Others may have better suggestions, but the only way I know to do it is through a general contact e-mail address for the park, which I'm sure is suboptimal.  If you go to 

https://www.algonquinpark.on.ca/news/algonquin_park_advisories.php

to see the advisories, at the bottom of the map there is a "Report a Problem in Algonquin Park" button.  That takes you to a screen where you have your choice of
1) "Submit an Algonquin Park Inquiry"
2) "Ontario Parks Reservation Service"
3) Contact The Friends of Algonquin Park"

I would go with Submit an Algonquin Park Inquiry.  Even though it tells you it is not monitored 24/7, that's probably the way to go.

In the past, when I've left the park with news like that, I've stopped by the nearest access point building to report it.  I don't know for sure that problems get addressed any faster that way, but the access point is a phone call away from whomever can help, and I always feel like it is less likely to get lost in the noise than with an e-mail.