Riverine transport and nutrient inputs affect phytoplankton communities in a coastal embayment

1. Rivers often transport phytoplankton to coastal embayments and introduce nutrients that can enrich coastal plankton communities. We investigated the effects of the Nottawasaga River on the nearshore (i.e. within 500 μm of shore) phytoplankton composition along a 10-km transect of Nottawasaga Bay, Lake Huron in 2015 and 2016. Imaging flow cytometry was used to identify and enumerate algal taxa, which were resolved at sizes larger than small nanoplankton (i.e. >5 μm). Multivariate analysis (perMANOVA and redundancy analysis) and a dilution model were used to examine how nutrients and the transport of algal taxa affected community composition in the bay.
2. Sampling stations with different percentages of river water had significantly different phytoplankton communities. Phytoplankton community composition was also strongly associated with nutrients, including total phosphorus, which also varied with the percentage of river water. The majority of the 51 phytoplankton taxa identified in 2016 had numerical abundances in the bay that could be explained simply by the dilution of incoming river water.
3. Phytoplankton transported from the river had a higher proportion of edible-sized cells (<30 μm), particularly in summer when colonial cyanobacteria were numerically dominant in the bay. Six taxa were more abundant than expected from the dilution of river water and included some cyanobacteria with late summer maxima. Five of the taxa that were transported from the river were less abundant than expected in the bay.
4. Whereas impacts of fertilisation due to the characteristically higher nutrient concentration in the river are to be expected, the strong and highly correlated effects of transport within the narrow coastal band of this study largely concealed any distinct fertilisation effects.
5. Riverine inputs may strongly influence the nearshore assemblage of phytoplankton in oligotrophic embayments in large lakes, creating hotspots for productivity, species turnover, and trophic dynamics.

     

Temporal organization of phytoplankton communities linked to physical forcing

The performance of individual phytoplankton species is strongly governed by the thermal stratification’s impact on vertical mixing within the water column, which alters the position of phytoplankton relative to nutrients and light. The present study documents shifts in phytoplankton structure and vertical positioning that have accompanied intensified long-term stratification in a natural ecosystem. Ordination analysis is used to extract gradients in phytoplankton composition in Lake Tahoe, an extremely nutrient-poor lake, over a 23-year period of records. Community structure in the 1980s was associated most strongly with resource availability (low nitrogen to phosphorus ratios, deeper euphotic zone depth), while intensified stratification dominated the phytoplankton structure since the late 1990s. Within diatoms, small-sized cells increased with reduced mixing, suggesting that suppressed turbulence provides them with a competitive advantage over large-sized cells. Among the morphologically diverse chlorophytes, filamentous and coenobial forms were favored under intensified stratification. The selection for small-sized diatoms is accompanied by a shoaling trend in their vertical position in the water column. In contrast, the motile flagellates displayed a deeper vertical positioning in recent years, indicating that optimal growth conditions shifted likely due to reduced upwelling of nutrients. As the thermal stratification of lakes and oceans is strongly linked to climate variables, the present study confirms that climate warming will alter phytoplankton structure and dynamics largely through effects on nutrient availability and sinking velocities. Intensified stratification should favor the expansion of small-sized species and species with the capability of buoyancy regulation, which may alter primary productivity, nutrient recycling, and higher trophic productivity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

应用光合色素研究广西钦州湾丰水期浮游植物群落结构

通过2010年6月现场航次19个站点的调查,应用反相高效液相色谱(RP - HPLC) 并结合二极管阵列检测器分析技术,分析了丰水期广西钦州湾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,钦州湾浮游植物光合色素含量以叶绿素a最高,其次为岩藻黄素;浮游植物的优势类群为硅藻,其次为蓝藻和青绿藻,它们分别平均占据了浮游植物生物量的70.2%、12.6%和9.4%,而其它藻类除了绿藻茅岭江河口占据较高的比例(40.2%)之外在其它站点所占比例很低。钦州湾浮游植物群落结构形成了茅岭江口、内湾、外湾和湾外近海共四种类型,茅岭江口以绿藻为优势类群,内湾以硅藻、蓝藻和青绿藻为主要优势类群,外湾以硅藻为单一优势类群,湾外相对于外湾硅藻比重略为下降。主要光合色素含量及浮游植物类群生物量的分布特征与盐度、营养盐关系密切,浮游植物群落结构的分布变化主要受径流及其输入导致的营养盐变化的影响,而这种影响导致了内湾和外湾之间浮游植物主要类群的生物量多寡及浮游植物群落结构的差异。     

九龙江西溪漳州段浮游植物组成与环境因子的关系

分别在2010年的丰、平、枯3个时期,从浮游植物种类组成、生物量(叶绿素a含量)及其粒级结构等指标对九龙江漳州段浮游植物组成进行了调查,同时分析了其与环境因子间的关系,评估了该河段的水质质量。结果表明:研究区域微微型浮游植物占总生物量比例与温度存在显著的正相关性(P<0.05),小型浮游植物占总生物量比例与总氮成显著的负相关(P<0.05)。共鉴定出浮游植物7门59种,且在各时期均有差异;丰水期与枯水期相比,蓝藻比例上升(P<0.05),甲藻比例下降(P<0.05);研究水域Shannon-Wiener指数在1.0～3.0,为中度污染。典型对应分析表明,温度和营养盐是影响九龙江西溪漳州段浮游植物群落变化的主要环境因子。     

Interactive effects of light and nutrients on phytoplankton stoichiometry

The stoichiometric composition of autotrophs can vary greatly in response to variation in light and nutrient availability, and can mediate ecological processes such as C sequestration, growth of herbivores, and nutrient cycling. We investigated light and nutrient effects on phytoplankton stoichiometry, employing five experiments on intact phytoplankton assemblages from three lakes varying in productivity and species composition. Each experiment employed two nutrient and eight irradiance levels in a fully factorial design. Light and nutrients interactively affected phytoplankton stoichiometry. Thus, phytoplankton C:N, C:P, and N:P ratios increased with irradiance, and slopes of the stoichiometric ratio versus irradiance relationships were steeper with ambient nutrients than with nutrients added. Our results support the light–nutrient hypothesis, which predicts that phytoplankton C:nutrient ratios are functions of the ratio of available light and nutrients; however, we observed considerable variation among lakes in the expression of this relationship. Phytoplankton species diversity was positively correlated with the slopes of the C:N and C:P versus irradiance relationships, suggesting that diverse assemblages may exhibit greater flexibility in the response of phytoplankton nutrient stoichiometry to light and nutrients. The interactive nature of light and nutrient effects may render it difficult to generate predictive models of stoichiometric responses to these two factors. Our results point to the need for future studies that examine stoichiometric responses across a wide range of phytoplankton communities.     

Nutrient limitation of phytoplankton in a central Arizona reservoir

The influence of added nutrients nitrogen, phosphorus and carbon on the phytoplankton of a small recreational reservoir in central Arizona was investigated during the summer, 1974. Polyethylene bags were used to isolate lake water and the natural populations for the addition of nitrogen, phosphorus and carbon individually and in combination. A large increase in phytoplankton numbers, extractable chlorophyll, pH and dissolved oxygen occurred only in bags to which both nitrogen and phosphorus were added, suggesting that both nitrogen and phosphorus levels were limiting to the primary producers. Little alteration in species composition resulted from the addition of the above nutrients.     

Ecological impact of a fresh-water “reef kill” in Kaneohe Bay,Oahu, Hawaii

Storm floods on the night of December 31, 1987 reduced salinity to 15 in the surface waters of Kaneohe Bay, resulting in massive mortality of coral reef organisms in shallow water. A spectacular phytoplankton bloom occurred in the following weeks. Phytoplankton growth was stimulated by high concentrations of plant nutrients derived partially from dissolved material transported into the bay by flood runoff and partially by decomposition of marine organisms killed by the flood. Within two weeks of the storm, chlorophyll a concentrations reached 40 mg m^-3, one of the highest values ever reported. The extremely rapid growth rate of phytoplankton depleted dissolved plant nutrients, leading to a dramatic decline or crash of the phytoplankton population. Water quality parameters returned to values approaching the long-term average within 2 to 3 months. Corals, echinoderms, crustaceans and other creatures suffered extremely high rates of mortality in shallow water. Virtually all coral was killed to depths of 1–2m in the western and southern portions of the bay. Elimination of coral species intolerant to lowered salinity during these rare flood events leads to dominance by the coral Porites compressa. After a reef kill, this species can eventually regenerate new colonies from undifferentiated tissues within the dead perforate skeleton. Catastrophic flood disturbances in Kaneohe Bay are infrequent, probably occurring once every 20 to 50 years, but play an important role in determination of coral community structure. The last major fresh water reef kill occurred in 1965 when sewage was being discharged into Kaneohe Bay. Coral communities did not recover until after sewage abatement in 1979. Comparison between recovery rate after the two flood events suggests that coral reefs can recover quickly from natural disturbances, but not under polluted conditions.     

三门湾浮游动物的季节变动及微型浮游动物摄食影响

2002年8月、11月、2003年2月和5月,在三门湾进行了4个航次生物、化学和水文等专业综合调查。根据采集的浮游动物样品的分析鉴定及海上现场实验结果,对浮游动物的群落组成、生物量、丰度、多样性指数的分布和季节变动及其浮游动物对浮游植物的摄食影响进行研究。结果表明,三门湾浮游动物有67属,89种,16类浮游幼体,主要可划分为4个生态类群：以近岸低盐类群为主,其优势种为中华哲水蚤Calanus sinicus、真刺唇角水蚤Labidocera etwhaeta、捷氏歪水蚤Tortanus derjugini、太平洋纺锤水蚤Acartiapacifica、中华假磷虾Pseudeuphausia sinica和百陶箭虫Sagitta bedoti等。半咸水河口类群、暖水性外海类群和广布种相对较少。浮游动物生物量和丰度的平面分布趋势除了夏季有所差异外,其它季节基本一致。2月份和5月份,浮游动物生物量和丰度,从湾顶向湾口呈逐渐增加趋势;8月份,湾口区生物量最高,而丰度高值区出现在湾顶部;11月份,生物量和丰度的平面分布相对均匀。浮游动物种类多样性指数有明显的季节变化,其动态变化与浮游动物种数和丰度的变化一致。微型浮游动物对浮游植物存在摄食压力,且有季节变化,摄食率的变化在0．18．0．68d^-1,微型浮游动物的摄食率低于相同季节的浮游植物生长率。微型浮游动物对浮游植物摄食压力的变化范围为16．1％-49．1％d^-1,对初级生产力摄食压力的变化在58．3％-83．6％d^-1。11月份,微型浮游动物对浮游植物和初级生产力的摄食压力均出现最高值。     

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

Biotic interactions may overrule direct climate effects on spring phytoplankton dynamics

To improve our mechanistic understanding and predictive capacities with respect to climate change effects on the spring phytoplankton bloom in temperate marine systems, we used a process‐driven dynamical model to disentangle the impact of potentially relevant factors which are often correlated in the field. The model was based on comprehensive indoor mesocosm experiments run at four temperature and three light regimes. It was driven by time‐series of water temperature and irradiance, considered edible and less edible phytoplankton separately, and accounted for density‐dependent grazing losses. It successfully reproduced the observed dynamics of well edible phytoplankton in the different temperature and light treatments. Four major factors influenced spring phytoplankton dynamics: temperature, light (cloudiness), grazing, and the success of overwintering phyto‐ and zooplankton providing the starting biomasses for spring growth. Our study predicts that increasing cloudiness as anticipated for warmer winters for the Baltic Sea region will retard phytoplankton net growth and reduce peak heights. Light had a strong direct effect in contrast to temperature. However, edible phytoplankton was indirectly strongly temperature‐sensitive via grazing which was already important in early spring at moderately high algal biomasses and counter‐intuitively provoked lower and later algal peaks at higher temperatures. Initial phyto‐ and zooplankton composition and biomass also had a strong effect on spring algal dynamics indicating a memory effect via the broadly under‐sampled overwintering plankton community. Unexpectedly, increased initial phytoplankton biomass did not necessarily lead to earlier or higher spring blooms since the effect was counteracted by subsequently enhanced grazing. Increasing temperature will likely exhibit complex indirect effects via changes in overwintering phytoplankton and grazer biomasses and current grazing pressure. Additionally, effects on the phytoplankton composition due to the species‐specific susceptibility to grazing are expected. Hence, we need to consider not only direct but also indirect effects, e.g. biotic interactions, when addressing climate change impacts.     

珠江口及毗邻海域营养盐对浮游植物生长的影响

基于2006年7月(夏季),10月(秋季)和2007年3月(春季)的现场调查数据,对珠江口及毗邻海域中的营养盐和叶绿素a等环境生态因子的时空分布特性进行了对比分析,研究了氮磷比与叶绿素a含量和种群多样性之间的联系,探讨了该海域营养盐对于浮游植物生长的影响。结果表明:(1)研究海域营养盐表现出较强的季节和空间差异性,总氮(TN)和总磷(TP)浓度均值春季(1.545 mg/L、0.056 mg/L)和夏季(1.570 mg/L、0.058 mg/L)均大于秋季(1.442 mg/L、0.034 mg/L),且春夏季浓度空间差异更明显。(2)调查期间海域营养盐含量超标现象突出,夏季尤为明显。无机氮(DIN)总体均值0.99 mg/L,超四类海水标准限值1倍,活性磷酸盐(PO4-P)总体均值0.021 mg/L,DIN∶PO4-P平均值为130;叶绿素a浓度与营养盐、p H、温度有较显著的相关性。(3)叶绿素a浓度较高的站位,具有较高的DIN∶PO4-P值,但浮游植物多样性指数偏低,优势种明显,主要为中肋骨条藻。氮磷比的改变会影响不同生长特性的浮游植物间的竞争和种群结构的改变;今后海洋污染治理中,在控制氮、磷污染时要注意氮磷比的改变可能造成的浮游生态影响。     

Peculiarities of summer phytoplankton spatial distribution in Onega Bay of the White Sea under local hydrophysical conditions

The species composition and phytoplankton biomass, concentrations of chlorophyll “a” (Chl) and nutrients in the surface water layer, and accompanying hydrophysical conditions were studied in Onega Bay of the White Sea in June 2015. The temperature and salinity of surface water layer and the water column stability varied greatly in the bay. The nutrients' concentrations exceeded the limiting threshold necessary for the phytoplankton development. The phytoplankton abundance was relatively low, averaged as 13.46 ± 9.00 mg C/m³ (total phytoplankton biomass), 0.78 ± 0.43 mg/m³ (concentration of chlorophyll “a”), and 0.18 ± 0.27 mg C/m³ (picophytoplankton biomass). The highest phytoplankton biomass has been registered along the frontal zones. Three phytoplankton communities that differed significantly in their structure have been found.     

钦州湾浮游植物周年生态特征

2008-2009年对钦州湾及附近海域进行4个季节航次的浮游植物调查,共鉴定出浮游植物131种,其中硅藻种数最多,达101种,占浮游植物总种数的77.1%;甲藻次之,23种;其他种类3门7种.浮游植物以广温性种和暖水性种为主.总种类数的季节变化与硅藻种类数均为春季最低,夏、秋、冬依次增加,冬季最高.各季节浮游植物丰度为232.28×10⁴～977.0×10⁴ cell·m^-3,平均为558.57×10⁴ cell·m^-3;各季节浮游植物丰度呈现夏、春、冬和秋依次减少的趋势;各区域浮游植物丰度四季均为由内湾至外湾先升高、到湾外逐渐降低的趋势,但在夏季其高丰度区由外湾南移至湾口附近.浮游植物群落的Shannon多样性指数和均匀度指数平均值分别为3.18和0.63,多样性水平较高.浮游植物丰度与温度、盐度、溶解性无机氮及活性磷酸盐的相关关系因季节而变化.     

横岗水库浮游植物种类组成与时空分布

于2005年5月和11月调查了横岗水库的水质和浮游植物,对浮游植物群落结构进行了分析.两次采样期间,浮游植物群落数量组成与种类结构发生明显变化.5月份浮游植物平均细胞数量高达275.41×10⁶cells·L^-1,平均生物量为31.07mg·L^-1;其中优美平裂藻(Oscillatoria.elegans)和银灰平列藻(Oscillatoria.glauca)为绝对优势种,占总细胞数的90%;平裂藻的大量出现导致浮游植物生物量由球体等效半径在10～20μm的种类占主导,但0～5μm区间的小型藻类仍维持较高的生物量.11月份,浮游植物平均细胞数量降低到12.69×10⁶cells·L^-1,平均生物量为20.78mg·L^-1.蓝藻门有16种,优美平裂藻、银灰平列藻的优势度明显下降;绿藻种类数量增加到48种,在新出现的25个种类中,鼓藻科(Desmidiaceae)、盘星藻属(Pediastrum)和栅藻属(Scenedesmus)有较高的细胞数;优势种类没有变化;大细胞种类的生物量明显增加,特别是等效半径大于20μm的种类.整体上看,浮游植物数量和生物量从河流入水口到大坝处有递减趋势,这种趋势在11月份更为明显,这与采样时期水库水动力过程的梯度相一致.     

Fish diversity as a determinant of ecosystem properties across multiple trophic levels

Biodiversity has been established as a potential determinant of function in many ecosystems; however, previous research has mostly focused on primary producers and effects at a single trophic level. A broader perspective that considers multiple components of food webs is necessary to understand natural systems. In particular, consumer diversity needs to be more thoroughly examined as trophic interactions and indirect effects can alter ecosystem properties. We test the potential for consumer diversity (fish richness and composition) to govern food web dynamics at two levels of environmental complexity (mesocosms and experimental ponds) and explore the consequences of removing individual species of fish on lower trophic levels. In mesocosms, both the richness and density of zooplankton were reduced when more fish species were present. No effects from the fish treatments were found on phytoplankton, but phosphorus levels increased with higher fish richness. Removing either generalist or specialist fish species increased the richness and density of zooplankton and the amount of phytoplankton, whereas all fish species had redundant effects on nutrients. In ponds, a dominant fish species (specialist shiner) determined the richness and density of zooplankton. In contrast, phytoplankton and nutrients were reduced by higher fish richness in the fall and spring. Overall, the specialist shiner had unique effects on the pond food web suggesting the key to understanding function is the presence of a dominant species and their biological interactions. Differences between mesocosms and ponds are likely due to increased heterogeneity of resources in the ponds allowing species to specialize on different prey. Our study links the biodiversity ecosystem function paradigm with food web concepts to improve predictions for conservation and management actions in response to changes in biodiversity.     

Why do phytoplankton species composition and “traditional” water quality parameters indicate different ecological status of a large shallow lake?

Long-term data on phytoplankton species composition in large and shallow Lake Võrtsjärv indicated a sharp deterioration of the ecological status at the end of the 1970s. The more traditional water quality indicators, such as the concentrations of nutrients and chlorophyll a, phytoplankton biomass, and Secchi depth, failed to capture this tipping point or even showed an improvement of the status at that time. As the shift coincided with a large increase of the lake’s water level (WL), we hypothesized that direct effect of the changing WL on traditional water quality indicators might have blurred the picture. We removed statistically the direct effect of the WL and the seasonality from the traditional water quality indicators in order to minimize the effects of natural variability. The average of the standardised water quality indicators, used as a proxy for the ecological status, distinguished a period of fast eutrophication in the first half of the 1970s (not captured by the phytoplankton species index), a fast improvement at the end of the 1970s (when the species index showed deterioration) followed by a continuous deterioration trend (when the species index remained rather constant). The causes of this inconsistency are discussed in the light of the alternative stable states theory and the priority of biotic indicators stipulated by the EU Water Framework Directive.     

Nutrient pulsing as a regulator of phytoplankton abundance and community composition in Galveston Bay, Texas

Galveston Bay, Texas, is a large shallow estuary with a watershed that includes 60% of the major industrial facilities of Texas. However, the system exhibits low to moderate (2-20 μg l⁻¹) microalgal biomass with sporadic phytoplankton blooms. Both nitrogen (N) and phosphate (P) limitation of phytoplankton growth have been proposed for the estuary. However, shifts between N and P limitation of algae growth may occur due to annual fluctuations in nutrient concentrations. The primary goal of this work was to determine the primary limiting nutrient for phytoplankton in Galveston Bay. Nutrient addition bioassays were used to assess short-term (1-2 days) phytoplankton responses (both biomass and community composition) to potentially limiting nutrients. The experimental bioassays were conducted over an annual cycle using natural water collected from the center to lower part of the estuary. Total phytoplankton biomass increased in the nitrate (10 μM) additions in 11 of the 13 bioassays, but no significant increases were detected in the phosphate (3 μM)-only additions. Bioassay results suggest that the phytoplankton community was usually not phosphate limited. All major groups increased in biomass following nitrate additions but diatoms increased in biomass at a faster rate than other groups, shifting the community composition toward higher relative abundance of diatoms. The results of this study suggest that pulsed N input events preferentially favor increases in diatom biomass in this estuary. The broader implications of this study are that N pulsing events, primarily due to river discharge, play an important role in structuring the phytoplankton community in the Galveston Bay estuary.     

Phytoplankton communities in the littoral zone of lakes: Observations on structure and dynamics in oligotrophic and eutrophic systems

Summary The structure and seasonal dynamics of phytoplankton communities in the littoral zone were compared between oligotrophic and eutrophic lakes in the southeastern United States. Differences in diversity and species composition between lakes could be ascribed to long-term variation in nutrients corresponding to trophic status. However, significant within-lake variation could not be accounted for by microstratification of nutrients or other abiotic variables. Local biotic factors, perhaps dominated by the spawning activities of centrarchid fishes, resuspend periphyton and generate tychoplankton which becomes a persistent and integral part of the phytoplankton community in eutrophic systems. The patchy distribution of these biotic factors and resultant tychoplankton may lead to the observed variation. Grazing by herbivorous zooplankton was considered to be the major factor affecting the relative abundance of phytoplankton in the littoral zone, completely overriding the effects of nutrient concentration and biotic interactions between phytoplankton species during spring and summer.The relative importance of tychoplankton and grazing as regulatory factors operates independently of the trophic status or geographical location of a lake, making comparisons of different studies difficult and perhaps meaningless if traditional analyses based only on nutrients and interactions between species of phytoplankton are used. Limnetic as well as littoral components must be considered in future studies of phytoplankton communities in the littoral zone.     

热带调水水库春季浮游植物群落的结构与动态

于2005年1-6月,对南亚热带调水水库—大镜山水库的水文、营养盐(N、P)和浮游植物进行了调查研究。共检测到浮游植物76种;浮游植物总的细胞数量在4.925×106-63.65×106cellsL-1之间,浮游植物的总生物量在1.632-20.420mgL-1之间。假鱼腥藻(Pseudanabaena sp.)是主要优势藻,但是早春(1-3月)假鱼腥藻和肘状针杆藻(Synedraulna)共同成为优势藻。浮游植物细胞的丰度晚春(4-6月)比早春的要高,但是生物量却比早春的低。在水文过程与其它水库有一定差别的典型调水水库,水温是引起浮游植物种类组成变化的主要原因。浮游植物组成与动态是受入库营养盐和水动力学的变化所影响的。     

Physical factors and dissolved reactive silica affect phytoplankton community structure and dynamics in a lowland eutrophic river (Po river,Italy)

We tested the hypothesis that species composition and persistence of phytoplankton communities in nutrient rich lowland rivers depends mainly on physical factors. The study aimed to analyse the effects of water discharge, temperature and chemistry on phytoplankton dynamic and species composition in the lowland reach of the eutrophic Po river (Italy). Both taxonomical and morpho-functional methods were used. True planktonic and tychoplanktic (i.e. detached taxa of benthic origin that remain in suspension) species were found, among which only a few taxa and functional groups prevailed. Diatoms were the most abundant, with a clear dominance of species either sensitive to the onset of water stratification or well adapted to turbid waters. Phytoplankton abundance, biomass and chlorophyll-a followed similar trends, attaining the highest values in summer, at low discharge rates. Correlation and multivariate analysis revealed that the development of a stable phytoplankton community was mainly controlled by water discharge rates. Namely, changes in water flow rates induced major variations in the community structure. The seasonal succession of phytoplankton assemblages was also related to water temperature and dissolved reactive silica availability to some extent overlapping flow effects.