Several microscopic organisms cause disease in humans, animals or both. For example, campylobacter, giardia, cryptosporidium and viruses can spread disease via water. They may be picked up through drinking, from recreation such as swimming or boating, or by eating contaminated fish and shellfish.
Diseases range from mild gastroenteric disorders to life-threatening conditions. Following international practice, New Zealand tests the level of Escherichia coli (E. coli) as an indicator of recent faecal contamination in fresh water, and E. enterococci in coastal waters. Water where people swim is monitored to ensure the levels of these ‘indicator’ bacteria comply with legislated standards.
Some of the worst damage to water quality in New Zealand rivers and lakes comes from excessive growths of algae (microscopic plants including diatoms, green algae and cyanobacteria). These algal blooms are usually caused by increased nutrients, but they also need certain conditions, including warm, calm weather. They can reduce water clarity in lakes, and smother river beds, especially in summer.
Rock snot and lake snot
Didymo or ‘rock snot’ (Didymosphenia geminata) has spread throughout several formerly pristine South Island rivers. It forms large blooms on the bottom of rivers and occasionally in lakes, clogging irrigation equipment and covering gravel-bottomed irrigation races, and damaging some recreation and fisheries sites. Slimy 'lake snot' (Lindavia intermedia) has also been found in several South Island lakes.
Milk – and manure
New Zealand’s dairy herds usually graze on pasture for much of the day. They are confined only for milking – 1.5 to 3 hours per day on tracks and in yards. The effluent from this short time comprises up to 12% of the total daily amount of manure. A 400-cow herd produces 132 kilograms of manure during a two-hour stint in the yard.
Water weed, water net and oxygen weed are problem plants. Some are rooted in the lake or river bed, others are floating mats that affect the water flow in drains and small streams, and may also block the intake of hydro station turbines. But they do provide food and habitat for some invertebrates and fish.
These plants release oxygen through photosynthesis in daylight, but their respiration in darkness can deplete oxygen in the water. Their eventual breakdown releases nutrients, raising the levels above normal, and may further reduce oxygen in some waterways.
Hornwort (Ceratophyllum demersum) is New Zealand’s most problematic water weed. It can grow up to 10 metres tall, submerged in still water, and can smother other aquatic plants. It is widespread in the North Island, but is currently found only at Timaru in the South Island.