Fill in the lakes

sed -r, 27 24 (27 with -r)

24 (27):

:;s/(#|@) ( *#)/\1@\2/;t

27 (30):

:e;s/([#@]) ( *#)/\1@\2/;te

Competes with the better of the two perl solutions

Perl, 25

s/# +#/$_=$&;y| |@|;$_/ge

Perl (>= v5.9.5), 24 chars

Run with perl -p:

1while s/#.*\K (?=\S)/@/

This requires Perl 5.9.5 or later to use the special escape \K.