Meaning of ‘thatching the ice’ and others

Meaning of ‘thatching the ice’ and others

The passage below is from Fathoms: The World in the Whale Hardcover by Rebecca Giggs.

Ecologically, krill matter most as biomass and the basis of a polar food web. There are many more krill on the planet than any other animal; some 400 trillion individuals. Their daylong industry is to scoff clouds of phytoplankton, high in the water column. They also consume the vast vegetable that is algae: an inverted, inch-thick pasture, thatching the ice, which the krill both graze on and huddle in, with their larvae. Krill are a faunal tide within the sea. They clamber up to glut on plankton, and then the krill descend again, jerky as little cocktail umbrellas, while they digest.

On this passage talking about krill’s ecology, I have five questions.

First, the sense subject of ‘high in the water column’. Is the subject of ‘high in the water column’, phytoplankton or krill? My guess is krill, but I’m not sure.

Second to fourth focus on 3rd sentence.

Second, why inverted are the algae? Algae, if inch-thick pasture, is supposed to put their roots in sea bottom. Then how they are inverted, I can’t figure out.

Third, this is the worst mystery. How an inverted, inch-thick pasture that is algae is thatching the ice. I can’t visualize anything on this INVERTED and THATCHING THE ICE part.

Last, I have two questions on the part of ‘which the krill both graze on and huddle in, with their larvae’. One, what does ‘which’ refers to? I thinks it’s algae, (but ‘the ice’ that is just in front of ‘which’ bugs me). Am I right, is it algae? Two, when it says the krill and their larvae huddle in algae (or the ice) what do they do? Do they gather together around algae, or do they hide in algae for some reason unknown in the context?

Thank you all as always.