Calcium carbonate is vulnerable to attack by acids, so as the amount of acid in the water increases, the rate of reactions between the acid and the shells of marine animals goes up. This in turn means that the marine animals have to dedicate more resources to replenishing the shells to avoid death - and in a competitive natural environment "more" isn't always available.

The reaction is as follows: CaCO3 (s) + 2 H^(+) (aq) -> Ca^(2+)(aq) + CO2 (g) + H2O (l)

Here’s a really good article that goes into the whole process and its real world consequences.

https://apps.seattletimes.com/reports/sea-change/2013/sep/11/oysters-hit-hard/

thank you very much

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Would the release of CO2 in this reaction add to the acidification of the seawater?

Very very slightly, yes. However, the quantity released is fairly minor. And it's diluted into quite a lot of water.

I think its basically this: excess CO2 + water=acidic water the more CO2 the more acidic. Acidic water + calcium carbonate can then form calcium bicarbonate which is more soluble in water than calcium carbonate & calcium carbonate shelled beings like clams,coral etc start to suffer from the above reactions.

i understand this problem perfectly from different point.. when i grow weed, the younglings are most vulnerable to ph changes (absorbtion balance of minerals, nutrients..) oh and balance of bacteria in medium aswell. Point is, tap water without letting it sit is enough to mess up the balance, and you notice the difference. growth might be hindered or the plant dies alltogether.. Ph is everything.

anyway, id imagine sea life being even more directly vulnerable since there is no medium (buffer) in between.