 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, such as the rabbitfish recorded in this study, to expand their range into other temperate seagrass meadows, where they could reduce seagrasses to the detriment of other species that depend on them, including sea turtles and marine mammals. Future studies will be necessary to understand how ocean warming will impact grazing fish populations, as these are important forces acting upon seagrasses.
Kate received her Ph.D. in Aquatic Ecology from the University of Notre Dame and she holds a Masters in Environmental Science & Biology from SUNY Brockport. She currently teaches at a small college in Indiana and is starting out her neophyte research career in aquatic community monitoring. Outside of lab and fieldwork, she enjoys running and kickboxing.