Prowe, Friederike (2011) Effects of the feeding functional response on phytoplankton diversity and ecosystem functioning in ecosystem models. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 188 pp.

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Abstract

Ocean ecosystems are under pressure from the needs of a growing human population and from global environmental change. A concurrent loss of diversity observed across ecosystems raises the question of how diversity influences ecological and biogeochemical processes of ecosystems. Little is known about controls of diversity and its role in shaping ecosystem processes in the global pelagic ocean and biogeochemical cycles of nutrients and carbon. Bottom-up controls by nutrient availability and use have previously been investigated using a novel global ecosystem model which resolves phytoplankton diversity. Top-down effects of zooplankton feeding as an important mechanism able to promote diversity have not yet been investigated on the global scale. Also, the influence of diversity on primary production and other indicators of ecosystem functioning in the global ocean are not well understood. The present thesis aims to extend our understanding of how zooplankton feeding influences phytoplankton diversity in ecosystem models. In addition, it addresses the question of how diversity may influence ecosystem functioning and biogeochemical cycling. The first part of the thesis examines the top-down control of zooplankton feeding on phytoplankton diversity in a global ocean ecosystem model with a self-assembling hytoplankton community. In simulations with different mathematical formulations for feeding, phytoplankton diversity differs by more than a factor of three. A sigmoidal Holling type 3 functional response implying preferential grazing on the most abundant prey creates refuges for phytoplankton at low abundances and punishes dominant types. The resulting seasonal succession is in better agreement with observations than for a type 2 functional response without preferential grazing. Simulations with different diversity also differ in primary production and net community production on the annual scale. The second part investigates the effects of phytoplankton diversity on primary production as a basic ecosystem function in the pelagic ocean. Global simulations with different levels of diversity are complemented by idealised simulations without environmental forcing. A positive relationship between diversity and productivity is found for simulations using type 3 feeding. In these simulations, the phytoplankton community is characterised by a complementary use of resources. Higher diversity increases primary production only in temperate, but not in oligotrophic oceanic regions, indicating a potentially important influence of the nutrient supply. No effect of diversity on primary production can be identified for a type 2 feeding functional response. The sensitivity of the simulated diversity to the feeding formulation motivates the development of an alternative zooplankton feeding model presented in the third part of this thesis. The commonly employed functional responses used in the previous parts are simplistic with regard to the feeding process and to the zooplankton community structure in the ocean. The model presented here addresses the first aspect by taking into account metabolic constraints on the feeding process. Energy obtained from predation is optimally allocated between foraging activity and the assimilation of food to maximise net growth. The model captures experimental feeding data for different zooplankton taxa. It provides an alternative approach for representing plankton dynamics in a seasonally stratified mixed layer without an otherwise required shift in community structure. The fourth part of this thesis complements the main model-based focus of zooplankton feeding and diversity with an experimental approach addressing the impact of feeding within a community. This study investigates herbivorous and carnivorous feeding relationships with a sea-ice community as a model system. It presents grazing experiments using natural communities of different algal taxa instead of individual predator-prey combinations, as well as predation experiments. The experiments reveal complex feeding relationships and estimate the feeding impact of sea-ice meiofauna on the sea-ice community. Similar experiments for plankton would provide valuable information for enhancing our understanding of feeding in a community context and ultimately help in developing future zooplankton feeding models. The different aspects of zooplankton feeding and phytoplankton diversity addressed in this study demonstrate the influence of top-down controls on diversity, and indicate consequences for ecosystem functioning and biogeochemical cycling. A better understanding and representation of the complex feeding processes in the plankton will enhance our ability to model diversity in the pelagic ocean. Resolving diversity may be an important component in predicting biogeochemical cycling in a future ocean.

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