Phytoplankton Production in Lake Arenguade (Hora-Hado), Ethiopia.
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Date
2010-06
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Addis Ababa University
Abstract
Dynamics of Chl a biomass, abundance amd photosynthetic production of phytoplankton
in relation to some phyico-chemical factors were studied in Lake Arenguade from
December, 2008 to October, 2009. Water transparency in Lake Aranguade varied from
0.21 to 0.37 indicating the shallowness of the trophogenic zone owing to high turbidity,
which seems to be largely of biological origin. Depth profiles of temperature showed the
occurrence of a superficial type of thermal stratification which is created by solar
heating and destroyed by nocturnal cooling although oxygen depth profiles determined
on two days seem to sugggsest the formation of stable and deep-seated thermal
stratification at times. Total Alkalinity of the lake ranged from 33 to 50.6 while pH varied
from 9.62 to 9.84. SRP, nitrate and silica varied temporally from about 702 to 2123 μg L-
, 1, 4.1 to 78 μg L-1 and 3.5 to 37.3 mg L-1, respectively. The phytoplankton community of
Lake Arenguade was co-dominated by two species of blue-green anlgae, Spirulina
platensis (Arthospira fusiformsi) and Anabaenopsis elenkinii, chl a biomass of
phytoplankton varied temporally ranging from 41.7 to 271 mg L-1 and with peaks
coinciding with those of phosphate and silicate and low levels of nitrate. The depthdistribution
of gross photosynthesis per unite water volume (A, mg C m-3 h-1) exhibited
the typical pattern for phytoplankton with maximum production rates at depths below the
surface on most sampling dates.The light-saturated rate of gross photosynthesis (Amax),
which ranged from 76.14 to 650 mg C (243.64–2080 mg O2) m-3 h-1. was positively but
modestly (r=0.53, r2=0.28 at p=0.061 ) correlated with Chl a biomass. Biomassspecific
rates at light-saturation (φmax, photosynthetic capacity) ranged from 2.3 to 13.2
mg O2 (mg Chl a)-1 h-1 with most values between 8 and 11 mg O2 (mg Chl a)-1 h-1. Hourly
integral rates of photosynthesis (ΣA) ranged from 0.13 to 1.67 g O2 m-2 h-1. The seasonal
peaks of hourly integral rates were associated with peaks of chlorophyll a concentration
and light-saturated rate of gross photosynthesis (Amax). The correlations between hourly
integral rates and chlorophyll a concentration (r=0.53, r2=0.28 at p=0.06 and Amax (r =
0.78, r2=0.62 at p=0.0014), were positive and strong. The causal relationship between
biological parameters and physico-chemical variables are examined and discussed
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Biology