Determining the health of river-dominated estuaries using microalgal biomass and community composition

Abstract

Microalgae, composed of phytoplankton and benthic microalgae, are used in estuary freshwater requirement studies as they respond quickly to changes in water quality and river flow. The aim of this paper is to show how microalgae are used to determine the present ecological status of river-dominated estuaries by determining the changes in biomass and community composition from the reference state, using the ecological water requirements studies of the uMzimvubu and Orange estuaries as case studies. Microalgal chlorophyll a was used as an index of biomass, phytoplankton counts were classified into functional groups and benthic diatoms were analysed using the software OMNIDIA to determine estuarine health.The slight decrease in river flow (4%) and substantial increase in nutrients from the reference state have resulted in an estimated 28% increase in phytoplankton biomass in the uMzimvubu Estuary from reference. Frequent flooding and high average river flow limit benthic microalgal growth, and changes to species richness, abundance and community composition have been estimated to be 15%, 17% and 15%, respectively. The estuary had high benthic diatom species richness with 228 identified taxa and challenges related to the use of diatom indices are discussed. Phytoplankton abundance has increased by an estimated 28% and community composition has shifted away, ~35% change, from being diatom-dominated to one with more chlorophytes and dinoflagellates. The present ecological status was estimated to be 65 (C); moderately modified.Damming and abstraction of river water in the Orange River catchment has resulted in a 60% reduction in flow into the estuary. During the reference state, the river flow exceeded 50m3 s−1 an estimated 73% of the time; freshwater dominated. Presently, river flow is between 5 and 20m3 s−1 for 52% of the time; marine dominated. In addition, agricultural practises in the river catchment and the increased intrusion of upwelled marine water have increased the concentration of nutrients in the estuary. These conditions support the development of a productive river–estuary interface zone in the middle reaches of the estuary where phytoplankton chlorophyll a exceeds 20μg L−1 and is dominated by flagellates and dinoflagellates; the diatom to flagellate ratio has decreased. As anoxic events are more likely to occur with a further reduction in river flow, an increase in the proportion of cyanobacteria is likely to occur. Fewer benthic diatom taxa (68), dominated by two species, resulted in a low species richness score. Of the variables considered, a 60% increase in phytoplankton biomass was the largest; giving a present ecological status based on microalgae a score of 40 (E); highly degraded.The use of benthic diatom indices, using the programme OMNIDIA, did not provide reliable results that could be used to determine estuarine health. The South African Diatom Index and Trophic Diatom Index gave mixed results that did not correlate well with the other more established methods, contrasting with phytoplankton abundance and community structure and water quality results. However, benthic diatom species diversity results were well aligned with these parameters and will continue to be a useful tool in freshwater requirement studies.