Freshwater algae

Algae are abundant in many freshwater habitats, especially in standing waters. The majority of species of algae in freshwater lochs are extremely small, though there are also algae which form larger growths or colonies. Microscopic algae may be free-floating or attached to the sediment, stones or higher plants. It is particularly impressive that such small plants, invisible to the human eye, can support entire ecosystems. They can do this as algal cells contain a variety of pigments with which they harvest light to drive the process of creating carbohydrates, which are then used as a source of energy. These tiny plants exist in a wide diversity of forms, some of which are described below, along with one of the groups of larger algae, the stoneworts.

Green algae

The green algae include many types, but stoneworts and desmids are of particular interest.

Desmids are remarkable because of their captivating forms and symmetrical cells, with many of these algae being symmetrical in more than one plane. The corners of the algae may be elongated, forming, typically, two or three 'arms' on each half cell. When looking at desmids, botanists originally thought they were looking at pairs of cells.

Stoneworts

Stoneworts, or charophytes, grow in freshwater habitats as well as salty lagoons. They are often the first large plants to colonise new or disturbed habitats. In freshwater lochs, where light penetration allows, they may form large carpets in deeper water, providing habitat for other species. Charophytes may also release chemicals which discourage the growth of certain microscopic algae, so helping to keep the water clear. Another advantage they have is that they produce oospores which can remain viable for a number of years. Oospores may be transported to new habitats by water movements, animals or birds and can lie dormant until conditions are suitable for growth.

A few types may be found in lochs with relatively low nutrient levels and comparatively soft water, but most species in the genus Chara are normally associated with harder waters and are also often found growing on sandy substrates such as machair lochs In the Western Isles.

Many of the UK's stoneworts are rare, so conservation of their habitats in Scotland is particularly important. Several of these occur in brackish water but least stonewort Nitella confervacea, which is associated with low to moderate nutrient levels, and slender stonewort Nitella gracilis, which is also associated with low nutrient waters, are found in freshwater lochs. Least stonewort is present in the Northern and Western Isles, Morar and Speyside, whilst slender stonewort survives in Ayrshire and Sutherland.

Blue-green algae

Also called cyanobacteria, as they are actually bacteria, rather than true plants. The contents of their cells are free within the cell, not packaged into structures, as they are in algae. Cyanobacteria have green, red and blue pigments, so look blue-green. There are species of cyanobacteria which can accumulate gas within their cells and they use this ability to move up the water column to better locations for collecting light and nutrients. Like certain terrestrial plants, there are species of cyanobacteria which can fix nitrogen. They have special cells called heterocysts, and the number of these cells increases when supplies of available nitrogen in the water are low. Blue-green algae have a bad reputation as they can form blooms and because certain species are toxic. However, they are normally present in many water bodies, in low numbers.

Yellow-brown algae

Diatoms are often associated particularly with spring, as at that time of year, there are nutrients in the water column and the water in lochs is being thoroughly mixed. The mixing is helpful to diatoms, as unlike many other algae, they have no active way of maintaining position in the light zone. Without mixing, they sink to deeper parts of the loch, where it is dark, so they are unable to survive. Diatoms are also often associated with shallow lochs for the same reason.

Diatoms exist in a variety of fascinating forms. Cells of the same species can come together to form attractive chains or star-shapes. Increasing their surface area in this way helps to slow the rate at which they sink. Diatoms are encased in silica shells called frustules, which remain in the environment long after the cells have died. When investigating the history of the ecology of a loch, cores of sediments can be taken and the different layers of sediment searched for these diatom remains. The lower the level at which the remains are found in the sediment, the further back in time conditions can be estimated.

Chrysophyte flagellates dominate the phytoplankton community in many of the lochs which have low levels of available nutrients. These algae may have one or two tails called flagellae, allowing them to maintain their position in the light zone, where they harvest light energy with their yellow pigments. Although they have these tails, chrysophyte flagellates may also be found in microscopic cellulose cups called lorica. In certain species these cups may come together to form colonies. These algae may also have scales made of silicon. Chrysophyte flagellates are suited to living in lochs with low concentrations of nutrients, as they can produce chemicals for breaking down complex substances into usable nutrients, but they can also engulf bacteria and small algae.

Dinoflagellates

Dinoflagellates generally have two flagellae, so like other flagellates can move around the water column to find optimal conditions. They are unusual in that one flagellum is directed backwards to allow forward propulsion, whilst the other lies across the cell and stabilises it during motion. Although there are 'naked' dinoflagellates, many in this algal group are distinctive because of their robust-looking 'armour plating'. Although the dinoflagellates tend to be associated with hard waters, the plates are made of polysaccarides, rather than calcium deposits. Dinoflagellates contain different pigments, including brown, for harvesting light.