Ecological Roles of Algae
First of all, algae are not plants (in fact they are technically in different kingdoms of life on earth). Algae and aquatic plants share, however, a similar role in pond ecosystems as primary producers. That is, they are green (resulting from an abundance of chlorophyll-a), and their photosynthesis produces oxygen, energy and nutrition used by a wide variety of other pond residents. In general, the greater the diversity of the types of algae ( phytoplankton, metaphyton and periphyton) and plants (often termed macrophytes), the greater will be the biodiversity of animals that depend on them. A typical food web is shown below.
The (usually) microscopic cells that inhabit the water column of ponds are termed the phytoplankton. They are individually barely visible to the naked eye. In "eutrophic" ponds with high nutrients, phytoplankton cells may become so abundant that the water color turns a murky green. Light is rapidly intercepted within the water column, reducing the growth of primary producers below. Phytoplankton cells come in all shapes and sizes. Although their nutritional quality varies, phytoplankton species are the principal food for zooplankton, which in turn serve as food for some kinds of fish and invertebrate predators. The zooplankton are collectively a highly diverse assemblage of species, barely visible to the naked eye. Bigger species such as the cladoceran Daphnia shown below are especially capable of reducing algal abundance, producing "clear water" phases in ponds at certain times of the year when they become especially abundant.
In marked contrast, the periphyton is an assemblage of algae adapted for life on the bottom of ponds. They too vary in their food value, but are widely consumed by aquatic insects, snails and other benthic invertebrates which, like the zooplankton, are consumed by fish and invertebrate predators. Benthic algae form carpets of cells on the sediments, as well as rock and plant surfaces. Like the phytoplankton they need light for photosynthesis, and may be at a disadvantage compared to the phytoplankton in ponds with poor light penetration.
Given high light and abundant nutrients, long hair-like filaments take over the benthic algal community. Photosynthesis by these filaments traps oxygen bubbles, causing them to rise to the surface, forming floating scums that slowly decay over time. Unlike the phytoplankton, and the benthic algae from which they arose, floating scums of metaphyton are unsightly, provide little nutritional value to most consumers, and their appearance frequently causes pond owners to consider alternative management options.