The macrobenthic communities of two estuary types with different ecological health status.

Abstract

The subtropical estuaries of KwaZulu-Natal are among the most ecologically compromised, subjected to pressures from pollution, intensive agricultural practices and disruptions in freshwater flow regimes. Consequently, 44% of KZN estuaries are categorised as having a health status ranging from moderately to severely modified. Macrozoobenthic organisms are considered reliable biological indicators of estuarine health due to their sedentary nature and sensitivity to natural and human-induced changes. Monitoring community attributes such as composition, abundance and diversity provide valuable insights into ecological changes over time. This study investigates spatio-temporal changes in macrozoobenthic communities within two estuary types in KwaZulu-Natal, which have contrasting ecological health statuses. These include the heavily modified, temporarily closed (TCE) uMdloti Estuary and the nearnatural, predominantly open (POE) uMlalazi Estuary. The analysis focuses on spatial and temporal variations in environmental parameters and community metrics such as species composition, abundance and diversity. The uMdloti Estuary, characterised by frequent mouth closures exhibited relatively homogenous environmental conditions yet displayed spatial and temporal shifts in macrozoobenthic community structure. The observed temporal disruptions in community composition, abundance and diversity can be linked to changes in mouth state between seasons. The system was dominated by a combination of estuarine and freshwater taxa, with notable numerical dominance of opportunistic and invasive species. The numerical dominance of opportunistic and invasive taxa in the uMdloti Estuary highlighted compromised health due to chronic pressures in the system. The uMlalazi Estuary, characterised by consistent freshwater inflow and a predominantly open mouth, exhibited well-defined salinity and sediment gradients. These gradients created diverse habitats that supported a wide range of taxa, with stable communities occupying distinct ecological niches along the estuarine gradient. A comparison of macrozoobenthic community attributes between the uMlalazi and uMdloti estuaries revealed differences linked to variations in morphological characteristics, pressure intensity and protection status. Despite variations in community attributes between the systems, the community compositions shared similarities in key estuarine taxa, such as Desdemona cf. ornata, Ceratonereis (Composetia) keiskama, Grandidierella lutosa and Grandidierella spp., indicating a biogeographical species pool. These similarities overlook the estuary type and its health status to a certain degree. Therefore, these taxa appear not to be robust indicators and should be interpreted with caution. The study findings contribute to macrobenthos taxonomic inventories of TCEs and POEs in the subtropical estuaries of KZN. In addition, this study highlights the need to consider factors that influence the environmental variables such as the location of the system, the protection status and anthropogenic pressure in and around the system to better understand the prevailing community composition and distribution patterns.