With autumn now in full swing it is likely the amount debris entering your pond has increased drastically in the form of fallen leaves. If these leaves are left to sink to the bottom of the pond they will decompose, eventually turning to sludge.
Initially this may not seem a massive issue with the debris only giving a pond a slight brown tint. However, the decomposed leaves will release rich micronutrients, acting as a catalyst for unwanted algae growth. Therefore if leaves are not removed successfully you could find excess growth of unwanted algae and blanket weed the following spring.
It is important that you regularly empty your skimmer basket to ensure it functions optimally this autumn, extracting fallen leaves and other debris and preventing a nutrient imbalance in your pond. If you are not fortunate enough to have an ecosystem pond with a skimmer we suggest you net the surface water of your pond regularly, although more labour intensive this will also help prevent leaf decomposition in your pond.