Sieving functional morphology of the gill raker feeding apparatus of atlantic menhaden

Menhaden occupy an important position in estuarine food webs, thus the rate processes associated with their feeding are critical to the ecosystem management of fishery and ecological resources. Atlantic menhaden feed on a wide range of plankton, the size and food quality of which change ontogenetically. We analyzed the functional morphology of the menhaden feeding apparatus in a size series of menhaden representative of juveniles and the adult migratory stock. The physical dimensions of gill arches and rakers increased isometrically with fish length; however, branchiospinule spacing, the dimension that forms the sieve apertures of the branchial basket, scaled allometrically with fish length. Juvenile menhaden from North Carolina have branchiospinule spacings that averaged 12 microm, with three arch subsections of average spacing < 10 microm. Spacings did not increase with juvenile growth until the first allometric inflection point at approximately 100 mm fork length (FL). Spacing data for juveniles from other locations suggests spacing increases with latitude. Spacings increase with fish length in adults until a second inflection at 200 mm FL, after which spacing averages 37 microm. These data suggest menhaden juveniles filter smaller plankton with higher filtration efficiency than previously considered and that regional recruitment may affect adult distribution through foraging preferences.     

Predictability and possible mechanisms of plankton response to reduction of planktivorous fish

The predictability of plankton response to reductions of planktivorous fish was investigated by comparing the plankton community in three biomanipulated lakes and ten unmanipulated lakes differing in intensity of fish predation. Data collected on total phosphorus, phytoplankton and zooplankton biomass and share of cyanobacteria and large grazers, as well as specific growth rate of phytoplankton, were further used to test some of the proposed underlying response-mechanisms. In the biomanipulated lakes the algal biomass and share of cyanobacteria decreased, specific growth rate of phytoplankton increased, and zooplankton biomass and share of large grazers increased or remained unchanged. This pattern was largely reflected in the differences in food-chain structure between the unmanipulated lakes with highversus those with low fish predation. The qualitative response to planktivorous fish reduction thus seems largely predictable. The biomanipulated lakes differed, however, in magnitude of response: the smallest hypertrophic, rotenone-treated lake (Helgetjern) showed the most dramatic response, whereas the large, deep mesotrophic lake (Gjersjøen), which was stocked with piscivorous fish, showed more moderate response, probably approaching a new steady state. These differences in response magnitude may be related to different perturbation intensity (rotenone-treatmentversus stocking with piscivores), food-chain complexity and trophic state. Both decreased phosphorus concentration and increased zooplankton grazing are probably important mechanisms underlying plankton response to biomanipulation in many lakes. The results provide tentative support to the hypothesis that under conditions of phosphorus limitation, increased zooplankton grazing can decrease algal biomassvia two separate mechanisms: reduction of the phosphorus pool in the phytoplankton, and reduction of the internal C:P-ratio in the phytoplankton cells.

     

Fish oil reduces cholesterol and arachidonic acid levels in plasma and lipoproteins from hypercholesterolemic chicks

The value of fish oil for prevention and/or treatment of human atherosclerosis has not been fully established. This study shows that replacement of saturated fat in young chick diet with menhaden oil produced a significant reversion of the hypercholesterolemia previously induced by coconut oil feeding. Fish oil also produced a clear decrease of plasma triacylglycerol levels. Coconut oil increased the percentages of 12:0 and 14:0 fatty acids, while menhaden oil increased those of 20:5 n-3 and 22:6 n-3. Percentages of 20:4 n-6, 18:2 n-6 and 18:1 n-9 significantly decreased by fish oil addition to the diet. Total cholesterol, phospholipid and protein contents of high and low density lipoproteins increased by coconut oil feeding. When coconut oil was replaced by menhaden oil, total cholesterol was significantly reduced in high, low and very low density lipoproteins. All chemical components of VLDL were decreased by menhaden oil feeding. Our results show a strong hypocholesterolemic effect of menhaden oil when this fat was supplemented to hypercholesterolemic chicks. The clear decrease found in arachidonic acid content of chick plasma and lipoproteins may contribute to the beneficial effects of fish oil consumption by lowering the production of its derived eicosanoids.     

The effect of different fish communities on the cladoceran plankton assemblages of the Kis-Balaton Reservoir,Hungary

In 1995 the authors studied the effect of differentfish communities on the structure of the cladoceranplankton in a shallow hypertrophic lake. After a fishkill of 1991, different fish communities developedin the Kazetta and the outer area of the Kis-Balatonreservoir. In the outer area of the reservoir, thedensities of plankton feeding fish species wereconsiderably higher than in the Kazetta. Thesedifferences induced changes in the structure anddynamics of the cladoceran plankton. The biomass ofsmall-bodied cladocerans (mainly Bosmina longispina)was higher and the biomass of the large-bodiedcladocerans (D. hyalina, D. magna) was lower in thewestern and eastern part of Kis-Balaton reservoir thanin the Kazetta. A peak in cladoceran biomass in theKis-Balaton reservoir was observed during the summer,close or during a bloom of filamentous cyanobacteria,whereas in the Kazetta a peak was observed during thespring, before the bloom of cyanobacteria. The adultfemales of D. hyalina were larger and produced moreeggs in the Kazetta than in the outer area of thereservoir.     

Myxosporean plasmodial infection associated with ulcerative lesions in young-of-the-year Atlantic menhaden in a tributary of the Chesapeake Bay,and possible links to Kudoa clupeidae

Ulcers in Atlantic menhaden Brevoortia tyrannus (Latrobe) (Clupeidae), observed along the USA east coast, have been attributed to diverse etiologies including bacterial, fungal and, recently, harmful algal blooms. To understand the early pathogenesis of these lesions, we examined juvenile Atlantic menhaden collected during their seasonal presence in Chesapeake Bay tributaries from April to October 1999 and from March to August 2000. We conducted histopathological examinations of young-of-the-year fish from the Pocomoke River tributary, which has a history of fish mortalities and high lesion prevalence. Kudoa clupeidae (Myxozoa: Myxosporea) spores were present in the muscles of fish collected in both years. Of the fish assessed by histology in April, 5 to 14% were infected, while in May 90 to 96% were infected. Infection rates remained high during the summer. Mature spores were primarily located within myomeres and caused little or no observable pathological changes. Ultrastructure showed spores with capsulogenic cells bearing filamentous projections, and a basal crescentic nucleus with mottled nucleoplasm containing cleaved, condensed chromatin. Also, a highly invasive plasmodial stage of a myxozoan was found in the lesions of juvenile Atlantic menhaden. The plasmodia were observed in fish collected between May and July, with the maximum occurrence in late June 1999 and late May 2000. Plasmodia penetrated and surrounded muscle bundles, causing grossly observable raised lesions in 73% of all fish infected with this invasive stage. Plasmodia were also detected in the visceral organs, branchial arches, and interocular muscles of some fish. Some of the invasive extrasporogonic plasmodial lesions were associated with ulcers and chronic inflammatory infiltrates. The plasmodial stage appeared to slough out of the tissue with subsequent evidence of wound healing. Ultrastructure showed plasmodia with an elaborate irregular surface, divided into distinct ectoplasm and endoplasm; the latter contained numerous spherical vegetative nuclei, secondary generative cells, and occasional cell doublets. Our ultrastructural studies indicate that the plasmodial organisms, which are important in the etiology of the skin lesions, are myxozoans, and they may represent early stages of K. clupeidae.     

Experimental study of the impacts of silver carp on plankton communities of eutrophic Villerest reservoir (France)

We examined the impact of five silver carp biomass levels (0, 8, 16, 20, and 32 g m⁻³) on plankton communities and water quality of Villerest eutrophic reservoir (France). We realized the experiments using outdoor mesocosms. The presence of silver carp led to changes in zooplankton and phytoplankton assemblages. High fish biomass strongly reduced cladoceran abundance (through predation). Silver carp inefficiently grazed down particles < 20 μm. More importantly, however, the suppression of herbivorous cladocerans resulted in the increase of small size algae which were relieved from grazing and benefit from high nutrient concentrations. In contrast, in mesocosms without fish, the dominance of cladocerans (mainly Daphnia) controlled small size algae and probably also larger size algae (colonial chlorophytes, cyanobacteria). Thus, the Secchi disc transparency increased markedly. Through cascade effects, the modification of grazers communities led to changes in the utilization patterns of the added nutrients by phytoplankton communities. In high fish biomass treatments, nutrients were more efficiently accumulated into particulate fractions compared with no-fish and low-fish biomass treatments that were characterized by higher dissolved nutrients concentrations. Zooplankton was an essential source of food for silver carp. The productivity of zooplankton sustained a moderate silver carp biomass (up to 16 g m⁻³). In the presence of the highest fish biomass, the productivity of zooplankton was not large enough and silver carps fed on additional phytoplankton. Although mesocosms with high fish biomass were characterized by a slight cyanobacteria development compared with other fish mesocosms, silver carp was not effective in reducing cyanobacteria dominance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interannual variability in the seasonal plankton succession of a shallow, warm-water lake

Common seasonal plankton succession patterns in temperate lakes are well understood, and were described in the popular PEG-model. Seasonal plankton succession in warm-water lakes, however, is not as well known. Recent theory suggests that some lake systems are characteristic of having alternate system-states, where one of the system-states is characterized by dominance of cyanobacteria, and transition between system-states can be abrupt and undeterminable. Lake Somerville, a shallow, well-mixed, warm-water reservoir located in eastern TX, U.S.A., experiences occasional periods of cyanobacteria dominance. To increase our understanding of seasonal plankton dynamics in warm-water systems, we analyzed 14-years of plankton data spanning a 22-year period. During this period, succession dynamics characteristic of those described by the PEG-model were observed, as well as succession dynamics expected during periods of cyanobacteria dominance, i.e., greater accumulated phytoplankton biovolume, low secondary productivity, and low light penetration. In addition to the PEG-model and cyanobacteria type system-states, other states of the system that were intermediate between these were observed. Therefore, we conclude the lake does not behave according to the alternate system-states model. The change from year to year in early-year cyanobacteria dominance was abrupt and non-monotonic during this period. In addition, the early year performance of cyanobacteria appeared to influence the plankton succession trajectory for the remainder of the season. While the magnitude of lake-flushing early in the year accounted for ～37% of variability in cyanobacteria prevalence, many of the traditional factors impacting cyanobacteria dominance appeared insignificant.     

Poor nutritional quality of primary producers and zooplankton driven by eutrophication is mitigated at upper trophic levels

Eutrophication and rising water temperature in freshwaters may increase the total production of a lake while simultaneously reducing the nutritional quality of food web components. We evaluated how cyanobacteria blooms, driven by agricultural eutrophication (in eutrophic Lake Köyliöjärvi) or global warming (in mesotrophic Lake Pyhäjärvi), influence the biomass and structure of phytoplankton, zooplankton, and fish communities. In terms of the nutritional value of food web components, we evaluated changes in the ω‐3 and ω‐6 polyunsaturated fatty acids (PUFA) of phytoplankton and consumers at different trophic levels. Meanwhile, the lakes did not differ in their biomasses of phytoplankton, zooplankton, and fish communities, lake trophic status greatly influenced the community structures. The eutrophic lake, with agricultural eutrophication, had cyanobacteria bloom throughout the summer months whereas cyanobacteria were abundant only occasionally in the mesotrophic lake, mainly in early summer. Phytoplankton community differences at genus level resulted in higher arachidonic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) content of seston in the mesotrophic than in the eutrophic lake. This was also reflected in the EPA and DHA content of herbivorous zooplankton (Daphnia and Bosmina) despite more efficient trophic retention of these biomolecules in a eutrophic lake than in the mesotrophic lake zooplankton. Planktivorous juvenile fish (perch and roach) in a eutrophic lake overcame the lower availability of DHA in their prey by more efficient trophic retention and biosynthesis from the precursors. However, the most efficient trophic retention of DHA was found with benthivorous perch which prey contained only a low amount of DHA. Long‐term cyanobacterial blooming decreased the nutritional quality of piscivorous perch; however, the difference was much less than previously anticipated. Our result shows that long‐term cyanobacteria blooming impacts the structure of plankton and fish communities and lowers the nutritional quality of seston and zooplankton, which, however, is mitigated at upper trophic levels.     

Absence of sterols constrains carbon transfer between cyanobacteria and a freshwater herbivore (Daphnia galeata)

A key process in freshwater plankton food webs is the regulation of the efficiency of energy and material transfer. Cyanobacterial carbon (C) in particular is transferred very inefficiently to herbivorous zooplankton, which leads to a decoupling of primary and secondary production and the accumulation of cyanobacterial biomass, which is associated with reduced recreational quality of water bodies and hazards to human health. A recent correlative field study suggested that the low transfer efficiency of cyanobacterial C is the result of the absence of long-chain polyunsaturated fatty acids (PUFA) in the diet of the zooplankton. By supplementation of single-lipid compounds in controlled growth experiments, we show here that the low C transfer efficiency of coccal and filamentous cyanobacteria to the keystone herbivore Daphnia is caused by the low sterol content in cyanobacteria, which constrains cholesterol synthesis and thereby growth and reproduction of the herbivore. Estimations of sterol requirement in Daphnia suggest that, when cyanobacteria comprise more than 80% of the grazed phytoplankton, growth of the herbivore may be limited by sterols and Daphnia may subsequently fail to control phytoplankton biomass. Dietary sterols therefore may play a key role in freshwater food webs and in the control of water quality in lakes dominated by cyanobacteria.     

水华蓝藻对鱼类的营养毒理学效应

水体富营养化导致蓝藻水华的发生已成为全球关注的水环境问题,很多鱼类处于水生态系统食物链的最高级,蓝藻水华的主要次级代谢产物-微囊藻毒素可通过鱼类的摄食活动或生物富集作用在鱼体组织中累积,并通过食物链危及人类健康。近年来,微囊藻毒素对鱼类的毒性效应引起众多科学家的关注。在天然水体中不少鱼类可以主动摄食蓝藻,所以,水华蓝藻对鱼类来说既具有营养物作用、也具有潜在的毒性作用。鉴于目前机械收获的水华蓝藻生物量资源化利用问题以及水产饲料业亟需大力开发鱼粉替代蛋白源的需要,从营养学和毒理学这两个角度来研究水华蓝藻对鱼类的营养作用和毒性效应具有较高的理论和现实意义。主要概述了蓝藻粉、蓝藻细胞对鱼类的营养学和毒理学效应,以期拓展水华蓝藻对鱼类毒性效应的研究视野,同时也为水华蓝藻的资源化利用提供新的思路。     

Hypoxia tolerance in two juvenile estuary-dependent fishes

Hypoxia events, or low dissolved oxygen (DO) conditions, occur frequently in North Carolina estuaries during the summer. These events may have harmful effects on important fish stocks, including spot (Leiostomus xanthurus) and Atlantic menhaden (Brevoortia tyrannus), but their consequences are not well understood. We investigated direct mortality due to hypoxia in juvenile spot and Atlantic menhaden to determine how the extent of mortality varies with the severity of hypoxia and the duration of exposure, and to explore how vulnerability to hypoxia changes across species, fish size, and temperature.Atlantic menhaden and spot were tested at two temperatures, 25 and 30 °C, and three dissolved oxygen concentrations, 0.6, 0.9, and 1.2 ppm. Survival analyses were performed on the data relating survival rate of each species to dissolved oxygen concentration, duration of exposure, fish size, and temperature. The data were analyzed using an LC₅₀ approach for comparative purposes, and 12-h LC₅₀ estimates ranged from 0.9 to 1.1 ppm O₂. Spot and menhaden exposed to 1.2 ppm O₂ showed no mortality in 24 h at 25 °C, and only 30-40% mortality at 30 °C. In contrast, both species experienced 100% mortality in 2-6 h at 0.6 ppm O₂. There was an effect of size on hypoxia tolerance, with small spot being less tolerant than large spot, while the converse size effect was observed for menhaden. Spot were consistently less tolerant to hypoxia than menhaden and both species were less tolerant to hypoxia at 30 °C than at 25 °C. Preliminary experiments showed a 24-h acclimation to sublethal levels of hypoxia significantly reduced mortality upon subsequent exposure to lethal hypoxia concentrations.Our results indicate that direct mortality due to hypoxia will vary with species, size, and temperature, but will likely only be substantial when these species are exposed to oxygen concentrations less than about 1 ppm O₂. Given the severity of hypoxia necessary to cause mortality and the ability of fish to behaviorally avoid hypoxia, direct mortality due to hypoxia may have limited impacts on fish population dynamics. Therefore, the greatest effects due to hypoxia may be caused by the stress imposed by sublethal hypoxic conditions alone or in concert with other stressors, or by indirect effects incurred by avoiding hypoxic areas.     

Comparison of plankton intake by Indian carp fingerlings under different feeding conditions

The influence of different culture systems on the amount of plankton consumed by fingerlings of three carp species (Labeo rohita, Catla catla and Cirrhinus mrigala) was examined in outdoor culture tanks during a 90 day growing period. The fish were raised under two basically different feeding conditions: (a) fed with allochthonous live plankton; (b) fed with plankton grown autochthonously in the fish growing tanks fertilized with manures. Both feeding regimes were compared with control groups. It was found that the plankton intake for carp held in the live food system was significantly higher than with the manured and control systems. Related to maximum intake, maximum abundance of plankton in the live food system was a consequence of improved water quality expressed in terms of lower values of BOD and COD and higher values of DO and pH; this was conducive to fast reproduction of some of the zooplankton which constituted the major food items for the test carp, as well as to the regular exogenous introduction of live plankton. The ingestion of plankton was found to be related to carp body weight raised to the power b. The exponent b ranged from 0.29 to 0.93 during the 90-day growing period.     

Effects of Biomanipulation on Fish and Plankton Communities in Ten Eutrophic Lakes of Southern Finland

The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha⁻¹ 3 yr⁻¹). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from ‘nutrient recycler’ to ‘nutrient storage’. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores. An erratum to this article is available at .     

Distribution fields for aquatic ecosystem components: method of identification of correlation zones

1. Data from the two multidisciplinary surveys of Lake Kinneret (Israel), including acoustic surveys of fish concentrations and concurrent sampling of plankton at stations, were used to reconstruct fish and plankton distributions. No significant lake-wide correlations for the distribution fields were found. With respect to patchiness of the fish and plankton distributions, we hypothesized that they might be correlated in localized zones. 2. A method is suggested for the identification of areas where there are strong correlations of two distribution fields. The method is based on outlining zones where the gradients of the two fields of interest are in the same direction (or are opposite). Only areas larger than the autocorrelation circles (or ellipses) for the fields are considered. The correlation of the fields is calculated for each of the zones selected. 3. The method was used in analysing data from the multidisciplinary surveys. We were able to detect areas of the lake where there were correlations for fish and plankton distributions. Analysis of specific conditions inside the correlation zones made it possible to construct hypotheses concerning the causes for the observed patterns of fish and plankton distributions.     

Reactive oxygen species function as second messenger during ischemic preconditioning of heart

The mechanisms involved in the hypolipidemic effects of fish oil have not been clearly established. This study shows that supplementation of 10% menhaden oil to the chick diet for 7 days produced a significant hypocholesterolemia and hypotriglyceridemia. Fatty acid composition of chick plasma drastically changed by the same dietary manipulation. Percentages of 20:5 and 22:6 n-3 fatty acids strongly increased, while percentages of 20:4 n-6, 18:2 n-6, and 18:1 n-9 significantly decreased. Changes observed in the relative percentages were parallel to those obtained in the amount of each fatty acid. Ratio of n-3/n-6 clearly decreased in plasma by fish oil feeding. Total cholesterol and triacylglycerol contents decreased in high density lipoprotein (HDL) but did not change in low density lipoprotein (LDL). All chemical constituents of very low density lipoprotein (VLDL) significantly decreased after the first week of menhaden oil supplementation to the diet. Similar modifications in fatty acid composition of the three lipoprotein fractions were also found. Our results suggest that the hypocholesterolemic effects of fish oil may be mediated by the depletion in VLDL synthesis and secretion into the chick plasma. On the other hand, the strong decrease found in the arachidonic acid (AA) content of chick plasma and lipoproteins may contribute to the beneficial effects of fish oil consumption by lowering the production of its derived eicosanoids.     

Effect of dietary n-3 fatty acids on HMG-CoA reductase and ACAT activities in liver and intestine of the rabbit

The regulation of hepatic and intestinal 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and acyl-CoA; cholesterol acyltransferase (ACAT) activities by dietary fish oil was examined in the rabbit. Rabbits were fed 10% menhaden oil or menhaden oil plus 1% cholesterol for 14 days. They were compared with animals fed a control diet or one enriched with long-chain saturated fats consisting of 10% cocoa butter oil or cocoa butter oil plus 1% cholesterol. Plasma cholesterol was increased in rabbits fed the fish oil and the two cholesterol-containing diets. In the liver, ACAT activity was increased and HMG-CoA reductase activity was decreased in rabbits ingesting the fish oil. The same was true for animals ingesting both cholesterol-containing diets. In the intestine, ACAT activity was not affected by the ingestion of the fish oil compared to control rabbits; however, it was significantly higher in animals fed the fish oil compared to animals ingesting the cocoa butter. HMG-CoA reductase activity was decreased in the distal two-thirds of the intestine in animals fed the menhaden oil compared to activities observed in controls. In animals ingesting the cholesterol diets, intestinal reductase was significantly decreased, whereas intestinal ACAT activity was increased in rabbits ingesting the cocoa butter and cholesterol diet when compared to their controls. Lipid analysis of hepatic and intestinal microsomes demonstrated an enrichment of n-3 polyunsaturated fatty acids in membranes from rabbits ingesting the menhaden oil.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recruitment of resting stages may induce blooms of Microcystis at low N:P ratios

Some species of cyanobacteria form resting stages at the sedimentsurface when environmental conditions become unfavourable. Asconditions turn more favourable, these resting stages hatchto the water phase, where the cells grow, reproduce, and sometimesform blooms. Since blooms of cyanobacteria have become an increasingthreat to inland and brackish waters, it is important to assessthe mechanisms and processes involved in the initiation of suchblooms. One such mechanism is recruitment from the sedimentsurface. Potential factors regulating the recruitment of restingstages include variations in nutrient concentrations and ratios,as well as variations in grazing. To investigate how the recruitmentof Microcystis responds to different levels of these factors,we performed an enclosure experiment (zooplankton abundanceswere regulated by predation from fish). We found that recruitmentand growth were most pronounced at the second highest nutrientconcentration (average concentrations were 498 µg l^-1of dissolved nitrogen and 134 µg l^-1 of total phosphorus),while no direct response to different grazing levels was detected.We also found that resting stages can be important for initiatingand sustaining blooms. The environmental conditions most importantin regulating the recruitment rate from resting stages correspondedto the requirements of the plankton cells, namely high nutrientaddition and low N:P ratio.     

Responses to fish predation and nutrients by plankton at different levels of taxonomic resolution

1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design. 2. Food chain theory accurately predicted total biomass development of both phyto‐ and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients. 3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume. 4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO₂) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability. 5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (<50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.     

Top-down or Bottom-up Effects by Fish: Issues of Concern in Biomanipulation of Lakes

A large-scale biomanipulation trial was carried out on Lake Vesijärvi in Finland during 1989–1993. Following the mass removal of coarse fish the biomass of cyanobacteria collapsed from 1.4 g/m^?3 to below 0.4 g/m^?3, while total phosphorus concentration declined from 45 μ g/L to 30 μ g/L. No relevant changes in zooplankton communities were observed. The results suggest that the success of food web manipulation as a tool for lake restoration is not necessarily dependent on the grazing rate of zooplankton. The effects of reduced fish-mediated internal loading and recycling of nutrients are in many cases stronger than those of reduced planktivory. Alternative stable states of water quality may also exist in lakes not covered by macrophytes, owing to the changes in the behavior of fish stocks. Year-to-year variation in the littoral zone may cause large oscillations in lake ecosystems—for example, through the recruitment of fish. In addition, the nutrients translocated by fish from the littoral zone may affect the nutrient dynamics of the pelagial plankton community. In terms of phytoplankton species composition and the ratio of phosphorus to chlorophyll a, the water quality in Lake Vesijärvi has improved in a stepwise fashion within the last 10 years. This is probably due to the fact that the five-year mass removal of fish in Enonselkä fulfilled the requirement of sustained management of fish stocks in order to maintain nonequilibrial conditions between alternate stable states. The prediction of the water quality development is obscured, however, by spatial and temporal within-lake variation, which sets high requirements for sampling programs.