Protecting seagrasses is essential to mitigate climate change because they store large amounts of carbon on the seafloor. However, effective conservation and its potential to provide solutions to climate change have been hampered by uncertainty about the extent and distribution of seagrasses. Scientists have recently studied the seagrass ecosystem located in the Bahamas, combining existing spatial estimates with remote sensing imagery and extensive seafloor ground measurements by divers. Do you believe that most of the data was obtained from tiger sharks?
Figure 1, Underwater seagrass
1. Seagrass ecosystems
Seagrass ecosystems play an increasingly important role in supporting biological productivity, carbon sequestration, marine biodiversity and fishery resources. Seagrass captures and permanently stores large amounts of carbon in sediments, accounting for 17% of the total carbon buried in marine sediments.
Figure 2, Mediterranean seagrass
Over the past few decades, the rapid loss of seagrass has reduced the sequestration capacity of seagrass ecosystems, while also releasing large amounts of carbon. Therefore, protecting seagrass ecosystems is essential for managing greenhouse gas emissions, protecting seagrass habitats alongside many endangered species and seafood resources supported by seagrass habitats.
Figure 3, Bahamas Beach
At a minimum, protecting seagrass ecosystems requires an understanding of their distribution and extent. However, their distribution in many areas remains poor, with we estimate the global seagrass area at 160,387 km. The global area of seagrasses is a major driver of uncertainty about their global carbon sequestration capacity, and as such, their value as a blue carbon resource is enormous.
Figure 4, Bahamas
Dominant seagrass species, such as the genus Seagrass, are sparsely covered on the seabed, protruding only one or a few centimeters above the sediment, as it is usually partially covered by pebble sand. As a result, small seagrass ecosystems are still being discovered, and it is likely that very large ecosystems have yet to be mapped.
Figure 5, Seagrass ecosystem
The coast of the Bahamas has the world's largest seagrass system
The Bahamas is an island nation located on the west coast of the Atlantic Ocean. The Bahamas is located in the southeast of Florida, United States, northeast of Cuba, extending from northwest to southeast, 1223 kilometers long and 96 kilometers wide. It consists of more than 700 islands and more than 2,400 coral reefs, 30 of which are inhabited. It has a subtropical climate.
Figure 6, Bahamas Marina
The banks of the Bahamas are large (> 135,000 square kilometres) covered by vast carbonate sediments, supporting a high biodiversity of large, highly mobile consumers (sharks, turtles, dolphins, manatees). They consist of two separate large banks, the Grand Bahama and the Little Bahamas, with an average depth of less than 10 meters, steep slopes at their borders and covered by some of the clearest waters in the ocean.
Figure 7, Paradise Island, Bahamas
Matrice, carbonate sands, warm temperatures and abundant light are suitable conditions for seagrasses, where the suitable plateau area is estimated at 112537 square kilometres, thus including most of the banks of the Bahamas (83%). As a result, the banks of the Bahamas are likely to host the world's largest seagrass ecosystem.
Figure 8, Bahamas Beach
Tiger sharks are used to assess seagrass range
Recent innovations in the development and application of biocamera tags present a unique opportunity for biologically relevant ground truths and habitat classification far beyond human capabilities. This is because highly mobile animals can move great distances in short periods of time and are not limited by human limitations (e.g., boat and bottom time and range, personnel, sea state, depth of investigation). In fact, green sea turtles have been shown to guide the discovery of seagrass ecosystems.
Figure 9, sea turtles
Similarly, fish that take advantage of the high mobility of shallow seagrass habitats around the world can serve as important allies to guide the discovery of new seagrass ecosystems and, more importantly, help with established methods for estimating the spatial extent of seagrass, such as remote sensing imagery obtained from Earth's satellites.
Figure 10, Great Barrier Reef
The tiger shark is a widely distributed apex carnivore that exhibits highly consistent connections to seagrass ecosystems worldwide. All life stages of the species are widespread throughout the vast marine ecosystem of the Bahamas, with large numbers of adult species migrating long distances on river banks and forming concentrated aggregates in seasonal hotspots.
Figure 11, Bahamian tiger shark
Previous monitoring of tiger sharks in the Bahamas has shown strong habitat options for seagrass habitats. Therefore, tiger sharks can be used as a useful research tool to guide and improve seagrass mapping.
Figure 12, Shaanxi Triassic shark fecal fossil
Fourth, how to use tiger sharks to establish seagrass ecosystems
Our assessment of the extent of the seagrass ecosystem in the Bahamas is based on several data streams, including independent remote sensing data, existing and empirical, and ground-based truths supported by human observers and tiger sharks. There are two innovative elements in our study that assess the Bahamian seagrass ecosystem.
Figure 13, Several fish intestine structures
First, using the ensemble voting method allows us to integrate estimates from different data streams. The second, and most innovative factor, is the use of tiger sharks to expand ground truths that support assessments. Tiger sharks, the largest apex predators in tropical oceans, collected innovative data on benthic habitats in our study as a unique investigative partner and tool to assess sparse and dense seagrasses.
Figure 14, Tiger shark
The data they obtained significantly expands the scale of terrestrial truth in our study area and thus helps to overcome the challenges surrounding terrestrial truth, which has so far hindered a reliable assessment of the extent of riparian seagrass ecosystems in the Bahamas. Although we provide here the largest amount of manual survey data on seagrass ecosystems in the ocean, ground-based data supported by human observers are still limited to a relatively small area and narrow depth range.
Figure 15, Paula, a 270-million-year-old fossil shark droppings from Brazil, 2013
Taken together, thanks to the efforts of tiger sharks and humans, scientists have discovered the largest and most innovative seagrass ecosystem to date, with strong ground-based truths supporting designating the Bahamian coast as the largest seagrass ecosystem on Earth. These findings have broad implications for estimating global carbon stocks.