Article: McSkimming, Chloe, et al. “Compensation of nutrient pollution by herbivores in seagrass meadows.” Journal of Experimental Marine Biology and Ecology 471 (2015): 112-118.
Recent research focusing on the impacts of climate change on marine ecosystems often paints a pretty bleak picture. Images come to mind of bleached corals covered in dense mats of algae and the surrounding waters completely devoid of life (Fig. 1). Climate factors like warming, acidification, and nutrient loading have been shown to favor the growth and spread of opportunistic algae (ones that can quickly take advantage of resources) at the expense of corals, kelps, and other long-lived species that help create and support complex habitats. However, the picture of future marine systems may not be as depressing when researchers start to take a broader look at response to climate change.
Seagrass ecosystems are some of the most important and most productive marine habitats. They provide a number of direct services (services we can see, like providing refuge for juvenile fish)(Fig. 2) and indirect services (services we can’t see, like storing carbon we put into the atmosphere). These habitats, though, are vulnerable to a changing climate. Although seagrasses themselves could benefit from increased nutrients and more carbon dioxide, it’s the smaller, opportunistic algal species that seem to take advantage and find success. Nutrient loading is a global problem. Runoff from sewage treatment facilities and agricultural lands floods aquatic ecosystems with excess nitrogen and phosphorous, both are limiting resources for photosynthetic organisms. But too much runoff can create problems. Many species of algae exploit these resources and multiply rapidly, overtaking a habitat, blocking other photosynthetic organisms from receiving light and smothering species that live on the sea floor.
With all the gloom and doom related to seagrass die-off and algal takeover, we often forget that there are other key players in these ecosystems. Plenty of marine herbivores will happily snack on algae. If they are eating quickly and in large quantities, that could help fight the blooms and preserve the seagrass. So researchers in Australia set out to investigate how marine herbivores and seagrasses respond to higher nutrient conditions. The did this by looking for changes in feeding and overall health of seagrasses.
Using a field site on the southern coast of Australia, researchers set up experimental plots within existing seagrass meadows. The dominant seagrass in this habitat is Posidonia angustifolia and is often found covered by small epiphytic algae (meaning growing on a plant or another alga), which can completely cover seagrass stems (Fig. 3), blocking them from light and nutrients when the conditions are right. Researchers wanted to test 3 different nutrient levels along with the presence or absence of small marine herbivores for their impact on both epiphytes and seagrass stems. The same type of techniques we would use in our backyard gardens to boost plant growth and remove insect pests were used to create the environmental treatments. Researchers used a slow release fertilizer to simulate a minor and moderate nutrient pulse and used a pesticide to ward off herbivores. After about 6 months, seagrass density and biomass were assessed along with epiphyte cover and biomass. Together, this data should reveal insight as to how actively herbivores are feeding on epiphytes and how seagrasses respond.
Overall, researchers found that epiphyte cover increased with increasing nutrients and ALSO increased further when herbivores weren’t present. It was shown that under moderate nutrient enrichment, epiphyte cover was around 70%, but when herbivores were there to eat, the percent cover dropped to almost 30% (Fig. 4)! Even under minor nutrient enrichment, percent cover of epiphytes was reduced by 25% when herbivores were present. It was also found that seagrass density and biomass were highest under minor nutrient enrichment when herbivores were present (Fig. 5). This points to not only the removal of epiphytes as an important factor for seagrass health but also the grasses’ ability to access the nutrients now available.
It is common to hear about climate change and the problems facing ecosystems and think that reversing the effects will require a massive, human-led effort. But studies like this one show just how important looking at a broader scope can be, highlighting existing relationships and forms of biological control. This work has shown that tiny herbivores can help critically important ecosystems from reaching the brink. By chewing away at opportunistic algae, that when left alone can take over an ecosystem; seagrass is going to be alright. Now, this shouldn’t mean we discount the fight against climate change, but it goes to show that ecosystems can be resilient, and have built in methods of dealing with change and maintaining a balance.