red-tinged and floating, deceptively tough

Size
Length: 1–2 cm
Lifespan
1–2 years
Diet
Floating aquatic fern that grows on still or slow-moving water surfaces. Requires still or slow-moving water with high nutrient levels. Tolerates a wide range of water conditions including slightly polluted water. Forms dense mats that cover entire ponds in summer, turning the water surface green.
Habitat
The water fern floats on the still surfaces of ponds, slow-moving streams, swamps, and irrigation channels throughout New Zealand. From Northland to Stewart Island. It is a fern of the water, a tiny plant that carpets the surface in summer, turning still water green. It does not stand tall. It drifts.
Range
Found throughout the North and South Islands, Stewart Island, and the Chatham Islands on still or slow-moving water. Most common in ponds, slow streams, swamps, and irrigation channels from sea level to 600 metres, from Northland to Southland. Also found in Australia and the Pacific.
Endemism
Native
Main Threats
None significant as this species is abundant and secure throughout its range. It is one of the most common floating plants in New Zealand, particularly abundant in summer when it can cover entire ponds. It can become a nuisance in farm dams and irrigation channels where it blocks water flow.
Population
Abundant and secure throughout its range. The water fern is one of the most common floating plants in New Zealand, particularly in still or slow-moving water. It can form dense mats that cover entire ponds in summer. No conservation concerns. It is not rare. It is just very good at making more of itself.
Conservation Status
Not Threatened
You have to get low to appreciate the water fern. Kneel by the edge of a pond. Look at the green film on the surface. At first glance, it looks like duckweed. A green scum. Nothing special. Then you look closer. Each tiny plant is a fern. A real fern. It has branching stems, overlapping leaves, and roots that dangle in the water. It is a miniature forest floating on the surface. Each plant is no bigger than your fingernail. The scale is deceptive. Size and colour make it special. The water fern is tiny. Each plant is one to two centimetres across. It grows in colonies. Hundreds of plants per square metre form a green carpet on the water. In summer, when conditions are right, it can double its population in a matter of days. And it changes colour. When the weather turns cold, when nutrients run low, or when the fern is stressed, it turns red. The leaves produce a pigment that protects them from harsh conditions. This turns the water surface from green to rusty red. It is the fern equivalent of a bad mood. Visible from across the pond. It floats. The water fern does not need soil. It does not need a trunk. It pulls its nutrients from the water and its energy from the sun. Its roots dangle in the water, absorbing dissolved minerals. Its leaves float on the surface, catching the light. It also has a superpower. The water fern hosts a symbiotic cyanobacterium, Anabaena azollae. This lives in cavities on its leaves. The bacterium fixes nitrogen from the air. It converts it into a form that the fern can use. In return, the fern gives the bacterium a home. This partnership allows the water fern to thrive in water that is poor in nitrogen. Which is most still water. The symbiosis is efficient. Reproduction occurs rapidly. In summer, it spreads by fragmentation. Pieces break off and grow into new plants. It also reproduces by spores. These produce tiny structures that sink to the bottom. They overwinter there, waiting for spring. The cycle is resilient. It survives the cold. It returns with the warmth. In a world of towering tree ferns and dramatic king ferns, the water fern is the small one. It is the fern you step over without noticing. The green scum on the pond. The plant that turns red when life gets hard. But it is also a survivor. It has been floating on still water for millions of years. Through ice ages and droughts and floods. It does not need to be tall. It does not need to be famous. It just needs a bit of still water. A bit of sun. And the patience to turn red when things get tough. No one told it otherwise.