Stoichiometric constraints in primary producers affect secondary consumers

This thesis investigates the effects of nutrient limitation in primary producers on higher trophic levels. To this aim the nutrient conditions and their effect on primary consumers in the North Sea were examined. Furthermore experiments were carried out to test the effect of nutrient limitations on tri-trophic food chains in controlled laboratory conditions. Despite the prevailing assumption that the herbivorous primary producers maintain strict homeostasis, thus buffering any nutrient imbalances at the base of the food web and providing their consumers with food of a high and constant quality, their nutrient stoichiometry and fitness was affected by the nutrient content of their food. The nutrient deficiency of the primary producers can thus be passed on through the food web and affect higher trophic levels. In a set of laboratory based experiments the nutrient contents of primary producers was manipulated to change their C:nutrient ratio and thereby their food quality. Primary consumers were reared on these producers and in turn were the food source for a higher consumer. The ctenophore Pleurobrachia pileus was exposed to copepods with manipulated C:P ratios along a gradient. The reaction of the ctenophore to food with a high C:P ratio, generally considered to be food of lower quality, was unexpected. P. pileus was negatively affected by nutrient replete food with a low C:P ratio and displayed higher levels of fitness when feeding on food with a high carbon:P ratio. This ctenophore is therefore more likely to be energy limited than nutrient limited. A possible explanation for this is the high body C:P ratio observed in P.pileus. The results are discussed in light of possible future scenarios in the aquatic environment. Freshly hatched larvae are very vulnerable and the mortality during this stage is particularly high. Therefore the effect of food with different C:N:P ratios was tested on larvae of the European lobster Homarus gammarus. Larvae were exposed to food with varying nutrient content either throughout their development or for the duration of one stage. The results confirm that the youngest larval stages are the most susceptible to a lack of nutrients in their diet, which had strong negative effects on their condition. The effect of P-limitation in particular was observed to change during ontogeny, with older larval stages being less affected. As nutrient-limitations reduce the fitness and reproductive output of primary consumers, this translates into a quantity effect for higher trophic levels by reducing the amount of food available to them. The combined effects of nutrient quality and nutrient quantity on a top predator were investigated in an experimental set-up. The consumer was negatively affected by the lack of nutrients in its diet even at very low nutrient quantities, indicating that quality of the food is more important than the quantity. Disentangling these two effects, the quality and quantity of food, remains a great challenge for future ecological studies in the marine environment. These findings have potentially far-reaching consequences for the trophic interactions and population dynamics of marine organisms.