grass communities will experience alterations in salinity, among other factors. Therefore, understanding how salinity and grazing are currently affecting these communities is important as we navigate the waters into the future.
Shark Bay, Australia
Sahira Bell and colleagues, of the University of Western Australia, investigated the impacts of grazing fish and their feeding choices on seagrasses in Shark Bay, a shallow coastal embayment of Western Australia. Shark Bay is a UNESCO World Heritage Site, as it has important conservation values because it holds the world’s largest and richest seagrass meadows.
Shark Bay is characterized by a range of salinities that span normal oceanic levels (~3% salinity) at the north end of the bay that opens up to the Indian Ocean, to hypersaline conditions (>3% salinity) at the southern end due to high rates of evaporation and limited mixing with the north end. The seagrass meadows in Shark Bay are dominated by large stands of temperate species, but smaller patches of fast-growing tropical grasses are also present.
To investigate how fish grazing intensity changed across the range of salinities experienced in Shark Bay, the researchers monitored fish grazing pressure by attaching equal amounts of 5 common live seagrasses to ropes suspended on the seafloor throughout the bay. The researchers made sure that they offered the same amount and species of seagrasses at various sites that spanned the low and high range of salinity.
Before the seagrasses were tied to the ropes, the researchers photographed each seagrass so that after the experiment they could estimate how much of the seagrass leaf was eaten by fish. The researchers also set up underwater GoPro cameras so that they could see which fish species were grazing on the seagrass ropes.
Some fish are choosy eaters; Often, they will show a preference for seagrasses that have high nitrogen concentrations. Nitrogen is important for building proteins necessary for daily life. Similar to the protein and energy information we find on our cereal boxes, the researchers measured the amount of nitrogen in the leaves of the 5 seagrass species used in the seagrass ropes.
What did they find?
Bell and colleagues found that seagrasses were grazed most intensively at the lowest salinity, while virtually no seagrasses were eaten at the sites where salinity was highest. They also found that not all seagrasses were grazed equally; the tropical species were chosen more frequently than the temperate grass species.
Among the grazers caught on video, rabbitfish and Western striped grunter were caught on video eating seagrasses. The authors also found that the tropical seagrasses had more nitrogen than the temperate species, which may explain why these grasses were grazed upon more frequently than temperate grasses. This finding indicates that while less nutritious temperate species of seagrass dominate in Shark Bay, the fish sought out the less abundant, but more nutritious, tropical seagrass species.
So why does salinity have this effect? The fish in Shark Bay likely hang out in the less salty areas because higher salinity areas are stressful for them, making it difficult to graze effectively. While the seagrasses can survive throughout the range of salinities of Shark Bay, fish were restricted to grazing in the lower-salinity meadows. This means that changes in salinity can restrict or expand grazing fish in Shark Bay.
The bigger picture
Seagrasses play a vital role in marine food webs. A majority of research on seagrasses before this study emphasized the role of seagrasses as primarily habitat for invertebrates and fish, not necessarily as food. Therefore, this study shows that not only are seagrasses important for structuring ecosystems, they are important sources of energy for fish, particularly the more nutritious tropical species.
This study shows how environmental factors and grazers interact to influence seagrass communities. This is important, especially within the context of climate change. Indeed, warming water temperatures may lead ravenous fishes, 