Pollution.

At this time of the year large areas of the estuary shore become covered in dense mats of green algae. They might look pretty but they are unnatural and a major ecological problem.
They develop because of the high levels of nutrients in the river water. The nutrients are from two sources; agricultural fertilisers washed into the river from fertilised fields and the effluent from sewage treatment plants up river.
The mats are problematical because they smother the mudflats and their invertebrates  and mussel beds which are a major food resource for the numerous birds that feed on the estuary.