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Biology

The seagrass isn’t always greener: how jellyfish and nutrients are impacting seagrass ecosystems

 

 

Article: Stoner, E. W., et al. (2014). “Modification of a seagrass community by benthic jellyfish blooms and nutrient enrichment.” Journal of Experimental Marine Biology and Ecology 461(0): 185-192.

DOI: 10.1016/j.jembe.2014.08.005

Background:

Certain aspects of global change, like ocean acidification and sea level rise have been dominating headlines; but there are other aspects that have flown under the radar. While there isn’t a great record of jellyfish populations in the past, there has been a noticeable increase in jellyfish populations around the world in the past few years. This increase has been linked to climate change and overfishing. Consequently, it is thought by scientists that future oceans may be swarming with jellyfish (Fig 1).

Fig 1: Jellyfish bloom. (Source: Discovery News)

Fig 1: Jellyfish bloom. (Source: Discovery News)

Jellyfish play interesting roles in marine ecosystems. They can control a system from the top down or from the bottom up. Top-down control means that an ecosystem’s structure and function is determined by species at higher trophic levels. In the case of the jellyfish, decline in larval fish populations have been directly linked to jellyfish abundance and predation. Bottom-up control indicates that an ecosystem structure and function are determined by factors influencing the lower trophic levels. When jellyfish bloom and subsequently die, their bodies leave behind large amounts of carbon and nutrients which can then impact the growth and success of primary producers. Jellyfish can impact ecosystems in many ways, and with populations expected to rise, they are a factor to be treated like other consequences of global change.

Nutrient addition to aquatic ecosystems is a global problem. Nutrients enter waterways via outflow from wastewater treatment facilities or run-off from land. Once in the system, nutrients stimulate algal growth which can lead to algal blooms (Fig 2). Algal blooms are detrimental to ecosystems as they can outcompete and smother most other primary producers. Bloom die off creates hypoxic zones, or zones with low oxygen levels, that is harmful for most other organisms in the community.

Fig 2: Nutrient addition can lead to algal blooms and can cause trouble for many other organisms. (Source: National Geographic)

Fig 2: Nutrient addition can lead to algal blooms and can cause trouble for many other organisms. (Source: National Geographic)

The Study:

Researchers at Florida International University set out to see how benthic jellyfish abundance in conjunction with other aspects of global change impacts coastal ecosystems. While the role and influence of pelagic jellyfish are better understood, there are large numbers of benthic jellyfish inhabiting valuable coastal ecosystems such as seagrass beds (Fig 3), coral reefs, and mangrove forests. Benthic jellyfish are of the genus Cassiopea and are also known as upside-down jellyfish (Fig 4). These creatures look a similar to other jellyfish but they are relatively sessile, spending their lives on the sea floor, tentacles up, as opposed to moving around pulsating in the water column. Because these organisms settle on the bottom, they often cover seagrasses, blocking them from obtaining light necessary for photosynthesis. Here, researchers were specifically looking at how Cassiopea abundance and increased nutrients (two coastal disturbances) affect the growth and productivity of seagrasses and the communities they support.

Fig 4: An upside-down jellyfish, Cassiopea spp. (Source: Keys Field Guide)

Fig 4: An upside-down jellyfish, Cassiopea spp. (Source: Keys Field Guide)

Fig 3: Seagrass meadow. (Source: The Nature Conservancy,  Jeff Yonover)

Fig 3: Seagrass meadow. (Source: The Nature Conservancy, Jeff Yonover)

 

 

 

 

 

 

 

 

 

Off the coast of the Bahamas, researchers set up 1×1 meter plots. They used four treatments: a control (no added jellyfish, no added nutrients), increased nutrients, increased jellyfish, and increased nutrients and jellyfish. Because Cassiopea are relatively sessile, it was easy for researchers to remove them from certain plots and add them to others. Nutrients were added to study plots by implanting the top layer of sediment with slow-release fertilizer. Nutrients typically act as a stressor for seagrasses due to the increased growth of algae that can outcompete seagrass for resources such as light and space. Over time, researchers tracked the abundance and growth of seagrass as well as benthic fauna and grazing on seagrasses.

Here’s what they found:

  • Increased nutrients, increased jellyfish abundance, and increases in both led to decreased seagrass cover (Fig 5).
  • With increased jellyfish abundance and nutrients, biomass of seagrass was much lower than in any other treatment (Fig 6).
  • Grazing on seagrass was significantly higher in the nutrient treatment compared to the jellyfish treatment (likely because the presence of jellyfish was deterring herbivores) (Fig 7).
  • Benthic fauna were negatively impacted by the presence of jellyfish, but not by increased nutrients, with lower numbers of organisms present in jellyfish treatments (Fig 8).
  • Fig 5: This plot show the percent cover of seagrass by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

    Fig 5: This plot show the percent cover of seagrass by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

    Fig 6: This plot shows the aboveground biomass of seagrass by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

    Fig 6: This plot shows the aboveground biomass of seagrass by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

 

Fig 7: This plot shows grazing impacts by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

Fig 7: This plot shows grazing impacts by treatment. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

This plot shows the density of animals associated with seagrass beds. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

Fig 8: This plot shows the density of animals associated with seagrass beds. C = control, N = nutrient addition, J = jellyfish addition, J+N = jellyfish and nutrient addition.

Significance:

Both increases in jellyfish abundance and increases in nutrients appear to have a detrimental affect on seagrasses. Seagrasses are important coastal ecosystems providing food, habitat, and ecosystems services. In fact, seagrass beds are often cited as systems that can withstand ocean acidification. These grasses are also great indicators of the ecosystem health. This study sheds light onto the impacts of jellyfish blooms, showing that this aspect of global change can be an important force in shaping future ecosystems. Seagrass beds are already highly susceptible to climate change, jellyfish blooms will only make life tougher on these ecosystems.

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