Orange debris in drainage ditch

Orange debris in drainage ditch

Observation - Orange debris in drainage ditch - UK and Ireland. Description: There is a spring-fed drainage ditch running though my field which by the early summer contains large amounts of flocculent orange debris carried along by the flow. In the early parts of the year normal green algae will grow in this section of the ditch a

There is a spring-fed drainage ditch running though my field which by the early summer contains large amounts of flocculent orange debris carried along by the flow. In the early parts of the year normal green algae will grow in this section of the ditch and upstream the ditch is more normal well-stocked with a variety of plants and insect larvae etc. (producing plenty of damsel/dragon flies frogs and newts). I presume that the debris is dead algae
My query is why ?
This section of the ditch is somewhat sheltered and after the trees to the north and west get into leaf there is no direct sunlight after midday - but it does get some sun as can be seen in one of the photographs. Otherwise the stream seem healthy and grass etc. grows in it and where it emerges from under the trees there is much more normal growth: reeds and watercress. However it does receive some inflow from the adjacent fields which do get a significant number of ducks and geese in a sizeable pond and the inflow to the ditch does take much of the overflow from the ducks' water so may be quite nutrient-rich.