Harmful Algal Blooms Negatively Impact Mugil cephalus Abundance in a Temperate Eutrophic Estuary

The increased frequency and magnitude of harmful algal blooms (HABs) from anthropogenic nutrient enrichment is of great concern in estuarine systems worldwide. Despite estuaries being important fish nurseries, a paucity of knowledge exists on the impacts of HABs on estuarine fishes due to the sporadic and unpredictable nature of HABs. Thus, this study aimed to determine the frequency and magnitude of HABs and the subsequent impact on the abundance of a common estuarine-dependent Mugilidae species, Mugil cephalus, in the shallow low inflow and agriculturally altered warm temperate Sundays Estuary in South Africa. The sampling design adopted a monthly (monitoring over 2 years between 2018 and 2020) and twice weekly (for 7 weeks during known spring/summer HABs in 2018) day and night sampling approach. It was hypothesised that Mugil cephalus abundance would be negatively related to phytoplankton biomass since HABs would result in unfavourable daytime supersaturated dissolved oxygen surface water and night-time bottom-water hypoxia. This study found that the Sundays Estuary was dominated by the HAB species, Heterosigma akashiwo, which significantly altered the dissolved oxygen in the estuary resulting in frequent supersaturated surface waters and hypoxic bottom waters. Mugil cephalus abundances were negatively related to phytoplankton biomass, with instances of high abundance (CPUE > 10) only being found at sites characterised as not experiencing severe bloom conditions (< 60 Chl-a mu g.L-1). This avoidance of HABs suggests that if unsustainable levels of agricultural fertilisation continue, resulting in more severe and frequent HABs, a major decline in fish nursery quality in the estuary may occur.

Automated summary

Harmful algal blooms (HABs) have a negative impact on the abundance of the estuarine-dependent fish species Mugil cephalus in Sundays Estuary. The presence of HAB species Heterosigma akashiwo alters the dissolved oxygen levels in the estuary, resulting in unfavorable conditions for the fish. The study suggests that unsustainable agricultural fertilization could lead to more severe and frequent HABs, causing a decline in fish nursery quality.