Photosynthesis and nitrogen fixation in a cyanobacterial bloom in the Baltic Sea

Daily integrals of photosynthesis by a cyanobacterial bloom in the Baltic Sea, during the summer of 1993, were calculated from the vertical distributions of light, temperature and the organisms in the water column and from photosynthesis/irradiance curves of picoplanktonic and diazotrophic cyanobacteria isolated from the community. The distribution of chlorophyll a in size-classes <20?µm and >20?µm was monitored over 9 days that included a deep mixing event followed by calm. Picocyanobacteria formed 70% of the cyanobacterial biomass and contributed 56% of the total primary production. Of the filamentous diazotrophs that formed the other 30%, Aphanizomenon contributed 28% and a Nodularia-containing fraction 16% of the primary production. For the whole population there was little change in standardized photosynthetic O₂ production, which remained at about 31?mmol?m^?2 before and after the mixing event. There were differences, however, between the classes of cyanobacteria: in picocyanobacteria primary production hardly changed, while in Aphanizomenon it increased by 2.6 and in Nodularia it fell below zero. Total phytoplankton photosynthesis was strongly dependent on total daily insolation with the compensation point at a photon insolation of 22.7?mol?m^?2?d^?1. Similar analyses of N₂ fixation showed much less dependence on depth distribution of light and biomass: Aphanizomenon fixed about twice as much N₂ as Nodularia their; their fixation exceeded their own N demand by about 12%. Together, these species contributed 49% of the total N demand of the phytoplankton population. Computer models based on the measured light attenuation and photosynthetic coefficients indicate that growth of the cyanobacterial population could occur only in the summer months when the critical depth of the cyanobacteria exceeds the depth of mixing.     

Food web structure variability in the surface layer, at a fixed station influenced by the North Western Mediterranean Current

The structure of the planktonic community and the influence of mesozooplankton migration on the microbial food web were investigated during six diel studies from June 92 to June 93 in the surface waters of a station in the North-Western Mediterranean Sea. Each diel study consisted of sampling at 5 and 40 m every 3 h over 24 h. Most of the times diel cycles did not show any convincing diel patterns in any of the variables studied. Clear zooplankton migration was evident in only two diel studies.The ratio of heterotrophic/autotrophic biomasses varied from 0.68 to 3.0, with a strong dominance of the heterotrophic biomass under oligotrophic conditions. Differences in food web structure were probably related to the influences of coastal water and the North-Western Mediterranean Current. Thus we found that the planktonic food web variability relatable to hydrodynamic variability, to be greater than diel variability. However, very large differences in food web structure among dates were evident. For example proportion of Chl a found in the <10 µm fraction varied from 18 to 96%.     

基于稳定同位素的湿地食物源判定和食物网构建研究进展

湿地生物营养动力学是湿地生态系统结构和功能评价研究的基础.碳、氮稳定同位素作为识别营养关系的方法,已在湿地生态系统食物来源、组成和食物链传递研究中得到广泛运用.本文系统综述了稳定同位素食物贡献度计算模型和营养级确定的基本方法和理论;讨论了动物营养分馏值和基线的选择依据;概括了湿地生态系统典型食物源及其稳定同位素变化特征;总结了草食、杂食和肉食等不同营养级动物的食物来源.指出了稳定同位素在湿地食物源溯源和食物网研究中的不足;基于国内外研究现状和发展趋势及需求,展望了未来同位素技术在湿地食物网生态学研究中的运用前景和研究重点,提出需要加强稳定同位素营养分馏和基线的影响因素、样品处理和保存方式研究以及胃含物、分子标记物和多元素同位素结合分析.     

Sedimentation of Nodularia spumigena and distribution of nodularin in the food web during transport of a cyanobacterial bloom from the Baltic Sea to the Kattegat

Nodularia spumigena is a toxic cyanobacteria that blooms in the Baltic Sea every year. In the brackish water of the Baltic Sea, its toxin, nodularin, mainly affects the biota in the surface water due to the natural buoyancy of this species. However, the fate of the toxin is unknown, once the cyanobacteria bloom enters the more saline waters of the Kattegat. In order to investigate this knowledge gap, a bloom of N. spumigena was followed during its passage, carried by surface currents, from the Baltic Sea into the Kattegat area, through the Öresund strait. N. spumigena cells showed an increased cell concentration through the water column during the passage of the bloom (up to 130 10³ cells ml⁻¹), and cells (4.2 10³ cells ml⁻¹) could be found down to 20 m depth, below a pycnocline. Sedimentation trap samples from below the pycnocline (10–12 m depth) also showed an increased sedimentation of N. spumigena filaments during the passage of the bloom. The toxin nodularin was detected both in water samples (0.3–6.0 μg l⁻¹), samples of sedimenting material (a toxin accumulation rate of 20 μg m^-2 day⁻¹), zooplankton (up to 0.1 ng ind.⁻¹ in copepods), blue mussels (70–230 μg kg⁻¹ DW), pelagic and benthic fish (herring (1.0–3.4 μg kg⁻¹ DW in herring muscle or liver) and flounder (1.3-6.2 μg kg⁻¹ DW in muscle, and 11.7-26.3 μg kg⁻¹ DW in liver). A laboratory experiment showed that N. spumigena filaments developed a decreased buoyancy at increased salinities and that they were even sinking with a rate of up to 1,7 m day⁻¹ at the highest salinity (32 PSU). This has implications for the fate of brackish water cyanobacterial blooms, when these reach more saline waters. It can be speculated that a significant part of the blooms content of nodularin will reach benthic organisms in this situation, compared to blooms decaying in brackish water, where most of the bloom is considered to be decomposed in the surface waters.     

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Metabolism and foraging strategies of mid‐latitude mesozooplankton during cyanobacterial blooms as revealed by fatty acids,amino acids,and their stable carbon isotopes

Increasing sea surface temperatures (SST) and blooms of lipid‐poor, filamentous cyanobacteria can change mesozooplankton metabolism and foraging strategies in marine systems. Lipid shortage and imbalanced diet may challenge the build‐up of energy pools of lipids and proteins, and access to essential fatty acids (FAs) and amino acids (AAs) by copepods. The impact of cyanobacterial blooms on individual energy pools was assessed for key species temperate Temora longicornis and boreal Pseudo‐/Paracalanus spp. that dominated field mesozooplankton communities isolated by seasonal stratification in the central Baltic Sea during the hot and the cold summer. We looked at (a) total lipid and protein levels, (b) FA trophic markers and AA composition, and (c) compound‐specific stable carbon isotopes (δ¹³C) in bulk mesozooplankton and in a subset of parameters in particulate organic matter. Despite lipid‐poor cyanobacterial blooms, the key species were largely able to cover both energy pools, yet a tendency of lipid reduction was observed in surface animals. Omni‐ and carnivory feeding modes, FA trophic makers, and δ¹³C patterns in essential compounds emphasized that cyanobacterial FAs and AAs have been incorporated into mesozooplankton mainly via feeding on mixo‐ and heterotrophic (dino‐) flagellates and detrital complexes during summer. Foraging for essential highly unsaturated FAs from (dino‐) flagellates may have caused night migration of Pseudo‐/Paracalanus spp. from the deep subhalocline waters into the upper waters. Only in the hot summer (SST>19.0°C) was T. longicornis submerged in the colder subthermocline water (~4°C). Thus, the continuous warming trend and simultaneous feeding can eventually lead to competition on the preferred diet by key copepod species below the thermocline in stratified systems. A comparison of δ¹³C patterns of essential AAs in surface mesozooplankton across sub‐basins of low and high cyanobacterial biomasses revealed the potential of δ¹³C‐AA isoscapes for studies of commercial fish feeding trails across the Baltic Sea food webs.     

Establishment of trophic continuum in the food web of the Yellow Sea and East China Sea ecosystem: Insight from carbon and nitrogen stable isotopes

Stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) are used to study the trophic structure of food web in the Yellow Sea and East China Sea ecosystem. The trophic continuum of pelagic food web from phytoplankton to top preyer was elementarily established, and a trophic structure diagram in the Yellow Sea and East China Sea was outlined in combination with carbon isotopic data of benthic organisms, which is basically consistent with and makes some improvements on the simplified Yellow Sea food web and the trophic structure diagram drawn based on the biomass of main resource population during 1985–1986. This result indicates that the stable isotope method is a potential useful means for further studying the complete marine food web trophic continuum from viruses to top predators and food web stability.     

Establishment of trophic continuum in the food web of the Yellow Sea and East China Sea ecosystem: Insight from carbon and nitrogen stable isotopes

The determination of trophic level for the biology in a marine ecosystem is very important as alteration of its structure and function may be reflected in the tro-phic level of component species. A change in trophic level indicates variation in an organism’s feeding bi-ology or in the pathway of energy flow from primary producers to the consumer. The gut content analysis is a traditional method for studying trophodynamics of food web in marine ecosystems. Species composition and amounts in al…     

长江上游弥陀漫滩水体鱼类食物网动态的季节性变化

长江上游是我国鱼类生物多样性最为丰富的地区之一,漫滩作为河流生态系统的重要组成部分对维持区域鱼类生物多样性具有重要作用。于2019年丰水期(8月份)和枯水期间(11月份)应用碳、氮稳定同位素技术并结合Bayesian混合模型和SIBER分析方法,对长江上游弥陀漫滩水体鱼类食物网结构动态的季节性变化特征进行了研究。结果显示：丰水期基础碳源的δ¹³C、δ¹⁵N平均值分别为-23.02‰和2.58‰,范围分别为-31.01‰—-11.2‰(δ¹³C)和-0.51‰—6.84‰(δ¹⁵N);枯水期其δ¹³C、δ¹⁵N平均值分别为-21.93‰、7.22‰,范围分别为-26.31‰—-15.36‰(δ¹³C)和4.89‰—8.81‰(δ¹⁵N)。丰水期鱼类食物网能量主要依赖于外源性营养物质,食物链长度达到3.6级;枯水期内源性营养物质是食物网能量的主要贡献者,食物链长度为2.6级。相对于枯水期,丰水期间鱼类群落拥有更大的生...     

Spatial and temporal variation in nitrogen fixation and its importance to phytoplankton in phosphorus‐rich lakes

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A food web analysis of the juvenile blue crab, Callinectes sapidus, using stable isotopes in whole animals and individual amino acids

The stable isotope compositions (C and N) of plants and animals of a marsh dominated by Spartina alterniflora in the Delaware Estuary were determined. The study focused on the juvenile stage of the Atlantic blue crab, Callinectes sapidus, and the importance of marsh-derived diets in supporting growth during this stage. Laboratory growth experiments and field data indicated that early juvenile blue crabs living in the Delaware Bay habitat fed primarily on zooplankton, while marsh-dwelling crabs, which were enriched in ¹³C relative to bay juveniles, utilized marsh-derived carbon for growth. In laboratory experiments, the degree to which juvenile blue crabs isotopically fractionated dietary nitrogen, as well as the growth rate, depended on the protein quality of the diet. The range of δ¹³C of amino acids in laboratory-reared crabs and their diets was almost 20‰, similar to the isotopic range of amino acids of other organisms. In laboratory studies, the δ¹³C of nonessential and essential amino acids in the diet were compared to those in juvenile crabs. Isotopic fractionation at the molecular level depended on diet quality and the crabs' physiological requirements. Comparison of whole-animal isotope data with individual amino acid C isotope measurements of wild juvenile blue crabs from the bay and marsh suggested a different source of total dietary carbon, yet a shared protein component, such as zooplankton. Received: 1 July 1998 / Accepted: 15 March 1999     

Vertical planktonic structure in the central basin of Lake Baikal in summer 1999, with special reference to the microbial food web

Planktonic microbial interactions in the central basin of Lake Baikal were examined on a summer day in 1999. The subsurface maxima of bacterial abundance and chlorophyll concentration were recorded at the same depth, whereas the vertical distribution of heterotrophic nanoflagellates was the inverse of those of bacteria and picophytoplankton. Release of extracellular organic car-bon (EOC) from phytoplankton was estimated by the NaH¹⁴CO₃ method as 2.4µgCl^–1day^–1. Bacterial production (4.3µgCl^–1day^–1), estimated in a bottle incubation experiment using size-fractionated water samples, exceeded the EOC released. Thus, other supplying sources of organic matter are needed for the bacterial production. Grazing (2.6µgCl^–1day^–1) was also estimated in the experiment and accounted for 60% of the bacterial production. This is the first report on the microbial food web in the central basin of Lake Baikal.     

Effects of modified nutrient concentrations and ratios on the structure and function of the native phytoplankton community in the Neuse River Estuary, North Carolina, USA

A variety of analyses were used to assess the structure (community composition) and function (assimilation number, nitrogen fixation) of phytoplankton in the Neuse River Estuary (NRE), NC under ambient and modified nutrient concentrations. Dilution bioassays were employed to reduce the concentration of nitrogen (N) or both N and phosphorus (P) and thus compare varied DIN:DIP ratios. Experimental manipulations created conditions that may result from mandated N load reductions to the estuary. We hypothesized that unilateral reduction of N loading to the NRE would increase the activity, abundance and diversity of N₂ fixing cyanobacteria. Changes in phytoplankton primary productivity, N₂ fixation (nitrogenase activity), genetic potential for N₂ fixation (presence of nifH), phytoplankton taxonomic composition (diagnostic photopigment concentration) and abundances of N₂ fixing cyanobacteria (microscopy) were determined. Decreasing ambient DIN:DIP ratios in NRE samples resulted in increased rates of N₂ fixation when seed populations were present and environmental conditions were amenable. Decreasing the DIN:DIP ratio did not lead to an increase in the abundance or diversity of N₂ fixing cyanobacteria. Because N₂ fixing cyanobacteria were only actively fixing nitrogen during periods of low riverine N discharge (summer and early autumn), lowering nutrient ratios may not have a major impact on the NRE. However, the maximum potential amount of N from N₂ fixation was calculated using rates from this study and was found to be approximately 3% of total riverine loading of N to the NRE. Because N₂ fixation occurs farther downstream and later in the year than riverine N loading to the NRE, there is potential for N₂ fixation to modify N dynamics. Analyses of the phytoplankton community as a whole in these relatively short term experiments indicated that reduced DIN:DIP may not have a major impact on their structure and function.     

Community assembly, stability and signatures of dynamical constraints on food web structure

To understand the dynamics of natural species communities, a major challenge is to quantify the relationship between their assembly, stability, and underlying food web structure. To this end, two complementary aspects of food web structure can be related to community stability: sign structure, which refers to the distributions of trophic links irrespective of interaction strengths, and interaction strength structure, which refers to the distributions of interaction strengths with or without consideration of sign structure. In this paper, using data from a set of relatively well documented community food webs, I show that natural communities generally exhibit a sign structure that renders their stability sensitive to interaction strengths. Using a Lotka-Volterra type population dynamical model, I then show that in such communities, individual consumer species with high values of a measure of their total biomass acquisition rate, which I term “weighted generality”, tend to undermine community stability. Thus consumer species’ trophic modules (a species and all its resource links) should be “selected” through repeated immigrations and extinctions during assembly into configurations that increase the probability of stable coexistence within the constraints of the community's trophic sign structure. The presence of such constraints can be detected by the incidence and strength of certain non-random structural characteristics. These structural signatures of dynamical constraints are readily measurable, and can be used to gauge the importance of interaction-driven dynamical constraints on communities during and after assembly in natural communities.     

The planktonic food web structure of a temperate zone estuary,and its alteration due to eutrophication

Hydrobiologia - Current conventional wisdom argues that human-induced excesses in nutrient loadings to estuaries often stimulate ‘excess’ algal production leading to hypoxia, via...     

Introduction into the ecosystem of the North Sea: Hydrography,biota, and food web relationships

The North Sea, one of the most productive of the earth's seas and oceans, is also surrounded by some of earth's most densely populated and heavily industrialized regions. A growing number of signals are being received which indicate that this valuable ecosystem is increasingly under stress. This has generated a corresponding increase in concern over the steps to be taken to protect the North Sea. While there are divergent views on what constitutes an ‘ideal’ North Sea, there is a general recognition that any decisions that are made should be based on a good understanding of this ecosystem. The intention of this paper is to give an overview of what is presently known, and to identify areas where more studies are needed. A brief summary of the hydrography and the biota of the North Sea is given. Biotic and abiotic structure justify partitioning the North Sea into three ecologically different regions: southern, central, and northern. For the most part, neither the top predators,e.g. marine birds and mammals, nor the macroalgae and sea grasses are included in this overview.     

基于稳定同位素技术的海州湾海洋牧场食物网基础及营养结构的季节性变化

基于稳定同位素技术对2015年春季海州湾海洋牧场海域采集的中小型生物消费者,包括鱼类、虾类、蟹类、头足类、螺类和双壳类等与其潜在碳源样品进行分析,利用IsoSource模型计算该海域消费者碳源贡献率,并对2014年夏季生物学样品与2015年春季样品比较,分析食物网营养结构的季节性变化,根据稳定同位素测定结果绘制二维双标图,计算出6种营养结构的量化指标.结果表明: 2015年春季海州湾海洋牧场海域消费者的δ¹³C值范围为-18.9‰～-17.1‰,3种潜在碳源[浮游植物、悬浮颗粒有机物(POM)、沉积物(SOM)]的δ¹³C值范围为-18.1‰～-23.4‰,根据模型计算得出浮游植物对消费者的平均碳源贡献最大,为80.8%,其余依次为SOM和POM,分别为10.8%和8.4%.2014年夏季生物样品与2015年春季样品的δ¹³C值存在显著差异,而δ¹⁵N值无显著性差异;6种量化指标表明群落营养结构存在季节性差异, 2014年夏季的δ¹³C比值范围(CR)、总面积(TA)、平均最邻近距离(NND)和平均最邻近距离标准差(SDNND)均大于2015年春季,δ¹⁵N比值范围(NR)和平均离心距离(CD)无明显变化,夏季群落营养结构冗余度小于春季,且食源多样性水平高于春季,存在季节差异.     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Comparison of acetylene-reduction and nitrogen-15 techniques for the determination of nitrogen fixation by field bean (Vicia faba) nodules

Field bean (Vicia faba L.) cv. Maris Bead seeds were inoculated with Rhizobium Catalogue No. 1001, supplied by Rothamsted Experimental Station, and grown in sand culture supplied with a complete nutrient solution which included nitrate at either 1.5 or 6.0 mM. Nodules were detached from the roots at intervals during plant development and their rates of nitrogen fixation estimated by both acetylene reduction and ¹⁵N gas technique. There was a constant relationship, independent of nitrate supply, between the results obtained by these two methods at all samplings. The amounts of acetylene reduced divided by a factor of 5.75 gave the amount of true nitrogen fixation; this factor is about twice the theoretical value. It is suggested that this discrepancy arose because, with acetylene, all the electrons available to the nitrogenase were used to form ethylene, whereas during normal fixation only about half the electron supply was used to fix nitrogen, the remainder having been consumed in the production of hydrogen gas.