Patterns of resource limitation of bacteria along a trophic gradient in Mediterranean inland waters.

The nature of the resource that limits heterotrophic bacteria, i.e. mineral nutrients or carbon (C), has consequences for biogeochemical cycles in aquatic ecosystems. Our aim was to identify the resource [C or phosphorus (P)] that mainly limits bacteria in a set of 31 Mediterranean inland water ecosystems spanning a wide trophic range. We followed an intersystem observational approach with three complementary perspectives, comparing the bacterial demand with the resource supply in terms of both the quantity (demand : supply ratio for C and P) and quality (C : P ratio of demand and supply), and assessing the relative strength of each resource in controlling bacterial production. The trophic gradient revealed a shift in the main limiting resource for bacteria, from C at the oligotrophic end (typically high-mountain, low-productivity lakes) to mainly P at the eutrophic end (typically nonmountain, high-productivity lakes). The patterns of resource limitation of bacteria found here may be related to the autotrophic nature of most of the studied ecosystems linked to a Mediterranean climate regime as representative of lakes with low inputs of allocthonous C. These patterns are consistent with the theoretical approaches and may potentially shape the contribution of this type of ecosystems to biogeochemical cycles.