It is a widely known fact by scientists that eutrophication (the leaching of excess nutrients from agricultural sites into bodies of water) causes excessive growth of algae, choking localised marine ecosystems. In recent years, experiments have suggested that herbivores such as fish, sea turtles and urchins could keep marine ecosystems healthy by consuming extra algae that grew in place of excess leached nutrients.
A new study, carried out by the University of Florida, sheds doubt on the degree of positive impact that herbivores have on this eutrophic process, underscoring the importance of sustainable growth in coastal areas. Specifically, the study found that herbivores had a significant positive impact only in small-scale experiments, and that nutrient pollution at larger, more realistic scales can overwhelm them.
Healthy reefs are important to everyone, not just those who are involved in marine recreational activities. In addition to sustaining sea turtles, whales and dolphins, these reefs provide a wide array of ecosystem services. These services include the provision of novel medical compounds, seafood and the protection of coastline from storm surges. Eutrophication can damage these reefs. As our population grows, paving and development dump runoff laden with nitrogen and phosphorus into nearby bodies of water. Fertilisers intended for lawns and crops at agricultural establishments find their way into the seas, where sewer pipes may also be disgorging waste. The resulting nutrient enrichment results in the excessive growth of algae that harms corals, sea grasses and kelp. This is an increasingly important problem in developing nations.
Researcher Mike Gil has seen coral reefs decline as algae increases during his field courses he runs in Mexico. This was regardless of the population of algae-eating fish remaining stable. As a result, Gil wondered whether herbivores alone made enough of a significant difference to help defend reefs. The field experiments that gave rise to this idea typically looked at areas of nutrient pollution of a square metre or less. With nutrient pollution zones covering zones typically covering hundreds of square kilometres, it was not ethically feasible for Gil to carry out a practical experiment on this scale. With this in mind, Gil and his co-authors (Jing Jiao and Craig Osenberg) turned to mathematical modelling.
The authors found that as the area affected by nutrient pollution increased, herbivores' ability to control the resulting algae decreased., suggesting that these ecosystems may be more vulnerable than scientists previously thought. The findings of this study could guide policy-makers in creating sustainable plans for industries such as tourism and fishing, that rely heavily on these reefs.
Original paper:
Michael A. Gil, Jing Jiao, Craig W. Osenberg. Enrichment scale determines herbivore control of primary producers. Oecologia, 2015; DOI: 10.1007/S00442-015-3505-1
