"My time at the Phoenix has been the highlight of my life; a fairytale that it was hard to believe was happening.
"I've had a few injuries lately and as you get older it becomes harder to shake them off.
"It has nothing to do with money; it is a very personal thing and it was difficult to tell Ernie what I wanted to do.
"I only went to the Phoenix for a year and ended up at the club for three years loving every minute of it and making some great friends.
"I wanted to finish with good memories and Ernie understood that."
Merrick said it was "a big surprise" when Riera said he wanted to finish so he "could do a lot of other things" and that he asked him to take some time to think it over.
"He did that but is convinced it is the right decision for him," the coach said.
"I have to respect what he has decided because he has been fantastic to have at the club.
"You can see why he was a paramedic because he wants to help people. Everyone liked him and he had time for everyone."
Riera played 68 games for the Phoenix, after originally being spotted while playing for Auckland City in the ASB Premiership.
"Albert was a pleasure to coach and I am so pleased we had that friendly game at Auckland City where I first saw him," said Merrick.
"He slotted straight in even though he was a 29-year-old and was our player of the year.
"You really have to respect and admire what he did late in his career."
Riera's absence will create a new overseas slot, with Merrick now needing an extra defensive reinforcement after the earlier departures of Ben Sigmund and Manny Muscat.