Resolving the role of zooplankton in the marine ecosystem with functional size spectra

Abstract

Zooplankton are the linchpin of the marine ecosystem, serving as the main energy pathway from primary producers to higher trophic level organisms, such as fish. Despite their critical role, zooplankton are typically oversimplified or ignored altogether in current marine ecosystem models, and we have limited understanding of how variation in the zooplankton affects energy transfer from phytoplankton to fish. An alternative to resolving the enormous taxonomic diversity of the zooplankton is to focus on functional traits, such as body size, feeding strategy and body composition, since these are the factors that determine an organism’s fitness in any given environment, and their role in the marine food web. Body size is the master trait, however zooplankton exhibit vast diversity in other important functional traits, such as predator-prey mass ratio (PPMR) and body composition. Energy transfer through the marine food web depends on these traits, so to better understand the role of zooplankton in marine ecosystems any realistic representation of the zooplankton must incorporate this diversity. In this thesis, I use recent developments in size spectrum modelling, coupled with the extensive literature on zooplankton physiology and size-based feeding characteristics, to explore the structure of the zooplankton community across the global ocean, and their role in mediating energy from phytoplankton to fish.We begin by reviewing the development of size spectrum modelling, particularly focusing on the last 10 years and the recent innovations in size spectrum modelling. In particular, we review the development of functional size spectrum modelling, which allows size spectrum models to resolve multiple groups by their functional traits, and how this has been used for various higher trophic level groups, especially fish. The focus on fish means that the unique dynamics of the plankton have been overlooked in these models. We use the functional size spectrum framework to demonstrate how changes in the size-based feeding behaviour of the zooplankton community affects the productivity of higher trophic levels, and their resilience to fishing pressure: the higher the PPMR of the zooplankton community, the more productive and resilient the fish community. However, higher zooplankton PPMR also increases the temporal variability of the zooplankton and fish communities.The most common modelling assumption with respect to the zooplankton community is that its composition does not change across environmental gradients. However, there is strong evidence that the composition of the zooplankton community is not static, but varies across the global ocean, and this will have implications for how energy moves from phytoplankton to fish. We explore the role of functional traits in structuring the zooplankton community across the global ocean. We develop a functional size spectrum model of the marine ecosystem, which resolves the body size ranges, size-based feeding characteristics and carbon content of nine of the most abundant zooplankton groups (heterotrophic flagellates and ciliates, larvaceans, omnivorous and carnivorous copepods, chaetognaths, euphausiids, salps and jellyfish). Zooplankton community composition emerges from the model across global environmental gradients, based on the functional traits of the nine groups. Across the global ocean, the emergent distributions of the zooplankton community broadly agreed with empirical observation and theory. Heterotrophic flagellates and ciliates, salps, larvaceans, carnivorous copepods and chaetognaths were a larger component of zooplankton biomass in oligotrophic waters, omnivorous copepods were prevalent everywhere, and euphausiids and jellyfish dominated the zooplankton in eutrophic waters.Traditionally, eutrophic systems are hypothesised to be more efficient at transferring energy from phytoplankton to fish, compared to oligotrophic systems. We use the zooplankton- resolved functional size spectrum model to test this hypothesis, and assess the role of zooplankton composition in mediating transfer efficiency from phytoplankton to fish. Changes in the composition of the zooplankton lead to eutrophic waters supporting three times more fish biomass per unit phytoplankton, compared to oligotrophic waters. However, we also found that salps and larvaceans – despite their low carbon content – were critical in oligotrophic waters, providing a direct energy pathway from the small pico-phytoplankton which dominates in oligotrophic regions, to planktivorous fish. Without salps and larvaceans, total fish biomass was up to 50% lower in oligotrophic waters, and 17% lower across the global ocean.The work presented here demonstrates the power of the functional traits to explain global patterns in the zooplankton community. The zooplankton-resolved size spectrum model developed here is a step forward in trait-based modelling, and our understanding of the role of zooplankton in the marine ecosystem.