I have two ImagesCollections. I can do mean() reducer before merging them. But after merging them I got error when I was trying to do mean() to the merged imagecollection. See below the code.
//The image collection of Chlorophyll from MODIS Aqua
var chla_Modis = ee.ImageCollection("NASA/OCEANDATA/MODIS-Aqua/L3SMI")
.filterDate('2010-7-04', '2011-3-14')
.select('chlor_a');
print(chla_Modis);
var seawifsData = ee.ImageCollection("NASA/OCEANDATA/SeaWiFS/L3SMI");
var chla_SeaWiFS = seawifsData
.filterDate('2010-1-04', '2010-12-11')
.select('chlor_a');
print(chla_SeaWiFS);
//merge the two imageCollections
var mergedChla=chla_Modis.merge(chla_SeaWiFS);
print(mergedChla);
var meanChla=mergedChla.mean();
print(meanChla);