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| In this study, a global data set on size-fractionated chlorophyll distributions collected
in the open ocean (depth >400 m) is used to investigate phytoplankton community size structure
in relation to temperature and inorganic nutrient availability in an attempt to identify the individual
and shared effects of these 2 factors. The macroecological patterns show an increase in the
fraction of large phytoplankton with increasing nutrient availability and a decrease with increasing
temperature. We empirically demonstrate that temperature has both a nutrient-independent
effect and a nutrient-shared effect on phytoplankton community size structure. We argue that the
nutrient-independent effect is likely a direct effect of temperature, whereas the nutrient-shared
effect may be an indirect effect of temperature (where thermal stratification influences the introduction
of nutrients to surface waters). When regional differences in the average contribution of
large cells were accounted for, the nutrient-independent effect of temperature explained 8% of
the variation in phytoplankton community size structure compared with the 23% explained by the
nutrient-shared effect. The results suggest that the relationship between phytoplankton community
size structure and temperature change is the same in all ocean regions and leads to a
decrease in the relative contribution of large cells in the community as temperature increases
regardless of ambient nutrient availability. As phytoplankton size is an important factor influencing
carbon transport to the deep ocean, it is important to incorporate any possible direct temperature
effect on phytoplankton community size composition in models addressing carbon flow and
metabolism in a warming ocean. | |
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