Cassells won the title with fellow Coleraine man Peter Chambers last year.
Chambers, meanwhile, helped the British four take silver behind defending world and European champions Switzerland.
Enniskillen's Holly Nixon and her GB team-mates were fifth in the quadruple sculls final with Alan Campbell third in the B single scull final.
London 2012 Olympic bronze medallist Campbell said afterwards that he had been suffering badly from hay fever over the weekend in Germany.
Cassells and Scrimgeour dominated the lightweight pair final and had over three seconds to spare from Denmark at the finish.
The lightweight pair is a non-Olympic boat class but Cassells and Scrimgeour will have reminded the Great Britain coaches of their selection claims for the lightweight four.
Chambers, Mark Aldred, Jonno Clegg and Chris Bartley led the fours final at the halfway stage before the Swiss took control to win by a length.
Enniskillen's Holly Nixon and her Great Britain crew mates Rosamund Bradbury, Jess Leyden and Kristina Stiller were going along well in fourth place before catching a crab just before the halfway mark which stalled the boat momentarily.
They eventually finished fifth behind winners Germany and will now finalise their preparations for the Olympic qualifying regatta in Lucerne in a fortnight's time.
There was success for Ireland in the men's lightweight double sculls when brothers Gary and Paul O'Donovan from County Cork rowed through leaders Norway in the closing 500m to clinch gold.
Reigning world champions France and silver medallists Great Britain - Coleraine's Richard Chambers and Will Fletcher - were missing from the field but that doesn't detract from a memorable moment for Irish rowing.
There was a second medal for Ireland in the women's single sculls when London Olympian Sanita Puspure took bronze behind scullers from Austria and Latvia.

Hamdi Alqudsi, 42, was found guilty of violating Australia's foreign incursion laws for making travel arrangements for a group of would-be jihadists in 2013.
Giving evidence for the first time at a sentence hearing, he expressed regret and said that he loved Australia.
Alqudsi also claimed he thought that IS would help civilians.
He faces a maximum 10 years in jail for seven counts of providing services with the intention of supporting hostile acts.
"As an Australian I should have minded my own business," Alqudsi told the New South Wales Supreme Court.
"I love Australia, I always have."
On Wednesday, Alqudsi admitted for the first time that he knew the seven men he assisted in 2013 planned to take up arms against Syrian government forces.
He said he should have limited his involvement to taking donations for Syrian civilians trough registered charity groups.
Alqudsi broke down in tears while giving evidence, saying he now realises he was breaking the law.
"At the time, I felt it was the right thing to do," he said.
"I am really, really sorry for what I have done."
Two of the seven men Alqudsi helped were killed in Syria. Two others have returned to Australia, another was arrested before he could leave the country, and the fate of two other men is unknown.
Alqudsi said his first impression of the Islamic State group was that they were a force for good.
The prosecutor put it to him that it was obvious the men intended to travel to Syria to fight.
"You knew these men were going to be out in the fields of Syria engaging in warfare," David Staehli SC said.
"Yes, I did," Alqudsi replied.
Coleraine rower Joel Cassells retained his European men's lightweight pair title as he and San Scrimgeour took gold for Britain in Brandenburg.
[NEXT_CONCEPT]
A Sydney man who helped seven young men travel from Australia to Syria to join the Islamic State conflict has told a court he is "not a terrorist".