The ecosystem of the sea region adjacent to the Antarctic Peninsula is undergoing remarkable physical and biological changes, in the context of global warming. However, understanding of the dynamics of phytoplankton taxonomic composition in this marginal ice zone remains unclear. In this study, seawater samples collected from 36 stations in the northeastern Antarctic Peninsula were analyzed for nutrients and phytoplankton pigments.Combining with CHEMTAX analysis, remote sensing data, and physico...     

The Effect of Salinity and Temperature Gradients on the Distribution of Littoral Microalgae in Experimental Solar Ponds, Dead Sea Area, Israel

Abstract. Several species of cyanobacteria, diatoms and one euglenid alga populated the littoral zone of hypersaline solar ponds investigated over a period of three years. The composition of the microalgae community changed with salinity and temperature. In the shallow marginal zone the diatoms predominated in winter and spring, at salinities of 30–72 g-1^-1 and at temperatures not exceeding 30o°C, whereas in summer, cyanobacteria were the most abundant. In the deeper zone, at higher salinities and temperatures, cyanobacteria predominated throughout the year. The depth limit of algal growth was 60-80cm, where salinity and temperature exceeded 211 g-1^-1 and 48°C.
The most euryhaline and eurythermal among cyanobacteria were Aphanolhece halophytica F remy and Phormidium hypolimneticum C ampbell , among diatoms Nitzschia sp. aff. N. rostellata H ustedt , Amphora coffeaeformis (A gardh ) K utzing , and Navicula massadaea E hrlich .     

Role of dissolved silicate in the occurrence of a phytoplankton bloom

The spring bloom of phytoplankton was studied in March in Funka Bay, Japan, to test the Tsunogai (1979)'s hypothesis regarding the role of silicate in the bloom. The hypothesis comprises two parts. 1) Diatoms are predominant when all the physical and chemical conditions are adequate for plankton growth. 2) Since the Si:P ratio of the diatom body is usually much larger than that of sea water, flagellates (non-siliceous phytoplankton) replace diatoms after dissolved silicate in the sea water has been almost completely consumed by diatoms. At the end of the bloom in late March phosphate still remained in the water but silicate was exhausted and the main species of phytoplankton changed from diatoms to flagellates. Grazing pressure by zooplankton at this time was not so great. A model using the data on assimilation rates of silicate showed a dramatic change of silicate uptake in late March. Poison in scallops caused byProtogonyaulux sp. (dinoflagellates) rapidly increased from mid-April at all stations along the coast of Funka Bay. All of these findings support Tsunogai's hypothesis.