Assessing and managing nutrient-enhanced eutrophication in estuarine and coastal waters: Interactive effects of human and climatic perturbations

Estuaries are among the most productive, resourceful, and dynamic aquatic ecosystems on Earth. Their productive nature is linked to the fact that they process much of the world's riverine and coastal watershed discharge. These watersheds support more than 75% of the human population and are sites of large increases in nutrient loading associated with urban and agricultural expansion. Increased nutrient loading has led to accelerated primary production, or eutrophication; symptoms include increased algal bloom activity (including harmful taxa), accumulation of organic matter, and excessive oxygen consumption (hypoxia and anoxia). While nutrient-enhanced eutrophication is a “driver” of hypoxia and anoxia, physical–chemical alterations due to climatic events, such as stormwater discharge, flooding, droughts, stagnancy, and elevated temperatures are also involved. The complex interactions of anthropogenic and climatic factors determine the magnitude, duration, and aerial extent of productivity, algal booms, hypoxia, and anoxia. Using the eutrophic Neuse River Estuary (NRE), North Carolina, USA, as a case study, the physical–chemical mechanisms controlling algal bloom and hypoxia dynamics were examined. Because primary production in the NRE and many other estuaries is largely nitrogen (N) limited, emphasis has been placed on reducing N inputs. Both the amounts and chemical forms of N play roles in determining the composition and extent of phytoplankton blooms that supply the bulk of the organic carbon fueling hypoxia. Biomass from bloom organisms that are readily grazed will be readily transferred up the planktonic and benthic food chain, while toxic or inedible blooms frequently promote sedimentary C flux, microbial mineralization, and hence may exacerbate hypoxia potential. From a watershed perspective, nutrient input reductions are the main options for reducing eutrophication. Being able to distinguish the individual and cumulative effects of physical, chemical and biotic controls of phytoplankton productivity and composition is key to understanding, predicting, and ultimately managing eutrophication. Long-term collaborative (University, State, Federal) monitoring, experimental assessments, and modeling of eutrophication dynamics over appropriate spatial and temporal scales is essential for developing realistic, ecologically sound, and cost-effective nutrient management strategies for estuarine and coastal ecosystems impacted by both anthropogenic and climatic perturbations.     

Sediment diatom assemblages and composition of pore-water dissolved organic matter reflect recent eutrophication history of Lake Peipsi (Estonia/Russia)

A paleolimnological approach was used for the assessment of the recent eutrophication history and identification of possible reference conditions in the large, shallow, eutrophic Lake Peipsi. Lake Peipsi is the fourth largest lake by area, and the largest transboundary lake in Europe, being shared between Estonia and Russia. Lake Peipsi has been anthropogenically impacted over a longer time-scale than that covered by instrumental limnological monitoring. The ²¹⁰Pb record and down-core distribution of fly-ash particles in the 40-cm core from the middle part of the lake suggest 130 years of sediment accumulation. Diatom assemblages indicate alkaline mesotrophic conditions and a well-illuminated water column, sediment pore-water fluorescence index values suggest low autochthonous productivity and a stable aquatic ecosystem similar to natural reference conditions during the second half of 19th and early 20th century. Near-synchronous stratigraphic changes including the expansion of the eutrophic planktonic diatom Stephanodiscus parvus, the appearance of new species associated with eutrophic lakes and the decrease in the relative abundance of littoral diatoms, together with changes in the fluorescence properties of sediment pore-water dissolved organic matter, imply increased nutrient availability, enlarged phytoplankton crops, reduced water-column transparency and the onset of human-induced disturbances in the lake since the mid-20th century. The most conspicuous expansion of eutrophic planktonic diatoms and maximum concentration of siliceous microfossils occur simultaneously with changes in the fluorescence indexes of pore-water dissolved organic matter, indicating a pronounced increase in the contribution of autochthonous organic matter to the lake sediment. This implies that nutrient loading and anthropogenic impact was at a maximum during the 1970s and 1980s. Sedimentary diatom flora may reflect a reduction of phosphorus loading since the 1990s. However, the absolute abundance of planktonic diatoms and sediment pore-water fluorescence index values vary greatly implying that the lake ecosystem is still rather unstable.     

Diatoms in surface sediments of the Gotland Basin in the Baltic Sea

A sediment core, 55 cm long, from station F81 in the Gotland Basin of the Baltic Sea was analysed for diatoms and ebridians. Chrysophyte stomatocysts found in the core were also counted but not identified. The aim was to trace environmental changes, e.g. eutrophication and salinity variations. There is evidence that eutrophication has been increasing in the Baltic Sea in recent decades.Brackish-marine plankton diatoms dominate the entire core and reflect the local planktonic taxa rather well. The dominant taxon is the polyhalobous Actinocyclus octonarius. The main biostratigraphical change within the core analysed takes place at a depth of about 22 cm, where the abundance of diatoms, and especially of Chaetoceros spp., Thalassiosira hyberborea var. pelagica and T. baltica start to increase. This may reflect eutrophication which can be estimated to have started c. 200 years ago.     

Small Players, Large Role: Microbial Influence on Biogeochemical Processes in Pelagic Aquatic Ecosystems

Although prokaryotes are small in size, they are a significant biomass component in aquatic planktonic ecosystems and play a major role in biogeochemical processes. A review of the recent literature shows that the relative importance of prokaryotes to material and energy fluxes is maximized in low-productivity (oligotrophic) ecosystems and decreases in high-productivity (eutrophic) ecosystems. We conclude that competition with eukaryotic autotrophs for dissolved nutrients and competition with phagotrophic heterotrophs and physical processes (sinking, photooxidation) for organic carbon (C) play important roles in determining the relative abundance and impact of prokaryotes in aquatic systems. Oligotrophic systems have low nutrient concentrations, with high proportions of dissolved nutrients in organic form, which favors prokaryotic heterotrophs over phytoplankton. Furthermore, a high proportion of the available organic C is dissolved rather than particulate, which favors prokaryotic heterotrophs over phagotrophic heterotrophs. In eutrophic systems, increased relative concentrations and loading of inorganic nutrients and increased relative concentrations of particulate organic C select for phytoplankton and phagotrophic heterotrophs over prokaryotic heterotrophs. Increased particle sinking fluxes and/or decreased excretion of organic carbon (EOC) may also decrease the relative importance of prokaryotic heterotrophs in eutrophic systems. In oligotrophic systems, interactions between autotrophs and heterotrophs are tightly coupled because the dominant heterotrophs are similar in size and growth rates, as well as having similar nutrient composition to the dominant autotrophs, small phytoplankton. In eutrophic systems, increased productivity passes through zooplankton that are larger and have slower growth rates than the autotrophs, leading to a greater potential for decoupled auto- and heterotrophic production and increased export production. Received 18 July 2000; Accepted 13 September 2001.     

沿岸海域富营养化与赤潮发生的关系

综述了赤潮的发生与沿岸海域富营养化的关系。近几十年来,人类活动使得天然水体的富营养化进程大大加速。营养负荷的增加与高生物量水华的增多相联系。控制营养输入后,浮游植物生物量或有害藻类水华事件也相应减少。营养的组成与浮游植物的种类组成及水华的形成有密切联系。有机营养对有害藻类水华的促进作用受到关注。营养输入时机影响浮游植物种间竞争的结果,因而对浮游植物的群落演替具有深远影响。由于浮游植物存在生理差异,因而对营养加富的反应因种而异。营养在调控某些有毒藻类的毒素产量方面也发挥着重要作用。此外,营养输入与藻类水华之间存在复杂的间接联系。当然,营养状况并非浮游植物群落演替的唯一决定因素。研究结果提示,控制营养输入、减缓水域富营养化是减少有害藻类水华发生的有效途径,而深入研究典型有害藻类的营养生理对策则为防治并最终消除有害藻类水华提供了理论基础。     

Environmental changes in Lake Taihu during the past century as recorded in sediment cores

The Lake Taihu drainage basin is an economically developed area with some of the highest population densities in China. The lake has deteriorated due to ecological destruction and eutrophication. Three short sediment cores from eastern, northeastern and southwestern Lake Taihu were collected. Total organic carbon (TOC), total nitrogen (TN), pigments, elements and particle size were analyzed for the purpose of understanding past trophic status and pollution levels. Sedimentation rates were based on ¹³⁷Cs or ²¹⁰Pb methods. Results indicated that sediment particle size became coarser since the 1920s, and the lake was contaminated by heavy metals, such as Cu and Zn, since the 1970s. A remarkable increase in eutrophication since the 1980s due to increased loading of untreated effluents from industry, agriculture and urbanization is reflected by total organic carbon, total nitrogen and pigments in the studied cores. However the onset times of eutrophication in different parts of Lake Taihu were not synchronous.     

The first decade of oligotrophication of Lake Constance

In Lake Constance, after several decades of cutrophication, a decrease in phosphorus loading over the last decade has lead to a partial recovery from eutrophication. Here we analyse the shift in the taxonomic composition of phytoplankton during the first decade of oligotrophication in Lake Constance. During the 1980s, spring total P concentrations decreased from ca. 130 to less than 50 ·l^–1. This decrease was reflected by an approximately proportional decrease in summer phytoplankton biomass while spring phytoplankton biomass seemed unresponsive. Major taxonomic changes occured during both growth seasons. In spring, the proportion of diatoms, green algae and Chrysophyta increased while the proportion of Cryptophyta decreased. The summer trend was very different: the relative importance of diatoms decreased and Cryptophyta and Chrysophyta increased, while Chlorophyta reached their peak around 1985. These trends are also analysed at the genus level. Comparison with taxonomic trends during the eutrophication period shows the expected reversals in most cases. Comparison with other lakes shows general similarities, with the notable exception that Planktothrix rubescens has never been important in Lake Constance. The increase of diatoms during spring is attributed to their improved competitive performance with increasing Si:P ratios. Their decrease during summer is explained by the increasing silicate removal from the epilimnion by increasing spring populations.     

Diatom assemblages in superficial sediments from the Gulf of Riga,eastern Baltic Sea

Twenty one superficial sediment samples from areas of high and moderate eutrophication in the Gulf of Riga were studied with respect to siliceous microfossils, mainly diatoms.The results seem to imply that the number of taxa and the abundance of the frustules are affected by runoff from rivers and the degree of eutrophication in different parts of the Gulf. Areas with high eutrophication, e.g. the river estuaries, have diatom assemblages of varying composition, while in areas with moderate eutrophication the composition is almost constant and the influx of freshwater diatoms and littoral periphyton small. The high abundance and low diversity of brackish-marine and brackish, planktonic diatoms seem to be a result of an influx of nutrients and pollutants in the water and bottom sediments. The sea ice diatoms occurring in the Baltic waters and also in the Gulf of Riga tend to be resistant to eutrophication or are even favoured by it.     

Response of a shallow boreal lake to recent nutrient enrichment – implications for diatom-based phosphorus reconstructions

Water quality of the shallow, mesotrophic, and macrophyte-dominated Lake Kaljasjärvi has been monitored at three to four year intervals since 1978. During the monitoring period, surface-water total phosphorus (TP) concentrations have typically varied between 20 and 25 g P l^–1. However, elevated total phosphorus concentrations were measured in 1987, 1991, and 1999. Diatom-based reconstruction of the historical lake-water TP concentrations was therefore employed to study the recent development of the lake. However, the diatom-TP model did not predict the high measured phosphorus concentrations despite the changes observable in diatom assemblages. In addition, the ratio of sedimentary diatom remains to chrysophycean stomatocysts declined towards the top of the sediment core, indicating decreasing trophy rather than eutrophication. Analysis of sedimentary pigments and phosphorus fractions, used to examine further the changes, also produced results that contradicted the simple eutrophication hypothesis. In particular, the proportion of chlorophyll derivatives instead of carotenoids increased and there was a rise in the concentration of refractory instead of NaOH-extracted phosphorus. These features appear to be related to the extensive littoral areas of the lake since enhanced littoral production can explain both the observed changes in sediment chemistry and the low diatom-inferred TP (DI-TP). Littoral primary producers are suggested to have benefited from the increased phosphorus inputs to the lake, transferring some of the phosphorus to the detrital pool and contributing to the increased pigment concentrations of sedimentary organic matter. High proportions of non-planktonic diatoms in the samples lower DI-TP because periphytic taxa are assigned low TP optima in the inference models used. Abundant aquatic macrophytes may also have made the lake resistant to eutrophication by assimilating nutrients, providing refuge for zooplankton, and having an allelopathic effect on phytoplankton. Since 1980, however, the sedimentary diatom assemblages also indicate increasingly eutrophic conditions. Additional loading from numerous cottages during the last 20 years seems to have caused observable changes in the phytoplankton communities.     

The ecology of the planktonic diatom Cyclotella and its implications for global environmental change studies

The fossil record of diatoms in lake sediments can be used to assess the effects of climate variability on lake ecosystems if ecological relationships between diatom community structure and environmental parameters are well understood. Cyclotella sensu lato taxa are a key group of diatoms that are frequently dominant members of phytoplankton communities in low‐ to moderate‐productivity lakes. Their relative abundances have fluctuated significantly in palaeolimnological records spanning over a century in arctic, alpine, boreal and temperate lakes. This suggests that these species are sensitive to environmental change and may serve as early indicators of ecosystem effects of global change. Yet patterns of change in Cyclotella species are not synchronous or unidirectional across, or even within, regions, raising the question of how to interpret these widespread changes in diatom community structure. We suggest that the path forward in resolving seemingly disparate records is to identify clearly the autecology of Cyclotella species, notably the role of nutrients, dissolved organic carbon and light, coupled with better consideration of both the mechanisms controlling lake thermal stratification processes and the resulting effects of changing lake thermal regimes on light and nutrients. Here we begin by reviewing the literature on the resource requirements of common Cyclotella taxa, illustrating that many studies reveal the importance of light, nitrogen, phosphorus, and interactions among these resources in controlling relative abundances. We then discuss how these resource requirements can be linked to shifts in limnological processes driven by environmental change, including climate‐driven change in lakewater temperature, thermal stratification and nutrient loading, as well as acidification‐driven shifts in nutrients and water clarity. We examine three case studies, each involving two lakes from the same region that have disparate trends in the relative abundances of the same species, and illustrate how the mechanisms by which these species abundances are changing can be deciphered. Ultimately, changes in resource availability and water clarity are key factors leading to shifts in Cyclotella abundances. Tighter integration of the autecology of this important group of diatoms with environmental change and subsequent alterations in limnological processes will improve interpretations of palaeolimnological records, and clarify the drivers of seemingly disparate patterns in fossil records showing widespread and rapid changes across the northern hemisphere.     

典型红树林生态系统藻类多样性及其在生态过程中的作用

藻类是红树林生态系统重要的生物类群, 根据生态习性可分为浮游植物、底栖微藻和大型藻类三个生态类群, 它们在红树林生态系统生物多样性、初级生产、元素循环等方面起着重要作用。但在红树林生态系统中, 关注重点多集中在红树植物和动物, 对其中的藻类重视不够, 且多数研究集中在近20年以及亚洲的红树林区。事实上, 红树林生态系统藻类非常丰富, 其多样性研究有助于深入揭示红树林生态系统的结构与功能。本文介绍了红树林生态系统藻类的组成类群及其重要性, 重点对红树林区浮游植物、底栖硅藻和大型海藻的种类组成、地理分布及其与初级生产力、水质污染、元素循环、碳库形成等生态过程中的作用的研究动态和进展等进行了总结。根据已有研究, 红树林区浮游植物和底栖硅藻的种类数一般为几十到上百种, 其中硅藻在种类和数量上都占绝对优势, 它们是重要的初级生产者、饵料生物和水质污染指示生物; 红树林区底栖大型藻类主要由红藻、绿藻、褐藻、蓝藻组成, 绿藻的种类较多, 红藻在数量上占优势; 藻类是红树林湿地碳库的重要贡献者, 在红树林湿地生态系统碳汇和碳循环中起重要作用。红树林生态系统是个高度动态和异质的系统, 今后应加强红树林藻类多样性的长周期、大尺度变化及不同生境藻类的综合研究, 关注大陆径流和潮汐对藻类多样性和蓝碳的影响, 借助沉积物藻类记录, 探明红树林区藻类的长周期变化, 反演气候变化和人类活动对红树林生态系统的影响过程和机制。     

Nitrogen-based symbioses,phosphorus availability,and accounting for a modern world more productive than the Paleozoic

Evolution of high-productivity angiosperms has been regarded as a driver of Mesozoic ecosystem restructuring. However, terrestrial productivity is limited by availability of rock-derived nutrients such as phosphorus for which permanent increases in weathering would violate mass balance requirements of the long-term carbon cycle. The potential reality of productivity increases sustained since the Mesozoic is supported here with documentation of a dramatic increase in the evolution of nitrogen-fixing or nitrogen-scavenging symbioses, including more than 100 lineages of ectomycorrhizal and lichen-forming fungi and plants with specialized microbial associations. Given this evidence of broadly increased nitrogen availability, we explore via carbon cycle modeling how enhanced phosphorus availability might be sustained without violating mass balance requirements. Volcanism is the dominant carbon input, dictating peaks in weathering outputs up to twice modern values. However, times of weathering rate suppression may be more important for setting system behavior, and the late Paleozoic was the only extended period over which rates are expected to have remained lower than modern. Modeling results are consistent with terrestrial organic matter deposition that accompanied Paleozoic vascular plant evolution having suppressed weathering fluxes by providing an alternative sink of atmospheric CO₂. Suppression would have then been progressively lifted as the crustal reservoir's holding capacity for terrestrial organic matter saturated back toward steady state with deposition of new organic matter balanced by erosion of older organic deposits. Although not an absolute increase, weathering fluxes returning to early Paleozoic conditions would represent a novel regime for the complex land biota that evolved in the interim. Volcanism-based peaks in Mesozoic weathering far surpass the modern rates that sustain a complex diversity of nitrogen-based symbioses; only in the late Paleozoic might these ecologies have been suppressed by significantly lower rates. Thus, angiosperms are posited to be another effect rather than proximal cause of Mesozoic upheaval.     

Seagrasses and eutrophication

This review summarizes the historic, correlative field evidence and experimental research that implicate cultural eutrophication as a major cause of seagrass disappearance. We summarize the underlying physiological responses of seagrass species, the potential utility of various parameters as indicators of nutrient enrichment in seagrasses, the relatively sparse available information about environmental conditions that exacerbate eutrophication effects, and the better known array of indirect stressors imposed by nutrient over-enrichment that influence seagrass growth and survival. Seagrass recovery following nutrient reductions is examined, as well as the status of modeling efforts to predict seagrass response to changing nutrient regimes.The most common mechanism invoked or demonstrated for seagrass decline under nutrient over-enrichment is light reduction through stimulation of high-biomass algal overgrowth as epiphytes and macroalgae in shallow coastal areas, and as phytoplankton in deeper coastal waters. Direct physiological responses such as ammonium toxicity and water-column nitrate inhibition through internal carbon limitation may also contribute. Seagrass decline under nutrient enrichment appears to involve indirect and feedback mechanisms, and is manifested as sudden shifts in seagrass abundance rather than continuous, gradual changes in parallel with rates of increased nutrient additions. Depending on the species, interactions of high salinity, high temperature, and low light have been shown to exacerbate the adverse effects of nutrient over-enrichment. An array of indirect effects of nutrient enrichment can accelerate seagrass disappearance, including sediment re-suspension from seagrass loss, increased system respiration and resulting oxygen stress, depressed advective water exchange from thick macroalgal growth, biogeochemical alterations such as sediment anoxia with increased hydrogen sulfide concentrations, and internal nutrient loading via enhanced nutrient fluxes from sediments to the overlying water. Indirect effects on trophic structure can also be critically important, for example, the loss of herbivores, through increased hypoxia/anoxia and other habitat shifts, that would have acted as “ecological engineers” in promoting seagrass survival by controlling algal overgrowth; and shifts favoring exotic grazers that out-compete seagrasses for space. Evidence suggests that natural seagrass population shifts are disrupted, slowed or indefinitely blocked by cultural eutrophication, and there are relatively few known examples of seagrass meadow recovery following nutrient reductions.Reliable biomarkers as early indicators of nutrient over-enriched seagrass meadows would benefit coastal resource managers in improving protective measures. Seagrasses can be considered as “long-term" integrators (days to weeks) of nutrient availability, especially through analyses of their tissue content, and of activities of enzymes such as nitrate reductase and alkaline phosphatase. The ratio of leaf nitrogen content to leaf mass has also shown promise as a “nutrient pollution indicator” for the seagrass Zostera marina, with potential application to other species. In modeling efforts, seagrass response to nutrient loading has proven difficult to quantify beyond localized areas because long-term data consistent in quality are generally lacking, and high inter-annual variability in abundance and productivity depending upon stochastic meteorological and hydrographic conditions.Efforts to protect remaining seagrass meadows from damage and loss under eutrophication, within countries and across regions, are generally lacking or weak and ineffective. Research needs to further understand about seagrasses and eutrophication should emphasize experimental studies to assess the response of a wider range of species to chronic, low-level as well as acute, pulsed nutrient enrichment. These experiments should be conducted in the field or in large-scale mesocosms following appropriate acclimation, and should emphasize factor interactions (N, P, C; turbidity; temperature; herbivory) to more closely simulate reality in seagrass ecosystems. They should scale up to address processes that occur over larger scales, including food-web dynamics that involve highly mobile predators and herbivores. Without any further research, however, one point is presently very clear: Concerted local and national actions, thus far mostly lacking, are needed worldwide to protect remaining seagrass meadows from accelerating cultural eutrophication in rapidly urbanizing coastal zones.     

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

Impacts of rising atmospheric CO₂ concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO₂ on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO₂ concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO₂ in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (F_v/F_m) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom‐forming Antarctic coastal diatoms might increase carbon contents under future pCO₂ conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO₂ sequestration potential of diatom communities in the future coastal Southern Ocean.     

SILICON METABOLISM IN DIATOMS: IMPLICATIONS FOR GROWTH

Diatoms are the world's largest contributors to biosilicification and are one of the predominant contributors to global carbon fixation. Silicon is a major limiting nutrient for diatom growth and hence is a controlling factor in primary productivity. Because our understanding of the cellular metabolism of silicon is limited, we are not fully knowledgeable about intracellular factors that may affect diatom productivity in the oceans. The goal of this review is to present an overview of silicon metabolism in diatoms and to identify areas for future research. Numerous studies have characterized parameters of silicic acid uptake by diatoms, and molecular characterization of transport has begun with the isolation of genes encoding the transporter proteins. Multiple types of silicic acid transporter gene have been identified in a single diatom species, and multiple types appear to be present in all diatom species. The controlled expression and perhaps localization of the transporters in the cell may be factors in the overall regulation of silicic acid uptake. Transport can also be regulated by the rate of silica incorporation into the cell wall, suggesting that an intracellular sensing and control mechanism couples transport with incorporation. Sizable intracellular pools of soluble silicon have been identified in diatoms, at levels well above saturation for silica solubility, yet the mechanism for maintenance of supersaturated levels has not been determined. The mechanism of intracellular transport of silicon is also unknown, but this must be an important part of the silicification process because of the close coupling between silica incorporation and uptake. Although detailed ultrastructural analyses of silica deposition have been reported, we know little about the molecular details of this process. However, proteins occluded within silica that promote silicification in vitro have recently been characterized, and the application of molecular techniques holds the promise of great advances in this area. Cellular energy for silicification and transport comes from aerobic respiration without any direct involvement of photosynthetic energy. As such, diatom silicon metabolism differs from that of other major limiting nutrients such as nitrogen and phosphorous, which are closely linked to photosynthetic metabolism. Cell wall silicification and silicic acid transport are tightly coupled to the cell cycle, which results in a dependency in the extent of silicification on growth rate. Silica dissolution is an important part of diatom cellular silicon metabolism, because dissolution must be prevented in the living cell, and because much of the raw material for mineralization in natural assemblages is supplied by dissolution of dead cells. Perhaps part of the reason for the ecological success of diatoms is due to their use of a silicified cell wall, which has been calculated to impart a substantial energy savings to organisms that have them. However, the growth of diatoms and other siliceous organisms has depleted the oceans of silicon, such that silicon availability is now a major factor in the control of primary productivity. Much new progress in understanding silicon metabolism in diatoms is expected because of the application of molecular approaches and sophisticated analytical techniques. Such insight is likely to lead to a greater understanding of the role of silicon in controlling diatom growth, and hence primary productivity, and of the mechanisms involved in the formation of the intricate silicified structures of the diatom cell wall.     

Carbon flow in a small turbid man-made impoundment

A shallow turbid man-made impoundment was studied intensively for five years. The carbon (C) budget indicated a well balanced system, where phytoplankton productivity and respiratory losses accounted for the major inputs and outputs. The carbon content was dominated by dissolved organic carbon, followed by detritus > fish > phytoplankton > bacteria > zooplankton > zoobenthos.From an analysis of a matrix flow model, three dominant components of C-flow in the system were identified, i.e. dissolved inorganic carbon (DIC), particulate organic carbon (POC) and fish. Phytoplankton and detritus were the important components of POC. The largest flow of C was through the largest pool, dissolved inorganic carbon (DIC), whilst the second largest flow was through the fifth largest pool, the phytoplankton. Phytoplankton was very important in determining the structure of the system, and variations in phytoplankton primary productivity influenced the entire system. This also applied to the input of organic material from macrophytes, but not to variations in the inflow and outflow of water from the impoundment.The input of detrital material from a littoral macrophyte community also markedly influenced the system. DOC was identified as a carbon buffer in the system, and differential flow occurred through this component upon variations in phytoplankton productivity.     

Mixed populations of marine microalgae in continuous culture: Factors affecting species dominance and biomass productivity

Marine microalgae were grown in multispecies continuous cultures. Under carbon dioxide limitation, blue-green algae dominated. Under nitrate and light limitation, species dominance depended on the initial conditions. When the inoculum consisted primarily of blue-green algae with smaller amounts of other species, blue-green algae and pennate diatoms dominated. When the inoculum consisted of equal amounts of all species, green flagellates and pennate diatoms dominated. Green flagellates and blue-green algae were incompatible and never shared dominance. When nutrient limitations were overcome, the productivity of seawater was increased 100-fold before light limitation occurred. The productivity could be further increased by reducing photorespiration in the culture. The dilution rates studied (0.1, 0.2, and 0.4 day(-1)) had no effect on species dominance, nor did the higher dilution rates select for smaller cells. The maximum productivity occurred at a dilution rate of 0.2 day(-1). Temperature had the greatest effect on species dominance, with green flagellates, pennate diatoms, and blue-green algae dominating at 20 degrees C and only blue-green algae dominating at 35 degrees C. The productivity at 35 degrees C was lower than that at 20 degrees C because of the lower solubility of carbon dioxide at higher temperatures. At 10% salinity, green flagellates and pennate diatoms dominated. The productivity at this salinity was 50% that obtained at the salinity of seawater (3.5%). At 25% salinity, only the green flagellate, Dunaliella salina, survived at a productivity of 1% that obtained at the salinity of seawater.     

太湖浮游植物优势种长期演化与富营养化进程的关系

利用1991年至2002年每月一次的监测资料,系统分析了浮游植物优势种和生物量的周年变化情况。同时,总氮、总磷和浮游植物叶绿素a含量等相关资料也被用于解释太湖富营养化演化与浮游植物的关系。结果显示,太湖总氮、总磷、叶绿素a和生物量均呈自梅梁湾底至湖心的逐步递减趋势。在20世纪80年代末太湖刚开始富营养化时,浮游植物优势种群从硅藻转变为蓝藻。之后,浮游植物优势种群一直是蓝藻,但各年的浮游植物总生物量有变化。总氮、总磷、叶绿素a和生物量的年均值持续增长至1996年,其后有逐步下降的趋势,究其原因可能和当地政府在太湖流域的控制排污行动有关。微囊藻在太湖的占优是太湖富营养化的标志之一。研究结果说明浮游植物在大型浅水湖泊中可以作为反映富营养化进程的生态指标。     

聚球藻硅质化作用初探

硅元素是全球生地化循环的重要组成成分之一,对海洋生态系统中以浮游植物主导的初级生产力和硅碳循环具有重要的意义。普遍认为硅藻主导着全球海洋的硅循环,成为海洋硅循环和碳循环交互作用的重要桥梁。海洋单细胞聚球藻对海洋食物网和能量流具有关键启动和支撑作用,是全球碳循环中固碳过程的主要贡献者,近年又被发现其具有重要的硅质化作用,为我们提供了一个在海洋中(特别是寡营养海域),除硅藻之外,连接硅碳循环交互作用的新视角,对硅藻在全球海洋硅碳循环的绝对地位具有重要的挑战意义。面对聚球藻在大洋中如此巨大的生物量,甚至高于硅藻,有必要弄清楚其碳沉降机制以及准确的模拟其硅循环,然而关于其在海洋硅循环的研究极少,硅质化作用的吸收和储存机理以及环境调节机制也不清楚;另外,其对世界海洋硅碳循环的调节作用也未见报道。为此,通过前人对海洋单细胞聚球藻硅质化作用研究的基础上进行有针对性的探讨,可望对海洋单细胞聚球藻硅质化作用及其对硅碳循环的调控机制有一个基本的认识,为深入研究聚球藻在全球海洋硅循环中的作用提供基础。     

Influence of ionic strength and conductivity on benthic diatom communities in a tropical river (Monjolinho), S?o Carlos-SP, Brazil

Benthic diatoms are important indicators of ecological conditions in lotic systems. The objective of this study was to elucidate the confounding effects of eutrophication, organic pollution and ionic strength and conductivity on benthic diatom communities. Benthic diatoms and water quality sampling was done at 10 sites during summer base flow period (2008 and 2009). Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to determine environmental gradients along which species vary with respect to ionic strength and conductivity and other environmental variables. Using variance partitioning, we assessed the individual importance of a set of environmental variables (eutrophication and organic pollution) versus ionic strength and conductivity on diatom community structure. The effects of ionic strength and conductivity and organic pollution, eutrophication and other environmental variables were integrated into overall resultant benthic diatom communities. Through partial CCA, we partitioned the variance in diatom data between two sets of exploratory variables, i.e. ionic strength and conductivity (26.9%); other variables, particularly eutrophication and organic pollution (23.0%); shared variance (11.3%) and unexplained variance (38.8%). Due to the interaction of the effects of ionic strength and conductivity and other variables in this study, laboratory experiments must be performed to confirm the observed effects of ionic strength and conductivity.