The green leaves are Asplenium scolopendrium, or Hart’s-tongue fern. The hart in question being a deer, I would think. The rest of the leaves are decaying Gunnera leaves that the gardeners cut down each year.
Here are more Gunnera leaves, that I photographed at the end of last November. You can see how in the first photo, that I took a few days ago, the thick stems have changed colour to purple. And the purple colour is leaching from the leaves into the water.
Photograph Works 