Abstract
; periodic turbulence with re-oxygenation of the whole water column, and high saturation of oxygen (80-100%); low retention time (< 20 days) and a extensive macrophyte grow in the headwaters that prevent high nutrient load. This is the picture for the last 44 years. The phytoplankton composition was consistent with the oligomesotrophic characteristic of the reservoir: predominance of diatoms and chlorophyceae with a maximum of 10µg/l chlorophyll. However in the winter of (july) 2014 the following changes were observed: a heavy bloom of cyanobacteria occurred for the first time in the reservoir. This cyanobacteria Cylindrospermopsis raciborskii, is a invasive species. Very high chlorophyll levels (up to 100µg/l) were measured and high concentration and input of phosphorus was also detected (Tundisi and Matsumura-Tundisi, 2014). The explanation for this sudden appearance of blooms, can be attributed to the following factors: increase of up to 2°C above the average water temperature during the winter; lower rainfall during summer (30% less of the yearly average of 1.500 mm). Increase in the retention time (from <20 days up to 60 days) in order to maintain volumes for hydroelectricity production. This cyanobacteria bloom can be attributed to changes in the regional climate and the hydrological regime, with consequences for the dam operation. As described by Paerl and Huisman (2008), global warming affects patterns of precipitation and drought. The changes in the hydrological cycle enhanced cyanobacterial dominance. Heavy rains after extensive drought periods, increase nutrient input promoting phytoplankton growth. During periods of drought ,residence time, increase, thus promoting blooms.
Highlights
The explanation for this sudden appearance of blooms, can be attributed to the following factors: increase of up to 2°C above the average water temperature during the winter; lower rainfall during summer (30% less of the yearly average of 1.500 mm)
As a further consequence of this process,extensive periods of drought prevent silica discharge into the reservoir reducing diatom growth; the Lobo/Broa reservoir had a predominance of Aulacoseira italica during many years due to silica concentrations of up to 5mg/l
Its is known from several sources and scientific research that cyanobacteria produces toxins that are harmful to human and animal health. (Carmichael 1992; Azevedo et al, 1994)
Summary
The explanation for this sudden appearance of blooms, can be attributed to the following factors: increase of up to 2°C above the average water temperature during the winter; lower rainfall during summer (30% less of the yearly average of 1.500 mm). As described by Paerl and Huisman (2008), global warming affects patterns of precipitation and drought. The changes in the hydrological cycle enhanced cyanobacterial dominance. Heavy rains after extensive drought periods, increase nutrient input promoting phytoplankton growth.
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