Network analysis indices reflect extreme hydrodynamic conditions in a shallow estuarine lake (Lake St Lucia), South Africa

Food web structure and function in ecosystems are a reflection of environmental conditions, this is especially apparent during extreme circumstances. The St Lucia estuarine-lake system in South Africa has recently experienced an unprecedented prolonged period of desiccation and hypersaline conditions caused by climatic variability and anthropogenic impacts. This has had a significant impact on species diversity, abundance and biomass. The system has received significant research attention over the past 70 years; however, little research has been conducted to understand how the system responds to changing environmental conditions as a whole. The aim of this study was to quantitatively assess the ecosystem response to different physico-chemical conditions and mouth states using ecological network analysis. The biomasses and trophic exchanges of various biotic species were estimated and used to establish ecological networks from which several ecosystem indices were calculated. Results indicate that the water level, salinity and mouth state have a significant impact on the total system biomass and productivity and the number and weight of energy flow pathways. These influence the biological structure and functioning of the St Lucia system. The substantial increase in the total living standing stock and species diversity during an intermittent open phase indicates that the system responds rapidly to such favourable conditions. This was reflected in the ecosystem indices calculated for before, during and after a breach which highlighted the importance of the short open mouth period after several years of mouth closure. Not only were biomass and productivity increased, but also the organisation of pathways to ensure more efficient energy transfer. At the same time adequate pathway redundancy was invigorated, resulting in a more robust and efficient functioning of the food web even after the re-closure of the inlet.