How to keep ducklings alive

Hi all

Just joined because i would like some tips and advice on what I can do to help.

A bit of background I moved onto this property in Jan 2022 and in the back there is a lake that has several mallard ducks and moor hens living it and some fish below the surface.

The lake is protected and maintained by the service company every 6 months.

The first summer I had bunch of ducklings from the mallards and the moorhens who all survived infancy and grew up just fine.

Every year since the moorhens ducklings are fine but there's 2 batches of mallard ducklings that hatch but only seem to live a few days. There's usually at least 7 and then they seem to disappear and reduce in number over the next few days. 

For example 7 ducklings hatched last week, and now from what I can tell there is only 2 left alive. I'm yet to see any remains from any around.

They don't seem to be leaving the lake and just remain in the water 

I was wondering if anyone could identify what the possible cause of death is and what I can do to keep them alive for longer.

There is a heron who is around the lake occasionally but I'm yet to see it eat any babies, foxes wouldn't go into the water to get the ducklings would they?

I've included a picture of the lake, i think the eggs are laid on the island on the right side but the ducklings don't live there after hatching but can't be certain.

 Thanks again