Nutrient limitation of autotrophic and mixotrophic phytoplankton in a temperate and tropical humic lake gradient

Nutrient enrichment experiments were carried out in three tropical (once) and three temperate (twice) lakes differing in humic content in order to examine whether there was a relationship between the limiting nutrient for algal growth [nitrogen (N) or phosphorus (P)] and humic content, and whether the prevailing limitation was connected to the relative abundance of autotrophic and phagotrophic phytoplankton (mixotrophs). In both climatic regions, there was a stronger tendency for total phytoplankton biomass accumulation to be N limited in lakes with a high humic content. However, in contrast to what we expected, there was no tendency for the mixotrophs to be more favored by the addition of N than of P. In the temperate lakes, the relative abundance of mixotrophs increased in the treatments receiving N or P separately or no nutrients (control) when exposed to a high light availability. In the following year, when the light availability was low, the mixotrophs increased relative to the obligate autotrophs in all treatments, irrespective of nutrient addition. Possibly, this was a result of their ability to supplement photosynthesis with the ingestion of prey. The results indicate that mixotrophy is an advantageous strategy when the availability of light and/or nutrients is low.