Thick mats of Zygnema filaments may indicate that a body of water is enriched with nitrates and phosphates, an attribute often caused by run-off from farmlands. Water chemistry can be greatly affected by a large presence of the algae and sometimes causes serious problems for other species inhabiting the same area. The concern may be greatest during the winter months in temperate regions when algal mats die and decompose, producing a substantial biological oxygen demand caused by respiring bacteria feeding on the decaying organic material. Intense competition for limited amounts of dissolved oxygen between the bacteria and other aquatic organisms may result in a phenomenon known as winterkill, the sudden death of large populations of fish.