As harmful algal blooms degrade the nursery habitat, recent findings indicate that multiple toxins have been found in young bull sharks in the Indian River lagoon.

Indian River Lagoon Harmful Algal Blooms

The Indian River Lagoon, which stretches 251 kilometers along the East Coast of Florida, has recently seen widespread, frequent blooms of toxic, harmful algae. When aquatic microalgae overabundance occurs in a body of water, it causes harmful algal blooms (HABs), which have a negative impact on people and wildlife and deteriorate aquatic habitats.

Phycotoxins, strong chemicals released by photosynthetic organisms, are accumulated and transmitted from prey to predator through various levels of aquatic food webs by HAB species. Sentinel species, also known as indicator species, can give a comprehensive picture of environmental contaminants.

Bull sharks (Carcharhinus leucas) inhabit the Indian River Lagoon, a significant estuary in the country. Florida Atlantic University's Harbor Branch Oceanographic Institute, in partnership with the Florida Institute of Technology and the University of Connecticut, is the first to measure multiple phycotoxin concentrations in bull sharks.

Bull shark recruitment and survival on the Atlantic coast depend on the Indian River Lagoon, according to a previous study by researchers from FAU Harbor Branch. The biological integrity of the lagoon has recently been compromised by human water pollution, and if environmental pollution in the area persists, the lagoon's biological integrity may continue to deteriorate, Science Daily reports.

Multiple Toxins in 50 Juvenile Bull Sharks

Phycotoxins were measured in samples taken from 50 immature or young bull sharks caught in the Indian River Lagoon from 2018 to 2020 for the current study. Toxins in shark gut contents, plasma or blood, and liver were measured using ultra-performance liquid chromatography tandem mass spectrometry.

The Indian River Lagoon may contain several phycotoxins that are common or persistent in the environment, according to the results. Numerous phycotoxins, including microcystin, nodularin, domoic acid, teleocidin, cylindrospermopsin, okadaic acid, and brevetoxin, were found in 123 samples of bull sharks.

The majority of toxins were found in the highest concentrations in gut content samples, indicating that dietary exposure is a significant mechanism of toxin transfer to bull sharks.

Michelle L. Edwards, a former field technician and FAU Harbor Branch marine science and oceanography graduate student, said that the fact that 82% of the bull sharks sampled for our study and the prey they consumed contained one or more phycotoxin outlines the potential threat that toxic algae pose to the ecosystem of the Indian River Lagoon and nearby human populations that may ingest the same prey species. Edwards is the corresponding author of the study.

All of the toxins that were found, excluding brevetoxin, were present in plasma samples: microcystin, nodularin, cylindrospermopsin, teleocidin, domoic acid, and okadaic acid.

Sampling Sentinel Species

Matt Ajemian, Ph.D., an assistant research professor and FAU Harbor Branch director for The Fish Ecology and Conservation Lab, said that bull sharks were a suitable sentinel species for the system because of their ecology in the Indian River Lagoon, which includes residence during early stages of life and use of distinct regional areas. Ajemian is the senior author of the study.

Additionally, the inclusive nature of using an upper-level predator's gut contents for assessment of toxin presence enabled their group to identify toxins across several lower trophic species, such as stingrays, catfish, and mullet.

Transfer of Toxins

The most frequently recognized prey species in the study, mullet, was one significant vector species for the transmission of phycotoxins to the systems of bull sharks in the Indian River Lagoon. Mullet was found in 60% of the gut contents where microcystin was found.

According to Edwards, mullets are found all over the Indian River Lagoon. They migrate to offshore areas to spawn from September to December every year, following a similar seasonal offshore migration pattern to that of young bull sharks, which spend time in offshore areas from October to March every year.

Even though resident bull sharks tend to spend the majority of their time in a few distinct areas with varying seasonal area usage, such as offshore area movements, the majority of toxins were found in samples gathered throughout the Indian River Lagoon as well as during both "wet" and "dry" seasons.

Further Studies

Ajemian pointed out that after a period of harmful algal bloom has passed, many phycotoxins may still be present in the environment. Both domoic acid and microcystin, an acid-type neurotoxin that was the most frequently discovered toxin in the bull sharks in their study, can adsorb to sediments where they may be consumed by benthic organisms or resuspended in the water column. This makes determining when someone was exposed to these toxins extremely difficult.

Although bull sharks living in the Indian River Lagoon may have a high frequency of exposure to phycotoxins, the higher concentrations in their gut contents compared to other shark tissues suggest that they may not accumulate them as much as lower trophic species.

Edwards said that it might be necessary to conduct more research on the likelihood of accumulation in shark tissues that weren't sampled for this study.

Ajemian claims that they have only begun to explore the implications of HAB exposure for the Indian River Lagoon bull sharks.

He added that this was a crucial first step in establishing crucial baselines as researchers continue to track these animals' reactions to HABs in the future, Phys Org reports.

The study by Ajemain, Edwards, and their colleagues is published in the journal Science of the Total Environment.