I noticed a while ago that in some of my own research, but also in a colleagues who is a dateset that is completely independent of mine (different organisms, different sequencing service, etc.) that there is a strong correlations between sequencing depth and ASV richness. Looking at the rarefaction curves, they all plateau out nicely and all samples have more than enough read coverage. Still, samples with higher read counts without fail have higher species richness, and lower coverage samples have lower. The correlations is definitely significantly strong.
Any ideas what could cause this?