Plants and decomposers play an important role in freshwater ecosystems. They are the organisms that produce and recycle the organic matter used as food by other organisms.
Algae: tiny but important
Phytoplankton are algae that live in water. They are vital because they convert sunlight to organic matter through photosynthesis, and so provide the base of the food chain.
Algae are classified into several groups, often according to their colour – for example, green algae and red algae. (Cyanobacteria are often referred to as blue-green algae, but they have a primitive cell structure and are more like bacteria than true algae.) Some types of algae grow independently as single cells, while others form colonies or filaments. Some are fixed to river and lake beds, and others live suspended in the water.
Trouble comes in threes
Cyanobacteria (blue-green algae) live in fresh water. Some cyanobacteria produce powerful toxins that can cause sickness and death in people or animals exposed to them. Neurotoxins target the nervous system, hepatotoxins damage the liver, and dermatoxins irritate the skin.
Diatoms are single-celled algae enclosed in a hard shell. They live in thin layers on river and stream beds, and are an important food source for mayfly nymphs and many other grazing invertebrates. Not all algae support the food chain to the same extent. The unsightly filaments of green algae often seen in nutrient-rich streams are not very palatable and can affect water quality.
Most of New Zealand’s common algae also occur overseas. But it is possible that with better understanding, more species unique to New Zealand will be identified.
Larger plants: macrophytes
A wide array of larger plants also live in fresh water for at least part of their life cycle. These plants, known as macrophytes, include macroalgae, mosses, liverworts, ferns, and vascular plants (angiosperms).
Most are restricted to lakes or lowland rivers with relatively slow-flowing water and/or stable beds. But mosses and liverworts are often found in fast-flowing streams, although only in areas with a stable bed.
Macrophytes have a variety of growth forms. Some species such as reeds are rooted to the river or lake bed, but push up above the water. Others grow entirely beneath the surface. Some have been recorded at depths of 70 metres in very clear lakes. Several species such as duckweed (Lemna minor) and ferny azolla float on the surface.
Although macrophytes are common and abundant, only two types of invertebrates (animals without backbones) will actually eat the living plants – freshwater kōura (crayfish) and an aquatic moth (Nymphula nitens).
After they die, macrophytes begin to decompose and their nutrients become available for other plants and animals to use.
Bacteria and fungi: recyclers
Many people associate bacteria with disease. But most bacteria found in natural systems play a very important role, breaking down organic matter and recycling its nutrients – these are then available for other plants and animals to use. Although they are important, there has been a limited amount of study on their diversity. Only a fraction of those found in natural systems have been scientifically described.
Fungi break down wood and other larger pieces of organic matter that become food for many invertebrates. Over 600 different types of fungi occur in fresh water.