Floating pennywort is a highly invasive, non-native species that is choking up the river Cam once again. It doesn’t particularly need excess nitrates or heat to grow like this, just plenty of sunlight and relatively still waters.
In 2012, the Ely Standard reported how the Environment Agency had acquired a floating weed harvester and had pulled out four tonnes from Ely south to Bottisham Lock, Waterbeach. From my photo taken a few days ago, it looks like Hydrocotyle ranunculoides is causing problems on the river once again. The lock itself was choked up, but as you can see from the photo the weed is growing from both banks of the river. (This is downstream from the lock wear and not navigable anyway.
The trouble with this invasive American species of water weed is that it chokes up waterways and bodies of water depriving fish and invertebrates of oxygen, it blocks drainage and filtering systems and crowds out native water plants.