Role of environmental variables and seasonal mixing in dynamics of the phytoplankton community in a Tropical Highland Lake Ardibo, Ethiopia

Abstract

This study aimed to assess the spatial and seasonal variations of the phytoplankton community in response to environmental variables in Lake Ardibo. The study was done from October 2020 to September 2021 and physicochemical parameters such as water temperature, pH, dissolved oxygen, electrical conductivity, turbidity, alkalinity, Secchi-depth, nitrate, ammonium, silicon dioxide, soluble reactive phosphorus, and total phosphorus were measured using standard methods. ANOVA results indicated that all physicochemical parameters except turbidity had significant variation (p < 0.05) in all seasons. A total of 66 phytoplankton species grouped into seven phyla, including Bacillariophyta (23 taxa), Chlorophyta (21 taxa), Cyanophyta (14 taxa), Euglenophyta (3 taxa), Charophyta (2 taxa), Dinophyta (2 taxa) and Cryptophyta (1 taxon) were identified. Phytoplankton biomass showed significant spatial and seasonal variation with maximum cell density during the pre-rainy season, and low density in the main rainy season. Redundancy analysis (RDA) revealed that nutrients had a positive affinity with the phytoplankton community, whereas electrical conductivity, water temperature, and pH showed a negative relationship with phytoplankton abundance. Temporal rather that abiotic factors far influenced phytoplankton dynamics in this deep tropical lake. The analysis postulates that partial atelomixis during the pre-rainy months favors higher phytoplankton biomass with functional groups such as desmids and heavy diatoms contributing most during this mixing period, while wet season mixing depressed algal biomass due to increased turbidity with nutrients not being limiting during these times.