Decadal changes in land-use, water quality and fish assemblages in a KwaZulu-Natal urban and non-urban estuary.

Abstract

Globally, estuaries are recognised as highly dynamic environments which support diverse and productive flora and fauna, and provide numerous goods and services for human wellbeing. These systems are under threat from growing coastal populations that demand the transformation of natural land to accommodate urbanisation, agriculture, harbour, and infrastructure developments. Direct and indirect land-use/land-cover (LULC) changes adversely affect estuarine water quality, directly impacting estuarine biota such as fishes. While the short links between these components are known, few studies have attempted to integrate these links, particularly within the South African context. Therefore, this study aimed to integrate different levels of assessment (LULC and estuarine water quality) to describe changes in estuarine fish ecology over space and time. The study compared two permanently open estuaries in KwaZulu-Natal with distinct land-cover types, namely the uMgeni and uMlalazi estuaries. The uMgeni Estuary is surrounded by urban features and is situated in a highly developed catchment, whereas the uMlalazi is near-natural, with some surrounding agricultural areas.

Historical land-cover imagery from the estuary to the 20 m topographical contour (inclusive of the Estuary Functional Zone), the estuary catchment land-cover, water quality and ichthyofauna data were collected. Land-cover types were grouped into eight categories and 42 feature classes based on modifications to the South African National Land Cover system, the United States Geological Survey and the Coastal Change Analysis Program. Multivariate statistical analysis identified two distinct groupings of land-cover types, Period 1 (before 1989) and Period 2 (during and after 1989). This period factor was then further used throughout the study to determine associated (temporal) responses in water quality and fish assemblages. Strong negative correlations were observed between dissolved oxygen and artificial land-cover types in the uMgeni Estuary. The results suggested that the uMgeni Estuary receives nutrient inputs from various urban activities, which reduce oxygen levels within the water column. Alterations to the hierarchical ichthyofauna structure in the uMgeni occurred on a year to year basis, relative to these anthropogenic impacts. In contrast, good water quality likely associated with fewer developments within the Estuarine Functional Zone has allowed the hierarchical fish structure in the uMlalazi Estuary to remain the same over time, although minor species level differences have occurred. The current state of each estuary was then investigated by measuring water quality parameters and sampling fish communities along the respective estuary gradients as determined by salinity penetration. Ichthyofauna and water quality data (salinity, turbidity, pH, dissolved oxygen) were collected within the lower, middle, upper and riverine reaches of each estuary. Each fish species sampled was assigned a functional guild depending on their use of the estuary and their feeding methods. The highly urbanised uMgeni Estuary was found to have an exceptionally degraded water quality in the upper reaches (average dissolved oxygen 3.78 mg l⁻¹). It supported a relatively low number of fish species (30) at a total catch per unit effort of 1488.7. In comparison, the less urbanised uMlalazi Estuary displayed much healthier water quality (average dissolved oxygen 6.94 mg l⁻¹), double the number of fish species (60) and higher total fish abundance (catch per unit effort of 2283.67). Salinity was primarily responsible for the longitudinal gradients in fish assemblage, reflecting the role of tidal influence in permanently open estuaries. Differences in fish communities between estuaries, however, were strongly influenced by oxygen levels within the water column. The uMgeni was primarily a detritus-based system that mostly supports small estuarine species (Ambassidae), detritivores (Mugilidae), and freshwater opportunists (Cichlidae). The uMlalazi Estuary is also a detritus based system, however, the diversity and fish assemblage suggests a more complex food web system across various feeding guilds, including piscivores, and is utilised extensively by marine and estuarine species. The results of this study highlighted the usefulness of incorporating guild and taxonomic distinctness tests in ecological studies. Additionally, they suggest that for the systems studied here, impacts of urban and land-use adjacent to estuaries are more detrimental to estuarine function than forestry or sugarcane cultivation. The results lend support to estuarine management and coastal development goals that aim to limit development in and around estuaries.