The influence of jellyfish on the nutrient cycling of coastal lagoons

A project undertaken at the Centre for Aquatic Processes and Pollution, and the School of Environmental and Applied Sciences, Griffith University, and supervised by Kylie Pitt and David Welsh

Background
Jellyfish are the most conspicuous animals in the coastal lagoons of eastern Australia. Frequently they form spectacular population blooms where the biomass may exceed 500 tonnes km-2 (Pitt & Kingsford 2003). Jellyfish are likely to contribute to nutrient cycling by taking up nutrients during feeding and releasing inorganic nutrients by excretion and, after they die, decomposition (Kingsford et al. 2000). Although the contribution of jellyfish to nutrient cycling may not differ substantially from other animals, the sheer biomass of jellyfish in coastal lagoons means that they are likely to be the major processes of nutrients in these systems. The role of jellyfish in nutrient cycling is, however, poorly understood.

Like corals, some species of jellyfish contain symbiotic algae (zooxanthellae) in their tissues. Symbiotic species derive their nutrition partly from feeding on planktonic prey and partly from the zooxanthellae transferring their photosynthetic products to the host jellyfish. Given that zooxanthellae (like all plants) take up and utilise inorganic nutrients, symbiotic and non-symbiotic jellyfish are likely to have contrasting roles in nutrient cycling (Muscatine and Marian 1982).

The jellyfishes, Catostylus mosaicus and Phyllorhiza punctata are the two most abundant species of jellyfish in the coastal lagoons of eastern Australia. Although they are morphologically similar, P. punctata contains dense concentrations of symbiotic zooxanthellae whereas C. mosaicus contains few or no zooxanthellae. A recent study that measured rates of excretion of inorganic nutrients by these two species found that C. mosaicus excreted enormous quantities of ammonia into the water column but P. punctata did not, suggesting that the zooxanthellae in P. punctata were probably sequestering the ammonia being excreted by the host jellyfish. Hence the nutrients taken in by C. mosaicus during predation are quickly released back into the water column by excretion but those taken in by P. punctata appear to be recycled internally between the jellyfish and its symbiotic algae, effectively trapping the nutrients and removing them from the water column. In this sense, populations of P. punctata, like macroalgae and seagrasses, may be considered to be a “sink” for nutrients and, while the population bloom is sustained, may have a role in mitigating the effects of eutrophication.

Objectives

Our project aims to study the influence of jellyfish in the nutrient cycling of coastal lagoons and, in particular, to compare P. punctata (which contains zooxanthellae and, therefore, partly functions like a plant) with C. mosaicus which functions solely as an animal.

Specific aims include:

  1. To identify the mechanism by which inorganic nutrients are incorporated into the tissues of Catostylus mosaicus and Phyllorhiza punctata.
  2. To use a combination of short-term and longer-term experiments to measure the duration for which inorganic nutrients are retained within C. mosaicus and P. punctata and, therefore, the degree to which they act as a sink for nutrients
  3. To measure the effects of decomposing jellyfish on biogeochemistry of the sediments and water column and the benthic infauna.

The outcomes of our research will have major benefits for all people concerned with the water quality of coastal lagoons, including environmental managers, tourist operators and recreational and commercial fishers.

 
Figure 1. A bloom of Catostylus mosaicus in Lake Illawarra, NSW, 1998

Figure 2. Catostylus mosaicus (Photo by M J Kingsford)

Figure 3. Phyllorhiza punctata