Do fish regulate phytoplankton in shallow eutrophic Northeast Brazilian reservoirs?

SUMMARY 1. In a comparative study, we examined the potential for fish to structure planktonic food webs in shallow mesotrophic to hypereutrophic Northeast Brazilian reservoirs. The food webs were dominated by three guilds of fish (facultative piscivores, generalist planktivores and omnivores), small herbivorous zooplankton and bloom‐forming cyanobacteria, with few littoral macrophytes. 2. A principal component's analysis on data from 13 reservoirs (27 sampling dates in 1995–99) revealed that euphotic depth, the relative density of phytoplankton (i.e. the percentage of overall phytoplankton density) represented by cyanobacteria, and the relative biomass of fish (i.e. percentage of overall biomass) represented by omnivores and facultative piscivores, explained most of the variance in the data. Physico‐chemical conditions, lake morphometry and rainfall were secondary factors. 3. Phytoplankton was related to fish guild structure. Chlorophyll concentration increased with total phosphorus and the relative biomass of omnivorous fish, decreased with the relative biomass of facultative piscivores, but was unrelated to the biomass and mean body size of herbivorous zooplankton. Chlorophyll concentration and the densities of filamentous and colonial cyanobacteria decreased with the ratio of the biomass of facultative piscivores to that of omnivores (FP : OM). 4. We propose two complementary mechanisms for the observed relationships between fish and phytoplankton. At a low biomass of facultative piscivores, juvenile zooplanktivorous fishes may induce a trophic cascade on zooplankton in the littoral zone. Regardless of piscivore biomass, piscivores and omnivores may regulate phytoplankton via multichannel omnivory because of the predominance of omnivorous or detritivorous foraging behaviour. 5. Manipulative experiments are needed to explore further whether, depending on priorities in the use of the reservoir, fisheries management could alter the FP : OM ratio either to enhance fish yields or to reduce phytoplankton densities and cyanobacterial blooms.     

The role of fish-poultry integration on fish growth performance,yields and economic benefits among smallholder farmers in sub-Saharan Africa,Tanzania

Aquaculture practices from sub-Saharan Africa are characterised by low production, owing to improper technology. Production can be increased through integrating fish farming with other existing on-farm activities, particularly livestock husbandry. We assessed the role of fish-poultry integration on all male Nile tilapia, Oreochromis niloticus growth performance, yields and economic benefits among smallholder farmers in sub-Saharan Africa, Tanzania. The study also compared phytoplankton species composition, abundance and biomass between the fish-poultry integration and non-integrated system. After 180 days of the experiment, all male O. niloticus cultured under fish-poultry integration exhibited significantly higher growth rates than those in the non-integrated system (p < 0.05). Gross fish yield (GFY), net fish yield (NFY) and net annual yields (NAY) obtained from fish-poultry integration were significantly higher than those from non-integrated system (p < 0.05). Partial enterprise budget analysis revealed that fish-poultry integration was more profitable than the non-integrated system. Moreover, fish-poultry integrated system produced significantly higher phytoplankton abundance and biomass than those from the non-integrated system. Results demonstrate that rural smallholder farmers can achieve higher growth rate, farm net yields and income by integrating all male O. niloticus with other on-farm activities than practising a stand-alone fish culture system.     

TN : TP ratio and planktivorous fish do not affect nutrient-chlorophyll relationships in shallow lakes

1. In previous work, phytoplankton regulation in freshwater lakes has been associated with many factors. Among these, the ratio of total nitrogen to total phosphorus (TN : TP) has been widely proposed as an index to identify whether phytoplankton are N‐ or P‐limited. From another point of view, it has been suggested that planktivorous fish can be used to control phytoplankton. 2. Large‐scale investigations of phytoplankton biomass [measured as chlorophyll a, (chl‐a)] were carried out in 45 mid‐lower Yangtze shallow lakes to test hypotheses concerning nutrient limitation (assessed with TN : TP ratios) and phytoplankton control by planktivorous fish. 3. Regression analyses indicated that TP was the primary regulating factor and TN the second regulating factor for both annual and summer phytoplankton chl‐a. In separate nutrient–chl‐a regression analyses for lakes of different TN : TP ratios, TP was also superior to TN in predicting chl‐a at all particular TN : TP ranges and over the entire TN : TP spectrum. Further analyses found that chl‐a : TP was not influenced by TN : TP, while chl‐a : TN was positively and highly correlated to TP : TN. 4. Based on these results, and others in the literature, we argue that the TN : TP ratio is inappropriate as an index to identify limiting nutrients. It is almost impossible to specify a ‘cut‐off’ TN : TP ratio to identify a limiting nutrient for a multi‐species community because optimal N : P ratios vary greatly among phytoplankton species. 5. Lakes with yields of planktivorous fish (silver and bighead carp, the species native to China) >100 kg ha^?1 had significantly higher chl‐a and lower Secchi depth than those with yields <100 kg ha^?1. TP–chl‐a and TP–Secchi depth relationships are not significantly different between lakes with yields >100 kg ha^?1 or <100 kg ha^?1. These results indicate that the fish failed to decrease chl‐a yield or enhance Z_SD. Therefore, silver carp and bighead carp are not recommended as a biotic agent for phytoplankton control in lake management if the goal is to control the entire phytoplankton and to enhance water quality.     

武汉东湖中浮游植物与鲢鱼相互作用的量的评价

以东湖郭郑湖为研究对象,将其生态系统中在质和量方面占重要地位的浮游植物-鲢作为亚系统,研究它们之间的相互作用。比较了夏季两个月中鲢的生长和让其充分摄食后的生长,发现最初20天没有差异,20天后两者开始出现差别。将两者的相互作用模型化,计算的结果是,浮游植物被鲢摄食后密度下降,而鲢得不到充足的饵料,导致生长缓慢。观察此期间的氮平衡,浮游植物所生产的氮几乎全部被鲢利用,但浮游植物所需要的78%的氮又是鲢提供的。这说明在湖区整个生态系统中,此相互作用系统在量的方面是很重要的。     

Omnivory by Planktivores Stabilizes Plankton Dynamics, but May Either Promote or Reduce Algal Biomass

Classical models of phytoplankton–zooplankton interaction show that with nutrient enrichment such systems may abruptly shift from limit cycles to stable phytoplankton domination due to zooplankton predation by planktivorous fish. Such models assume that planktivorous fish eat only zooplankton, but there are various species of filter-feeding fish that may also feed on phytoplankton. Here, we extend these classical models to systematically explore the effects of omnivory by planktivorous fish. Our analysis indicates that if fish forage on phytoplankton in addition to zooplankton, the alternative attractors predicted by the classical models disappear for all realistic parameter settings, even if omnivorous fish have a strong preference for zooplankton. Our model also shows that the level of fish biomass above which zooplankton collapse should be higher when fish are omnivorous than when fish are zooplanktivorous. We also used the model to explore the potential effects of the now increasingly common practice of stocking lakes with filter-feeding fish to control cyanobacteria. Because omnivorous filter-feeding fish forage on phytoplankton as well as on the main grazers of phytoplankton, the net effect of such fish on the phytoplankton biomass is not obvious. Our model suggests that there may be a unimodal relationship between the biomass of omnivorous filter-feeding fish and the biomass of phytoplankton. This implies that to manage for reductions in phytoplankton biomass, heavy stocking or strong reduction of such fish is best.     

Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha⁻¹. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha⁻¹ fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

     

Interactions between Nile tilapia (Oreochromis niloticus) and cladocerans in ponds (Khartoum,Sudan)

Juvenile Nile tilapia (Oreochromis niloticus) are omnivorous, and the question asked in this study is how they affect on their environment? Do they mainly act as predators on the cladoceran zooplankton or do they compete with the cladocerans for phytoplankton? This problem was studied in three ponds with and three ponds without small tilapia (3–5 cm). The fish growth rate, the succession of plankton species and the changes in abiotic conditions, were monitored over a period of 67 days. The fish biomass was kept low and the mean was approximately constant (12.6 g m^?2) during the experiment. Phosphate was added to avoid phytoplankton nutrient limitation. Although the diet of Nile tilapia contained both phytoplankton and zooplankton, the fish affected the ecosystem in a similar way as zooplanktivorous fish. The fish ponds got more phytoplankton due to increase of Chlorophyta. Effects on the other phytoplankton groups Euglenophyta, Bacillariophyta, Cryptophyta and Cyanophyta could not be registered. The ponds without fish had higher densities of Daphnia lumholtzi and D. barbata. The other Cladocerans seemed less influenced by fish presence. The relative fish growth rate was most positively correlated with the density of Daphnia lumholtzi, Diaphanosmoa excisum and Bosmina longirostris. Tilapia seemes to have two feeding modes: (1) preying on large zooplankton and (2) unselective filtration of small planktonic organisms such as phytoplankton. In our experiment the first feeding mode affected the ecosystem more than the second.     

The impact of planktivorous flsh on the structure of a plankton community

SUMMARY. 1. The abundance of pianktivorous juvenile yellow perch, Perca flavescens , was manipulated in three 750 m³ enclosures in a eutrophic lake.
2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter-feeding cladocera. primarily bosmi- nids. At low fish densities the zooplankton community was dominated by large filter-feeding cladocera, primarily daphnids.
3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid-summer.
4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely ineffective.     

The effect of a lake fertilization on the stability and material utilization of a limnetic ecosystem

The interaction between the phytoplankton, zooplankton and fish populations and certain abiotic environmental factors, was investigated in an oligotrophic Norwegian lake during a 5-yr period (1974–1978). The effects of adding artificial fertilizer in 1975 and 1976 were also studied. When cladoceran dominated, the zooplankton community was able to maintain a more or less constant phytoplankton biomass and a rather low phytoplankton production even when nutrient levels were raised. In years when rotifers were dominant, algal biomass and productivity increased, despite the amount of added nutrients being lower. The regression for the relationship between daily phytoplankton P/B and daily herbivore zooplankton P/B indicated that these trophic levels were highly interdependent. A change, from large-sized to smaller herbivorous zooplankton, due to fish predation, also led to an increase in phytoplankton turnover. The investigations show that planktivorous fish may be the key factor which determines the stability of limnetic systems and controls the material transfer from the algae to the higher trophic level.     

Long Term Effect of Cyprinid Fishes on Phytoplankton and Zooplankton Communities in a Shallow Water Protection Reservoir

The effects of fish kill and different fish stocks on the phytoplankton and zooplankton dynamics were studied in a shallow hypertrophic reservoir system. When fish stock was below 100 kg ha⁻¹, nutrient availability was not the main limiting factor for growth of phytoplankton. Consequently top‐down forces controlled phytoplankton. In the years with high fish stock (>100 kg ha⁻¹) the bottom‐up forces dominated as nutrient availability was the main limiting factor for growth of phytoplankton. We can conclude that significant water quality improvement can be achieved in the reservoir system by decreasing fish stock below 100 kg ha⁻¹. Although clear‐water phase could be stabilised temporary by macrophytes, stabilisation of good water quality requires continuous regulation of fish community. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ecosystem size structure response to 21st century climate projection: large fish abundance decreases in the central North Pacific and increases in the California Current

Output from an earth system model is paired with a size‐based food web model to investigate the effects of climate change on the abundance of large fish over the 21st century. The earth system model, forced by the Intergovernmental Panel on Climate Change (IPCC) Special report on emission scenario A2, combines a coupled climate model with a biogeochemical model including major nutrients, three phytoplankton functional groups, and zooplankton grazing. The size‐based food web model includes linkages between two size‐structured pelagic communities: primary producers and consumers. Our investigation focuses on seven sites in the North Pacific, each highlighting a specific aspect of projected climate change, and includes top‐down ecosystem depletion through fishing. We project declines in large fish abundance ranging from 0 to 75.8% in the central North Pacific and increases of up to 43.0% in the California Current (CC) region over the 21st century in response to change in phytoplankton size structure and direct physiological effects. We find that fish abundance is especially sensitive to projected changes in large phytoplankton density and our model projects changes in the abundance of large fish being of the same order of magnitude as changes in the abundance of large phytoplankton. Thus, studies that address only climate‐induced impacts to primary production without including changes to phytoplankton size structure may not adequately project ecosystem responses.     

Fish-zooplankton interactions and their effects on water quality of a tropical Brazilian reservoir

The influence of zooplanktivorous fishes on the plankton community and water quality of Americana Reservoir, Brazil was studied experimentally in 4 floating enclosures during the dry seasons (July–September) of 1982 and 1983. Two enclosures were stocked with adult fish (Astyanax bimaculatus in 1982;A. fasciatus in 1983) at near maximal densities measured in the reservoir upper surface waters (35 m^–2) and two were fish-free during each experiment lasting about one month. Marked differences were evident between the fish and fish-free enclosures after a 2–3 week period in each experiment, particularly with respect to water transparency, phytoplankton biomass, and zooplankton abundance as well as species and size composition. By the end of each experiment water transparencies were lower and phytoplankton biomass higher in the fish enclosures compared to those without fish. Also at that time Rotifera were the prominent zooplankters in the fish enclosures and Cladocera in the fish-free ones. Larger or more conspicuous species of Cladocera asDaphnia gessneri, D. ambigua, andMoina micrura were present in the fish-free enclosures but not in the fish enclosures. The interactions between fish predation, zooplankton grazing, phytoplankton biomass and water quality conditions are discussed in relation to eutrophication of a tropical aquatic ecosystem.     

基于稳定同位素技术的保安湖食物网结构特征研究

文章利用碳、氮稳定同位素技术对江湖阻隔典型湖泊-保安湖的食物网结构进行了研究。结果表明保安湖中鱼类消费者的主要营养级范围为2.1—3.3, 在调查到的16种鱼类中, 顶级肉食性鱼类种类很少, 杂食性鱼类的种类最多。保安湖食物网主要由两条营养传递途径构成, 即由POM、浮游植物为主要食物源的浮游牧食链与沉积物为主要食物源的底栖食物链。POM、浮游植物、浮游动物和底栖动物是保安湖水域食物网中鱼类的主要食物来源, 其次是沉积物中的碎屑和水生植物等。此外, 从基于理论食性数据的食物网与BIMM模型预测的食物网结构可以看出, 从POM、浮游植物、浮游动物到杂食性鱼类的浮游牧食链在整个食物网中具有主导性, 而从水生植物、沉积物和底栖动物到杂食性鱼类的底栖食物链相对重要性较低。     

Decoupling of pelagic and littoral food webs in oligotrophic Canadian Shield lakes

The importance of top-down effects of piscivorous fish on phytoplankton in natural oligotrophic lakes is still debated. In this study, we analyzed patterns in phytoplankton and zooplankton abundance in 37 oligotrophic Canadian Shield lakes in relation to variations in both piscivorous fish predation and resources (total phosphorus; TP). Zooplankton community structure (but not total biomass) was partially affected by the variation in fish predation while the phytoplankton community structure and total biomass showed no response. Carbon isotope analyses revealed that the lack of top-down effects is due to the uncoupling of the littoral and the pelagic food webs. We found that the fish community depends mostly on benthic resources, suggesting that only low planktivory occurred in our study lakes. Due to the absence of specialized zooplanktivorous fish, zooplankton is poorly exploited in these lakes and thus able to control phytoplankton by grazing. A comparison of our data with published studies on the TP–chlorophyll a relationships in both natural and manipulated systems shows that the phytoplankton biomass per unit of TP is relatively low in Canadian Shield lakes.     

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.     

罗非鱼对水质的影响

鱼类是影响水库等水质的重要因素,以我国南方主要养殖鱼类之一的罗非鱼为对象,通过广东大镜山水库原位围隔实验,研究了鱼类对水质的影响。结果表明,与对照组相比罗非鱼围隔中总氮,总磷和叶绿素a浓度分别升高了42%,129%和347%。罗非鱼的排泄增加了水体营养负荷,为浮游植物生长提供大量的营养盐,引起浮游植物生物量增加,水体透明度降低。罗非鱼通过营养盐释放所产生的上行效应明显大于由其牧食导致的下行效应。     

Phytoplankton blooms and fish recruitment rate

The relationships between phytoplankton and zooplankton productionand fish larval survival to recruitment are examined by linkingtwo generic models. It is first demonstrated that the phytoplankton–zooplanktonmodels can be appropriately combined with a zooplankton–larvae–recruitmentmodel. The combined model reveals some general principles. Recruitmenttends to be a domed-shaped function of initial fish egg production.‘Bloom’ phytoplankton conditions are important forhigh recruitments. The timing and duration of fish egg productionis important in determining recruitment through their impacton the phytoplankton bloom. It is argued that optimal recruitmentwould be obtained if the duration of larval feeding was lessthan the duration of the phytoplankton bloom; a hypothesis whichis testable.     

三门湾健跳港网箱养殖区浮游生物多样性的夏季调查

调查了三门湾健跳港海水养殖区夏季浮游生物的种类组成、密度和多样性指数,并将浮游生物群落指标与水质理化因子进行相关分析。共发现浮游生物29种,其中浮游植物14种,第一优势种为中勒骨条藻(Skeletonema costatum);浮游动物15种,第一优势种为太平洋纺锤水蚤(Acartia pacifica)。表层浮游植物的平均密度为43 328ind·L^-1,Shannon-Weiner多样性指数为0.826;浮游动物平均密度为389ind.m^-3,Shannon-Weiner多样性指数为2.964。相关分析结果表明:无机氮、无机磷促使了浮游植物大量繁殖,并使浮游动物多样性指数提高。网箱养鱼区水体富营养化指数与浮游植物多样性指数之间有一定的正相关关系,与浮游动物多样性指数之间呈显著的正相关关系。     

营养盐加富和鱼类添加对浮游植物群落演替和多样性的影响

浮游植物是水体生态系统敞水区最重要的初级生产者,其组成与多样性反映了群落的结构类型和存在状态。通过围隔实验,模拟水库春季发生的营养盐加富和鱼类放养的干扰,分析在这两种干扰下的浮游植物群落演替过程中优势种和稀有种的变化,并通过以丰度与生物量为变量的香农和辛普森多样性指数的计算,分析浮游植物群落演替过程中的多样性变化特征。结果表明,营养盐加富干扰下的浮游植物群落的优势种变化和演替更为明显,营养盐加富与鱼类添加对浮游植物群落多样性变化的影响符合中度干扰理论。在优势种优势度变化较大的浮游植物群落演替过程中,多样性指数与浮游植物生物量有较高的负相关性。在浮游植物群落演替过程中,香农和辛普森多样性指数的变化趋势基本一致,采用丰度与生物量为变量的两种多样性指数的计算结果对实验系统中浮游植物群落多样性的分析结果没有明显的影响。     

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.