Well according to Google, one anchovy school can contain up to 100 million fish, so 1bn marine animals could mean all the whales died or ten schools of anchovies.

Without further information, I'm unsure what to make of the metric. Of course, microscopic animals are also animals, and important parts of the food chain, but 1billion zooplankton probably would be smaller than the anchovy schools.

I think "1bn" it makes for an attention getting but not an especially useful headline.

It seems like the real story is that mussels can't survive more than a few hours at 40 degrees and died en masse. This has many ripple effects in the ecosystem.

- mussels are a primary food source for many species, like sea stars

- their shells are used as habitats by hermit crabs and other crustaceans and worms

- they help filter the water and keep it clear.

- low oxygen in the water as they decompose

This isn't a longitudinal study, 1bn is just back of napkin math, but it sounds like it is fair to assume that there will be dramatically reduced biodiversity and biomass in the area for years to come. And if another such anomalous temperature event takes place in the next few years it will compound the issue.

% of organisms seems like the right place to start

Like all metrics, it depends on what you are trying to accomplish.

As others have said, the marine bio systems contains organisms from unicellular to whale scale. For most purposes a count is meaningless.

For a headline, you need a heart-tugging number.