Application of trophic state indicators and nutrient ratios for the characterization of trophic state and primary productivity of the Volta Lake, Ghana

The trophic state and limiting nutrient of Volta Lake were assessed using trophic state indicators and nutrient ratios, respectively. The objective was to categorize the Lake's trophic state and identify the limiting nutrient responsible for its primary productivity. A total of 240 water samples were collected for quality assessment. Results show that Chl-a concentrations ranged from 0.09 to 13.6 µg/L and a mean value of 6.7 ± 7.2 µg/L. TN concentrations ranged from 0.04 to 0.34 mg/L and a mean value of 0.11 ± 0.01 mg/L. TP concentrations ranged from 0.05 to 0.23 mg/L and a mean value of 0.15 ± 0.12 mg/L. TN:TP ratios ranged 0.26–6.8 and a mean value of 1.34 ± 1.8. Secchi depth levels (transparency) ranged from 0.12 to 3.01 m and a mean value of 1.45 ± 0.78 m and the TSS levels ranged 6.57–55.7 mg/L and a mean value of 23.9 ± 15.6 mg/L. Based on Carlson’s multivariate approach to reporting deviations in the TSI’s of Chl, Tp and Sd, the limiting nutrient in the Volta Lake is phosphorus with predominantly smaller particles and dissolved colour clay particles. Spearman’s Correlations among the trophic state indices and nutrient loadings show moderate to strong correlations indicating expected trophic state and primary productivity dynamics in the Lake. PCA resulted in the extraction of four main principal components which accounted for approximately 81.66% of the total variance. Component 1(25.4%) explains light penetration, whilst Component 2 (22.98%) explains the limiting nutrients, Component 3 (20.21%) explains the nutrient contributing significantly to light impairment and Component 4 (13.08%) explains the algal chlorophyll loadings in the trophic state dynamics of the Volta Lake ecosystem. Based on the nutrient loadings in the Volta Lake and the general conditions of the lake (the CTSI), the lake is generally oligotrophic and therefore, may be productively low due to lack of plant nutrients and may contain oxygen at all depths with clarity in its waters. It is thus, recommended that future underlying research needs for the Volta Lake should be directed towards protection for sustainable and efficient management.