Green algae as a structural element of phytoperiphyton communities in streams of NW Russia

Observations were made on the development and distribution of phytoperiphyton communities in 66 lake-river systems in NW Russia from Lake Ladoga to the Barents Sea. In total, 130 genera and 648 species were identified from different substrates, belonging to Cyanophyta (19.1%), Bacillariophyta (59.6%), Chlorophyta (18.7%), and algae from other orders (2.6%). In all streams diatoms dominated by species richness, but they were surpassed by green algae in terms of biomass. The green algae ranged from small planktonic forms to large filamentous species and produced easily visible algal communities. Among the planktonic forms the desmids were the most diverse group. They occurred in attached communities of all rivers and, while never abundant, were widespread. The attached community’s biomass was dominated by green algae. Among these, the filamentous algae Mougeotia sp., Oedogonium sp., Zygnema sp., Spirogyra sp. and Ulothrix zonata exhibited mass development in streams. Their distribution was patchy in the basin, with a total cover varying from less than 1% to 90% of the stream bottom. In some river stretches the diversity and predominance of green algae could be due, in part, to poorly developed riparian canopies. The term periphyton adopted here follows the definition of Odum (1971): “Assemblages which include both plant and animal organisms growing attached to submerged objects”. The prefix phyto- is added to indicate that of the whole biocoenoses only phototrophs are considered in this study. Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007, Slovakia.     

空间层次及人工基质对着生藻类建群特性的影响

为全面了解着生藻类在建群中群落变化的生态学特性,揭示着生藻类的建群规律,在以丝状藻类为优势藻的生态塘中,采用花岗岩和瓷砖为附着材料,设置水体底部和中部为附着位点,进行频次为10d的采样分析。结果表明,生态塘中共检出8门73属117种着生藻类,其中以硅藻、蓝藻、绿藻为优势类群。同时不同人工基质和不同空间层次条件下着生藻类的建群特征较一致,早期以单细胞硅藻如舟形藻(Navicula sp.)、脆杆藻(Fragilaria sp.)、曲壳藻(Achnanthes sp.)等为优势,后期以丝状藻类如鞘丝藻(Lyngbya sp.)、颤藻(Oscillatoria sp.)、伪鱼腥藻(Pseudanabaena sp.)等为优势;研究结果发现不同人工基质(花岗岩和瓷砖)对着生藻类的种类组成、细胞密度、生物量和藻类多样性无显著影响,花岗岩和瓷砖上附着的着生藻类具有较高的相似性;但不同的空间层次对着生藻类建群特征影响明显,水体底部具有更多的硅藻种类数,中部具有更多的绿藻,随着建群时间的发展,蓝藻比例不断增加;就生物量而言,底部的着生藻类叶绿素a显著高于水体中部,但两者的细胞密度无显著性差异;随着建群过程的发展,水体底部的着生藻类生物量达峰值所需的时间比中部更长。通过相关性分析,生态塘中着生藻类的生长主要受总磷的影响。     

Mechanisms responsible for the development of periphyton community structure during seasonal succession: the role of interspecies competition and plankton sedimentation

The development of periphyton community structure by exchange of organisms between substratum and water column (noninteractive mechanism) and by interspecific competition for surface (interactive mechanism) was studied during seasonal succession in Akulovsky water supply channel (the Upper Volga basin). The influence of exchange was assumed by similarity between the species composition of plankton and periphyton. At early stages of succession when the diatoms dominated in periphyton the community was formed mainly by phytoplankton sedimentation, while the competition for substratum didn't result in decrease of species diversity because the poor competitors were partly displaced by new colonists from the water column. Later when the green filamentous algae abundantly developed in periphyton, their numbers were probably controlled by factors not related to exchange of propagules. At the same time, the species structure of secondary periphyton cover developing on the thallus of filamentous algae depended mainly on the plankton sedimentation. At the last stages of seasonal succession when periphyton was represented by colonies of cyanobacteria and diatoms closely covering the substratum, the exchange of organisms between substratum and water column was not so important as interspecific competition for surface. As one could suppose, increase in biomass in this period resulted in the decrease of specificity as it was predicted by hypothesis of interactive community. In such a way, both mechanisms (interactive and noninteractive ones) took part in development of periphyton structure. Their relative influence changed in the course of seasonal succession.     

Laboratory experiments on the impact of biofilms on the plankton of a large river

1. Benthic organisms can have a strong effect on the plankton in rivers, although normally only members of the macrofauna are considered as important consumers. In the present study we conducted experiments on four different dates (in December, March, June and September) to assess the potential role of periphytic heterotrophic flagellates (HF), ciliates and rotifers in the control of potamoplankton (bacteria, algae, HF and ciliates). 2. Natural periphyton was established on the walls of circular flow channels by exposing them to river water (River Rhine, Germany). The experimental channels (with periphyton) and control channels (without periphyton) were filled with riverine water and the increase rates of planktonic bacteria, algae, HF and oligotrich ciliates were determined for the two treatments. 3. The abundance of periphytic ciliates and rotifers at the beginning of the four experiments showed large differences with low values in December and March, and high values in June and September. Dominant potential consumers of plankton were the heterotrich ciliate Stentor sp. and bdelloid rotifers. 4. The rates of increase of planktonic algae, HF and ciliates were significantly smaller in the presence of periphyton compared with those in their absence. Significant interactions between the treatment (with and without periphyton) and the time of experiment were found for the planktonic HF and algae, indicating that the impact of the periphyton varied temporarily. The planktonic groups responded differently to the periphyton with the planktonic HF showing the highest loss rate. Significant differences were also found among the loss rates of different HF groups and different diatom size classes. 5. These laboratory experiments demonstrate that periphytic ciliates and rotifers are potentially important consumers of different planktonic groups. The quantitative impact of periphyton on plankton with respect to the selective feeding needs further attention.     

Periphytic and drifting microalgae in a tributary stream of Øvre Heimdalsvatn

Samples of drifting and periphytic microalgae were collected during 1972 from a fast-flowing, stony stream (Brurskardsbekken) in the Jotunheimen mountain area, central southern Norway. The predominant algal groups in the drift and the periphyton were diatoms and green algae, while only a few species were recorded in both communities. A considerable number of species from Chrysophyceae, Cryptophyceae and other algal classes were also recorded in the drift samples. The species composition was in good agreement with microalgal communities earlier described from mountain areas in Scandinavia, although species which probably are new to Norway, were also recorded. A quantitatively important fraction of planktonic species in the drift is interpreted as a contribution from lacustrine habitats in the watercourse. A general change in the periphyton during the summer, from green algae to diatoms, was observed, Altitudinal differences in the periphyton included a delayed green algal maximum at higher altitudes compared to lower. In the zone around the upper birch limit, a transition in species composition as one goes up stream, described in other investigations, was not observed.     

COLONIZATION AND COMMUNITY STRUCTURE OF TWO PERIPHYTON ASSEMBLAGES,WITH EMPHASIS ON THE DIATOMS (BACILLARIOPHYCEAE)

Examination of the three-dimensional structure of periphyton communities through time indicated that their microsuccession is analogous to higher plant succession. The development of attached diatom communities in two reservoirs was studied using artificial substrates, and the morphology of dominant organisms, patterns of spatial heterogeneity, and community interactions were documented with scanning electron microscopy. Of 93 taxa found, Gomphonema parvulum, G. olivaceum, Navicula graciloides, Nitzschia palea, and N. dissipata were dominant, depending upon season and reservoir. Comparisons of community diversity (SIMI) between reservoirs within seasons ranged from 0.023–0.843 (median = 0.254), indicating that the reservoirs were quite different with respect to diatoms present and their apportionments. Colonization was slow the first 2 wk in spring and fall, and throughout winter, but rapid during summer. Shifts in numerical dominance between certain species occurred in fall, spring, and summer. These inverse correlations of abundances and the functional dominance of overgrowth suggested competition for substrate surface area in the periphyton. The colonization sequence was often predictable—a presumably organic coating and a variety of bacteria, followed by low profile diatoms, and finally an upperstory of long-stalked and large-rosette diatoms and filamentous green algae. Periphyton microsuccession is similar to higher plant succession in the consistent change in vertical community structure from low to high physical stature, in the association of numerical dominance with large stature (via cell size or long mucilaginous stalks), and in the progressive slow-down in the rate of succession. Diatom mucilage also contributed to community structure by binding particulates and entrapping other algae and serving as the mechanism for substrate attachment.     

Feeding habits of Schizothorax oconnori Lloyd, 1908 in the Yarlung Zangbo River,Tibet

Seasonal, ontogenetic and sexual feeding habits of Schizothorax oconnori in the Yarlung Zangbo River were studied using percentages by number, weight and index of relative importance (IRI). The Schoener overlap index was applied to compare diets related to season, fish size and sex. The feeding intensity exhibited seasonal trends, with a minimum food intake in summer. Overall, 103 prey taxa belonging to seven prey categories (diatoms, green algae, cyanophytes, other algae, small invertebrates, macroinvertebrates and remains) were identified in 136 guts. Highest feeding diversity and evenness were recorded in summer, while lowest values of both indices occurred in autumn. S. oconnori fed almost exclusively on periphyton. Diatoms were the most important prey in terms of number, weight and IRI, followed by macroinvertebrates; other prey were also occasionally ingested. S. oconnori is a generalized and opportunistic feeder relying upon a wide trophic spectrum, but mainly feeds on diatoms as they are the most prevalent periphyton. There were high diet overlaps among seasons, fish sizes and between sexes. S. oconnori showed no dietary preference for any of the other prey available in the periphyton community.     

The effect of fetch on periphyton spatial variation

The marked spatial variation in periphyton could reflect differences in exposure, grazing or substratum. To determine if any of these factors was significant, I studied the relationship between the degree of exposure to waves (measured as fetch), grazing intensity (measured as invertebrate biomass) and spatial variation in periphyton biomass in sites of similar substratum along an island in the central mesotrophic basin of Lake Memphremagog (Québec). In spring, there was a positive relationship between fetch and periphyton biomass both on stones and on artificial substrata. This effect was especially strong for diatoms, for algae between 100–1000 µm³, for planktonic forms and for filamentous and long-stalked species. In spring, water renewal encouraged growth of these forms, but the effect disappeared in summer, coincident with silica depletion. Grazers, which were not important in the spatial variation observed in spring, probably contributed to the sudden decline of periphyton in the exposed sites in summer. Observations in nearby lakes indicate that these patterns may be general.Contribution of the Lake Memphremagog Project, Limnology Research Centre.     

Architecture and development of periphyton on reed-stems in Lake Maarsseveen

Summary This study forms part of investigations on the importance of periphyton in the lake ecosystem. The substratum consists of submerged parts ofPhragmites australis in their growing season as well as older ones. Significant differences in occurrence of several species of algae were found between lakeside and bankside of the stems, peripheral stems and central stems in the reed stand, stems of the current year and stems of the previous year, as well as between upper, middle and lower parts of the submerged stems.There are differences inAchnanthes/Gomphonema ratio at various localities. This is most evident in early winter, being obscured in spring and summer by an overall rise in number ofAchnanthes.In the month of May the young sprouts are rapidly colonised by green algae and diatoms. The green algae offer an additional substratum, and therefore enlarge the available substratum for diatom growth. It appears that the green algae are better colonisers than the diatoms. During the summer the green algaeOedogonium andBulbochaete become covered by a layer of lime, which becomes densely packed with diatoms. A significant rise in the number of diatoms is also due to the additional substratum offered by stalks and tubes ofCymbella species. In December the lime disappears as do nearly all green algae.Cymbella lanceolata andC. prostrata then begin an explosive bloom, which continues to the end of the winter. The total number of diatoms can rise to nearly 2,000,000/cm², the half of which is situated on stalks and tubes ofCymbella. When in April mostCymbella disappears, the total number of diatoms decreases also abruptly. The stem, now nearly one year old, is then barely grown over with periphyton. Soon the development of the periphyton on this stem corresponds to the settlement of epiphytes on the new sprouts.The presence of the periphyton itself offers a considerable temporal additional substratum. Great numbers of diatoms may occur onCymbella stalks. Multitudes of diatoms may be present onBulbochaete andOedogonium. The winter situation reveals a storied growth, depending on the stalk length of the various diatoms. There also is a zonation in diatom growth on large stalks,Achnanthes growing near the reed surface, andSynedra preferring the free space. In between the sessile algae the chains and guirlandes of the araphid genera are woven.In this way the periphyton forms a firm unity, not only functional, but structural as well. *** DIRECT SUPPORT *** A0692010 00005     

Effect of pollution on phytoplankton in the Ashar Canal,a highly polluted canal of the Shatt al-Arab Estuary at Basrah,Iraq

The effect of pollution on the phytoplankton in the Ashar Canal was studied during March and June 1976. Higher temperatures were recorded in June. The canal water was more turbid than that of the adjoining Shatt al-Arab estuary due to high algal populations and sewage disposal. The chlorosity values were remarkably high in the canal, due to sewage disposal. The flood waters in June lowered the chlorosity in both estuary and canal. Sewage disposal into the canal decreased its dissolved oxygen content. The lower oxygen values in the estuary in June were accompanied with higher temperatures. The pH values were generally higher in the estuary and were also higher in June than in March.Eighty-six algal taxa were recorded, 45 of which were diatoms. The phytoplankton in the estuary was more abundant in March, due to the increase in currents and turbidity in June. The Chlorophyta, Conjugatophyta, Myxophyta and Bacillariophyta in the estuary showed reverse seasonal trends from those of the total phytoplankton. These algal seemed to flourish during periods of high temperatures. The dense blue-green algae recorded in June is attributed to the high temperature and dissolved organic matter. The Chrysophyta recorded high populations in the estuary in March and were totally absent in June. Lower temperatures and high transparency in March might have led to their relative abundance.The total phytoplankton in the Ashar Canal followed two different seasonal trends, one resembling and the other differing from those in the estuary. The disposal of domestic sewage and agricultural wastes in the canal seemed to lead to such adverse seasonal variations. The diatoms only dominated the phytoplankton at one station in both months. The sewage and agricultural wastes disposal seemed to favour the abundance of the other phyla than the diatoms. Increased organic load in the canal has led to a considerable increase in the Myxophyta in June. Blooms of Chlamydomonas sp. and Euglena acus increased the phytoplankton densities in March at stations II and IV, respectively. Benthic or periphytic algae were found in the phytoplankton populations. Raised salinity seemed to restrict the distribution of the planktonic diatom Bacillaria paradoxa. Among the noticeable widespread planktonic algae in the study area are Dinobryon sertularia, Pediastrum duplex and P. simplex.     

Periphyton distribution and abundance on substrata of different size along a gradient of stream trophy de Montréal

In stream, substrata of different size present different degree ofstability, current, erosion, and deposition to colonizingorganisms. In this study, we tested the importance of substratumsize ranging from sand to small boulders for periphytondistribution and abundance. Because trophy strongly affect streamorganisms, we sampled at nine sites chosen to represent the rangeof nutrients typical of Eastern Ontario and Western Québec. Alarge part of the variability in algal biomass (as chlorophyll)among sites was explained by trophy (as seston or totalphosphorus). However, there was also an effect of substratum size.Cobbles had the highest biomass, and gravel the lowest; sand andboulders were intermediate. Assemblages on different substrata weredifferent in taxonomy and life forms. Cyanobacterial colonies andmotile diatoms dominated the finer substrata while adnate andfilamentous algae were more developed on the larger ones.Consequently periphyton on fine sediments was more loosely attachedthan on rocks. Average algal size was not related to substratumsize but increased significantly with trophy confirmingobservations in benthic and planktonic assemblages inlakes.     

基于着生藻类的太子河流域水生态系统健康评价

本研究以辽宁省太子河流域为研究范例,调查了全流域范围内69个样点的着生藻类群落和水环境理化特征,并在此基础上应用硅藻生物评价指数(DBI)和生物完整性评价指数(P-IBI),同时结合栖息地环境质量评价指数(QHEI),对太子河流域水生态系统进行健康评价。结果表明,太子河流域着生藻类群落结构具有明显的空间异质性,CCA结果显示驱动着生藻类群落结构形成的水环境因子为电导率、总溶解固体和总氮。虽然DBI、P-IBI和QHEI在太子河流域某些河段上的评价结果有较大出入,但从全流域尺度上看,DBI、P-IBI和QHEI的评价结果基本一致,表现为太子河上游健康状况较好,中游健康状况一般,而下游健康状况较差。文中讨论了水环境理化因子与着生藻类群落结构的相互关系,并对比分析了DBI、P-IBI和QHEI这三种河流健康评价方法。     

Seasonal variations of diatoms and dinoflagellates in a shallow,temperate estuary,with emphasis on neritic assemblages

Seasonal changes in the diatom and dinoflagellate assemblages were examined in the neritic zone of the Urdaibai estuary (north Spain) with regard to some major physical and chemical variables during an annual cycle. A total of 81 diatoms and 38 dinoflagellates were identified and quantified during the study period. Both groups displayed a distinctive pattern of seasonal succession. The seasonal distribution of the Shannon index showed a trend of increasing values from the upper estuary to the lower neritic segment. The diatom diversity maxima were observed in February, April and September, and dinoflagellate maxima in April–May, July and October. Diatoms dominated the assemblages, reaching 1×10⁶ cells l^–1 from April to September. A shift from large diatoms and dinoflagellates to small bloom-forming taxa was observed during winter–early spring. A spring diatom bloom composed of Rhizosolenia spp. was observed in April, while small chain-forming taxa (chiefly Chaetoceros spp.) dominated from June to September. Cell maxima for both groups in late summer were produced by the diatoms Chaetoceros salsugineum and Skeletonema costatum, and by the dinoflagellates Heterocapsa pygmaea and Peridinium quinquecorne. Silicate availability by river supply and strong tidal-mixing of the water column seem to determine the year-round dominance of diatoms over dinoflagellates.     

Herbivorous animals can mitigate unfavourable ratios of energy and material supplies by enhancing nutrient recycling

Recent evidence shows that high supply ratios of light and nutrients limit planktonic herbivore growth by lowering the nutritional quality of algae. Over longer time scales, however, grazers may ameliorate this effect by their impact on nutrient cycling. We examine this possibility using two species of the herbivorous zooplankter Daphnia and its algal prey under different light intensities and low phosphorus supply in laboratory microcosms. At high light, Daphnia biomass was limited for a substantial period because of low P content of algal cells. However, a gradual increase in Daphnia density eventually improved food quality through grazing and nutrient cycling and via a novel process involving positive density dependence. Competitive exclusion of one of the two Daphnia species occurred under low light but not under high light when algae were nutritionally unsuitable. Such stoichiometrically mediated interactions among herbivorous animals may represent important mechanisms that affect community structure and material flows in ecosystems.     

Periphyton in Lake Vechten,with emphasis on biomass and production of epiphytic algae

Summary Characteristics of the periphyton associated with the submerged macrophytesCeratophyllum, Elodea andMyriophyllum were similar. During the whole growing season the periphyton dry weight often exceeded that of the macrophyte. The periphyton contained much detritus and carbonates, and the epiphytic algae were accompanied by large numbers of bacteria, protozoans and rotifers. Attached algae were scarce, but motile diatoms were abundant throughout the year. Thus the periphyton showed the features of a benthic community. Consequently, the role of the macrophytes would be rather the extension of the bottom area instead of actively promoting the growth of microorganisms by providing minerals or excreting organic substances.During the peak of the littoral development the epiphytic algae expressed a high photosynthetic capacity, while at the same time the periphyton dark respiration was intense. After measurements of the depth distribution of biomass and light in aCeratophyllum stand, it was concluded that the contribution of the epiphytic algae to the production in the lake's littoral zone was substantial. Furthermore, on a daily basis, the total association of macrophytes and periphyton would be oxygen consuming due to heterotrophic activity, notably mineralization of the detritus.     

西湖湖西浮游与着生藻类季节变化及相互关系

2014年11月至2015年8月调查了西湖湖西沿岸带浮游藻类和不同基质(植物、石块和底泥)上着生藻类的群落结构及季节变化, 分析了着生丝状藻与浮游丝状藻的相互关系以及它们与环境因子的相关性, 探讨湖西生态修复过程中季节性暴发的丝状藻水华的原因。结果表明浮游藻类和植物、底泥、石头上着生藻类中均以硅藻门种类数(分别占52.5%、60.4%、86.7%和72.7%)最多, 蓝藻门(分别占10.1%、8.9%、6.7%和15.2%)和绿藻门(分别占26.3%、19.8%、5.6%和10.6%)次之, 其他门类相对较少, 浮游藻类与着生藻类优势种季节差异较大。附植丝状藻密度显著高于附泥和附石丝状藻, 且狐尾藻上着生丝状藻密度与浮游丝状藻密度呈显著正相关, 表明狐尾藻着生丝状藻可能是浮游丝状藻较重要的来源之一, 该结果可为西湖丝状藻水华的控制提供一些参考。相关性分析表明, 着生藻类和丝状藻与各理化因子(水深、透明度、溶解氧、水温、pH、TN、SRP、TP等)均无显著相关性。     

Uncoupling of omnivore-mediated positive and negative effects on periphyton mats

The riverine grass shrimp (Palaemonetes paludosus) and eastern mosquitofish (Gambusia holbrooki) consume periphyton and small invertebrates, potentially affecting periphyton through negative effects (i.e., consumption) and/or positive effects such as nutrient regeneration, physical stimulation, and trophic cascades. We performed field experiments in the Everglades in which omnivores and periphyton were maintained in cages, with a fraction of the periphyton held in omnivore-exclusion bags that allowed passage of nutrients but prevented its consumption or physical disturbance. In some instances, periphyton growth rate increased with increasing omnivore biomass. Omnivores probably stimulated periphyton growth through nutrient regeneration, possibly subsidizing periphyton with nutrients derived from ingested animal prey. The net balance of omnivore-mediated negative and positive effects varied among experiments because of seasonal and spatial differences in periphyton characteristics. Consumption of periphyton mats might have been reduced by the arrangement of palatable algae (green algae and diatoms) within a matrix of unpalatable ones (CaCO(3)-encrusting filamentous cyanobacteria). In a laboratory feeding experiment, mosquitofish consumed more green algae and diatoms in treatments with disrupted mat structure than in those with intact mats. No difference in diet was observed for shrimp. Our study underscores the complexity of consumer-periphyton interactions in which periphyton edibility affects herbivory and consumers influence periphyton through multiple routes that cannot be fully appreciated in experiments that only investigate net effects.     

Periphytic algal community adaptive strategies in N and P enriched experiments in a tropical oligotrophic reservoir

This study aimed at evaluating periphytic algae adaptive strategies, including size class, growth, and adherence forms, and the CRS functional groups model under nitrogen and phosphorus artificial enrichment in a Brazilian tropical shallow and oligotrophic reservoir. Four treatments were designed using enclosures (n = 3) filled with 185 L of reservoir water: control (no nutrient addition), P+ (isolated P addition, N-limiting condition); N+ (isolated N addition, P-limiting condition); and NP+ (combined addition, no limitation). N:P ratios were maintained throughout the experiment. Glass slides were used for periphyton growth, and sampling campaigns were carried out at short regular intervals (3–5 days) over a 31-day succession. Enrichment favored replacement of flagellates and loosely attached (mobile) forms by firmly attached mainly prostrate and entangled forms over succession. Isolated or combined P addition favored C–S-strategists green algae, whereas P limitation kept R-strategists over succession, promoting R-Cyanobacteria under high P limitation. Our results were supported by the species density and biovolume due to the dominance of small size classes (small-sized classes) in the community (nano and picoperiphyton). Overall, only the CRS strategies were more predictive of the experimental nutritional conditions.     

Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms

1. Anthropogenic activities in prairie streams are increasing nutrient inputs and altering stream communities. Understanding the role of large consumers such as fish in regulating periphyton structure and nutritional content is necessary to predict how changing diversity will interact with nutrient enrichment to regulate stream nutrient processing and retention. 2. We characterised the importance of grazing fish on stream nutrient storage and cycling following a simulated flood under different nutrient regimes by crossing six nutrient concentrations with six densities of a grazing minnow (southern redbelly dace, Phoxinus erythrogaster) in large outdoor mesocosms. We measured the biomass and stoichiometry of overstory and understory periphyton layers, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles to evaluate whether fish were selectively foraging within or among habitats. 3. Model selection indicated nutrient loading and fish density were important to algal composition and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favoured diatom growth over green algae and decreased periphyton C : N. Increasing grazer density did not affect biomass and reduced the C : N of overstory, but not understory periphyton. Algal composition of dace diet was correlated with available algae, but there were proportionately more diatoms present in dace guts. We found no correlation between fish egestion/excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. 4. Large grazers and nutrient availability can have a spatially distinct influence at a microhabitat scale on the nutrient status of primary producers in streams.     

Fish-mediated indirect effects in a littoral food web

The strength of the direct effect by scraping cladocerans and the indirect effect of nutrient regeneration by filtering herbivorous cladocerans on periphyton growth was investigated in a littoral food web. Ten enclosures were erected in a lake in an area with artificial vegetation. Five enclosures were stocked with juvenile perch ( Perca fluviatilis ) and five lacked fish. In addition, a reference area in the artificial vegetation was sampled. The mesh size of the net surrounding the cages was chosen to allow an inflow of phytoplankton into the cages from the surrounding water. The periphyton and filtering herbivorous cladoceran biomasses were highest in the fish-free treatment. There was no difference in phytoplankton biomass between treatments despite the large difference in filtering herbivorous cladoceran biomass, suggesting that the inflow of phytoplankton into enclosures completely compensated for the loss due to filtering. The reference area and the enclosure with fish showed the same patterns in developments with respect to filtering herbivorous cladocerans and periphyton. Scraping cladoceran biomass was higher in the fish-free treatment resulting in a positive correlation between scraping cladoceran and periphyton biomass. Our results suggest that the positive indirect effect of filtering herbivorous cladoceran nutrient regeneration on periphyton was stronger than the negative direct effect of grazing by scraping cladocerans on periphyton in this semi-open system, and that pelagic production by phytoplankton may foster periphyton growth in the littoral habitat via filtering herbivorous cladocerans. Furthermore, heterogeneity within trophic levels involving primary producers of different growth forms such as phytoplankton and periphyton may enhance the potential for compensatory responses via nutrient recycling.