Phytoplankton is composed mainly of microscopic free floating or suspended algae and is generally referred to as greenwater by aquarium hobbyists. The word phytoplankton is derived from the Greek language (phyto = plant; plankton = wanderer). It is a term used to describe plants that are so small that their movement is primarily controlled by the motion of the water. These plants called algae (alga = singular) include a number of microscopic, single and multiple cell forms. In freshwater, large numbers of phytoplankton often occur in lakes, rivers and ponds. Various phytoplankton species bloom in response to certain conditions such as changes in temperature, photoperiod, light intensity and nutrients.
The majority of phytoplankton is made up of holoplankton, organisms that spend most of their life cycle in the planktonic community. However, many phytoplankton species are capable of producing resting spores, which can to be found in deeper water or in the bottom sediment of freshwater habitats. These 'resting stage' spores are generally what cause the algal blooms that we often see in freshwater environments. Phytoplankton is usually rich in green algae. However, it also includes diatoms, flagellates and blue-green algae. Although, it is generally accepted that blue-green algae are actually bacteria and not algae.
The diet of most rainbowfish larvae consists of a wide diversity of phytoplankton and zooplankton organisms that are found in great abundance in their natural habitats. This abundance and diversity of food organisms of different sizes and nutritional composition usually provide all the dietary requirements of the feeding larvae. Phytoplankton contains proteins, starches, fatty acids and oils and forms the base of the food chain in aquatic environments. As such, it is an excellent primary food source for newly-hatched rainbowfish larvae.
Although several alternatives for feeding rainbowfish larvae exist, such as commercially available fine powder-based feeds, live greenwater is still the best and the preferred food source. Most phytoplankton is very small, often-minute organisms ranging in size from 2 to 100 µm and stay suspended in the water column. Phytoplankton also serves as food for zooplankton, which in turn are fed upon by the fish larvae. Besides, rearing rainbowfish larvae using the "greenwater technique" directly in the larval tanks is believed to play a role in stabilising the water quality. It would appear that phytoplankton can use up a lot of nutrients, and, instead of being over supplied with nutrients; an aquarium with greenwater is more often low in nutrients, at least in nitrogenous substances.
Phytoplankton species can vary significantly in their nutritional value, and this may also change under different growth conditions. Most phytoplankton contain 20~25% protein, 5~30% carbohydrate and 5~25% lipid. Phytoplankton is rich in essential amino acids and polyunsaturated fatty acids. Phytoplankton also provides vital bio-pigments that are required for the development of improved colouration for captive bred rainbowfishes, and can also be a rich source of ascorbic acid (0.11~1.62% of dry weight).
Cultivation of phytoplankton for feeding rainbowfish larvae is very simple. All that is required is water, light and nutrients. Natural sunlight is best, if not mandatory, as without strong light the phytoplankton will simply not grow. Phytoplankton use energy from the sun to make their own food through photosynthesis. In the process of photosynthesis, plants use carbon dioxide, water, nutrients, and sunlight to produce oxygen, sugar and energy. The sugar molecules then combine to form starch and cellulose, energy-rich organic molecules that are food for the plants. One of the critical chemicals for photosynthesis is chlorophyll, which is bright green.
Phytoplankton does not appear overnight, it may take a week or two to appear. However, once you have a culture established, they will bloom much quicker when you use some old greenwater to start a new culture, often within 3 days. Under favourable conditions, phytoplankton grows continuously by a process known as cell division. Each cell enlarges and divides into two daughter cells that subsequently grow and divide yielding a culture that increases exponentially (e.g., 8, 16, 32, 64...etc.). Growth slows as the algal population becomes more crowded. Nutrients are depleted, metabolites build, and light penetration decreases because of self-shading. The culture will then go into a stationary phase for the current conditions and will not increase in density. Phytoplankton contain their best nutritional value when their growth is still within the "exponential growth" phase.
Perhaps the simplest way to culture greenwater is just to place some old aquarium water (that hasn't been chemically treated) from one of your waterchanges into a suitable container and store it outdoors. When it turns green you can then collect a portion of the culture each day to feed to your fishes. Outdoor cultures should be protected from heavy rain and screened to prevent entry of predacious aquatic insects. Filamentous algae and predators of fish larvae can be troublesome in outdoor cultures.
One of the problems with outdoor cultures is that you have no control over the sunlight. Generally, the more light the greater the greenwater density. While phytoplankton will grow all year round in one region, they may not grow in all regions. Their light requirement remains the same even when the days are shorter and the air temperature is colder. Another requirement for successfully growing phytoplankton outdoors is the shape, size and colour of the culture container. Deep containers need more light than shallower ones. The deeper containers get less light penetration and therefore fewer algae will grow. Shallow white or clear translucent plastic containers are the best. They allow the sunlight to penetrate most of the water column. Decreasing the depth of the container can have the same effect as higher light intensity. Shallower containers require much lower light levels initially. You could use a standard all-glass aquarium. Water circulation can be also be used to keep the greenwater in suspension, thus exposing as much surface area as possible to the sunlight.
If on the other hand you want to culture your greenwater indoors, then the best way is to use a bare 50-litre aquarium placed near a window that receives several hours of natural sunlight each day. Fill the tank with old aquarium water and it should turn green within a few days (depending on light and nutrients). Mechanical circulation can be used to keep the algal cells in suspension, thus exposing as much surface area as possible to the light for photosynthesis, while making sure they don't remain in the same spot too long for photo-inhibition to become a factor. Cultures are generally maintained at a temperature range of 20~24°C, pH 7.5-8.5. High-density cultures may require a pH buffer to prevent pH drift.
Unless you are maintaining pure cultures in sterile conditions, your culture will have both phytoplankton and zooplankton in greater or lesser degree. Cultures maintained indoors without sufficient light will probably contain more zooplankton than phytoplankton but outdoors the situation is usually reversed. Introducing more light will encourage the existing phytoplankton to grow.
Providing phytoplankton with zooplankton as food for the newly hatched rainbowfishes has several advantages. The larvae are easily able to switch to different sized prey, a feature not present in monocultures of organisms. Phytoplankton also enables the resident zooplankton to feed on the algae and microbes, thus retaining their nutritional value for greater periods of time. Studies performed in aquaculture have shown that fish larvae fed on diets composed of multiple phytoplankton species have higher survival rates and quicker growth rates than those fed with a single type of phytoplankton.
Feed 5~10 ml (per 20~50 fry) of greenwater suspension three or four times per day. You can tell if they are feeding well as the larvae should have nice "swollen stomachs" after feeding. As the fish larvae grow, the amount of food they need increases and they prefer to eat larger-sized prey. Larvae older than 14 days can start to be fed live brine shrimp nauplii and/or microworm. Brine shrimp and microworm will accelerate growth so it is important to commence feeding as soon as possible. The smallest larvae will continue to eat the greenwater while the larger ones will start feeding on the brine shrimp nauplii or microworm. Gradually increase the proportion of brine shrimp nauplii or microworm and phase out the greenwater - this should be around day 14 to 21. This feeding regime should generally result in greater than 90% larval survival.
Alternatively, commercially available phytoplankton products such as Phytoplan or PhycoPure can be incorporated into their diets. Phytoplan is a spray dried blend of several strains of phytoplankton. PhycoPure contains 7 different types of microalgae as well as a dinoflagellate and zooxanthellae. It is grown in natural seawater that has been ozonated, charcoal filtered and UV treated. It offers a wide range of particle sizes and nutritional content that, individually, have been proven very effective in various aquaculture projects. However, research has shown clear difference in nutritional value between non-living and living diets, and among commercial diets advertised as containing live algae. Overall, results showed that fresh cultures displayed the best growth and lowest mortality rates. Also, a mixed algal diet promotes faster growth and higher survival than single algal diets. Results also suggest that phytoplankton present in some commercial diets may lose their nutritional value during processing or refrigerated storage.
Artificial Greenwater Recipe
Mix a suspension of 1 level tablespoon spirulina powder per litre of distilled water and fed at a rate of about 10~50 mil per 20 litres - but only if the water has cleared from any previous feeding. The amount of the powder is not so important that you need exact measures. It should not be so much that it won't go into solution and should be enough to make the mixture a dark green colour.
© Copyright Adrian R. Tappin
Updated December, 2008.
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