Assessing the trophic structure and functioning of a large tropical lagoon. Case study: Keta Lagoon, Ghana

Abstract

A model of trophic interactions in the Keta Lagoon was developed to assess the current state and the energy flow within the food web structure of the system components using an ECOPATH with ECOSIM modelling approach. Seventeen (17) functional groups were identified for the Keta Lagoon model construction. Ecosystem health and maturity parameters were derived using the ecological network analysis. The functional groups identified in the food web were of varied trophic levels ranging from primary producers (trophic level (TL) = 1) to top predators (TL = 3.216). Most fish groups had higher eco-trophic efficiencies (EE > 0.9), indicating their high utilisation within the system. The total system throughput was estimated at 10,287.920 t/km²/year, with the system depending on the consumption of primary producers. The mean trophic level of the catch (2.762) indicated that the fisheries target fish groups with higher trophic levels. Ecological indices such as the net system production (1,480.452 t/km²/year), total primary production/total biomass (7.207), total biomass/total system throughput (0.054 t/km²/year), Finn’s cycling index (4.933%), system omnivory index (0.155), ascendency (26.29%) and system overhead (73.71%) indicated that the ecosystem is in its developing stage and prone to environmental disturbance hence, the need for management. The mixed trophic impact routine indicated that the lower trophic level groups positively impacted most of the higher trophic level groups. Also, predatory birds and macro-invertebrates were the most influential functional groups structuring the lagoon. Management strategies that could be implemented include habitat protection, stock enhancement, alternative livelihoods (aquaculture), closed seasons, and enforcement of fisheries regulations.