Evidence for extensive but variable nutrient limitation in New Zealand lakes

Nutrient limitation causes reduced growth of organisms, which can translate into far-reaching consequences for populations, communities, and ecosystems. Phosphorus (P) limitation, in particular, is associated with reductions in organismal growth because ribosomes, upon which growth depends, require abundant phosphorus to be produced. Chromosomes are also relatively rich in P, meaning that organisms with relatively high chromosome complements (e.g. polyploids) might be especially dependent on abundant environmental P. Here we address the likelihood of nutrient limitation in multiple populations of Potamopyrgus antipodarum, a New Zealand freshwater snail featuring wide ploidy variation. We found that some form of P limitation is very likely in many, but not all, populations of this snail that we surveyed. We also detected extensive across-population variation in P and nitrogen (N) content and N and P limitation and co-limitation in the algae that P. antipodarum eat. Accordingly, we then experimentally evaluated how P and N alone and together influenced growth rate in P. antipodarum. We found that response to nutrients differed by lineage and that dietary P content was more important than dietary N content as a determinant of growth rate, a trait closely tied to fitness in P. antipodarum. The widespread likelihood of (1) P limitation and (2) variation in dietary P availability across New Zealand lakes, along with (3), evidence for lineage-level variation in sensitivity to P limitation, sets the stage for the possibility that variation in nutrient availability contributes to the distribution and maintenance of ploidy variation in P. antipodarum.