Turbidity and salinity influence trophic cascades on oyster reefs through modification of sensory performance and facilitation of different predator types

Abiotic factors can influence the distribution of organisms through physiological tolerance limits and by affecting their sensory performance in critical life history functions such as foraging or predator avoidance. In estuaries, salinity and turbidity directly influence the distribution of organisms but the indirect, synergistic effects of these factors on trophic interactions and community structure remain obscure. We investigated the effects of salinity and turbidity on oyster reef communities by comparing oyster reef community structure in low vs. high turbidity in consecutive years that varied considerably in rainfall and ambient salinity levels. Turbidity had significant effects in both 2016 and 2017 by interfering with fish foraging ability and consumption. In turbid sites, fish predation decreased by ~21%, crab mesopredators were 11% larger and nearly 5 times more abundant due to reduced top-down control by fish, and oyster reef biodiversity was 12% lower. In 2016, oysters were 350% less abundant in sites with abundant crab predators. However, in 2017, salinity increased, facilitating a new predator (oyster drills Stramonita haemastoma) to emerge onto reefs, and oysters were 7 times less abundant in sites with oyster drills despite having fewer crab predators. Thus, salinity and turbidity can indirectly affect food webs by facilitating different predators and influencing their sensory performance. Turbidity had significant effects on estuarine food webs regardless of salinity levels, and like salinity, turbidity should also be considered in oyster reef restoration and management of estuarine ecosystems.