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Climate Change

Local disturbance and global warming unite to make seagrasses taste better to predators

Seagrasses form some of the most important habitats in the marine world. Under threat from global climate change as well as local disturbances, they’re also the subject of wide-spread investigation. Field and laboratory studies have shown that nutrient pollution, temperature changes, acidification, and other disturbances will negatively affect seagrass health, at the individual and community level. Existing research also suggests that for seagrass patches to be resilient to these disturbances they must be large, contiguous, and genetically diverse in the makeup of individuals belonging to that patch (or population). Less is known, however, about how these pressures will interact with other factors that may affect long-term seagrass success, like being eaten.

Seagrasses are important staple foods for a number of marine herbivores. Fish, shellfish, and some mammals all include seagrass as major or complementary parts of their diets. Some of these animals are also known to feed preferentially on seagrass with higher nutrient contents, in order to maximize their benefits while minimizing energy costs. (That’s also why we grab for the chips and candy before we cut up some veggies for lunch).

The seagrass Ucria (Yu Ito, 2012)

Because nutrient pollution can cause seagrass to briefly increase their nutrient contents, pollution from nearby settled areas can cause major changes to these nearby vegetated estuaries and seabeds, both by directly affecting the grass itself and by increasing herbivory. Jimenez-Ramos et. al. from the University of Cadiz studied a common seagrass herbivore, the purple sea urchin, in different seagrass populations to see if they had preferences for seagrass exposed to different disturbances.

The researchers collected specimens of the seagrass Cymodocea nodosa, or Ucria, from random patches in meadows surrounding Cadiz Bay, Spain. They were transferred to a lab where they acclimatized to artificial seawater environments and were then subjected to treatments simulating different types of disturbances. Climate change and local disturbances can cause significant changes in water temperature, light, and other water conditions, and many of these factors have set levels scientists expect them to reach under climate change. The researchers subjected the Ucria to temperature, nutrient, and acidity level changes expected under climate change while keeping another group of grasses under the existing seawater conditions as a control. They also combined all treatments with one of the other treatments to determine what effect each disturbance and combination of disturbances had on the seagrass. After one month, they presented the different seagrasses to purple sea urchins. Some urchins were also given an alternative choice of Ulva sp., another plant the urchins are known to like just as much if not more than Ucria., to see if their preferences would change given this added factor.

Temperature, acidity, and nutrient level changes, independently and combined, all resulted in increased herbivory by sea urchins, compared to the control grasses. These results aren’t surprising, as increased temperature and nutrient levels can make plants more productive and therefore better food choices for predators. Acidic environments can also make seagrass blades easier to break down, which is great if you’re an urchin and have to work for hours just to chew your food. What is interesting is how differently the urchins acted depending on what food was offered to them. If the urchins were only fed Ucria, it was the warming temperature that made them consume more of it. But if the urchins were fed a choice of Ucria and Ulva, it was the nutrient pollution that made them eat more Ucria. This could have important implications for conservation of seagrasses in different communities. Given a choice between seagrass and another preferred food source, predators may not have a significant effect on important seagrass beds. Under local disturbance and the pressures of climate change however, everything changes, and single and combined pressures may work in surprising ways to affect seagrasses. Jimenez-Ramos’ study shows that predicting seagrass resilience into the future will require intimate knowledge of the local and larger-scale changes that are occurring in different meadows. Conserving important seagrass habitat therefore requires detailed monitoring of environmental conditions in different meadows, because they vary so much. Then of course, we need to reduce the threats.

Jimenez-Ramos R, Egea LG, Ortega MJ, Hernandez I, Vergara JJ, Brun FG (2017) Global and local disturbances interact to modify seagrass palatability. PLoS ONE 12(8): e0183256. https:// doi.org/10.1371/journal.pone.0183256

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