Variations In Phytoplankton Community Structure Research Articles

Seasonal variations in phytoplankton community structure in the Sanggou, Ailian, and Lidao Bays

The seasonal variations in phytoplankton community structure were investigated for the Sanggou Bay (SGB) and the adjacent Ailian Bay (ALB) and Lidao Bay (LDB) in Shandong Peninsula, eastern China. The species composition and cell abundance of phytoplankton in the bay waters in spring (April 2011), summer (August 2011), autumn (October 2011), and winter (January 2012) were examined using the Utermohl method. A total of 80 taxa of phytoplankton that belong to 39 genera of 3 phyla were identified. These included 64 species of 30 genera in the Phylum Bacillariophyta, 13 species of 8 genera in the Phylum Dinophyta, and 3 species of 1 genus in the Phylum Chrysophyta. During the four seasons, the number of phytoplankton species (43) was the highest in spring, followed by summer and autumn (40), and the lowest number of phytoplankton species (35) was found in winter. Diatoms, especially Paralia sulcata (Ehrenberg) Cleve and Coscinodiscus oculus-iridis Ehrenberg, were predominant in the phytoplankton community throughout the study period, whereas the dominance of dinoflagellate appeared in summer only. The maximum cell abundance of phytoplankton was detected in summer (average 8.08 × 103 cells L−1) whereas their minimum abundance was found in autumn (average 2.60 × 103 cells L−1). The phytoplankton abundance was generally higher in the outer bay than in the inner bay in spring and autumn. In summer, the phytoplankton cells were mainly concentrated in the south of inner SGB, with peak abundance observed along the western coast. In winter, the distribution of phytoplankton cells showed 3 patches, with peak abundance along the western coast as well. On seasonal average, the Shannon-Wiener diversity indices of phytoplankton community ranged from 1.17 to 1.78 (autumn > summer > spring > winter), and the Pielou’s evenness indices of phytoplankton ranged from 0.45 to 0.65 (autumn > spring > summer > winter). According to the results of canonical correspondence analysis, phosphate level was the major factor that limited the occurrence of P. sulcata and C. oculus-iridis, whereas optimal temperature and low salinity were responsible for Prorocentrum blooms in summer. The detailed description of seasonal variations in phytoplankton community structure in the three bays provide reference data for future studies on marine ecosystems and mariculture in adjacent areas.

Seasonal variability of phytoplankton community structure in relation to different nitrogen-phosphorus ratios in the southern coastal waters of Zhejiang, China

With the rapid development of economy and increase of population in the drainage areas, the nutrient loading has increased dramatically in the Changjiang estuary and adjacent coastal waters. To properly assess the impact of nutrient enrichment on phytoplankton community, seasonal microcosm experiments were conducted during August 2010–July 2011 in the coastal waters of Zhejiang Province. The results of the present study indicated that the chl a concentration, cell abundance, diversity indices, species composition and community succession of the phytoplankton varied significantly with different N/P ratios and seasons. Higher growth was observed in the 64:1 (spring), 32:1 (summer), 16:1 (autumn) and 128:1, 256:1 (winter) treatments, respectively. The values of Shannon-Wiener index (H′) and Pielou evenness index (J) were lower in the 8:1 and 16:1 treatments in autumn test, while H′ value was higher in the 128:1 and 8:1 treatments in winter test. A definite community succession order from diatoms to dinoflagellates was observed in the autumn and winter tests, while the diatoms dominated the community throughout the culture in the spring and summer tests.

Spatio-temporal variations in phytoplankton community structure in small water bodies within Lake Victoria basin, Kenya

Spatio-temporal variations of phytoplankton, expressed in terms of species composition and diversity collected at various sampling sites in small water bodies (SWBs) within Lake Victoria basin, Kenya, were investigated monthly from November 2010 to June 2011, in relation to selected physical and chemical water quality parameters. Temperature, D.O, TN and TP revealed a significant difference between the dams (p<0.005) unlike pH and BOD 5 . These SWBs were built during the pre-independence era and stocked with various species of fish. The dams provide water for both domestic and agricultural use. A total of 1392 phytoplankton species belonging to four families and 20 genera were identified in Kesses dam whereas in Kerita dam, a total of 376 phytoplankton species belonging to four families and 10 genera were identified. In Siaya dams, Yenga dam had three families of phytoplankton; Chlorophyceae, Euglenophyceae and Cyanophyceae with Mauna also recording four families: Chlorophyceae, Bacillariophyceae, Desmidiaceae and Cyanophyceae. All the SWBs generally registered low species diversity with majority of them recording a value of < 2. Seasonal variations in phytoplankton species composition and diversity were significant (P < 0.05) with low species composition and diversity occurring during the dry season, and being maximum following the end of the rainy season from November 2010 to March 2011, suggesting the possible influence of various environmental factors on the SWBs. Overall, water quality seemed to have had effect on the species diversity, dominance and richness of phytoplankton community structure. Key words: Community structure, small water bodies (SWBs), phytoplankton, species composition and diversity.

浙江紧水滩水库浮游植物群落结构季节变化特征

于2010年1、3、5、7、9、11月6次对紧水滩水库采样调查,并对浮游植物种类鉴定与数量统计,分析了浮游植物的优势种、多样性和群落结构季节变化特征。其结果为:共鉴定浮游植物284种,隶属7门105属。绿藻门最多,共51属139种,其次是硅藻门19属67种,蓝藻门22属52种,金藻门4属9种,甲藻门5属8种,裸藻门2属5种,隐藻门2属4种。浮游植物细胞丰度在1.04×10⁵-3.70×10⁶ 个/L之间,平均丰度9.63×10⁵ 个/L。多样性指数<em>H'</em>值为1.76-4.64,平均值3.09,丰富度指数<em>D</em>为0.48-2.80,平均值1.62,均匀度指数<em>J</em>为0.51-1.26,平均值0.91。根据TSI(Σ)并结合浮游植物群落结构对水质评价,紧水滩水库水质属于中-富营养状态。;Jinshuitan Reservoir is an important hydroelectric project in Oujiang River which is the second largest man-made river in Zhejiang Province. Its eco-environment and water management have a great significance on sustainable development of this area. With the fast economical development of the area, the water quality has been declined as an amount of pollutants discharged. Most of previous research works on Jinshuitan Reservoir watershed mainly focused on its meteorology, climatology and aquaculture, lacking of phytoplankton community structure, seasonal variation and water quality assessment. To explore the characteristics of the phytoplankton community structure and the response of water quality, which can provide database for its water utilization and protection, a survey was carried out every alternative month from January to November of 2010. Based on the analysis of phytoplankton samples from six sites in Jinshuitan Reservoir , the characteristics of community structure and abundance distribution were studied. Cell enumeration and species identification were performed by light microscope (Olympus BX-51). The dominant population, species diversity and community structure of phytoplankton were determined as well. A total of 284 species in 105 genera and 7 divisions were identified, of which the most abundant group was Chlorophyta, with 139 species in 51 genera that account for 48.94% of total species, followed by Bacillariophyta (67 species in 19 genera), Cyanophyta (52 species in 22 genera), Chrysophyta (9 species in 4 genera), Dinophyta (8 species in 5 genera), Euglenophyta (5 species in 2 genera), and Crytophyta (4 species in 2 genera). The abundance of phytoplankton cells varied from 1.04×10⁵to 3.70×10⁶ cells/L (averaging 9.63×10⁵ cells/L). The indices of Shannon-Wiener, Margalef's and Pielou Evenness were 1.76-4.64 (averaging 3.09), 0.48-2.80 (averaging 1.62) and 0.51-1.26 (averaging 0.91), respectively. The dominant species of each season were identified and their contribution to the total abundance was analyzed. The seasonal variation of phytoplankton community structure was significant. The relationship between phytoplankton and environmental factors were discussed. By using Pearson correlation analysis, nutrients and temperature were the essential factors to determine the seasonal changes of phytoplankton species biodiversity and cell abundance. The nutrients were the key factor to affect the horizontal distribution and cell abundance of phytoplankton. Cell abundance is postively correlated with total phosphorus(TP) and Chlorophyll-a(<em>r</em>=0.292 and 0.143, respectively, <em>P</em>< 0.01). It is negatively correlated with temperature, total nitrogen(TN) and transparency(<em>r</em>=-0.605, -0.384 and -0.18 respectively, <em>P</em>< 0.01). The number of species exhibited a negative correlation with temperature, TN and TP(<em>r</em>=-0.187, -0.3 and -0.466 respectively, <em>P</em>< 0.01). TSI(Σ), dominant species, Shannon-Weaver diversity index, Margalef index and Pielou evenness index were integrated to assess the water quality. The result indicated that the water quality of Jinshuitan Reservoir was medium-eutrophicated.

Trace metals Cd, Co, Cu, Ni, and Zn in waters of the subantarctic and Polar Frontal Zones south of Tasmania during the ‘SAZ-Sense’ project

Trace metal micronutrients regulate primary production in oceanic surface waters, particularly those characterised as ‘high nutrient, low chlorophyll’, such as the Subantarctic Zone (SAZ). Our goal was to evaluate the distribution and biogeochemistry of Cd, Co, Cu, Ni and Zn in the upper 1000m of this zone to the south of Australia during the SAZ-Sense Project (Jan–Feb 2007). 13 depth profiles were sampled for dissolved, labile metal measurements through subtropical and SAZ waters – west and east of Tasmania – and southward into the Polar Frontal Zone. We determined Cd from detection limit <4pM to 900pM, Co 5–43pM, Cu 0.22–1.79nM, Ni 2.51–7.31nM and Zn <0.22–7.9. Throughout the study area, Cd, Cu and Ni correlated closely with the macronutrient phosphorus, but their depletion (relative to winter levels) in surface waters differed: Cd (64–>96%), Cu (2–35%), Ni (7–20%). The profile south of the Polar Front contrasted markedly with all others, having the lowest depletions in surface waters. Another discordancy was some anomalously high Cu concentrations in surface waters close to Tasmania. Co behaved as a recycled (nutrient) element correlating with P and Chl a in the top 200m of the water column, but deeper it was different with a distinct subsurface peak. The characteristics of this peak suggested lateral supply of dissolved, labile Co from polar surface waters to lower-latitude ocean depths via Antarctic Intermediate Waters. Zn also typified a recycled micronutrient. It was loosely correlated with the macronutrient silicon—more strongly to the south and in the Polar Frontal Zone (PFZ). In the vicinity of the Subtropical Front to the east of Tasmania, dissolved Zn was decoupled from other micro- and macro-nutrients. Its irregular distribution suggested this region's dynamic eddy field and possibly variable supply and a phytoplankton community dominated by non-diatom species as influential factors. Distributions of Cd, Cu, Ni and Co were much more uniform in the vicinity of the Subtropical Front (only dissolved Fe data from SAZ-Sense exemplified similar patchiness in distribution as Zn), distinguished solely by being either side of the Front, in subtropical or subantarctic waters. These new data for Cd, Co, Cu, Ni and Zn improve scant coverage for them in the Australian sector of the Southern Ocean, and provide a basis for linking their distribution to regional primary productivity and variations in phytoplankton community structure.

Variability in phytoplankton community structure in response to the North Atlantic Oscillation and implications for organic carbon flux

The North Atlantic Oscillation (NAO) is a major mode of variability in the North Atlantic, dominating atmospheric and oceanic conditions. Here, we examine the phytoplankton community‐structure response to the NAO using the Continuous Plankton Recorder data set. In the Northeast Atlantic, in the transition region between the gyres, variability in the relative influence of subpolar or subtropical‐like conditions is reflected in the physical environment. During positive NAO periods, the region experiences subpolar‐like conditions, with strong wind stress and deep mixed layers. In contrast, during negative NAO periods, the region shifts toward more subtropical‐like conditions. Diatoms dominate the phytoplankton community in positive NAO periods, whereas in negative NAO periods, dinoflagellates outcompete diatoms. The implications for interannual variability in deep ocean carbon flux are examined using data from the Porcupine Abyssal Plain time‐series station. Contrary to expectations, carbon flux to 3000 m is enhanced when diatoms are outcompeted by other phytoplankton functional types. Additionally, highest carbon fluxes were not associated with an increase in biomineral content, which implies that ballasting is not playing a dominant role in controlling the flux of material to the deep ocean in this region. In transition zones between gyre systems, phytoplankton populations can change in response to forcing induced by opposing NAO phases.

Phytoplankton communities and water quality in the Nhue - Day river system

The Nhue-Day river system is a good example of peri-urban river which is strongly influenced by the human activites in the whole basin (basin area: 7949 km2). Recently, the water quality and biodiversity of this river system has been reduced due to receiving the wastewater from point and non-point sources in its basin. In this paper, the actual status of water environment quality and the variation of phytoplankton community structure have been investigated, in terms of species composition in the Nhue-Day river system. The monthly sampling surveys were organized during the period 2007 - 2008 at 5 sites along the Day river and at the Khe Tang in the Nhue river. In terms of water quality, the physico-chemical variables such as water temperature, pH, dissolved oxygen, turbidity, nitrate-nitrogen, nitrite-nitrogen, ammonia-nitrogen, total phosphorus, silica and Chl a were analyzed. The results showed that the water quality tended to decrease from the upstream to the downstream of river system, notably in the Day river section after receiving the Nhue river or receiving the industrial and domestic wastewater. During this period, the phytoplankton community investigation showed that a total of 170 phytoplankton species belonging Bacilariophyceae, Euglenophyceae, Chlorophyceae, Dinophyceae and Cyanobacteria were observed. Aulacoseira, Cyclotella, Fragilaria, Pediastrum, Staurastrum species were abundant in upstream and whereas Euglena, Phacus, Oscillatoria, Spirulina, Merismopedia, Microcystis, Lyngbya were clearly dominant in the polluted sites (downstream) of the Nhue - Day river system. The cell number of phytoplankton species varied much in both temporal and spatial ways, especially in the rainy season. Our results clearly revealed the close relationship between water quality, in terms of physico-chemical variables and the phytoplankton community structure, in terms of species composition in the Nhue-Day river system. The results of this study opened the posibility of utilization of phytoplankton community structure as bioindicator for assessing the water quality in Vietnam.

Annual dynamics of phytoplankton abundance and community structure (2010) in Lake Caizi, Anhui Province

2010年对菜子湖浮游植物群落结构进行了调查和分析,结果显示:(1)共鉴定浮游植物8门110属285种,不同月份浮游植物的种类组成存在极显著差异,3月份种类数最多,173种,1月份最少,105种.优势度分析显示:蓝藻存在全年高峰;硅藻存在1、5、9、11月的高峰;黄藻存在1、3、5月的高峰;绿藻存在11月的高峰,隐藻存在5月份的高峰;金藻存在1月份的高峰.不同月份浮游植物的细胞密度亦存在极显著差异,7月份最高,为(66.13±8.58)×10⁵ cells/L,1月份最低,为(12.78±0.61)×10⁵ cells/L,夏、秋季较高,冬、春季较低;不同月份浮游植物的生物量差异极显著,9月份最高,为2.80±0.17 mg/L,5月份最低,为0.72±0.03 mg/L.(2)Margalef丰度指数为1.51~3、Shannon-Weaver多样性指数为1.41~3.01、Pielou均匀度指数为0.39~0.66,各指数表现为冬、春季节大于夏、秋季节,3月份最高,7月份最低.(3)聚类分析的结果显示,月份不同影响因素不同,菜子湖水域浮游植物按群落结构特征的分组不同.(4)2007年相比,2010年浮游植物物种数有明显下降,由340种下降到285种,细胞密度明显上升,由(5.91±0.90)×10⁵ cells/L上升到(33.81±10.10)×10⁵ cells/L,群落结构变化较大,贫营养型和固着型藻类都有所减少,富营养型藻类、丝状藻类和浮游性藻类增多.;The annual dynamics of phytoplankton community structure in Lake Caizi has been investigated in 2010. The results indicated that:(1) A total of 285 phytoplankton species from 110 genera of 8 phylum were identified. Phytoplankton species composition in different months showed significant difference. The maximal number of phytoplankton species (173) occurred in March while the minimum (105) in January. The dominant class of phytoplankton changed seasonally. Cyanophyta peaked all the year round; diatoms had an apparent dominance in January, May, September and November; Xanthophyta also played an important role in January, March and May; Chlorophyta dominated in November, Cryptophyta dominated in May while Chrysophyta dominated in January. The cell density and biomass of phytoplankton were higher in summer and autumn than that in winter and spring. The cell density of phytoplankton between different months had significant difference. The maximal density, with the value (66.13 ± 8.58)×10⁵ cells/L occurred in July while the minimal value of (12.78 ± 0.61)×10⁵ cells/L appeared in January. Whereas, the maximal biomass of phytoplankton(2.80 ± 0.17 mg/L) occurred in September and the minimal(0.72 ± 0.03 mg/L) appeared in May, and the biomass in different months was also significant different. (2) Temporal variations of three indices (including Margalef index, Shannon-Wiener index and Pielou evenness index) were obvious. All the indices were higher in winter and spring than those in summer and autumn. The maximal value occurred in March while the minimal one appeared in July. (3) The phytoplankton community structure was influenced by different factors in different months. As a result, the groups of sampling station changed seasonally according to cluster analysis. (4) The obvious variation of phytoplankton community structure was present in 2010 compared with that in 2007. The number of species decreased from 340 in 2007 to 285 in 2010, however, the cell density increased obviously from (5.91 ± 0.90)×10⁵ cells/L in 2007 to (33.81 ± 10.10)×10⁵ cells/L in 2010. At the same time, both the numbers of oligotrophic algae and perphytic algae decreased to the same extent and eutrophic, filamentous and planktonic algae increased.

Remote sensing of phytoplankton community composition along the northeast coast of the United States

Satellite imagery has proven to be a powerful tool for measuring chlorophyll a in surface waters. While this provides an estimate of total phytoplankton biomass, it does not distinguish between phytoplankton groups, many of which have functional differences and therefore affect biogeochemical cycles differently. Phytoplankton pigment analysis has been used to quantify a wide range of photosynthetic and accessory pigments, and chemotaxonomic analysis (e.g. CHEMTAX) has been used to successfully quantify functional taxonomic groups in nature based on pigment distributions. Here, we combine CHEMTAX analysis with satellite-derived distributions of specific phytoplankton pigments to describe the distributions of particular components of the phytoplankton community in the northeast coast of the United States from space. The spatial and seasonal variations in phytoplankton community structure elucidated through satellite remote sensing methods generally agreed with observations of abundance estimates of cell counts. Diatoms were generally the most abundant phytoplankton in this region, especially during Winter–Spring and in the inner shelf, but phytoplankton populations shifted to increasing abundance of other taxa during Summer, especially offshore. While still preliminary, satellite-derived taxa-specific information with proper regional controls holds promise for providing information on phytoplankton abundance to a taxonomic group level which would greatly improve our understanding of the impacts of human activity and climate change on ecosystems.

Modeled biogeochemical responses to mesoscale eddies in the South China Sea

[1] Mesoscale eddies are observed each year in the South China Sea (SCS); however, their contributions to the biogeochemical cycles have never been systematically quantified. Here, we use a coupled three-dimensional physical-biogeochemical model to evaluate the eddy impact. We first track the modeled mesoscale eddies in the SCS and then analyze the biogeochemical responses to these eddies individually. Compared with the SCS basin mean, modeled depth-integrated (0–125 m) chlorophyll, zooplankton, new production, and silicate uptake are significantly enhanced in the cyclonic eddies and reduced in the anticyclonic eddies. Following the movements of the eddy center, temporal variations of phytoplankton community structure suggest that diatoms respond to cyclonic eddies strongly first and the responses last longer; then picoplankton grow after the diatoms. In the cyclonic eddies, modeled new production is 1.87 ± 0.37 mmol N m−2 d−1, which is 28% higher than the SCS basin-averaged value, while in the anticyclonic eddies, modeled new production is about 32% lower than the SCS basin mean. As a consequence, modeled detrital nitrogen export for cyclonic eddies is 41% higher than the SCS basin mean, and that for anticyclonic eddies is 31% lower than the SCS basin mean. These values experience strong interannual variations with anomalously low magnitudes found during El Nino conditions for both of the eddies and the SCS basin mean. Our results indicate that cyclonic eddies in the SCS are important sources of nutrients to the euphotic zone and therefore play a significant role in regulating biological productivity and the carbon cycle.

Temporal and spatial variations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the East China Sea and the Yellow Sea

Temporal and spatial distributions of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) were determined in the East China Sea and the Yellow Sea during June–July, 2006 and January–February, 2007. The concentrations of DMS and total DMSP in surface water in the study area were 5.64 (1.79–12.24) and 28.25 (13.98–44.93) nmol L −1 in summer, and were 1.79 (1.02–3.51) and 11.01 (6.90–17.98) nmol L −1 in winter, respectively. The distributions of DMS and DMSP in the study area were obviously influenced by the Yangtze River effluent and the Kuroshio water. Even under highly variable hydrographic conditions, a significant relationship was observed between DMS and chlorophyll a concentrations in summer as well as in winter, suggesting that phytoplankton biomass might play an important role in controlling DMS distribution in the study area. The summer ratios of DMS/chlorophyll a and DMSP/chlorophyll a were approximately twofold higher than winter values, corresponding with the temporal variation in phytoplankton community structure between summer and winter. The sea-to-air fluxes of DMS were estimated to be 5.32 and 11.92 μmol m −2 d −1 using the equations of Liss and Merlivat (1986) and Wanninkhof (1992), respectively.

Phytoplankton responses to changes in macrophyte density in a tropical artificial pond in Zaria, Nigeria

The response of phytoplankton population dynamics to changes in densities of Nymphaea lotus L. and Polygonum limbatum Meisn. was studied in an artificial pond in Zaria, Nigeria, from June to November 2007. Antagonistic effects of these macrophytes on Netrium sp., Staurastrum sp., Ulothrix sp., Marssionella sp. and Closterium sp. were collinear and clearly separated from other environmental effects. Dermatophyton sp., Cyanoderma sp. and Pernia sp. were sensitive to high densities of both macrophytes, and were closely positively associated with DO and pH. Qualitative analysis confirmed the presence of glycosides, anthraquinone, saponins, steroids, triterpenes, flavonoids and alkaloids in N. lotus and P. limbatum. Fourier transform infra-red analysis peaks were recorded, confirming the presence of these allelochemical groups in both species. The observed variations in phytoplankton community structure and dynamics were closely linked to the presence of these macrophytes as well as to environmental conditions in the pond.

Seasonal variations of phytoplankton community structure in relation to physico-chemical factors in Lake Baiyangdian, China

In order to study the seasonal variations in the structure and dynamics of phytoplankton community of the Lake Baiyangdian in China, quantitative samples were collected in each season during 2005. A total of 152 taxa were identified by microscope, phytoplankton had an abundance ranging from 131.25×104 ind. L-1 to 745.88×104 ind. L-1. The highest phytoplankton abundance were observed in spring and the lowest in autumn. Cyanophyta, Euglenophyta and Cryptophyta were the most important groups in terms of abundance and biomass. The preponderant and sub-preponderant species changed in accordance with the seasons. Canonical Correspondence Analysis (CCA) was used to investigate the relationship between environmental factors and phytoplankton community. The results indicated that nutrient gradient had a positive correlation with axis 1 in spring, summer and autumn. But in winter, the nutrient gradient had a positive correlation with axis 2. The varieties of environmental factors affected the structure of phytoplankton community in each season. CCA shows a slight, but significant method to understand the spatial distribution of phytoplankton community in lake systems.

Determinants of beta diversity: the relative importance of environmental and spatial processes in structuring phytoplankton communities in an Amazonian floodplain

AIM: Beta diversity is defined as the change in species composition along environmental gradients, and in the present study, we investigated the influence of local (i.e., environmental) and regional (i.e., dispersal) factors in community structure. The aims of this study were to evaluate the beta diversity of phytoplankton communities in the Curuai floodplain and to determine the relative importance of environmental and spatial processes in shaping phytoplankton community structure; METHOD: The phytoplankton communities were sampled in 16 lakes of the Curuai floodplain (Amazon Basin) during high-water periods in 2002 and 2003. We used partial redundancy analysis (pRDA) to evaluate the pure effect of environmental (six variables) and spatial (spatial filter) variability on phytoplankton community composition; RESULTS: There were 156 taxa recorded in the two study years, including 122 algae species in 2002 and 66 algae species in 2003. The beta diversity that we measured (βSIM index) was 0.889 in 2002 and 0.789 in 2003. The partitioning variation demonstrated that the majority of variation in phytoplankton community structure was not significantly explained by pure environmental and pure spatial components. However, environmental variables presented a larger coefficient of determination than the spatial variable; CONCLUSION: Other factors than those we measured in this study, such as local variables (i.e., biotic interactions, hydrology, etc.) and stochastic events, affected the absence of significant results in our data. Therefore, we suggest that additional variables, such as biological interactions and other local factors, should be considered in this type of analysis to increase its explanatory power for understanding the variation of diversity in these communities.

Spatial and seasonal variations in phytoplankton community structure in alkaline–saline Lake Nakuru, Kenya

AbstractSpatial and seasonal variations of phytoplankton, expressed in terms of species composition, cell density, biovolumes and biomass, collected at 10 sampling sites in alkaline–saline Lake Nakuru, Kenya, were investigated monthly from March 2004 to February 2005, in relation to selected physical and chemical water quality parameters. A total of eight species, belonging to five genera and three classes, were recorded. The Cyanophyceae comprised the bulk of the phytoplankton, followed by Euglenophyceae, while Bacillariophyceae were the least abundant. Arthrospira fusiformis was the most dominant species, in terms of density, among the Cyanophyceae, while Euglena spp. and Navicula elkabs dominated the Euglenophyceae and Bacillariophyceae density, respectively. Seasonal variations in phytoplankton species composition, density and biomass were significant (P &lt; 0.05) with minimum cell density and biomass occurring during the dry season, and being maximum following the end of the two rainy seasons from May to November 2004, suggesting the possible influence of various environmental factors on the lake. Sampling sites located close to the inlets of inflowing rivers exhibited significantly (P &lt; 0.05) higher phytoplankton cell density and biomass than those by the inshore sites. Soluble reactive phosphorus, ammonia‐nitrogen, conductivity and total alkalinity accounted for most of the variations in the cell densities of phytoplankton species.

Effect of Thermal Stratification and Mixing on Phytoplankton Community Structure in the Western Channel of the Korea Strait

The profile of a fixed site at station M (<TEX>$34.77^{\circ}N,\;129.13^{\circ}E$</TEX>) in the Korea Strait was studied from March 2006 to February 2007. The aim was to understand the relationship between the annual thermal stratification pattern and seasonal variation in phytoplankton community structure. Physicochemical factors including temperature, salinity and nutrient concentrations, which strongly influence the proliferation and diversity of phytoplankton, were measured. The study period was divided into three due to the characteristic of thermohaline structures; mixed I (March-May 2006), stratified (June-November 2006) and mixed II(December 2006-Feburuary 2007). Diatoms dominated during the mixed I (89%) and II (48%) periods, while nanoplankton group occupied over 83% of total population during the stratified period. The dominant species during the mixed I and II was Chaetoceros socialis (47% and 29%, respectively), while during the stratified period Gyrodinium sp.(4%) was the most dominant. Averaged total chl a concentrations during the mixed I and II periods were 0.61 mg <TEX>$m^{-3}$</TEX> and 0.72 mg <TEX>$m^{-3}$</TEX>, respectively, which were at least two-fold higher than that during the stratified period (0.30 mg <TEX>$m^{-3}$</TEX>). The vertical mixing and convection process of the water column induced nutrient supply from the bottom layer to the euphotic zone. It also led to the dominance of diatoms during the mixed periods, whereas small phytoplankton prevailed over large phytoplankton as stratification blocked the upward movement of nutrients to subsurface during the stratified period. During the mixed I and II periods, microplanktonic chl a dominated concentrations (50% and 48%, respectively), while picoplanktonic chl a occupied over 37% of total chl a during the stratified period.

Spatial and temporal variation in phytoplankton community structure in a southeastern U.S. reservoir determined by HPLC and light microscopy

Spatial and temporal variation in phytoplankton community structure within a large flood-control reservoir (Sardis Reservoir, MS, USA) was investigated in relation to variation in physicochemical properties, location within the reservoir, hydraulic residence time (HRT), nutrient concentrations, temperature, and light conditions over a 14-month period. During periods of short HRT, phytoplankton communities throughout the reservoir were homogeneous in biomass, composition, and production. With a gradual increase in HRT from spring to summer, spatially heterogeneous phytoplankton communities developed along the longitudinal axis of the reservoir. During this period of longer HRT, diatoms and chlorophytes were a larger proportion of total phytoplankton biomass at shallow and more turbid locations near the head of the reservoir, whereas cyanobacteria were a larger proportion of the community at deeper and less turbid locations closer to the outflow. Seasonal succession of the phytoplankton community was represented by high abundance of diatoms in spring, increasing biomass of cyanobacteria through summer, and a secondary bloom of diatoms in fall. Species of Cyclotella, Asterionella, Nitzschia, and Ankistrodesmus were among the first colonizers in the early growing season, closely followed by Aulacoseira, whereas species of Staurastrum and Tetraedron appeared later in the spring. Species of Synedra, Crucigenia, Selenastrum, Scenedesmus, and Merismopedia occurred throughout the sampling period. As the diatoms started to decrease during mid-spring, cryptophytes increased, prior to dominance of species of Pseudanabaena in summer. Reservoir management of HRT, in combination with spatial variation in reservoir morphology and seasonal variation in temperature and riverine nutrient inputs, creates seasonally variable yet distinct spatial patterns in phytoplankton community biomass, composition, and production.

Seasonal and interannual variations of phytoplankton in the Gulf of Venice (Northern Adriatic Sea)

Spatial and temporal variations of phytoplankton community structure were studied in the Gulf of Venice (Northern Adriatic Sea) from February 1999 to December 2001 (INTERREG II, Italia–Slovenia project). Phytoplankton samples were collected, with a monthly frequency, at 11 stations in the basin. Abundance, biomass, and species composition were considered. Synthetic descriptors were used in order to evaluate the main succession pattern and to cluster communities that typify different hydrological conditions. A decreasing gradient of abundance and biomass was generally observed from west to east, passing from the land-influenced waters to offshore. The seasonal pattern and inter-annual variability of the main phytoplankton taxa are described and discussed in relation with the occurrence of a massive mucilage event and of a Po River flood.

Seasonal Variation of Phytoplankton Community Structure in NortheasternCoastal Waters off the Korean Peninsula

Phytoplankton community in the coastal waters off the northeastern Korean Peninsula were characterized from May 2002 to August 2003. Taxonomic composition, abundance and biomass were determined at two water depths at 10 sample sites. A total of 153 phytoplankton species including 121 diatoms, 28 dinoflagellates, 7 green algae and 7 other species were identified. The mean abundance of phytoplankton varied from 15 to 430 cells mL–1 in the surface layer and from 11 to 545 cells mL–1 in the bottom layer, respectively. Phytoplankton was more abundant in coastal stations relative to those in more open ocean. The most dominant species were marine diatoms such as Thalassionema nitzschioides, Licmorphora abbreviata, Chaetoceros affinis and Chaetoceros socialis. In addition, a few limnotic diatoms including Fragilaria capucina v. rumpens, the green alga Scenedesmus dimorphus, some marine dinoflagellates and Cryptomonas sp. appeared as dominant species. Mean concentration of total chlorophyll-a varied from 0.22 to 7.87 μg chl-a L–1 and from 0.45 to 6.79 μg chl-a L–1 in the surface and bottom layers, respectively. The contribution of phytoplankton each size-fractionated varied highly with season. The contribution of microphytoplankton to total biomass of phytoplankton in the surface and bottom layer was high in February and August 2003, and that of nano-phytoplankton was high in May 2002 in both surface and bottom layers.

Changes in DMS production and flux in relation to decadal shifts in ocean circulation

A fundamental question is are the biological processes regulating dimethylsulphide (DMS) production by the marine ecosystem interconnected and responding to atmospheric or ocean signals at decadal timescales? Related to this is a need to quantify how climate change affects these interconnections and understand the expected levels of natural variability on decadal timescales. To explore this we have used indicators of climate variability [the Gulf Stream North Wall (GSNW) and the North Atlantic Oscillation (NAO) indices] as probes to demonstrate that a marine ecosystem model, incorporating DMS production, can extract and amplify a climatic signal, which is spread across a variety of meteorological variables. The GSNW signal is imparted through the wind and cloud forcing, despite the fact there was not significant relationship observed between the GSNW index and the meteorological forcing data. The model simulations appear to reproduce observed decadal variability in phytoplankton community structure in the eastern North Atlantic and imply that DMS(P) biogeochemistry may vary on decadal timescales as a consequence of changes in community structure. The GSNW index is a potential indicator of such changes and there may have been a regime shift in DMSP production in the eastern North Atlantic coincident with that observed for plankton. Sensitivity analysis indicates that the impact of climate variability on DMS biogeochemistry may potentially be damped by the ability of microbial communities to adapt physiologically to the effects of changes in light and nutrients.