We all love eau-de-pond-sludge! That black foul mud that builds up at the bottom of the pond is mostly decaying organic matter; faeces, feathers, uneaten food and rotting plant material.
The first we cannot control, unless we keep fewer birds, but with care we can reduce the others. In a planted enclosure or close to trees, there is plenty to be done. Leaves float for a short time, so can be scooped off the water surface. Leaves on the grass will leave bare patches if they are left to lie or may blow into the water.
Although it is tempting to scatter food to make sure everyone has plenty, anything not eaten will sink and add to that aroma.