we are analyzing pollutants [µg/kg] in mussels with regression models. Let's say we have only the two predictors (1) "year" and (2) "month". However, the effect of "year" on the outcome is assumed to be additive, i.e., in avergage a constant amount of poullutants is added each year, independent of the total concentration. However, the effect of "month" is rather multiplicative, since it depends on seasonal mussel growth. I.e., the pollutant concentration changes relative to the total amount.

In case every predictor would be additive in nature, I would use the identity-link function. If all predictors are multiplicative, I would use the log-link function or log transform the outcome with a id-link function. But what can I do with this "mixed" situation??

Thank you very much in advance