By and large the answer is yes. But there are some variations to be included.
Algae and cyanobacteria are tiny organisms that occur naturally in saltwater and freshwater. Individual organisms can often only be seen under a microscope, although with some species, individuals can join together to form colonies visible to the naked eye. It is important to understand the similarities and differences between algae and cyanobacteria as both groups can have distinct impacts on surface water quality.
Algae is beneficial in the simplest manner because algae release oxygen as part of their metabolism, they serve to oxygenate the water. Green algae are preferred because blue-green algae float on the surface and are associated with water quality problems. Algae also play an important part in the aquatic food chain, as they are the main food source for zooplankton and small fish, which in turn serve as food for larger fish and other wildlife. To have more algae in the water means that more carbon dioxide is used from the atmosphere and that more oxygen is released into the atmosphere. Algae require warmth, sunlight, and nutrients to grow and reproduce. They are found in the upper 200-300 feet of ocean water. The upper layer of water, known as the epipelagic zone, is rich in oxygen, penetrated by sunlight, and warmer than water at lower levels.
Algae, specifically the type of algae that exists in a specific water source can also be beneficial in another manner. Algae are good indicators of the atrophic status of a water body, that is, the degree of pollution and nutrients in that water. A lake dominated by green algae and diatoms is relatively "clean" oligotrophic water, whereas dominance by bloom-forming blue-green algae indicates a more polluted or eutrophic condition, typically caused by the troublesome Cyanobacteria.
Cyanobacteria are members of a group known as eubacteria or true bacteria. For a long time they were not recognized as bacteria, more often being referred to as blue-green algae. All bacteria belong to a group of organisms known as prokaryotes, a Latin word meaning 'before nucleus'. Bacteria have no organized nucleus. Cyanobacteria are classified as bacteria, not algae, since their genetic material is not organized in a membrane-bound nucleus. Unlike other bacteria, they have chlorophyll and use the sun as an energy source. They are often referred to as 'blue-green' since the first cyanobacteria identified were bluish-green in color. However, not all members are in this color. Some are olive or dark green, and others are even purplish in color.
Cyanobacteria occur naturally in surface waters. Although their size is usually microscopic, when conditions are ideal, both can undergo a phenomenon known as bloom. This results when the algae reproduce rapidly and the individuals form clumps visible to the naked eye.
Heavy blooms can overtake water bodies, and even choke out portions of streams or rivers. It is difficult to predict when a bloom will occur. Some species bloom only in spring, others more frequently in the fall. These organisms can bloom in flowing or standing water. Blooms may even occur under the ice in the middle of winter. As seawater freezes, algae living in the water are frozen in the ice, where they later can be released during a thaw. These algae are a vital source of food for krill and shrimp-like organisms. Large, nuisance blooms commonly form following periods of hot, calm weather when the water is warm. They are also more likely to occur when water nutrient levels, and in particular phosphorus, are high. Heightened nutrient levels result when water bodies receive runoff or leaching from such sources as: fertilized fields, lawns, poorly managed manure, storm drain discharges, poorly contained septic systems, or soil and sediment transport in runoff water. These nutrients lead to blooms in coastal waters to a greater extent than in the open ocean. Algae blooms in the open ocean are not usually harmful. Instead, Algae blooms provide benefits, largely from the fact that the open ocean is relatively low in nutrients.
Large blooms of algae and cyanobacteria can clog intake pipes and filter lines, and are aesthetically unappealing. During these blooms, a liter of water can contain millions of algae! When blooms of algae or cyanobacteria die and decay, the dead cells often produce objectionable odors as a result of oxygen depletion in the surrounding water. When a bloom dies in a pond or shallow lake, severe oxygen depletion can even cause fish deaths. Algae do not produce substances that are toxic to humans or animals. In contrast, some cyanobacteria produce substances that are extremely toxic, and are capable of causing serious illness or even death if consumed. These substances are called cyanotoxins. There are currently over 70 different cyanotoxins, which are grouped by their method of toxicity. One cannot tell if a cyanobacterial bloom is producing toxins simply by looking at the bloom. Instead, you should assume toxins are present and avoid using the water.
So as algae is a very normal and naturally positive component of the aquatic arena, benefiting overall water quality, aquatic life and even humans, it can also pose many water quality issues if the wrong type of algae forms under the ideal conditions. In this case, the saying is true that too much of something good is not always for the best! Moderation is the key and the right balance of good is what makes for a stable and fruitful life. Find your balance!