If the controversial citations in the article you cited are true (which I haven't read in detail, but seem questionable at a glance.) - unless the predominant amount of individuals immigrating into Spain/Norway are coming from countries with lower average IQs I would not expect it to have an effect on the average IQ of Spain/Norway.

It also stands to reason that there is no reason to assume Spain is at the top of the chart when it comes to national average - so they are most likely also having immigration from countries with a higher average. This would mitigate the effect of countries with a lower average, would it not?

Returning to the original point:

Because immigration usually entails an amount of merit and ability - the successful immigrants are usually at the higher end of the capability spectrum of the originating countries population.

Doesn't it stand to reason immigration would have either a minuscule or positive effect on national average IQ?