Analysis of the Relationship between Abundance Surrogates of Phytoplankton of Lake Bosomtwe (Ghana), And Their Relationship with the Physicochemical Environment

phytoplankton abundance surrogates of Lake Bosomtwe, Ghana

Abstract

Parallel determination of phytoplankton abundance surrogates viz, cell numbers, wet biomass, chlorophyll a and their derivatives were investigated in Lake Bosomtwe (Ghana) by collecting samples from an index station. In the laboratory cells were counted and fitted to geometric shapes for wet weight determination while chlorophyll a was estimated by fluorometry. No significant correlation was observed between the phytoplankton crop of cells and wet biomass (r=+0.26, n=33), chlorophyll a and cells (r= -0.23, n=33) and chlorophyll a and wet biomass (r= -0.16, n=33) respectively (p˃0.05). Chlorophyll a content per unit cell (0.34±0.24 µg cell-1, n=33; CV=71.8%) and per unit wet biomass (0.45±0.32 %, n=33; CV=71.3%) were low and both decreased significantly with increase in cells (r = -0.68) and wet biomass (r = -0.49) respectively (p˂0.05, n=33). Factors that significantly affected the variability of both chlorophyll a per unit cell and per unit wet biomass were mixed layer depth, ratio of mixed layer depth to euphotic depth, secchi depth and the extinction coefficient (p˂0.05). In both cases, ratio of mixed layer depth to euphotic depth caused the most significant variability in chlorophyll a per cell (r2=59%; n=33) and chlorophyll a per unit wet biomass (r2=67%; n=33) respectively. The low chlorophyll a content per unit phytoplankton cell or wet biomass is attributed to cyanobacteria dominance which generally has low chlorophyll a content compared to other phytoplankton. The negative relationships between phytoplankton crop of cells or wet biomass and chlorophyll a per unit cell or per unit wet biomass is attributed to cell size reduction as abundance increase, physiological changes, light and hydrodynamic variability in the lake. Also, the chlorophyll a content per unit phytoplankton crop of cells or wet biomass seem to be controlled mostly by a combination of energy-related and hydrographic factors whiles nutrient-related factors appeared unimportant. Our study shows that the phytoplankton abundance surrogates of Lake Bosomtwe do not correlate well with each other and that the use of chlorophyll a as an abundance measure should be done with caution and all abundance surrogate measures should be measured in any study of the lake.