Effects of food quality and stoichiometric mismatch on copepod growth rates
Date Issued
2015
Date
2015
Author(s)
Wong, Esther
Abstract
Trophic interactions between producers and consumers determine community structure and ecological function. The efficiency of energy and materials transfer at this interface through grazing critically depends on the quality of producers as food for consumers. Existing evidences showing the effect of food quality on consumers were mainly gained from controlled experiments and studies on freshwater species. However, the effect of stoichiometric mismatch between food supply and consumer demand has not been examined quantitatively in natural marine systems. Here, we investigate whether food quality and stoichiometric mismatch can impact on growth rates of copepods, using field data in the East China Sea. We use C:N ratios of particulate organic matter (POM) and two zooplankton community size fractions (nauplii and copepodites) to represent stoichiometry of phytoplankton and copepods respectively, and measure copepod in situ weight-specific growth rates using the artificial cohort method. We remove temperature and body size effects on growth rates a priori according to the Metabolic Theory of Ecology to test the effect of stoichiometry. The C:N ratios of POM (5.94 ± 0.77) were lower than the Redfield ratio with small variation. In contrast, the C:N ratios of nauplii (7.55 ± 1.20) and copepodites (6.83 ± 1.34) vary substantially and are on average higher than the Redfield ratio, and are not correlated with the C:N ratios of POM. In addition, the C:N ratios of nauplii are generally higher than those of copepodites, indicating ontogenetic change. Our results contradict the generally accepted perception of stoichiometric homeostasis of zooplankton. Surprisingly, growth rates of both size fractions were not affected by food quality. Rather, growth rates of copepodites increased significantly with the increasing stoichiometric mismatch (C:Ncopepod - C:NPOM). We propose a hypothesis to explain this observation: higher stoichiometric mismatch triggered higher copepodite growth rates. That is, for the copepodites having high C:N relative to the C:N of their food, they would increase their consumption rates in order to reach the Redfield ratio; as such, their growth rate elevated. Nevertheless, we cannot rule out the possibility that the positive correlation between growth rates and C:N of copepodites could simply occur when higher consumption rates (and thus higher growth rate) are associated with high carbon growth and lipid accumulation.
Subjects
food quality
stoichiometric mismatch
C:N
copepod
growth rate
homeostasis
SDGs
Type
thesis
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