Phytoplankton Production In Lake Arenguade (hora-hado) Ethiopia.

Dynamics of Chl a biomass, abundance amd photosynthetic production of phytoplanktonrnin relation to some phyico-chemical factors were studied in Lake Arenguade fromrnDecember, 2008 to October, 2009. Water transparency in Lake Aranguade varied fromrn0.21 to 0.37 indicating the shallowness of the trophogenic zone owing to high turbidity,rnwhich seems to be largely of biological origin. Depth profiles of temperature showed thernoccurrence of a superficial type of thermal stratification which is created by solarrnheating and destroyed by nocturnal cooling although oxygen depth profiles determinedrnon two days seem to sugggsest the formation of stable and deep-seated thermalrnstratification at times. Total Alkalinity of the lake ranged from 33 to 50.6 while pH variedrnfrom 9.62 to 9.84. SRP, nitrate and silica varied temporally from about 702 to 2123 μg L-rn, 1, 4.1 to 78 μg L-1 and 3.5 to 37.3 mg L-1, respectively. The phytoplankton community ofrnLake Arenguade was co-dominated by two species of blue-green anlgae, Spirulinarnplatensis (Arthospira fusiformsi) and Anabaenopsis elenkinii, chl a biomass ofrnphytoplankton varied temporally ranging from 41.7 to 271 mg L-1 and with peaksrncoinciding with those of phosphate and silicate and low levels of nitrate. The depthdistributionrnof gross photosynthesis per unite water volume (A, mg C m-3 h-1) exhibitedrnthe typical pattern for phytoplankton with maximum production rates at depths below thernsurface on most sampling dates.The light-saturated rate of gross photosynthesis (Amax),rnwhich ranged from 76.14 to 650 mg C (243.64–2080 mg O2) m-3 h-1. was positively butrnmodestly (r=0.53, r2=0.28 at p=0.061 ) correlated with Chl a biomass. Biomassspecificrnrates at light-saturation (φmax, photosynthetic capacity) ranged from 2.3 to 13.2rnmg O2 (mg Chl a)-1 h-1 with most values between 8 and 11 mg O2 (mg Chl a)-1 h-1. Hourlyrnintegral rates of photosynthesis (ΣA) ranged from 0.13 to 1.67 g O2 m-2 h-1. The seasonalrnpeaks of hourly integral rates were associated with peaks of chlorophyll a concentrationrnand light-saturated rate of gross photosynthesis (Amax). The correlations between hourlyrnintegral rates and chlorophyll a concentration (r=0.53, r2=0.28 at p=0.06 and Amax (r =rn0.78, r2=0.62 at p=0.0014), were positive and strong. The causal relationship betweenrnbiological parameters and physico-chemical variables are examined and discussed

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