Geoscience Reference
In-Depth Information
IV. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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ABSTRACT
We develop a spatially explicit, continuous, time-dependent model of size
spectra to predict how the active movement and passive transport
of individuals can influence individual growth and size spectra. Active move-
ments are 'prey-seeking' behaviour, with individuals moving locally towards
areas with high concentrations of favoured prey, and 'predator-avoiding'
behaviour, with prey moving away from areas of high predator density.
Passive transport represents the effects of turbulent mixing on small indivi-
duals. The model was used to explore the individual and community effects
of these biotic and abiotic processes and their interactions, and to predict
how energy from local sources of primary production is propagated through
the food web. Prey-seeking and predator-avoiding behaviour led to system-
atic changes in the relative abundance of different-sized individuals in rela-
tion to centres of primary production and associated changes in size-spectra
slopes. In areas of high phytoplankton abundance, community size-spectrum
slopes were shallower and larger individuals were present, whereas in low
production areas, slopes were steeper and size spectra truncated. Variations
in size-spectra slopes were much reduced by spatial aggregation across the
gradient of phytoplankton abundance, and regional slopes most closely
approximated the slopes close to centres of high primary production. Indi-
vidual growth was faster when closer to centres of production. The extent to
which stability is apparent in size spectra depended on the scale of aggrega-
tion. This implied that sampling at relatively large space and time scales in
relation to those of phytoplankton 'blooms' was necessary to compare
emergent properties, such as size spectra, among regions or ecosystems.
Further, at larger scales, responses to human impacts will be clearer and
less likely to be masked by variability induced by smaller scale processes.
I. INTRODUCTION
Spatial processes influence the structure and dynamics of populations,
communities and ecosystems. These are driven by interactions among and
between individuals and the abiotic environment. In marine environments,
spatio-temporal heterogeneity in physical processes results in spatial hetero-
geneity of primary production at multiple scales (
Behrenfield et al., 2002;
Falkowski et al., 1998
). Examples are mesoscale upwellings that drive prima-
ry production in otherwise oligotrophic waters at
scales 1-100 km
