Lakes are home to waterfowl, fish and smaller invertebrates that live in fresh water or burrow in mud on the lake floor. Plant life is varied, including rooted and floating ferns, and flowering plants.
The food chain
Most lake life feeds on phytoplankton. These tiny floating organisms are mostly types of algae, diatoms and cyanobacteria that grow in the sunlit upper layers, where they make food by photosynthesis.
Phytoplankton are eaten by zooplankton (including minute rotifers, water fleas and copepods) as well as juvenile forms of larger animals. It is the beginning of a food chain leading to crustaceans, fish and waterfowl.
Many plants are rooted near the lake shore. If there is adequate shelter, they grow like verdant underwater gardens. Some rooted and floating plants can survive at considerable depths, depending on the amount of light that filters through. The deepest known plants are a few species of bryophyte which have been found at depths of more than 70 metres in the clear waters of Lake Colerdige in Canterbury.
Shellfish and insects
The invertebrates (animals without backbones) that live in lakes are those that prefer water with little movement. The most obvious are freshwater crayfish or kōura, which grow larger than those in rivers. A variety of insects live in or on the water, including caddisflies, stoneflies, midges, mosquitoes and pondskaters.
Lakes and streams contain native fish (galaxiids), known to Māori as īnanga, kōaro and kōkopu, which do their best to stay hidden. The fish most likely to be seen are rainbow and brown trout, first introduced to New Zealand in the late 19th century. Both species of trout have thrived in the cool clear waters of most lakes, sometimes decimating galaxiid populations, but co-existing with other fish such as eels.
Red-billed and black-billed gulls have nesting colonies around Lake Rotorua. They often gather around the lake’s thermal areas, enjoying the warmth in winter. You can spot a Rotorua gull that has lived there a long time, because the webs of its feet have become corroded by the warm, acid waters.
The birdlife around lakes includes many birds found in other wetlands such as swamps and estuaries. Native birds that are only found in or near ponds and lakes include grey ducks and grey teal, and skilled divers such as the scaup, New Zealand dabchick (North Island) and crested grebe (South Island).
The larger birds are mostly introduced species, including mallard ducks, black swans and Canada geese. Other birds include the Australian coot and different species of shag.
It can be difficult to identify waterfowl at a distance. As a rule of thumb, ducks tend to favour open water whereas dabchicks, coots and grebes prefer the vegetation on lake margins.
Human impact: nutrient overload
Plant growth requires nutrients, especially soluble forms of nitrogen and phosphorus. These are naturally washed into lakes by streams and groundwater flow, but have greatly increased because of the impact of human activity, especially:
- runoff from fertilised farmland
- sewage and septic tank effluent (treated and untreated)
- eroded soil from cleared land.
If the nutrient level is too high, there will be rapid growth of lake weed and algae, including toxic algal blooms. The decay of this excess plant material uses up the available oxygen in the water, and fish and other animals can die.
Ultimately a lake may become anoxic – lacking in oxygen – and effectively dead. This has been happening to many of the lakes near Rotorua, as well as lowland lakes such as Te Waihora (Lake Ellesmere) in Canterbury, since the 1950s.
Some exotic lake weeds, accidentally introduced, have thrived in lakes with unnaturally high nutrient levels. Piles of rotting African oxygen weed (Lagarosiphon major) around the margins of Lake Rotorua in the late 1950s were the first sign of nutrient enrichment that was obvious to the general public.