On the course where they won the gold medal at last year’s International Rowing Federation (FISA) World Championships, Croatia’s men’s pair of Martin and Valent Sinković will be seeking a better start to the season than they had in 2018, where their early preparations were undermined by illness and injury.

They arrive in Bulgaria having dominated at the World Championships but they will face a challenge of a different order in Romania’s up-and-coming world silver medallists Marius-Vasile Cozmiuc and Ciprian Tudosa.

Meanwhile, the Sinković brothers’ compatriot, Damir Martin, has also got something to prove in Bulgaria.

Martin, who chased Mahe Drysdale home in taking the Olympic silver medal at Rio 2016, had a miserable season with injuries last year, missing the World Championships and only making the C final at the Europeans.

He will be seeking to re-establish himself at the top of his event.

Competition, meanwhile, in the new Olympic women’s boat class, the four, is getting stronger as teams begin their final preparations for Tokyo 2020 qualification.

This race seems to be acting as national selection for China and the United States, who have both entered multiple crews, some who are also competing in the women’s pair.

America's crews are packed full of talent, although there is only one remaining member of their 2018 World Champion women’s four.

Molly Bruggeman is joined by Olympic champion Meghan Musnicki, who is back after a two-year break following the gold medal at Rio 2016.

Musnicki also won a gold medal at London 2012.

The Netherlands and Romania are also expected to challenge strongly.

The US has entered four women’s pairs, who will also race in the women’s four.

A late withdrawal of the top crew of Tracy Eisser and Megan Kalmoe for medical reasons has meant a reshuffle and more opportunities for their team-mates.

Romania has three women’s pairs using combinations from their women’s eight.

However, last year’s European Champions, Madalina Beres and Iuliana Popa, are not among them.

Spain’s Aina Cid has a bronze medal from last year’s World Championships and is now paired up with new partner, Virginia Diaz Rivas.

The women’s single sculls sees the return of two rowing legends in Ekaterina Karsten from Belarus and Miroslava Knapková of the Czech Republic.

The 46-year-old Karsten is now back to the women’s single after competing last year in the women’s quadruple sculls and this marks the start of her 29th season of World Rowing events.

Knapková, the London 2012 champion, is working her way back after taking the 2017 season off to have a baby.

She finished well down at last year’s World Championships where she raced in her country’s women’s quad.

There are strong home hopes too for rising talent Desislava Georgieva, while China will be looking to Yan Jiang, part of the women’s quad that finished sixth at Rio 2016 and second at the 2017 World Rowing Championships.