Predation induced defense simultaneously increases metabolic cost and decreases resource acquisition rates, shifting ecosystem-level carbon partitioning and residence time in a multi-trophic level system

Background/Question/Methods

Prey species subjected to prolonged predation pressure often exhibit induced predation-avoidance traits.  Allocation to defensive traits has the potential to alter resource requirements and nutrient acquisition in prey species prompting a trade-off between predator avoidance and growth rates. Such trade-offs, which can alter physiology of primary producers, may contribute to changes in whole-ecosystem properties such as primary production, nutrient cycling and gross respiration.When subjected to grazing pressure Chlamydomonas reinhardtii, a wide-ranging freshwater alga, develops palmelloid cell complexes, consisting of individual C. reinhardtii cells bound together by a mucus-like matrix that enables a release from grazing.  We grew grazed (by Daphnia magna) and non-grazed populations of two strains of C. reinhardtiiin semi-continuous culture in order to examine how grazing induced palmelloid colonies affect photosynthetic and respiration rates as well as nitrogen and phosphorus uptake.  Once cultures reached steady state we performed inorganic N and P uptake to determine if there were mass specific differences in N and P uptake and utilization. We quantified gross primary production (GPP) and respiration, normalized to culture biomass, from dissolved oxygen measurements made under dark and light conditions. 

Results/Conclusions

While grazed (palmelloid-forming) and non-grazed cultures had marked differences in overall biomass, population-level respiration rates did not vary greatly. There was no significant effect of C. reinhardtii strain on biomass or respiration rates.  A reduction of biomass in grazed cultures while maintaining similar rates of mass-specific respiration to control cultures sugggests a reduction in physiological efficiency and possibly increased carbon demand in palmelloid-forming cultures.  Nitrogen and phosphorus uptake rates were lower in the palmelloid colonies. These results provide support for the idea that palmelloid-forming populations show a reduced physiological efficiency resulting from a trade-off between defense and production.  Grazed populations have greater carbon costs associated with growth and maintenance, which in addition to limiting C acquisition may also alter ecosystem-level carbon availability and residence time.  Quantifying individual trophic level respiration as well as aggregate mesocosm respiration will allow us to identify whether population-level changes in C pool sizes and residence times will be apparent across multiple trophic levels.