You can't calculate the amount of rain a freshwater ecosystem gets in a year because it varies wildly depending on the specific location and ecosystem.

Here's why:

* Freshwater ecosystems are diverse: They include everything from small ponds and streams to vast lakes and rivers. Each has its own unique climate and rainfall patterns.

* Rainfall varies greatly: Even within a single region, rainfall can differ significantly depending on elevation, proximity to bodies of water, and other factors.

* No single average: There's no global average rainfall for freshwater ecosystems. You'd need to specify a particular location or type of ecosystem to estimate rainfall.

To get a more specific answer, you would need to:

1. Specify the freshwater ecosystem you're interested in: For example, a specific lake, river, or type of wetland.

2. Find data on average annual rainfall for that location: This could be found through local meteorological records, scientific studies, or online databases.

Let me know if you have a particular freshwater ecosystem in mind, and I can help you find some relevant rainfall data.