| dc.description.abstract | Potentially toxic cyanobacterial blooms have been frequent in Brazilian lakes
and reservoirs, causing economic issues as well as public health risks. In Rio
Grande do Sul, a series of five reservoirs in the high course of the River Jacuí
form a cascade system. One of the reservoirs, Passo Real, mainly accumulates
water for hydroelectrical power generation and also regulates water flow to
downstream reservoirs, amongst other uses. Blooms of cyanobacteria have
been frequently recorded in this water body, possibly due to the long water
residence time, associated to weather conditions and nutrient runoff from
intense agricultural activities in the surrounding area. The study described
herein aimed to: a) verify patterns of climatic, hydrological and limnological
conditions, as well as their relationship with blooms of cyanobacteria; b)
describe the yearly circulation regime, and thermal stratification of the water
column; c) define the most favourable environmental conditions for the
incidence of both dispersive and surface accumulative blooms, and also which
species are adapted to each bloom type. 60 phytoplankton samples were taken
from Passo Real reservoir between October 2009 and January 2013.
Cyanobacteria species were identified and counted to estimate specific density
and specific biovolume. Each bloom was assigned to a class: dispersive or
surface accumulative. Measurements of solar radiation availability, solar shine,
air temperature, rainfall, direction, and velocity of wind were taken, and also
weather conditions for the sampling period were recorded. Hydrological (inflow,
outflow, altitude, usable volume and residence time) and limnological
(transparency, water temperature, pH, electric conductivity, total dissolved
solids and chlorophyll-a) variables were also measured. The most influential
variables favouring bloom formation were: intense rainfall (>20mm) up to ten
days before sampling; air temperature mainly above 15ºC thirty days before
sampling; low wind speed (<2m·s-1); high inflow caused by intense rainfall,
coinciding with agricultural off season; long water residence time, 132 days on
average on months with blooms; warmer temperatures and thermal stratification
of the water between October and March. The reservoir was classified as warm
monomictic, with only one complete vertical circulation in the winter and thermal
stratification in the summer. Dispersive blooms were found when wind speed
was above 7,9m·s-1, and surface accumulative blooms occurred when wind
speed was below 5,5m·s-1. Species of the genus Dolichospermum were more
adapted to produce dispersive blooms, mainly D. crassum and D. circinalis. On
the other hand, species of Microcystis were better adapted to surface
accumulative blooms, namely M. aeruginosa and M. panniformis, which were
also indicator species for this type of bloom. | |
