Dissertations / Theses on the topic 'Planktonic food web'

In situ microcosms of a planktonic community were exposed to the triazine herbicide simazine. Randomly selected sets of microcosms were collected and sampled each week for three weeks (plus Week 0). Samples of ambient water were collected each week for evaluation of enclosure effects. Physical and chemical parameters were measured per microcosm. Simazine was measured at Weeks 0 and 3 only. The following organisms were preserved and quantified: phytoplankton, bacteria, and zooplankton, including ciliates, copepod nauplii, cladocerans and rotifers. Simazine decreased dissolved oxygen and pH, but increased nitrate and ammonia concentrations compared to control microcosms. A temporary decrease in temperature occurred at Week 1. Phytoplankton were differentially affected by simazine. Sensitive taxa included Trachelomonas, Glenodinium, diatoms and several species of relatively minor significance. Dinobryon and miscellaneous coccoids were not significantly affected. Phytoplankton ≥9 um were more affected by simazine than phytoplankton <9 um. Many cells <9 um may be facultative or obligate heterotrophs and not susceptible to simazine. Although data were variable, bacteria were also not affected by phytoplankton changes or simazine. Rotifers were the major zooplankters and the two dominant species, Kellicottia bostoniensis and Keratella cochlearis, were reported to graze exclusively on cells <9 um. Polyarthra vulgaris and Synchaeta pectinata also graze in this size range but are not limited to it. Copepod nauplii/copepodids were present, but adult copepods and cladocerans were rare. The tintinnid ciliate Codonella exhibited a temporary population increase during the study. Zooplankton were not affected by simazine-induced changes in the phytoplankton. Kellicottia bostoniensis was the only zooplankter affected by simazine: it had lesser mortality in higher concentrations of simazine. Possible reasons for this enhanced survival were discussed. The zooplankton (primarily rotifers) appeared to feed more on heterotrophic cells than on autotrophic cells, largely as a function of food size, and may have been more closely associated with the detrital food chain than the autotrophic food chain.
M.S.

Nitrogen fixation is no longer considered to be a minor factor of the nitrogen cycle in oceanic ecosystems. Recent geochemical and biological efforts have led to a significant increase in the estimated input of nitrogen to marine ecosystems by biological fixation, while molecular studies have increased our knowledge of the number and diversity of nitrogen fixers known to be active in the ocean. Although Trichodesmium spp. have long been viewed as the primary marine nitrogen fixers, recent efforts have shown that various members of the picoplankton community are also actively involved in nitrogen fixation. The relative abundance of different nitrogen fixers is an important ecosystem parameter since nitrogen fixers may differ significantly in their physiology, life history and ecology. Here we combine rate measurements and stable isotope natural abundance measurements to constrain the impact of N2 fixation in the waters north of Australia. Samples were collected in the Coral, Arafura, and East Timor Seas, thus spanning three distinct hydrographic regions. Our data show that Trichodesmium has a significant influence on the stable nitrogen isotope ratios of particulate and zooplankton biomass and suggest that Trichodesmium is a significant source of nitrogen for the pelagic ecosystem. Based on stable carbon isotope ratios, it is also likely that the pathways are indirect and nitrogen fixed by Trichodesmium enters the higher trophic levels via decomposition as dissolved organic and inorganic nitrogen. Picocyanobacteria showed high diazotrophic activity at some stations, but unlike Trichodesmium, their N2 fixation rate was not reflected in the stable N isotope ratios of particulate and zooplankton biomass. Our results suggest an important N contribution to biomass by diazotrophs in the Coral Sea, Arafura Sea and East Timor Sea.

Publisher version
As part of a larger investigation, the effect of apex predation on estuarine bacterial community structure, through trophic cascading, was investigated using experimental in situ mesocosms. Through either the removal (filtration) or addition of specific size classes of planktonic groups, four different trophic scenarios were established using estuarine water and its associated plankton. One such treatment represented a “natural” scenario in which stable apex predatory pressure was qualified. Water samples were collected over time from each of the treatments for bacterial community evaluation. These samples were assessed through pyrosequencing of the variable regions 4 and 5 of the bacterial 16S rRNA gene and analysed at the species operational taxonomic unit (OTU) level using a community procedure. The blue-green group dominated the samples, followed by Proteobacteria and Bacteroidetes. Samples were the most similar among treatments at the commencement of the experiment. While the bacterial communities sampled within each treatment changed over time, the deviation from initial appeared to be linked to the treatment trophic scenarios. The least temporal deviation-from-initial in bacterial community was found within the stable apex predatory pressure treatment. These findings are consistent with trophic cascade theory, whereby predators mediate interactions at multiple lower trophic levels with consequent repercussions for diversity.

Consumers in marine and estuarine environments have a strong reliance on planktonic and benthic primary production. These two basal resources form the foundation of aquatic food webs, yet the abundance of phytoplankton and benthic algae are frequently inversely related due to competition for light and nutrients. As a result, optimal habitats for benthic and planktonic consumers vary spatially and temporally. To investigate these trends, three studies were conducted focusing on light attenuation and basal resources in a bay, river, and on a continental shelf. δ13C and δ15N stable isotopes can be used as endogenous tracers to determine both the trophic level and basal resource use of consumers. δ13C values of primary producers are determined by the isotopic values of available CO2 and by the degree of photosynthetic fractionation (εp) that occurs during photosynthesis. εp by aquatic algae is greater in high CO2concentrations, high light, during slow growth rates, and for cells with a small surface area to volume ratio. Interaction among these parameters complicates prediction of algal εp in a natural setting, prompting the investigation as to which factors would impact εp and δ13C in a dynamic estuary. Community-level fractionation of an assemblage of filamentous algae, pennate diatoms, and centric diatoms grown on glass plates was found to be positively correlated with photosynthetically active radiation (PAR), resulting in higher δ13C values for organic matter in low-light conditions. These results support the concept that the low-light benthic environment may contribute to the widely observed phenomenon of ~5 / higher δ13C values in benthic algae compared to phytoplankton. Spatial and temporal variability in the isotopic baseline provides evidence of shifting biogeochemical controls on primary production. The West Florida Shelf in the eastern Gulf of Mexico transitions from a eutrophic ecosystem near the Mississippi River to an oligotrophic ecosystem in offshore continental shelf waters. Spatiotemporal variability in the δ13C and δ15N signatures of primary producers and fish populations were examined along this gradient. Muscle δ15N from three widely distributed fish species exhibited strong longitudinal isotopic gradients that coincided with the principal trophic gradient, whereas δ13C values of fish muscle and benthic algae were correlated with depth. The three fish species had relatively high site fidelity, as isotopic gradients were consistent between seasons and years. Isotopic mixing models showed all three fish species had a significant reliance on benthic algae as a basal resource. Dynamic models of the West Florida Shelf isotopic baseline were created using spatial data and satellite-derived water quality characteristics as predictors. Models were constructed using data from three fish species and tested on four other species to determine if the models could be extrapolated to new taxa. Both dynamic and static δ15N models had similar predictive capabilities, indicating a fairly stable δ15N baseline. The satellite-derived dynamic variables explained more variation in baseline δ13C than static spatial descriptors. Planktonic primary production can directly impact benthic food chains through phytoplankton deposition. A novel phytoplankton deposition detection method that combined water-column and benthic fluorometry with surficial sediment sampling was developed and assessed in a two-year study of the Caloosahatchee River estuary. Classifications based upon this detection method showed phytoplankton deposition dominated the upstream region and deposition was associated with reduced dissolved oxygen concentrations. Benthic algae dominated in downstream regions, particularly during low freshwater flow conditions when light absorption by colored dissolved organic matter was low. This same Caloosahatchee River estuary study was used to determine if zooplankton aggregate in regions with optimal basal resource availability. The isopod Edotia triloba was found to associate with chlorophyll peaks when freshwater velocity was constant. Chlorophyll peaks were offset downstream or upstream from isopod aggregations when freshwater flow was accelerating or decelerating, implying that phytoplankton and isopods have different response times to changes in flow. Temporal and spatial fluctuations in water quality and primary production introduce instability to aquatic consumers that primarily rely on one basal resource. The current global trends in eutrophication and increasing planktonic production are likely to be a liability for benthic consumers due to increased benthic hypoxia and light attenuation. The results of these studies indicate that both the location of consumers and their isotopic signatures can be impacted by factors, such as light attenuation, that control benthic and planktonic primary production.

Les interaccions basades en la mida individual dels organismes tenen un paper important en la dinàmica de la comunitat i en els processos dels ecosistemes aquàtics, ja que la mida d’un individu és un tret clau dels organismes molt vinculat a la seva taxa metabòlica. El canvi climàtic i les pertorbacions afecten a les xarxes tròfiques planctòniques d'aigua dolça, debilitant les interaccions basades en la mida dels individus. En aquesta tesi es van desenvolupar quatre estudis relacionats però independents per obtenir una comprensió més profunda de les interaccions tròfiques basades en la mida individual dels organismes. L’objectiu era demostrar com els factors biòtics i ambientals afecten les interaccions basades en la mida dels individus de xarxes tròfiques planctòniques utilitzant aproximacions observacionals i experimentals en diferents localitats a Europa. Generalitzant, les nostres troballes clau d'aquesta tesi ressalten la importància d’integrar les relacions basades en la mida dels organismes i la resiliència de les comunitats amb la variació intraespecífica dels organismes a l’hora d’estudiar les interaccions tròfiques entre els organismes d’un ecosistema aquàtic. La comprensió d'aquestes interaccions ens permetrà gestionar i restaurar millor els ecosistemes aquàtics davant el canvi climàtic i altres pertorbacions induïdes per l'home.
Individual size-based interactions play a significant role in the community dynamics and ecosystem processes of aquatic ecosystems, because body size is a key trait of organisms that is highly linked to metabolic rates. Climate change and disturbances influence freshwater planktonic food webs, weakening the strength of size-based interactions. In this thesis, we conducted four related but independent studies to obtain a deeper understanding of the size-based trophic interactions. We aimed to demonstrate biotic and environmental factors influencing size-based interactions in planktonic food webs using observational and experimental approaches at different locations in Europe. Overall, our key findings from this thesis suggest that integrating size-based relationships and resilience of communities together with intraspecific variation is important while studying trophic interactions. Understanding these interactions will allow us to better manage and restore aquatic ecosystems in the face of climate change and other human-induced disturbances.

Lakes are important global ecosystems and many of them are nutrient-poor (unproductive). Especially in northern boreal latitudes, lakes may be heavily subsidized by terrestrial organic material (t-OM) from peat layers in the catchment. Thus, in addition to heterotrophic bacteria and phytoplankton, zooplankton may also use the particulate fraction of peat layer t-OM (t-POM) as a potential food source in those systems. Inputs of t-OM in northern latitudes are anticipated to increase in the future due to increasing precipitation and temperature. As t-OM is a good substrate for bacterial growth and as bacteria can often outcompete phytoplankton for inorganic nutrients, the proportions of heterotrophic bacteria and phytoplankton are expected to change in unproductive lakes. This may have pronounced impacts on zooplankton population dynamics. The aim of my thesis was to investigate how changes in food quality and quantity will affect metazoan zooplankton performance in unproductive lakes. Three laboratory studies assessed the quality of specific food components (phytoplankton, bacteria and peat layer t-POM) and their effects on Daphnia survival, growth and reproduction. Further, a mesocosm study with a full natural plankton community tested the predictions of the Light:Nutrient-Hypothesis in an unproductive clear water lake in situ by adding carbon and inorganic nutrients and changing light availability. I found that pure bacterial (Pseudomonas sp.) or t-POM diets could not sustain Daphnia populations, even though both were readily ingested. Daphnids needed at least 10-20% phytoplankton (Rhodomonas) in the diet to survive and even higher proportions (≥ 50%) were necessary for the production of viable offspring. Further, I showed that the dilution of non-limiting concentrations of Rhodomonas with increasing proportions of Pseudomonas or t-POM led to decreased Daphnia performance. Both Pseudomonas and t-POM lack essential biochemicals (fatty acids and sterols). In contrast, mineral nutrient limitation only occurred on t-POM-dominated diets as evidenced by a labeling experiment that showed Daphnia can incorporate carbon and phosphorus from Rhodomonas and Pseudomonas with similar efficiencies. Thus, peat layer t-POM was a lower quality food than Pseudomonas. This was corroborated by the finding that intermediate additions of Pseudomonas to limiting amounts of Rhodomonas supported increased Daphnia survival, growth and reproduction while t-POM additions had no beneficial effect. My results suggest that high terrestrial stable isotope signals in metazoan zooplankton are most likely derived from t-OM that is channeled tohigher trophic levels via the microbial loop (i.e. heterotrophic bacteria and/or bacterivorous protozoa) but not from direct metazoan feeding on t-POM. Furthermore, bacteria may serve as an important supplement to zooplankton diets when phytoplankton abundance is low. However, a sufficient proportion of high quality phytoplankton is always necessary to fulfil mineral and especially biochemical requirements of zooplankton in unproductive aquatic systems. The results of the mesocosm study showed that the Light:Nutrient-Hypothesis is not applicable to unproductive clear water systems in which the phytoplankton community is dominated by mixotrophs. In the face of the theoretical predictions, low light levels led to decreased zooplankton biomass. This was most likely caused by a shift in the algal community composition towards less edible taxa. Another reason may have been a weakening of the microbial loop. This is in line with the results of the laboratory studies that point out the importance of the microbial food web for zooplankton nutrition in unproductive lakes.

Thesis (doctoral)--Ludwig-Maximilians-Universität München, 2006.
Title from PDF t.p. (viewed on May 14, 2006). Includes reprints of papers co-authored with others. Vita. Includes bibliographical references.

Empirical studies have shown that algal- and detrital-based food web components are coupled in many pelagic systems as algal carbon enhances bacterioplankton production and growth efficiencies. Such phyto-bacterioplankton coupling impacts carbon flow through plankton food webs, yet the extent of coupling is poorly understood in systems receiving large amounts of allochthonous carbon. To investigate this issue, bacterioplankton abundance (BA) and community composition were compared to chlorophyll a concentrations and phytoplankton production in the tidal-freshwater James River (VA). BA averaged 107 cells mL-1 and was significantly related to chlorophyll a, phytoplankton production, and DOC concentrations. Analysis of DOC quality using fluorescence spectroscopy revealed that the fulvic DOC fraction was dominated by allochthonous compounds. However, estimates of DOC C:N and DOC turnover rates indicated that DOC was more labile in the lower part of the study reach where BA was highest. T-RFLP analysis of 16s rDNA showed that bacterioplankton community composition significantly varied between the upper and lower portions of the sampling reach. These findings suggest that coupling of food web components is an important pathway affecting carbon cycling within the tidal-fresh water James River.

Phytoplankton produce allelochemicals; excreted chemical substances that are affecting other microorganisms in their direct environment. In my thesis, I investigated strain specific variability in the expression of allelochemicals of the harmful flagellate Prymnesium parvum, that is euryhaline but mainly bloom forming in brackish water. I found a large variation among strains, but further showed that all strains of P. parvum were more allelopathic in brackish water compared to marine water. In a marine microbial community, allelochemicals can affect prey, competitors and grazers both, directly and indirectly. For instance, in a food web where grazing controls prey abundance, the negative direct effect of allelochemicals on grazers will positive affect their prey. During my thesis, I investigated how marine microbial communities respond to the addition of allelochemicals. I performed field experiments with microbial communities from seawater collected from different places over Europe, and tested how this communities respond to the addition of allelochemicals from the dinoflagellate Alexandrium tamarense. Before I incubated the microbial communities for several days with A. tamarense algal filtrate, I evaluated the allelopathic efficiency of the algal filtrates with an algal monoculture of Rhodomonas spp. This allowed me to compare the effect of A. tamarense filtrate between the different microbial communities. In general, bacteria reached higher abundances when allelochemicals were present. As allelochemicals also inhibited nanoflagellates and ciliates, we concluded, that allelochemicals indirectly benefit bacteria by reducing grazing pressure. In microbial food webs with many heterotrophic grazers, allelochemicals further benefitted other phytoplankton by inhibiting grazers. It was also shown that bioavailable DOM is released from a microbial community when allelochemicals are present. As most DOM was released from the seawater fraction > 60 μm, we concluded, that larger microorganisms are more affected by allelochemicals than smaller microorganisms. The results can be explained by the surface to volume ratio of microorganisms: Larger organisms provide more contact surface for allelochemicals, and therefore, are probably more vulnerable towards allelochemicals. In conclusion, the effect of allelochemicals on a microbial community depends among others on the structure of the microbial food web, the amount of available DOM, the particle density in the seawater and the composition of the phytoplankton community.

Os ecossistemas aquáticos têm sido afetados pelo uso intensivo de agrotóxicos e pela introdução de espécies exóticas. Para melhor avaliar os efeitos diretos e indiretos dessas atividades sobre as comunidades aquáticas, é essencial a utilização de experimentos em escala mais próxima a real. O objetivo desta pesquisa foi analisar os efeitos do agrotóxico Vertimec® 18 CE, cujo princípio ativo é a abamectina, sobre a comunidade planctônica através de um experimento em mesocosmos com duração de 20 dias. Para reproduzir relações biológicas mais complexas e analisar o impacto da introdução de uma espécie exótica sobre o plâncton, inseriram-se girinos de Lithobates catesbeianus em alguns tratamentos. Foram consideradas duas concentrações do agrotóxico (2 μL L-1 em dose múltipla e 8 μL L-1 em dose única) na ausência e presença de girinos, além de dois tratamentos não contaminados (controle com e sem girinos). Variáveis físicas, químicas e biológicas (clorofila a, aspectos qualitativos e quantitativos do fitoplâncton e do zooplâncton) da água e do sedimento foram monitoradas. A partir dos resultados obtidos, verificou-se que os girinos causaram um aumento na concentração de nutrientes na água e uma diminuição do oxigênio dissolvido e da densidade planctônica. Nos tratamentos contaminados, como um efeito indireto do incremento da concentração de nutrientes e da mortalidade das espécies de Cladocera e Copepoda, a densidade do fitoplâncton e de algumas espécies tolerantes de Rotifera aumentou. No tratamento com a concentração mais elevada do agrotóxico, registrou-se a morte de todos os girinos, proporcionando um aumento na concentração de nutrientes na água e na densidade de algumas espécies fitoplanctônicas, que alcançaram uma densidade total de 96.919 ind mL-1. Estes resultados indicam que a inserção dos girinos e a adição do agrotóxico causaram efeitos diretos e indiretos sobre a comunidade planctônica e a qualidade da água. Conclui-se que os mesocosmos podem contribuir para a compreensão de tais efeitos e oferecer subsídios ao manejo de ecossistemas de água doce impactados por atividades antrópicas.
Aquatic ecosystems have been impacted due to the intensive use of pesticides and the introduction of exotic species. It is essential to use more realistic experiments for assessing direct and indirect effects of these activities on aquatic communities. This thesis aimed to analyze the effects of the pesticide Vertimec® 18 EC, whose active ingredient is abamectin, on plankton community through a mesocosm experiment during 20 days. Tadpoles of the species Lithobates catesbeianus were introduced in some treatments to simulate complex biological relationships and to evaluate the impact of the introduction of such exotic species on plankton. Different treatments were analyzed, including one case of no contamination (control) and two cases of contamination: 2 μL L-1 (multiple dose) and 8 μL L-1 (single dose). Such conditions were considered both with absence and presence of tadpoles. Physical, chemical and biological (chlorophyll a, qualitative and quantitative aspects of phytoplankton and zooplankton) variables of water and sediment were also monitored. The tadpoles caused an increase in the concentration of nutrients and a decrease of dissolved oxygen concentration and plankton density. In the contaminated treatments, as an indirect effect of the nutrient increase and the mortality of species of Cladocera and Copepoda, the density of phytoplankton and some tolerant species of rotifers increased. In the treatment with the highest concentration of the pesticide, all the tadpoles died. As a result of the death of the tadpoles, there was nutrient enrichment in the water, with an associated increase in the density of some phytoplankton species, reaching a total density of 96,919 ind mL-1. These results suggested that direct and indirect effects occurred on plankton community and water quality through the addition of the contaminant and the tadpoles. We conclude that the mesocosms might contribute to the understanding of these effects and provide subsidies for management of freshwater ecosystems impacted by human activities.

As comunidades mudam no tempo e espaço em função de pressões decorrentes da estrutura do habitat e disponibilidade de recursos. O entendimento das relações ecológicas e interações das comunidades são essenciais para o manejo com objetivo de melhorar a qualidade da água através de interferências na cadeia trófica aquática e na dinâmica do ecossistema através de modelagem ecológica. O presente estudo foi desenvolvido em dois sistemas límicos subtropicais no extremo sul do Brasil: banhados temporários (‘campos de cultivo de arroz’) e um grande lago raso, lagoa Mangueira. O objetivo do trabalho nos banhados temporários constituiu-se em avaliar a dinâmica das comunidades planctônicas e íctica durante o desenvolvimento desses ecossitemas (um ciclo produtivo de cultivo de arroz). Nossos resultados evidenciaram um claro gradiente temporal direcionado pela disponibilidade de nutrientes no banhado temporário. Entretanto, as variáveis limnológicas não apresentaram nenhuma tendência espacial entre as estações amostrais. Através de regressão linear verificou-se correlação positiva entre clorofila a, nutrientes, biomassa zooplanctônica e, em especial, biomassa de copépodos. Por outro lado, biomassa de peixes e, particularmente, peixes planctívoros foram inversamente relacionados à concentração de clorofila a. Relações significativas entre o DOC com nutrientes, biomassa de plâncton e peixes foram igualmente identificadas ao longo do ciclo produtivo do arroz. Na lagoa Mangueira, sistema raso (zmed 3m), de 90 km de comprimento, o objetivo foi avaliar a distribuição espacial e temporal de plâncton e peixes em função da presença da macrófita emergente Zizaniopsis bonariensis. Além disso, foi verificada a existência de um gradiente longitudinal na lagoa e a estrutura da teia trófica do sistema. Análise de componentes principais e análise de redundância evidenciaram a presença de gradientes espacial (Norte/Sul) e temporal (sazonal) durante o estudo. Análise de similaridade (ANOSIM) aplicada às variáveis ambientais evidenciou diferenças significativas entre os extremos da lagoa (Norte e Sul). Transparência Secchi, DOC e clorofila a foram significativamente diferentes entre os pontos amostrais do Norte e Sul. Através de ANOVA verificou-se que os efeitos temporais sazonais são mais intensos no Norte da lagoa Mangueira. Com relação à estrutura da comunidade na lagoa Mangueira, a biomassa do bacterioplâncton, clorofila a, biomassa zooplanctônica e captura de peixes foram diferentes sazonalmente após a extração do efeito da estrutura de habitat, enquanto que no Sul apenas a clorofila a apresentou variação sazonal significativa. A estrutura de habitat (junto à Z. bonariensis e em água aberta) induziu diferenças significativas na concentração de clorofila a tanto no Norte como no Sul da lagoa Mangueira, após a extração dos efeitos decorrentes da sazonalidade. A captura de peixes também foi influenciada pela estrutura de habitat no Norte, após a extração do efeito sazonal. Análise de similaridade (ANOSIM) aplicada à comunidade de peixes também demonstrou diferenças significativas entre o Norte e o Sul da lagoa Mangueira, embora diferenças entre as áreas vegetadas e águas abertas não tenham sido verificadas. Biomassa dos peixes, número de capturas e riqueza específica foram diferentes entre os pontos amostrais. Baseado nas razões de δ13C and δ15N identificou-se a estrutura trófica da lagoa, bem como a posição trófica de espécies de peixes e as fontes de carbono do sitema. Análise de isótopos estáveis de nitrogênio permitiu a identificação de dois níveis tróficos de peixes na lagoa Mangueira. Macrófitas emergentes e perifíton foram identificadas como importantes fontes de carbono que sustentam a estrutura trófica do sistema. Análise de grupamento a partir de dados de δ13C e δ15N e de conteúdo estomacal identificou um arranjo perfeitamente filogenético entre as espécies de peixes analisadas, refletindo que, em sentido amplo, nichos alimentares são compartilhados por espécies taxonomicamente relacionadas. A dinâmica trófica da espécie de peixe dominante na lagoa, Oligosarcus jenynsii foi também analisada. Foram identificadas mudanças sazonais e ontogenéticas na dinâmica alimentar da espécie, evidenciando a grande plasticidade trófica associada com uma estratégia oportunista, característica de espécies carnívoras generalistas. As informações obtidas neste estudo permitiram identificar grande heterogeneidade espacial e claro gradiente longitudinal na lagoa Mangueira, tanto de fatores bióticos quanto abióticos, com reflexos na distribuição e abundância de plâncton e peixes. De posse destas informações foi possível alcançar um entedimento sobre a estrutura atual do sistema, bem como os direcionadores da sua dinâmica. A abordagem direcionada neste trabalho sobre o entendimento da estrutura atual de um lago raso subtropical possibilita futuros estudos na área da modelagem ecológica. A dinâmica das comunidades abordadas neste estudo vem subsidiar a parametrização de um modelo ecológico, com capacidade de estabelecer prognósticos, tendo como contorno, estressores naturais e antrópicos.
Biological communities change in time and space following driving pressures from differences in habitat structure and resource availability. Understanding the ecological role and interactions within aquatic community is essential for any management action trying to improve water quality by interferences in lake food web, ecosystem dynamics and modeling. The present study was carried out in two freshwater systems in Southern Brazil: an irrigated rice field and in a large shallow system, Mangueira Lake. The goal of this study in temporary wetlands was to evaluate the plankton and fish dynamics during a productive cycle. Our results evidenced a temporal gradient by nutrients availability in the temporary wetland studied. However, the limnological variables did not display any horizontal pattern among sampling stations. Linear regression showed a positive relationship between chlorophyll a and nutrients, zooplankton biomass and copepod biomass. In contrast, fish biomass and planktivorous fish biomass were inversely related to chlorophyll a. Statistically significant relationships between DOC with nutrients, plankton and fish biomass were also identified during the rice production cycle. In Mangueira Lake, a shallow system (zmed 3m) and 90 km long, the goal was to evaluate the spatial and temporal distribution of plankton and fish biomass as a function of the presence of the emergent macrophytes Zizaniopsis bonariensis. We also analyzed the existence of longitudinal gradient in lake and the food web structure in system. The PCA and RDA analyses showed the temporal (seasonal) and spatial (North/South) gradient during the study. Analysis of similarity (ANOSIM) applied to environmental variables showed significant differences between sampling sites (North and South). Secchi transparency, DOC and chlorophyll a were significantly different in North and South sampling sites. ANOVA results showed that season effects are stronger in the Northern sampling site. Concerning the community structure in the Mangueira Lake, bacterioplankton biomass, chlorophyll a, zooplankton biomass and fish captures were different seasonally after extracting the habitat structure effect, while in the Southern sampling site only chlorophyll a presented a seasonal significant variation. Habitat structure (by Z. bonariensis stands and open water samples) induced significant differences in chlorophyll a both in North and South samples after extracting seasonal effect. Fish captures do also responded to habitat structure in the North, after extracting the season effect. Analysis of similarity (ANOSIM) applied to fish community also showed significant differences between North and South, although no significant differences between vegetated and open water zones was observed. Fish biomass, number of captures, and richness were different between sampling sites. Based on the δ13C and δ15N ratios and analysis of stomach content of fish species, we provide also a description of food web structure, trophic positions of fish species and primary producers of system. Analysis of nitrogen isotope ratios yielded two fish trophic levels in Mangueira Lake. Emergent macrophytes and periphyton were important carbon source that sustain the food web structure of the system. Cluster analysis of δ13C and δ15N values and stomach content yielded a perfect phylogenetic arrangement of species. This result reflects that major feeding niches are shared by taxonomically related species. The feeding dynamics of the dominant fish species, Oligosarcus jenynsii was analysed by using analyses of stomach contents and stable isotopes signature. Seasonal and ontogenetic change in the feeding biology of O. jenynsii was identified, showing the high feeding plasticity, in addition to an opportunistic strategy of this generalist carnivore species. This study allows identifying a large spatial heterogeneity and clear longitudinal gradient both in biotic and abiotic factors, reflecting fish and plankton distribution and abundance. The evaluated parameters are intended to feed a mathematical model for the Mangueira Lake, looking for predictable scenarios from natural and anthropogenic stressors.

Periodic monitoring of contaminant levels in fish from the Yukon Territory indicated that organochlorine (OC) contaminants had rapidly declined since the early 1990s. This study examined OC concentrations, including chlordane (sigma-CHL), sigma-DDT, hexachlorocyclohexane (sigma-HCH), toxaphene (sigma-CHB), sigma-PCB and chlorinated benzenes (sigma-CBz) in sentinel fish (species of consistent annual observation and collection) from two Yukon lakes (Kusawa, Quiet), and from the aquatic food web of a focus lake (Lake Laberge) across several temporal points between 1993 and 2003. OC analysis and phytoplankton counts from dated sediment cores as well as climate data were also collected. Population, morphological (length, weight, age), biochemical (lipid content, delta-13C, delta-15N) and OC contaminant data for fish and invertebrates (zooplankton, snails, clams) were reviewed to elucidate the primary causes for these OC declines. Although some spatial differences in contaminant levels exist between the Yukon lakes, OC concentrations were declining for lake trout in all three lakes, with declines also noted for burbot from Lake Laberge. Several other fish species as well as zooplankton from Lake Laberge exhibited decreases in contaminant levels except northern pike, which registered consistently higher levels from 1993 to 2001. There was no evidence to support the hypotheses of changes in fish trophic levels or food sources with the exception of burbot, which marginally decreased, and northern pike, which climbed a half trophic level. Through OC flux analysis in dated sediments, the hypothesis that declines in abiotic deposition affected the contaminant levels in the food web was also negated. The closure of the Lake Laberge commercial fishery resulted in faster fish growth and larger fish populations, which are contributing to biomass dilution of OC concentrations, higher OC biomagnification factors for some species and likely changes in predator-prey interactions as resource competition increases. The large ratio of OC decreases in the lower vs. higher trophic levels of Lake Laberge have increased food web magnification factors (FWMF) for all six OC groups. It is also suspected that above-average temperatures and below-average precipitation in the lower Yukon region over the 1990s may have contributed towards an increase in lake primary production resulting in biomass dilution of contaminants in zooplankton for all three study lakes. Concurrently, shifts in the Lake Laberge zooplankton community, from climate fluctuations or increased fish predation, have gone from an abundance of Cyclops scutifer in 1993 to dominance by Diaptomus pribilofensis in 2001, although sample sites were limited. Characteristics specific to each species (e.g. body size, composition and metabolism) likely play a role in the significant OC declines measured in zooplankton. Fluctuations in population dynamics, species characteristics and OC contaminant concentrations in the Lake Laberge ecosystem may continue for several years to come. Sentinel species such as lake trout, burbot, whitefish, cisco and plankton should continue to be monitored in all three Yukon lakes for future temporal correlations with contaminants or climate change.
May 2006