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A rainbow spectrum of zooplankton water content

Published: 16 August 2017
The wide range of body size, form and function in the zooplankton is fascinating but poses some serious challenges to modellers

A MERP publication has provided a fresh approach to how we should incorporate the diversity of jellyfish and other zooplankton into our understanding and models of zooplankton.

Most of us know about the wonderful variety of shapes and body forms within the plankton, from the massive barrel jellyfish that we have been seeing in the Westountry during the MERP field seasons, right down to the minute forms only visible through a microscope. But for modellers this variety is also a bit of a headache. How can our numerical models capture such a stunning array of body form and functions into a series of simplified equations that help us understand food webs?

A recent development among plankton modellers, both in MERP and more generally, is to use “trait-based models”. This is jargon for an approach where form and function are seen in terms of trade-offs. For example a larger body size has advantages for increasing swimming and migration ability, but disadvantages for how quickly the animal can grow.

The degree of gelatinousness (i.e. carbon mass as a percentage of wet mass) of zooplankton taxa vary enormously and almost continuously, in a spectrum ranging from 0.01%25 to 19%25. Top panel shows the individual taxa, bottom panel shows medians, quartiles and range for the broader groupsOf all the traits, body size is described as the “master trait” because it governs the pace of life at about every level of organisation, from an individual cell to whole ecosystems. But what do we actually mean by body size? The barrel jellyfish is large in wet mass but has a relatively low percentage of metabolically active tissue, compared to the numerous small but carbon-rich copepods.

So is it sheer size that we should be measuring, or amount of carbon, or both? This question has been hampered by the frequent habit of zooplankton ecologists simply to lump zooplankton into two categories; it is either a jelly or it is a carbon-rich crustacean. This paper has broken down these barriers to show that water content can vary greatly even among the gelatinous taxa, and like carbon mass, can be better appreciated if it is seen as a continuous rainbow spectrum of variability.

By compiling numerous data from the literature, this paper has broken down size into its two components, namely wet mass and carbon mass. By doing this they present a single, unified equation that is much better at predicting zooplankton growth than those used previously. This approach, and the compilation of zooplankton water content data will also allow modellers within and outside MERP to use the water content of zooplankton as a variable trait to examine the benefits and trade-offs of a gelatinous body form.


Related information

McConville, K. et al (2017) Disentangling the counteracting effects of water content and carbon mass on zooplankton growth. Journal of Plankton Research, Volume 39, Issue 2, 1 March 2017, Pages 246–256. 

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