Water quality in reservoirs is often compromised in many regions worldwide by nutrients and trace metals. This demands continuous monitoring; however, analyses of large data sets collected during regular monitoring remain a difficult task. Multivariate techniques offer a fast and robust approach for interpreting complex results. The objective of this study was to check the efficacy of selforganizing maps (SOMs) as a tool to investigate biogeochemical processes. This tool can also help to illustrate influences of land use patterns on the water quality of reservoirs. Here we use the Itupararanga Reservoir in Brazil as a subtropical example. Vertical profiles were sampled from seven sites in the reservoir in a total of seven campaigns over 24 months. Next to physicochemical parameters in the water column (dissolved oxygen, Eh, pH, and temperature), levels of nutrients (NO 3 − , NH 4 + , and PO 4 3− ), transition and trace metals (Al, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, and Zn), and chlorophyll-a (Chla) were measured. These variables were correlated with land use using SOM. With this technique samples were classified into 17 distinct groups that showed distinct influences of spatial heterogeneity and seasonality. The analyses helped to reveal a seasonal stratification period, where Fe, Mn, and P were released from sediments. Nutrients and some metal inputs (Al and Fe) were related to agricultural, urban, and grass/pasture areas around the reservoir. Our approach also helped to explain physical and biogeochemical seasonality in the reservoir.
RESUMODentre as diversas atividades desenvolvidas no entorno de corpos de água, a poluição por agroquímicos e o lançamento de efluentes acarretam a carga excessiva de nutrientes que contribuem para o processo de eutrofização artificial e degradação destes ecossistemas. Neste contexto, os objetivos deste trabalho foram: determinar a heterogeneidade sazonal e espacial nas águas superficiais do reservatório Itupararanga e verificar a influência dos usos do entorno sobre a qualidade da água do reservatório localizado na cidade de Ibiúna, São Paulo, Brasil. Analisou-se o Índice de Estado Trófico (IET) e variáveis limnológicas em duas estações distintas -período seco e chuvoso de 2013. Os dados foram comparados com mapa de usos do solo e com parâmetros estabelecidos pela resolução CONAMA n° 357 de 2005. Os resultados demostraram que o fósforo total não esteve em conformidade com as especificações CONAMA. O reservatório apresentou heterogeneidade sazonal e espacial em relação ao IET e às variáveis limnológicas. Por meio da análise de agrupamento com os dados de qualidade de água e uso e ocupação dos solos foi comprovada a heterogeneidade espacial. A análise de agrupamento indicou que a cobertura florestal é um importante fator para a contenção da entrada de sólidos em suspensão. Indicou também que o uso agrícola é a principal fonte de nitrogênio para o meio enquanto que os rios formadores contribuem, principalmente com cargas de fósforo. Assim, por se tratar de uma área de importância social e econômica, medidas de preservação devem ser tomadas a fim de mitigar os avanços das atividades potencialmente impactantes.
Anthropic eutrophication is one of the most widespread problems affecting water quality worldwide. This condition is caused by excessive nutrient inputs to aquatic systems, and one of the main consequences is accelerated phytoplankton growth. Eutrophication can lead to damage to human health, the environment, society, and the economy. One of the most serious consequences of eutrophication is the proliferation of cyanobacteria that can release toxins into the water. The aim of this research was to evaluate the trophic condition of a tropical reservoir over the course of time, using a database extending over 15 years to investigate relationships with environmental conditions, considering spatial heterogeneity and seasonality, as well as inter-relations between trophic state indicators. Data for chlorophyll-a, total phosphorus, and total nitrogen were collected from 2000 to 2014, and cyanobacteria abundance was determined from 2004 to 2014. The trophic state index was also calculated. The results demonstrated the existence of two distinct compartments in the reservoir: one lotic and the other lentic. No relationship was observed between chlorophyll-a and phosphorus. The results suggested that phytoplankton growth was mainly controlled by nitrogen concentrations. These conditions favored cyanobacteria predominance, resulting in increasing abundance of these potentially toxic bacteria over time. The model obtained indicated hypereutrophic conditions, with high phytoplankton biomass and cyanobacteria abundance during the next years likely to affect the uses of the water of the reservoir.
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