Algae are a diverse set of organisms that can be found just about anywhere around the globe, from the artic circle to the underwater volcanic vents. These organisms conduct photosynthesis but do not have roots or leaves. Rather they are collections of cells that remain individual or connect together into colonies that can form mats or even resemble plants. Though unsightly at times, algae are an essential part of the food web and contribute significantly to dissolved oxygen in the water column.
Algae populations are driven, largely, by the amount of nitrogen and phosphorus in the water. But depending on what species of algae is present, green water can be a sign of a healthy and productive aquatic ecosystem.
Though this is a major oversimplification, we will primarily classify algae into four groups: (1) Unicellular (green), (2) filamentous, (3) blue-green, and (4) charophytes (plant-like).
Unicellular, green algae grow suspended throughout the water column and can indicate high levels of nitrogen and phosphorus, and high biological productivity. These are unicellular and don’t have any noticeable structure (unless you have a microscope), but will cause the water to take on a greenish or gritty appearance. These nutrient-rich environments are not necessarily bad as they will support a wide variety of wildlife, from plankton to invertebrates, to fish. However, the overgrowth of green algae can prevent light penetration through the water that is needed to support submersed plants. If no plants are present, as the algae populations cycle through the season, dissolved oxygen can range from high to low in a matter of days. This rapid cycling of oxygen can be troublesome to the aquatic ecosystem and fish kills may result.
Filamentous algae (Cladophora, Pithophora, and Spirogyra spp., and others) often form dense floating mats that can range from green to brown, to red. These mats can be thick and have hair-like quality to them. Generally speaking, these form on the bottom of relatively shallow or clear lakes that have sufficient light penetration to the bottom to support growth. Gases formed during photosynthesis get trapped in the mats, causing them to float to the surface. These algae can be particularly troublesome to desirable submersed plants as the algae cover them over and compete for sunlight. They are also unsightly and complicate fishing, swimming, and other recreation. Particularly as the older mats begin to rot the smell can be unpleasant, further adding to the nuisance of these algae.
Blue-green algae is a cyanobacteria that develops a recognizable blue-ish hue when blooming. If the bloom persists, the cells can begin to clump together and develop a noxious odor. Of primary concern, the blue-green algae microsystis can produce a powerful toxin (microsystin) that can be harmful to pets and humans. These algae are often managed in drinking water reservoirs as some of the compounds they produce have a bad taste at very small levels, in the parts-per-trillion range.
Charophytes, or plant-like algae, form very structured colonies that will appear rooted in the sediment with branch-like structures, very much looking like a plant. These are still algae; they are just very structured in their colonies. Kelp, a marine algae, looks very much like a plant but is a clonal algae. In freshwater some of these algae can grow to nuisance levels, impeding navigation and swimming. One example, chara algae, will often smell like garlic or onions and has an abrasive texture that can impede recreation.
In general, depending on the species, algae can:
Management of algae can be quite difficult and nuanced. Since the blooms are fueled by high nutrients, failing to reduce nutrient concentrations will not ultimately remedy the problem. Therefore, the best way to manage unwanted algae blooms is to have a two-pronged strategy: 1. Ensure a healthy submersed plant population to compete for these nutrients, and 2. be cautious about adding unneeded nitrogen and phosphorus. Regularly feeding, or over-feeding, fish in a small pond can add significant nutrients that ultimately result in continual blooms.
Certain pond dyes can be used to shade the water and prevent light penetration through the water column. Less light can result in slower algae growth and reproduction. Another option is to add aeration. Ponds/lakes can often stratify, with low oxygen conditions near the bottom. Aeration will stir the water and result in more consistent oxygenation throughout the water column, leading to more activity/reproduction of oxygen-loving bacteria. The bacteria will compete with the algae for nutrients and potentially result in lower algae density.
Harvesters can be used to remove floating mats. Unfortunately, this does little to prevent future blooms since the mats form on the lake bottom before floating to the top.
There are fish species that consume algae, but many of these fish are invasive in Florida. Please speak with a professional before adding a non-native fish to any water.
There are a wide variety of algaecides that are toxic to algae. There are also chemicals that reduce algae growth by binding nutrients and making them unavailable. If this level of algae management is desired, it is best to speak with a lake/pond management professional since these decisions are nuanced and need to be made with great care and consideration.