Effects of poor food quality on copepod growth are dose dependent and non-reversible

Herbivores regularly face imbalanced diets on a variety of timescales. They respond to such diets with reductions in growth and reproduction. The effect of exposure time to poor quality diets and recovery potential in herbivores has yet not been intensively studied. In order to investigate the response of herbivores growth and nutritional condition to phosphorus limited algal food, we fed the copepod Acartia tonsa with the autotrophic flagellate Rhodomonas salina, which was cultured towards high and low phosphorus content. To test the effect of the duration of the exposure to low and high P food on copepod growth, we also switched a subset of the copepods from high P to low P food for two to 12 days before putting them back on their original diet. Phosphorus limited prey clearly reduced copepod nutritional condition, expressed as their RNA:DNA ratio and consequently their growth rates. Several days after re-feeding on high P algae, the copepods were able to reach growth rates comparable to the control groups, but they were in no case able to over shoot the growth rates of the control groups. A dose of two days feeding on low P algae resulted in slightly less than one stage growth reduction compared to copepods exclusively reared on high P algae. Our results show that phosphorus limited food drastically reduces secondary production, and no compensatory growth occurs in the investigated copepod species. Hence, even short term exposure of herbivores to poor food has a lasting effect on secondary production.