Compared with many other countries, New Zealand is blessed with plenty of fresh water and a wide range of freshwater habitats. The wild, clear rivers and large scenic lakes attract visitors from all over the world. These freshwater habitats support a diverse range of life, much of which is not obvious to the casual observer.
Life in the water flow of rivers and streams is challenging, and the organisms that flourish there have specific adaptations for dealing with the flow. For example, many insects that live in rivers have flattened bodies so they can seek shelter in the layer of slow-moving water near the riverbed. Other organisms have special suckers or hooks that attach them to the bottom, while some caddisflies have ‘ballast’ stones in their outer casing, making them less likely to be swept downstream.
Life in flowing water also has advantages. Aquatic insects use the flow to drift downstream and find new places to forage. Drift-feeding fish can sit in one spot and catch these tasty morsels as they go past. Some aquatic insects use a similar strategy, building nets in the current to filter food from the water as it passes downstream.
Because of New Zealand’s variable climate, the rivers tend to have very changeable flows. Organisms have to cope with large floods, and with long periods of low or intermittent flow.
In lakes, life is concentrated near the edges, making the most of the sunlight reaching the lake bed in the shallows. Aquatic plants often grow profusely, providing cover and habitat for other species further up the food chain.
In the middle of a lake, the community is normally fuelled by phytoplankton (floating microscopic plants) that live near the surface, where there is sufficient light for photosynthesis. Larger microscopic animals called zooplankton graze on these tiny plants. To stop them sinking below the productive sunlit zone, they have various features or behaviours: many can swim, while others use air bubbles for buoyancy, or have spines and other outgrowths to slow their sinking rate.
In the darkness near the bottom of deep lakes, there is a more limited range of organisms. Life depends on organic matter raining down from above. In this zone, low oxygen levels are common in lakes that are high in nutrients, making life even more challenging.
Wetlands and groundwater
When people think of freshwater habitats, they mostly think of rivers and lakes. But wetlands and the species found there are also reliant on fresh water. A distinct range of organisms can also be found underground. For example, tiny crustaceans known as amphipods live in alluvial groundwater beneath the Canterbury Plains.