The independent and interactive effects of snail grazing and nutrient enrichment on structuring periphyton communities

We investigated the independent and interactive effects of nutrient enrichment and snail grazing on structuring periphyton communities in a northern temperate lake. Nutrient releasing substrates and grazer enclosures were used to simultaneously manipulate nutrient availability and herbivory. Periphyton was allowed 18 days to accrue before grazers (Elimia livescens = Goniobasis livescens) were introduced.Addition of nitrogen and phosphorus caused a significant increase in biovolume (p < 0.001), whereas grazing had no significant effect on biovolume but resulted in a shift in species composition. Four taxa were largely responsible for the increase in biovolume on the nutrient enriched substrates: Oedogonium sp, Stigeoclonium tenue, Navicula radiosa var. radiosa and Navicula radiosa var. tenella. By the 28th day, nutrient enrichment caused a shift from a community dominated by diatoms (Bacillariophyceae) to a community dominated by green algae (Chlorophyceae). Blue green algae (Myxophyceae) maintained an equal proportion in high and low-nutrient regimes.Grazing had a more pronounced effect on altering community composition on the nutrient enriched substrates than on the unenriched substrates. Grazing caused a decrease in diversity and an increase in dominance by green algae on the nutrient enriched substrates. The relative biovolume of green algae increased from 64% to 93% on grazed substrates, due to the significant increase in relative abundance of Stigeoclonium tenue. This taxon has both prostate basal cells and erect filamentous cells. The ratio of basal: filamentous cells increased from 4.7 to 5.2 with grazing, suggesting that the heretotrichous growth form of Stigeoclonium tenue is adapted to grazing by virtue of the basal cells which are able to adhere to the substratum and resist being grazed.     

Alterations in the biomass-specific productivity of periphyton assemblages mediated by fish grazing

1. In an experimental flume, we examined the effects of a biomass reduction and alteration of taxonomic composition, because of grazing by the fish Plecoglossus altivelis, on the net biomass accumulation of periphyton. 2. Grazed and ungrazed assemblages with different biomass and taxonomic composition were first prepared in fish enclosures and exclosures, respectively. These assemblages were then set out in the flume and incubated for 2 days under grazing‐free conditions to examine (i) the relationship between biomass and biomass accumulation rate and (ii) the effect of taxonomic composition on the relationship between these two. 3. The grazed and ungrazed assemblages were dominated by upright filamentous cyanobacteria and diatoms, respectively. The rate of biomass accumulation decreased with increasing periphyton biomass in both the grazed and ungrazed assemblages, and was lower in the grazed than the ungrazed assemblages at any biomass level. 4. The results showed that the reduction in biomass and the alteration of taxonomic composition due to fish grazing have opposite effects on biomass‐specific productivity. Biomass accumulation rate increased in response to biomass reduction, although a shift in dominance from diatoms to upright filamentous cyanobacteria decreased the overall productivity of the periphyton.     

Effects of nutrient (N,P, C) enrichment,grazing and depth upon littoral periphyton of a softwater lake

Three field experiments were performed in Lake Lacawac, PA to determine the importance of potentially limiting nutrients relative to other factors (grazing, depth) in structuring shallow water algal periphyton communities. All three experiments measured periphyton growth (as chlorophyll-a, AFDM or biovolumes of the algal taxa) on artificial clay flower pot substrates which released specified nutrients to their outer surfaces.Control of standing crop by nutrient supply rate vs. grazing was examined in Expt. I. Substrates releasing excess N and P, together with one of 4 levels of C (as bicarbonate) were placed either inside or outside exclosures designed to reduce grazer densities. Chlorophyll-a rose from 1.1–25.6 µg.cm^–2, and some dominant taxa (e.g., Oedogonium, Nostoc, Anacystis) were replaced by others (e.g., Scenedesmus, Cryptomonas) as bicarbonate supply increased. Reductions in invertebrate density did not significantly affect chlorophyll-a at any of the nutrient levels.Reasons for the species shift were further evaluated in Expt. II, using a minielectrode to measure the elevation of pH within the periphyton mat through photosynthetic utilization of bicarbonate. The pH adjacent to pots diffusing N, P and large quantities of bicarbonate, and supporting high chlorophyll-a densities of 32 µg cm^–2, averaged 10.0 compared to 6.3 in the water column. Pots diffusing only N and P supported 0.7 µg chlorophyll-a cm^–2 and elevated pH to 8.2. We suspect that bicarbonate addition favored efficient bicarbonate users (e.g., Scenedesmus), while inhibiting other taxa (e.g., Oedogonium) because of the attendant high pH.Expt. III was designed to test effects of depth (0.1 m vs. 0.5 m) and N (NH₄ ⁺ vs. NO₃ ^–) upon the growth response to bicarbonate observed in Expts. I and II. Similar standing crop and species composition were noted on pots at 0.1 m vs. 0.5 m. Enrichment with NH₄ ⁺ vs. NO₃ ^– also appeared to have little effect upon the periphyton community.Shallow water periphyton communities in Lake Lacawac, when supplied with sufficient N and P, appear to show a distinctive response to increasing bicarbonate concentration and pH which is robust to moderate variation in grazer densities, distance from the water surface, and the form of N enrichment.     

Controls on periphyton biomass in heterotrophic streams

1. Headwater streams of the Hubbard Brook Experimental Forest (HBEF) are typically characterised by a periphyton assemblage of low biomass and diversity. However, periphyton blooms have been observed following catchment deforestation experiments and occasionally during the annual spring thaw before canopy leaf‐out. 2. There is pronounced seasonal variation in both nutrient and light availability in HBEF streams. Stream water nitrogen (N) concentrations and light levels are higher before canopy leaf‐out and after leaf senescence and are lower during the growing season. Periphyton accrual rates also change seasonally; they are highest in spring prior to leaf‐out and significantly lower during summer and in autumn. 3. Periphyton biomass rarely responded positively to in‐situ experimental enrichment with nitrogen or phosphorus. In the summer, nutrient enrichment overall had no effect on periphyton biomass, while outside the growing season N enrichment had inhibitory effects on periphyton. 4. Despite these experimental results, surveys of ambient chlorophyll a concentrations in streams across the HBEF demonstrated no relationship between streamwater dissolved inorganic N or P concentrations and benthic chlorophyll a. 5. Our results suggest that HBEF periphyton communities are not closely regulated by nutrient availability, even during periods of high light availability. The inhibitory effects of nutrient enrichment outside the growing season are interesting, but further research is necessary to elucidate the mechanisms driving these responses.     

The influence of sediment mobility and channel geomorphology on periphyton abundance

相似文献   

Effect of periphyton biomass on hydraulic characteristics and nutrient cycling in streams

The effect of periphyton biomass on hydraulic characteristics and nutrient cycling was studied in laboratory streams with and without snail herbivores. Hydraulic characteristics, such as average water velocity, dispersion coefficients, and relative volume of transient storage zones (zones of stationary water), were quantified by performing short-term injections of a conservative tracer and fitting an advection-dispersion model to the conservative tracer concentration profile downstream from the injection site. Nutrient cycling was quantified by measuring two indices: (1) uptake rate of phosphorus from stream water normalized to gross primary production (GPP), a surrogate measure of total P demand, and (2) turnover rate of phosphorus in the periphyton matrix. These measures indicate the importance of internal cycling (within the periphyton matrix) in meeting the P demands of periphyton. Dense growths of filamentous diatoms and blue-green algae accumulated in the streams with no snails (high-biomass streams), whereas the periphyton communities in streams with snails consisted almost entirely of a thin layer of basal cells of Stigeoclonium sp. (low-biomass streams). Dispersion coefficients were significantly greater and transient storage zones were significantly larger in the high-biomass streams compared to the low-biomass streams. Rates of GPP-normalized P uptake from water and rates of P turnover in periphyton were significantly lower in high biomass than in low biomass periphyton communities, suggesting that a greater fraction of the P demand was met by recycling in the high biomass communities. Increases in streamwater P concentration significantly increased GPP-normalized P uptake in high biomass communities, suggesting diffusion limitation of nutrient transfer from stream water to algal cells in these communities. Our results demonstrate that accumulations of periphyton biomass can alter the hydraulic characteristics of streams, particularly by increasing transient storage zones, and can increase internal nutrient cycling. They suggest a close coupling of hydraulic characteristics and nutrient cycling processes in stream ecosystems.     

Sequential colonization by river periphyton analysed by microscopy and molecular fingerprinting

1. An artificial glass substratum was incubated in the River Danube for a period of 28 days in order to detect the sequential colonization of microorganisms.
2. Light and fluorescent microscopy showed that microalgae and the picoalgal fraction on the slides increased rapidly over the first 2 weeks of colonization. Diatoms were numerically the most abundant component of the periphyton and their species richness and diversity increased rapidly in the early phase of colonization whereas diversity subsequently increased moderately.
3. Evenness of the diatom community was initially high, lower in the intermediate phase and again higher later on. Succession involving early, intermediate and late colonizer species was observed. Community composition during the first 5 days of colonization was very different from later stages whereas there were only minor changes subsequently.
4. Molecular community analysis by means of terminal restriction fragment length polymorphism analysis of PCR amplified 16S rRNA and 18S rRNA genes pointed to even larger differences between the composition of samples obtained early and late in the period.
5. The number of 18S rRNA and 16S rRNA terminal restriction fragments (T-RF-s) was variable over the colonization period and the fragment patterns of both the bacterial and eukaryotic portion of the microbial community were variable, with most T-RF-s unique to a single sample, suggesting a wide diversity and dynamic properties of periphytic organisms.     

Water chemistry and periphyton in an alpine wetland

Remote high elevation sites are thought to be good sites to monitor global change and anthropogenic effects on ecosystems. This study was conducted during 1987–1990 in a high elevation wetland (3593 m) located in the Green Lakes Valley, Front Range, Colorado (USA). Salix spp. was the dominant riparian species in this 2 ha. wetland. Small shallow pools (<0.5 m depth) constituted a water area of 236 m³. The major source of water during the study period was snowmelt. The wetland had a well defined outlet and inlet, although an undetermined amount of water entered as groundwater from the snow patch above. Outlet discharge was 424–460 m³ during the month of July and declined thereafter as water input from the snowpatch declined. Inlet discharge was 67% of outlet discharge. Water temperatures in the outlet were always less than 6.8°C, pH 6.0–6.3, and mean conductivity 30.8 µS cm^–1. Both NO _inf3 ^sup– and SO _inf4 ^sup–2 were higher in the inlet thanin the outlet. Dominant cations in the inlet and outlet waters were Ca⁺² Mg⁺² > K⁺ + Na⁺; dominant anions were SO _inf4 ^sup–2 HCO _inf3 ^sup– > NO _inf3 ^sup– Cl^–. Nutrient limitation by P was demonstrated once using nutrient diffusing substrata. No limitation could be shown for NO _inf3 ^sup– , HCO _inf3 ^sup– , or Fe+EDTA. Slow colonization rates of periphyton on tiles were attributed to low temperatures and/or ultraviolet radiation. However, interannual differences in biomass on tiles were as much as 300% after 35 days. A minimum of 16–54 samples would be needed to detect a significant interannual change in biomass on tiles after 35 days assuming that the extreme case for periphyton patchiness. Global climate change is likely to affect discharge and water temperature in this wetland which hill have direct and indirect affects on population dynamics and ecosystem function.     

围栏放牧下土壤-植被碳密度空间分布格局

对围封13年且放牧的冷季高寒矮嵩草草甸,进行了从围栏入口到内部不同距离植被和土壤碳密度状况的调查.结果表明:1)入口到50 m植被现存碳密度平均为1298.0gC·m-2,60～180m有所下降(平均为997.3 g C·m-2),200～300 m反而升高(平均为1285.5 g C·m-2).当年净初级生产碳密度分布趋势与其相同,0～50 m、60 ～180 m和200～300 m平均分别为742.5、571.0和745.7 g C·m-2.这种分布趋势与放牧过程中绵羊觅食频度和强度有关.一般在中央地带放牧强度大,绵羊觅食时间长,边缘地带受围栏效应或围栏外环境因素影响,放牧强度相对较弱,一定程度上对植被生长发育起到了保护作用,使边缘地带植被碳密度得到提高.2)从围栏入口到草场内部土壤碳密度变化趋势表现复杂,入口到100 m增加,100～170 m减小,然后略有升高.土壤碳密度最高值出现在95 m处(15.42 g C·m-2),最低值出现在170 m处(14.12 gC· m-2).目前尚不清楚为何出现这种格局,但至少认为,土壤有机质的动态转化过程受多种因素影响,与植被碳密度相比具有一定的迟滞效应.具体如何影响有机质的动态转化及其迟滞效应,有待进一步研究.     

放牧及土壤斑块质量对白三叶密度及分枝格局的影响

对白三叶长期禁牧草地及土壤斑块质量不同的春季休牧草地的研究结果显示,４月份,在中等放牧强度草地上,约有７６．５％￣８０．４％的白三叶植株为茎部出现断折的残株,放牧草地植株密度明显高于对照草地。放牧草地较好的土壤营养条件使白三叶单株构件数量明显上升,但使分枝强度略有下降。白三叶各构件密度之间存在显著正相关。不同样地上白三叶的分枝均以２、３级分枝为主。放牧草地的分枝角度大小顺序为：１级分枝角＞２级分枝     

Effects of nutrients and light on periphyton biomass and nitrogen uptake in Mediterranean streams with contrasting land uses

1. Nutrient diffusing substrata (NDS) were used to determine the relative importance of nutrients and light as potential limiting factors of periphyton biomass and nitrogen (N) uptake in Mediterranean streams subjected to different human impacts. The nutrients examined were phosphorus (P) and N, and we also further differentiated between the response of periphyton communities to N species (i.e. NO₃‐N and NH₄‐N). To examine the effect of light and nutrients on periphyton biomass, chlorophyll a accrual rates on NDS located at open and closed canopy sites were compared. The effect of nutrient availability on periphyton uptake was measured by ¹⁵N changes on the NDS after NO₃‐¹⁵N short‐term nutrient additions. 2. Results show that light was the main factor affecting algal biomass in the study streams. Algal biomass was in general higher at open than at closed canopy sites. Nutrient availability, as simulated with the NDS experiments, did not enhance algal biomass accrual in either of the 2 light conditions. 3. In the control treatments (i.e. ambient concentrations), periphyton NO₃‐N uptake rates increased and C : N molar ratios decreased consistently with increases in N availability across streams. NO₃‐N uptake rates were altered when ambient N concentrations were increased artificially in the N amended NDS. Periphyton assemblages growing on N enriched substrata seemed to preferentially take up N diffusing from the substratum rather than N from the water column. This response differed among streams, and depended on ambient N availability. 4. Periphyton biomass was not significantly different between substrata exposed to the two forms of available N sources. Nonetheless, we found differences in the effects of both N sources on the uptake of N from the water column. NH₄‐N seemed to be the preferred source of N for periphyton growing on NDS. 5. Results suggest that the effect of riparian zones on light availability, although seldom considered by water managers, may be more important than nutrients in controlling eutrophication effects derived from human activities. Finally, our results confirm that not only increases in concentration, but also stoichiometric imbalances should be considered when examining N retention in human altered streams.     

管理措施对牧鸡生长性能及草地蝗虫数量的影响

蝗灾作为威胁草地生态系统的主要灾害之一,给草地畜牧业发展带来严重影响,如何经济、有效和安全抑制蝗虫数量显得尤为紧迫。通过野外调查就草地牧鸡放牧强度、补饲量以及放牧方式对牧鸡生长性能和蝗虫发生数量的影响进行探讨。结果发现,高山草地利用牧鸡放牧可有效控制蝗虫发生数量,长期连续牧鸡捕食能有效将蝗虫数量控制在经济阈值之下;不同放牧强度(15只/hm2和13只/hm2)和补饲程序对蝗虫种群数量没有显著差异,但对牧鸡生长性能有明显影响;自由放牧和限时放牧均能控制蝗虫数量,但限时放牧牧鸡日增重和最终活重均高于自由放牧。综合防治蝗虫和养育牧鸡来看,每公顷草场初次放牧15只牧鸡,限时放牧和适当增加补饲量可达到控制蝗虫和获得较高禽肉生产之双赢目的。     

Seasonal and artificially elevated temperatures influence bioenergetic allocation patterns in the common pond snail, Physella virgata

Allocation of organic carbon (OC) to primary energetic pathways was estimated under seasonal and artificially elevated ambient temperatures for a field population of a freshwater pulmonate snail, Physella virgata. Allocation to respiration increased with temperature. Snails allocated most assimilated OC to reproduction within their natural temperature range (15 degrees -35 degrees C), where assimilation efficiencies remained relatively stable at 25%-35%. However, in artificially heated waters exceeding 35 degrees C, declining assimilation rates and increasing respiratory demands inhibited allocation to reproduction and growth. At the species' 40 degrees C upper thermal limit, assimilation efficiencies fell below 10%, while average consumption levels more than doubled relative to snails unaffected by the thermal effluent. Ambient temperature substantially influenced OC allocation over P. virgata's natural temperature range and negatively affected growth and reproduction at temperatures approaching or exceeding maximum natural levels.     

The potential of fish production based on periphyton

Periphyton is composed of attached plant andanimal organisms embedded in amucopolysaccharide matrix. This reviewsummarizes research on periphyton-based fishproduction and on periphyton productivity andingestion by fish, and explores the potentialof developing periphyton-based aquaculture.Important systems with periphyton arebrush-parks in lagoon areas and freshwaterponds with maximum extrapolated fish productionof 8 t ha^–1 y^–1 and 7 t ha^–1y^–1, respectively. Experiments with avariety of substrates and fish species havebeen done, sometimes with supplemental feeding.In most experiments, fish production wasgreater with additional substrates compared tocontrols without substrates. Colonization ofsubstrates starts with the deposition oforganic substances and attraction of bacteria,followed by algae and invertebrates. Afterinitial colonization, biomass density increasesto a maximum when competition for light andnutrients prevents a further increase. Often,more than 50% of the periphyton ash-free drymatter is of non-algal origin. Highest biomass(dm) in natural systems ranges from 0 to 700g m^–2 and in aquaculture experiments wasaround 100 g m^–2. Highest productivity wasfound on bamboo in brush-parks (7.9 gC m^–2 d^–1) and on coral reefs (3 gC m^–2 d^–1). Inorganic and organicnutrients stimulate periphyton production.Grazing is the main factor determiningperiphyton density, while substrate type alsoaffects productivity and biomass. Better growthwas observed on natural (tree branches andbamboo) than on artifical materials (plasticand PVC). Many herbivorous and omnivorous fishcan utilize periphyton. Estimates of periphytoningestion by fish range from 0.24 to 112 mg dm(g fish)^–1 d^–1. Ingestion rates areinfluenced by temperature, fish size, fishspecies and the nutritional quality of theperiphyton. Periphyton composition is generallysimilar to that of natural feeds in fishponds,with a higher ash content due to the entrapmentof sand particles and formation of carbonates.Protein/Metabolizable Energy (P/ME) ratios ofperiphyton vary from 10 to 40 kJ g^–1.Overall assimilation efficiency of fish growingon periphyton was 20–50%. The limited work onfeed conversion ratios resulted in valuesbetween 2 and 3. A simple simulation model ofperiphyton-based fish production estimates fishproduction at approximately 2.8 t ha^–1y^–1. Together with other food resources infishponds, total fish production with thecurrent technology level is estimated at about5 t ha^–1 y^–1. Because grazingpressure is determined by fish stocking rates,productivity of periphyton is currently themain factor limiting fish production. Weconclude that periphyton can increase theproductivity and efficiency of aquaculturesystems, but more research is needed foroptimization. Areas for attention include theimplementation and control of periphytonproduction (nutrient levels, substate types andconformations), the ratio of fish to periphytonbiomass, options for utilizing periphyton inintensive aquaculture systems and with marinefish, and possibilities for periphyton-basedshrimp culture.     

Responses of a lake outlet community to light and nutrient manipulation: effects on periphyton and invertebrate biomass and composition

1.  Experimental channels were installed at the outlet of a small Canadian Shield lake to study the role of light and nutrients on the regulation of periphyton and invertebrate biomass and taxonomic composition. Light (93% reduction) and nutrients (four-fold increase of ambient total phosphorus (TP) concentration) were manipulated in a factorial design experiment.
2.  Periphyton chlorophyll a (Chl a ), measured four times during the 12-week experiment, increased due to higher irradiance but was unaffected by phosphorus enrichment. Over the experiment, periphyton biomass was, on average, three times higher in open than in shaded channels.
3.  Algal taxonomic and growth form composition were affected by light and phosphorus enrichment. The proportion of cyanophytes was significantly higher in unenriched, shaded channels (45%) compared to the three other treatments. Single cells and colonial forms were dominant in shaded channels, whereas filamentous and chain-forming algae were prevalent in open channels.
4.  Total invertebrate biomass remained unchanged over time and among treatments. At the beginning of the experiment, all the channels were dominated by Chironomidae and filter feeders (Simuliidae and Hydropsychidae). After 55 days, filter feeders became clearly dominant (60%) in all treatments except in the enriched, open channels where there was a significant shift in the functional group composition toward grazers (snails and oligochaetes). Among filter feeders, Simuliidae increased in shaded channels.     

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

相似文献   

The influence of mating system,demography, parasites and colonization on the population structure of Biomphalaria pfeifferi in Madagascar

Current evolutionary forces and historical processes interact to shape the distribution of neutral genetic variability within and among populations. Focusing on the genetics of recently introduced organisms offers a good opportunity to understand the relative importance of these factors. This study concerns variation at 8 polymorphic microsatellite loci in 30 populations of Biomphalaria pfeifferi. The sampling area spans most of the species' range in Madagascar where it was probably introduced recently. Extremely low variation was found within all populations studied, which may partly result from high selfing rates. However, this cannot account for the variance of variation across populations, which is better explained by habitat openness (that reflects environmental stochasticity), the prevalence of the parasitic trematode Schistosoma mansoni and historical demography (colonization and subsequent bottlenecks). Large global differentiation was also observed, suggesting that current gene flow among populations is limited to small distances, within watersheds and to few individuals. Our data set also allowed us to test several hypotheses regarding colonization, based on bottleneck and admixture tests. The observed pattern requires at least two independent introductions from slightly differentiated genetic sources in the western part of Madagascar. Another introduction, from a very different genetic origin, should also be postulated to explain the genetic composition of eastern populations. That this introduction occurred recently suggests that the colonization of Madagascar by B. pfeifferi is an ongoing process.     

Alteration of trophic interactions between periphyton and invertebrates in an acidified stream: a stable carbon isotope study

The hypothesis according to which proliferation of periphytic algae under acid conditions results from a release of grazing pressure is tested. Stable carbon isotope analysis is used to investigate the autochthonous/allochthonous balance of invertebrate feeding in streamside artificial channels that were experimentally acidified. We find that the relative contribution of autochthonous food sources (epilithon) to total invertebrate biomass was slightly lower (after 1 mo of acidification) or not altered (after 2 mo) under acidified conditions when compared with a control. Feeding shifts were exhibited by some invertebrate taxa and provided evidence that acidification modifies trophic interactions between attached algae and primary consumers. Cross-treatment calculations showed that reduction of grazing pressure after the first month of acidification was an effect rather than the cause of periphyton proliferation. Our approach using stable carbon isotope analysis and biomass measurements of macroinvertebrates allows the quantification of the trophic base of lotic secondary producer communities under both experimental and natural conditions.