Abstract
Human activities in coastal areas are accelerating ecosystem changes at an unprecedented pace, resulting in habitat loss, hydrological modifications, and predatory species declines. Understanding how these changes potentially cascade across marine and freshwater ecosystems requires knowing how mobile euryhaline species link these seemingly disparate systems. As upper trophic level predators, bull sharks (Carcharhinus leucas) play a crucial role in marine and freshwater ecosystem health. Telemetry studies in Mobile Bay, Alabama, suggest that bull sharks extensively use the northern portions of the bay, an estuarine–freshwater interface known as the Mobile‐Tensaw Delta. To assess whether bull sharks use freshwater habitats in this region, environmental DNA surveys were conducted during the dry summer and wet winter seasons in 2018. In each season, 5 × 1 L water samples were collected at each of 21 sites: five sites in Mobile Bay, six sites in the Mobile‐Tensaw Delta, and ten sites throughout the Mobile‐Tombigbee and Tensaw‐Alabama Rivers. Water samples were vacuum‐filtered, DNA extractions were performed on the particulate, and DNA extracts were analyzed with Droplet Digital™ Polymerase Chain Reaction using species‐specific primers and an internal probe to amplify a 237‐base pair fragment of the mitochondrial NADH dehydrogenase subunit 2 gene in bull sharks. One water sample collected during the summer in the Alabama River met the criteria for a positive detection, thereby confirming the presence of bull shark DNA. While preliminary, this finding suggests that bull sharks use less‐urbanized, riverine habitats up to 120 km upriver during Alabama's dry summer season.
Highlights
Human alterations to the global landscape are accelerating shifts in ecosystem structure, function, and service at an unprecedented pace (Halpern et al, 2019)
The headwaters that pass through the Mobile-Tensaw Delta (MTD) and feed the Mobile Bay estuary encompass the richest freshwater fauna in North America (Boschung & Mayden, 2004; Lydeard & Mayden, 1995), including many rare and endemic species
Our findings provide evidence that bull sharks can occupy freshwater upstream habitat in the Alabama River and further demonstrate the ability of environmental DNA (eDNA) to identify rare species in Alabama rivers (e.g., Alabama Sturgeon Scaphirhynchus suttkusi, Pfleger et al, 2016)
Summary
Human alterations to the global landscape are accelerating shifts in ecosystem structure, function, and service at an unprecedented pace (Halpern et al, 2019). These trends are evident in coastal areas marked by reductions in predatory species and losses of critical spawning and nursery habitats (Lotze et al, 2006). Increased urbanization of these coastal areas further contributes to changes in habitat by modifying hydrological processes and nutrient dynamics (Lee et al, 2006). The nutrient-rich discharge into Mobile Bay supports critical habitat, both for primary consumers like white shrimp (Litopenaeus setiferus, Linnaeus, 1767) and blue crab (Callinectes sapidus, Rathbun, 1896) (Rozas et al, 2013) and higher-order consumers such as young-of-the-year (YOY) bull sharks (Carcharhinus leucas, Müller and Henle, 1839) (Drymon et al, 2014)
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