Why do some particle systems not show up in the depth buffer?

I’m doing some compositing stuff with particles which relies on the depth buffer. I’ve noticed that for some reason there are certain particle systems that don’t appear in the depth buffer.

Here is an example, with two particle systems, shown in both the final colour and depth buffer (scaled for visibility):

As you can see, the system on the right shows up in depth, but not the one on the left. It has Rendering->Render in Depth Pass set to true, and occludes correctly with other objects, so I just don’t understand why it’s not showing up in the depth buffer.

Anyone know what’s going on here?

Is the one on the left using translucent materials? I don’t believe translucent objects appear in the depth buffer since everything behind them must also be rendered.

The water is translucent, which doesnt write to depth.

Aaaaaah, you’re both right, yes. No depth from translucency - thanks!