Algae are a group of microscopic plants, which live in a variety of habitats including oceans, estuaries, rivers, streams, lakes, ponds, and even puddles. They are an important food source for fish, insects, and birds, and produce a large proportion of the earth’s oxygen.

Algal populations rise and fall in accordance with variables such as temperature, sunlight, and nutrient availability. Under optimum growth conditions, algae populations can increase rapidly and cause “blooms”. Persistent blooms can look unsightly, impede navigation, and reduce water quality. Blooms may "crash" when a large number of algal cells die off rapidly. This can result in a high rate of removal of oxygen from the water as the cells decompose.

There are many different types of algae, the three main types found in the Bay are diatoms, green algae, and blue-green algae.

Diatoms are algae. Most diatoms are unicellular, although some form chains or simple colonies. Most are free-floating (planktonic) whilst others attach themselves to surfaces. Because they are photosynthetic they are an important primary food source.  Whilst they contain chlorophyll they also often contain pigments which give rise to their characteristic brown or golden colour. Diatoms possess silica cell walls, a characteristic unique to this group of algae. These walls show a wide diversity in form but usually consist of two asymmetrical sides with a split between them giving rise to the group name. A Wales-wide survey of diatoms is currently being undertaken by the National Museums and Galleries of Wales. Cardiff Bay is one of the sample sites included in this study and the Cardiff Harbour Authority Environment Team assist Museum staff with the collection of samples from Cardiff Bay .

Green algae are also known as ‘Chlorophytes’. As their name suggests they are green in colour. Some types float alone whilst others join together to create colonies. Some form long filaments which can clump together into dense tangled mats clearly visible in the water body. It is believed that higher plants may have evolved from this group of algae as they exhibit many similar characteristics.

Blue-green algae are also known as ‘Cyanobacteria’ as they exhibit characteristics common to both algae and bacteria. Blue-green algae can produce toxins which may be released into the water when  algal cells die. Dying cells will rise to the surface and form a ‘scum’. The scum may look like paint, jelly or mousse and will vary in colour according to the species present.

Although very rare, there have been reports from other locations that these toxins have caused the death of dogs, farm livestock, and wild animals, after drinking substantial quantities of water containing blue-green algal scums.

Thankfully, no human deaths have been recorded but algal toxins have been known to cause skin rashes, vomiting, diarrhoea, fever, and muscle pain in people who have swallowed or swam through blue-green algal scums.

Whilst many blue-green algal species are capable of producing toxins they do not always do so. The conditions required for toxin production are not clearly understood hence it is not possible to predict whether a scum contains toxins.

A comprehensive monitoring programme has been developed in conjunction with Professor Codd of Dundee University, a world renowned expert on algae. Water samples are taken on a regular basis to monitor levels of different algal types including the blue-greens. An Alert Level Framework System is in place which outlines actions to be taken if blue-green algae numbers begin to cause concern. If blue-green algal populations are high, warning signs will be placed around the Bay. If blue-green populations continue to increase, Bay users will be formally notified and restrictions may be imposed on certain activities within the Bay.

Should problematic scums form, Cardiff Harbour Authority takes samples to test for the presence of toxins and removes the scum from the water surface using specialist equipment. The resulting waste is disposed of in accordance with current legislation.

Zooplankton are a group of microscopic animals which live in open water. They are free-floating or weakly-swimming planktonic organisms  incapable of photosynthesis. They are an important link in the aquatic food chain. Zooplankton feed on microscopic plants and bacteria, and in turn, are eaten by larger animals including fish. The Harbour Authority is funding a 3 year Phd which will study the spatial and temporal distribution of zooplankton in the Bay. Although zooplankton populations have been studied quite extensively, few studies have been undertaken in newly created freshwater environments such as Cardiff Bay. To date, results have shown there to be a very diverse zooplankton community in the Bay, with 47 different species identified so far.

Zooplankton are particularly important in the economy and function of lakes, for example by grazing on algae and in transferring energy to predators such as fishes. Around forty different species of crustacean zooplankton have been found in Cardiff Bay to date through fortnightly surveys over an 18 month period.  This is a large number of species for a lake of this size. Several species of the water flea, Daphnia, are particularly common. Initial findings indicate there is a strong seasonal influence on the zooplankton population, with the highest densities occurring in spring and summer and much lower densities during winter.

Populations throughout the main body of the Bay appear to be very similar, both in the number of different species and the density of individuals at any particular site. This could be due to the well-mixed nature of the Bay, with water quality varying very little between sites. Fortnightly surveys are on-going, whilst larger-scale surveys are planned for the future in order to further investigate any spatial patterns.