Abstract Aquatic humic substances (HS) represent about 60-95% of the dissolved organic carbon (DOC) present in coastal environments of northern Rio de Janeiro state, Brazil. Although they are important regulators of processes involving aquatic communities, the response of the zooplankton community to their presence remains poorly understood, especially in natural tropical environments. Therefore, our objective was to elucidate zooplankton community responses along a natural gradient of HS. Such natural humic gradient was obtained in coastal freshwater environments with distinct DOC concentrations (20-200 mg L-1). Results show a decrease in zooplankton density and biomass along the HS gradient. However, microphages organisms (e.g. non-predatory rotifers and smaller testate amoeba, such as Difflugia) were most present in environments with higher concentration of HS, probably due to a stronger importance of the microbial-loop in these environments. Some species – such as Scapholeberis armata (Cladocera) and Lecane boettgeri (Rotifera) were only accounted for environments with high HS concentration, illustrating their potential as bioindicators for HS presence. Nevertheless, we were able to observe the effects of HS on the structure and composition of primary consumers and how these substances might indirectly affect species dynamics. We point out to new findings in highly humic tropical environments, which are still poorly studied and understood.

Abstract Studies of the limnetic zooplankton in Brazilian freshwater have been done using several sampling strategies which are frequently associated to personal preferences, system constraints and objectives of the studies. It is known that a better sampling technique exists for each group of investigated zooplankton. So, the challenge is to provide a good sampling method for all groups at once. Regarding this theme we are contributing by pointing out the merits and demerits of two collecting methods of zooplankton widely used in many studies in Brazilian reservoirs to clarify the information about communities’ structure in past published and non-published biological data and also find a more useful sampler for future studies. Two simultaneous strategies were used with a 68μm-plankton net: filtration of water collected at sub-surface by bucket and vertical hauls throughout the euphotic zone. A total of 133 taxa was found with both strategies. Rotifers (69 taxa) comprised the group with the highest richness, followed by protozoans (31 taxa), cladocerans (30 taxa) and copepods (5 taxa). Paired t-tests showed significant difference for richness despite none for density in whole period. Spatially, sampling strategies differed in abundance and composition of zooplankton at most sites sampled (except for L1- shallowest site and L4 – deepest site). Temporally, both samplers showed the highest mean abundances at thermal stratification period. In general, the extent of spatial data collected throughout the vertical gradient provided better estimations in the limnetic area for the attributes analyzed: zooplankton density, richness, specific diversity, equitability and cumulative richness. Zooplankton sampling by vertical hauls through the euphotic zone was more efficient, but in order to obtain integrated data and according to the collector’s curve it might be useful to adopt both methodologies.

Resumo A flexibilidade alimentar e onivoria são características das espécies de peixes de água doce neotropicais, especialmente em espécies de Astyanax. Tradicionalmente a maioria dos recursos na dieta de peixes é reconhecidamente originária do ambiente aquático, porém estudos recentes têm apontado para a importância das fontes alóctones. Além disso, a colonização de macrófi tas, comuns em vários reservatórios tropicais, pode melhorar os aspectos de insetivoria na dieta dos peixes expandindo ou concentrando a área disponível para a alimentação. Aqui analisamos o conteúdo estomacal de duas espécies de Astyanax simpátricas para avaliar a importância das diferenças de habitats, como a complexidade espacial, em dois reservatórios tropicais com diferentes características ambientais. A análise NMDS indicou separação na dieta destas espécies entre reservatórios (stress = 8.28%). Além disso, a análise de variância revelou uma diferença signifi cativa no volume de itens alimentares consumidos entre os reservatórios (Anova one-way F (1. 132) = 4,4446; p = 0.037), salientando a importância das condições ambientais na composição da dieta de peixes. O estudo destacou o hábito alimentar insetívoro das espécies de Astyanax e revelou diferentes estratégias de alimentação entre esses peixes simpátricos apesar da alta sobreposição de nicho nos dois ambientes. A heterogeneidade de hábitat aumentando a disponibilidade de recursos alimentares teve importância sobre a estratégia alimentar das espécies de Astyanax e consequentemente na manutenção constante de ambas nos dois diferentes reservatórios.

Abstract Food flexibility and omnivory are important features pronounced in Neotropical freshwater fish species, particularly for Astyanax species. Traditionally most fish diet resources are known to be originated in the aquatic environment, however recent studies have pointed to the importance of allochthonous sources. Besides, the colonization of macrophytes, common at several tropical reservoirs, may enhance insectivory in fish diet expanding or concentrating the area of available resources for feeding. Here we employed stomach contents analysis of two sympatric Astyanax species to access the importance of habitat differentiations as spatial complexity in two tropical reservoirs with different environmental features. The NMDS analysis indicated separation in the diet of these species between reservoirs (Stress= 8.28%). Additionally, analysis of variance revealed a significative difference in the volume of food itens ingested between the reservoirs (Anova one-way F(1, 132)=4.4446; p= 0.037). This points out the importance of environmental conditions on the composition of the diet of fishes. This study highlighted the insectivorous feeding habit of Astyanax species and revealed different feeding strategies between sympatric fishes despite high niche overlap in both environments. Habitat heterogeneity increasing food resources availability plays an important role in the diet strategy of these Astyanax species and on their constant maintenance in the two different reservoirs.

Os rotíferos têm sido utilizados com indicadores de mudanças súbitas de características físicas e químicas do ambiente aquático. Tais características sofrem grandes variações durante o pulso de inundação em pequenos lagos associados a planícies de inundação. No entanto, poucos estudos investigam quais são as consequências deste pulso de inundação sobre a composição, abundância, riqueza e diversidade de rotíferos, especialmente em lagos amazônicos. Neste estudo, foram analisadas amostras de um pequeno lago de águas escuras de um igarapé conectado permanentemente com o rio Negro, na Amazônia Central. As amostras foram coletadas duas vezes por ano durante dois anos, compreendendo os períodos de enchente e vazante do pulso. A abundância de rotíferos aumentou significantemente após eventos de seca e a condutividade elétrica e a turbidez foram intrinsecamente relacionadas a esta variação. A composição de espécies também variou entre os períodos de enchente e vazante. Alguns táxons, como Brachionus zahniseri reductus e Lecane remanei foram restritos a períodos de vazante, enquanto B. zahniseri, B. gillardi e L. proiecta foram presentes apenas durante fases de enchente. Uma mudança na composição de famílias de rotíferos foi observada entre um período e outro, ilustrando o efeito de renovação de águas do pulso de inundação. Estes resultados sugerem que o pulso de inundação age como força direcionada e condição estressante que altera consideravelmente a dinâmica da comunidade de rotíferos, seja modificando a composição de espécies ou diminuindo sua abundância.

Rotifers have often been used as indicators of sudden changes in physical and chemical features of the aquatic environment. Such features vary greatly during flood pulse events in small lakes connected to major floodplains. However, few are the studies that investigate the consequences of the flood pulse in rotifer species composition, abundance, richness and diversity, especially in Amazonian lakes. We analyzed samples from a small blackwater lake of an “igarapé” connected permanently to the Negro river, in Central Amazonia. Samples were taken twice a year for two years, comprising flooding and receding periods of the flood pulse. Rotifer abundance increased significantly after draught events, and electrical conductivity and turbidity were intrinsically related to such variation. Species composition also changed from flooding to receding periods. Some taxa, such as Brachionus zahniseri reductus and Lecane remanei were restricted to receding periods, while Brachionus zahniseri, Brachionus gillardi and Lecane proiecta were only present during flooding. A shift in the composition of rotifer families was observed from one period to another, showing the effect of renewing waters of the flood pulse. These results suggest that the flood pulse acts as a driving force and stressing condition, considerably altering rotifer community dynamics, either changing species composition or decreasing abundance.