Algae

Some species of algae can occur in extreme environments. For example, species of green algae have been observed in hot-water springs at Yellowstone National Park, in highly acidic volcanic lakes, in the extremely saline Great Salt Lake and Dead Sea, and on the surfaces of glaciers and snow. Some algae even survive suspended in the atmosphere as spores or in droplets of moisture.

These are, however, extremely stressful environmental conditions. Most algae occur in less stressful habitats, where their productivity tends to be limited by the availability of nutrients (assuming that sufficient light is available to support the photosynthetic process). In general, the productivity of freshwater algae is primarily limited by the availability of the nutrient phosphate (PO₄^-3), while that of marine algae is limited by nitrate (NO₃-) or ammonium (NH₄+). Some algal species, however, may have unusual nutrient requirements, and their productivity may be limited by certain micronutrients, such as silica, in the case of diatoms.

The structure of algal communities may also be greatly influenced by ecological interactions, such as competition and herbivory. For example, when herbivorous sea urchins are abundant, they can sometimes over-graze species of kelps in subtidal ecosystems of the west coast of North America, degrading the kelp forests. However, where sea otters (Enhydra lutris) are abundant this does not happen because the otters are effective predators of the urchins.

Another example of a biological influence on the structure of an algal community concerns the zebra mussel (Dreissena polymorpha). This is a bivalve mollusk that has been accidentally introduced by ocean-going ships to the Great Lakes of North America, where it has become an important pest because it clogs water pipes with its prolific growths, and can displace native species by competitively appropriating hard-substrate habitats. More to the present point, however, the zebra mussel is such an effective filter-feeder on phytoplankton, that its large populations in parts of the Great Lakes are apparently responsible for some of the clarification of the water that has occurred in recent years. Grazing by the zebra mussel has actually resulted in decreased standing crops of phytoplankton, even in well-fertilized waters.