I would rather suggest converse. You calculate the saturation index for Calcite precipitation for the ambient conditions in a tropical estuary of your interest.
I also agree that the best way to focus it is to reverse the question.
Anyway, we can assume an alkaline, high temperature and medium salinity tropical estuary. Obviously, salinity is extremely controlled by mixing conditions of your estuary, which I should expect to be an inverted-circulating type (¿¿??). Under such range of conditions, we might expect that apparent solubility product of calcite oscillates among 0.5x10-8 (20ºC and freshwater) and 45x10-8 (30ºC and 32º/ºº of salinity) mol2 l-2. Overall, precipitation is enhanced by high temperature (less CO2 solubility) and low salinity.
Another question is which other chemical species can compete with calcite (ie aragonite, MgCO3, etc...).