I am interested in modelling the impact of some environmental parameters on a concentration of measured phytoplankton pigment. The concentration of pigment is skewed so that low concentrations are more frequent than high ones and there are occasions of 0 (zero) concentration. I was hoping to use a GLM model with a Gamma distribution, but the zero values prevent such a fitting. My question is if there are other options for me.
I thought to simply add a very small number to all concentrations (e.g. 0.00001) and proceed, but perhaps this is ill-advised. Given that the response variable is continuous, I am not sure what my options are - Poisson and Negative Binomial distributions are only for count data?
Any advise would be greatly appreciated.