TL;DR the tree provides a ton of stuff that just an ocean doesn't, so expect it to be a hub of biodiversity.
1) Expect aquatic plants and animals everywhere, with some -very- large species
Many theories state that abiogenesis (life first starting from a mix of chemicals, minerals etc) occurred in the ocean, so it's reasonable to say that life could still develop in the ocean independent of the tree.
Expect some things like fish, plankton, etc - anything possible in our oceans. Perhaps these creatures can grow larger if the water is mineral-rich, as you said. - If a tree can get this large, there might be Cthulus and Krakens and all sorts of monstrosities supported by your ecosystem.
Additionally, if nutrients can support one continent tree, why can't they support a host of other massive, floating plants?
2) Below the tree, decomposers everywhere
If this tree is continent sized, there will be an extremely large dark area beneath it - that will stay dark for a very, very long time. It's possible that some species similar to the ones we see at the bottom of the ocean will develop here, but instead of feeding on energy from thermal vents, they will consume decaying tree matter. This will power its own ecosystem relying on dead tree matter, fish that feed on decomposers, etc.
3) "Land" animals abundant
Image is an artist's interpretation of an alien animal and may not reflect what is present on the tree
Just as life moved from the oceans to land on Earth, so too will some species transition to live on and in the tree. It must have began as an environment without predators, with plant life to eat as far as the eye could see; this would fuel evolution on land just like it has on Earth.
4) Burrowers abundant
If the tree is as thick as you describe, some of the inside may eventually decompose into soil. Some land animals may take advantage of the protection offered inside the tree, as well as the nutrients - expect things similar to worms, burrowing mice, etc.
5) A strong ecosystem is now possible
The problem with basing an ecosystem just off of the tree is that everything else will have the same nutrients to work with. For example, what if the tree doesn't take in the minerals required for strong bones, because it has other structures instead? Animals feeding off of it will not have the nutrients required for strong bones! This applies to almost all continental creatures - if they feed off of the tree, they will be severely limited nutrientwise, and will develop to be relatively weak.
Burrowers solve this problem. If they dig to the bottom of the tree and feed on decomposers, they bring a whole different set of nutrients into the cycle - and in turn, if carnivores feed on burrowers, a healthy ecosystem can be maintained.
Coastal areas will probably be the most diverse, as any animals can fish for food.
6) Diverse climate-adapted organisms are possible
If the tree is continent sized, areas may be at significantly different latitudes - meaning some parts of the tree are hotter, some are colder, etc. 445
This could produce conditions for species to adapt to - as long as the tree can survive, too - and lead to some variation.
7) More bladetrees (?)
One of the most important requirements for a living thing is that it reproduces. Only one tree won't work because unless it makes more of itself, it will go extinct upon death. A possible solution is that this is not "just one tree", but instead "all one tree". Let me explain.
Pando, one of the largest organisms in the world, happens to be a giant meshy tree! How convenient. It is not "one individual tree", but instead a network of the same tree, reproduced through new stems asexually. This applies to the bladetree. Technically, may or may not be "one tree" - it may have consistent genetic material, so it appears to be "one" but there may be multiple copies of the same thing woven together. If a part of the tree breaks off and attempts to sprout, making a second Bladetree, it can be consumed by fish.
8) Tree viruses and bacteria
Just as trees on Earth can be infected by viruses, bacteria and other small pests, so can the Bladetree. Large sections of it may decay due to this and fall off of the actual tree, recycling minerals into the ocean.
9) Birds
Are completely possible. They may evolve practically as they did on Earth, but they will stay within the boundaries of the continent for the most part 445