AIM: The aim of this work was to study the diel stratification parameters temperature, dissolved oxygen and conductivity, providing a more detailed description of a friagem event. METHODS: the mixing behavior of the limnetic water column in Lago Catalão, an Amazon floodplain lake flooded by two of the world’s largest rivers (Solimões and Negro), was studied through diel profiling of temperature, conductivity and dissolved oxygen, between 2006 and 2007. Measurements were made every four hours at 1m depth intervals to the lake bottom. RESULTS: The water column remained stratified during most of the year with the exception of one month with low water level, when nocturnal mixing occurred and one friagem event during high water (May 2007) when an isothermal temperature profile and an atypical orthograde oxygen profile were observed. CONCLUSIONS: The friagem event, a large drop in air temperatures due to the intrusion of a polar air mass in the Amazon, constitutes an important force in the dynamics of the water column, which produces a profound redistribution of dissolved gases and solutes in the system. This is the first time that the effects of the friagem on the mixing patterns in a lowland Amazon lake have been documented in detail. The decline in air temperature, observed during the friagem event destabilized the normally stratified water column, which can be associated with an environmental disturb.
RESUMOEste estudo analisou os atributos da comunidade fitoplanctônica (riqueza de espécies, diversidade, equitabilidade, densidade, estrutura de tamanho e taxa de modificação da comunidade) associados com algumas características físicas e químicas de um lago de água mista (branca e preta) durante quatro semanas consecutivas na cheia e na seca, em quatro estações amostrais e três profundidades (superfície, meio e fundo). O lago apresentou diferentes características limnológicas entre os períodos de seca e cheia. O fitoplâncton foi composto principalmente por algas nanoplanctônicas, especialmente Chlorophyceae e Cyanobacteria. Em ambos os períodos, o grupo funcional X1 foi a associação predominante no que se refere à densidade de organismos. Com relação aos períodos de amostragem, os maiores valores de riqueza, equitabilidade e diversidade foram registrados no período de seca, sendo Chlorophyceae, Euglenophyceae, Bacillariophyceae e Cyanobacteria os grupos com maior número de espécies, enquanto, no período de cheia, Chlorophyceae e Cyanobacteria foram os grupos com maior riqueza de espécies. As variáveis físicas e químicas determinantes na distribuição das espécies foram oxigênio dissolvido, temperatura, condutividade elétrica e razão zona de mistura:zona eufótica.Palavras-chave: fitoplâncton, planície de inundação, lago amazônico. ABSTRACTThis study analyzed the attributes of the phytoplankton community (species richness, diversity, equitability, density, size structure and rate of community compositional change) associated with some physical and chemical characteristics of a mixed water lake (white and black water) for four consecutive weeks in the dry and rain season, in four sampling sites and at three depths (surface, middle and bottom). The presented differences in limnological characteristicas in the rainy and the dry season. The phytoplankton was composed, mainly by nanoplanktonic algae, especially Chlorophyceae and Cyanobacteria. In both periods the functional group X1 was the predominant association in terms of organism density. Considering the sampling seasons, the highest values of richness, equitability and diversity were recorded in the dry season. Chlorophyceae, Euglenophyceae, Bacillariophyceae and Cyanobacteria were the groups with the highest number of species in the dry season, while Chlorophyceae and Cyanobacteria were the groups with the highest species richness in the rainy season. The main physical and chemical variables that determine the distribution of the species were dissolved oxygen, temperature, electrical conductivity and ratio of mixing zone to euphotic depth.Key words: phytoplankton, floodplain, Amazon Lake. Estrutura da comunidade fitoplanctônica de um lago de inundação amazônico (Lago Catalão, Amazonas, Brasil) Phytoplankton community structure in an Amazon floodplain lake (Lago Catalão, Amazonas, Brazil) 113 Neotropical Biology and Conservation Estrutura da comunidade fi toplanctônica de um lago de inundação amazônico (Lago Catalão, Amazonas, Brasil)
Este estudo aborda a variação temporal (sazonal, semanal e nictemeral) e espacial de algumas variáveis limnológicas na região limnética do lago Catalão, um lago da planície de inundação do rio Amazonas. Foram analisados os valores de temperatura, zona eufótica, zona de mistura, condutividade elétrica, oxigênio dissolvido, transparência, nitrogênio e fósforo total, amostrados em quatro estações, em escala semanal, durante um mês no período de seca de 2006 e um mês no período de cheia de 2007. Regressões simples foram realizadas para testar a hipótese da homogeneização de ambientes aquáticos em função da inundação. Foram analisados, também, os valores de temperatura, oxigênio dissolvido e condutividade elétrica em um perfil vertical ao longo de um ciclo nictemeral na seca e outro na cheia. Os resultados confirmam a importância do pulso de inundação na dinâmica das variáveis físicas e químicas no lago. A ACP indicou que o maior gradiente das variáveis físicas e químicas na seca foi o espacial. Já na cheia, o padrão de comportamento foi temporal. As regressões indicaram homogeneização do ambiente em função da inundação. Os resultados das análises nictemerais indicam que o lago Catalão apresenta comportamento polimítico na seca e oligomítico na cheia. Palavras-chave: variáveis limnológicas, variação nictemeral, planície de inundação, variação sazonal, pulso de inundação, Amazônia Brasileira. ABSTRACT. Limnological considerations about an Amazonian floodplain lake (Catalão lake-Amazonas State, Brazil). The temporal and spatial variation of some limnological variables in the limnetic zone of an Amazonian floodplain lake, Catalão Lake, was studied. The following parameters were measured weekly during the dry period of 2006 and the flood period of 2007: water temperature, euphotic zone, mixing zone, electrical conductivity, dissolved oxygen, water transparency and total phosphorus and nitrogen. Simple regressions were carried out to test the hypothesis of environmental homogeneity as a function of flooding. Also, diel temperature, dissolved oxygen and electric conductivity profiles were studied during the dry period and in the flood period. The results show the importance of the flood pulse in the variation of the physical and chemical variables in the lake. The PCA indicated that, during the dry period, the physical and chemical variables had a stronger spatial pattern of distribution, while during the flood period, the pattern was temporal. The regressions indicated the homogeneity of the environment as a function of flooding. The results of the diel cycles seem to indicate that Catalão lake has a polimictic and oligomictic behavior during the dry and flood periods, respectively.
The transfer of Encyonopsis frequentiformis Metzeltin & Krammer to the genus Gomphonema Ehrenberg is proposed based on observations using light and scanning electron microscopy. The taxon is characterized by lanceolate heteropolar valves, with both apices strongly capitate, two apical pore fields, and elongated areolae slit-like and the internal stigma opens in a slit. Heteropolarity of the specimens measured and other shared similarities with the genus Gomphonema (APF, areolae, external and internal aperture of the isolated pore, pseudosepta at both apices) allows us to classify this species into the genus Gomphonema. Morphological, ecological and distribution data in South America are presented and discussed based on available literature for this Neotropical taxon.
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