Potential ecology of a magic hot spring?

Question

My fantasy world has an ecology hole.

• The northern areas are, on average, -5 degrees Celsius.
• Gravity is 90% that of Earth.
• Residual magic from tectonic phenomena is released as heat into the Earth.
• This energy travels to the surface through water and pools in hot springs that are notably warmer than their surroundings.
• Water temperature at the surface is about 25 degrees Celsius.
• Temperature increases slowly as you go down.
• Limited foliage and grasses around springs.
• They can be quite deep.
• Higher oxygen level than our world.
• The soil can support life.
• Springs are safe to drink from.
• Springs vary in size from pools to small lakes.
• There are no trees.
• Snowstorms are uncommon but expected.

Base ecology already has

1. Generic catfish
2. Copious amounts of seaweed
3. Large crab that feeds on seaweed
4. Large carnivorous turtles
5. Small fish that consume algae and insects
6. Insects that eat algae and seaweed
7. Some form of crustaceous scavenger

I am looking to diversify and improve the biosphere, particularly by adding insects and reptiles.

(Edits welcome, I'm new to this)

Answer 1

The biggest issue with populating your springs is whether the world was always the way it is now, or if at one time things were warmer. If your cold world was once more hospitable and the sort of life now found at the springs was once widespread, then your springs are refugia.

from http://cba.anu.edu.au/research/highlights/ecological-speciation-role-climatic-refugia

Refugia are sanctuary sites where creatures can survive and ride out widepread changes in conditions. One of my favorites are the tiny watering holes in the Sahara desert where Nile crocodiles live - these were stranded here as the world changes and the Sahara dried up. They are little and inbred but they survive.

from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0014734

That opens the most possibilities for a variety of creatures in your warm pools and ponds. Imagine what lived in your world when it was warm, and then roll time forward and imagine how a stranded population of these things would change over time.

If your world has always been cold with warm pools then your pools are more like deep hydrothermal sea vents: islands of life surrounded by inhospitable wastes.

from microbewiki.kenyon.edu

Some of these have ancient lineages of creatures specific only to that site. It turns out that some creatures that specialize in these sites have the ability to colonize other similar sites at a distance - probably by virtue of a planktonic form that by good luck winds up in the right place. I think the purely endemic evolved on site lineages are mostly slimy little things and microbes. Most of the big creatures at these sites are descendants of more widespread lineages (worms, crabs etc).

Since you already have catfish and crabs and seaweeds in your ponds I think you have by default opted for #1: the world was more hospitable once and now has condensed down to your warm pools refugia. In your mind, populate your world as it once was and then populate your springs with the degenerate descendants of the prior world. Some of these holdovers might have the ability to migrate across the wastes from from pool to pool. I think of polar bears which go from hole to hole in the ice, attacking and eating whales and other animals who are restricted to these holes to breathe.

https://www.natgeocreative.com/comp/04/496/1143099.jpg

Answer 2

Most hot spring life is bacterial due to the high temperatures. Animals can live comfortably under 40°C, some insects and crustaceans are comfortable up to 50°C and some plants and fungi survive up to 60°C. Above this temperature the only organisms that can survive the heat are some groups of bacteria and archaea.

As the water is only 25°C, most organisms you would find in underwater cave structures would be plausible... but underwater cave life hasn't been explored in any great detail. There are tens of thousands of underwater caves scattered around the world, but less than 5 percent of these have ever been explored and scientifically investigated.

Stygofauna are found in groundwater systems or aquifers, and are divided into three groups: stygophiles, stygoxenes, and stygobites, depending on how much time they spend underwater. Stygofauna of Earth include flatworms, gastropods, isopods, amphipods, decapods, fish and salamanders, and would likely get more alien the further down you go.

Large, carnivorous turtles sound like the apex predator in your underwater environment, unless the larger bodies of water could support enough food for any of these beauties. On the surface, however, a consistent watering hole that will likely never freeze over will be well guarded by any large fauna whose territory includes them. In addition, a primarily terrestrial animal would likely evolve the ability to dive and hunt underwater (I'd like to think this large carnivore would resemble a sub-arctic version of a polar bear).