I have seen a lot of talk about phytoplankton, and notice that there are a lot of new supplements showing up in pet shops that advertise they contain phytoplankton. What exactly is this stuff, and do I need it for my tank? What exactly is it supposed to do for my aquarium?
Let’s start with the first question – what are phytoplankton? Well, the term phytoplankton is made up of two roots "phyto" – pertaining to plants, and "plankton" – meaning basically tiny stuff that floats around in the ocean currents. Phytoplankton is just a fancy word for tiny floating plants (such as diatoms and dinoflagellates) which serve the same role in the food chains of the oceans as grass and shrubs serve on land; namely small herbivorous things eat them, and are in turn eaten by bigger things and so on down the line. They form the basis of the marine food chain, and ultimately almost all life in the ocean is supported in one way or another by phytoplankton.
The second question – do you need it for your tank – is much harder to answer. Need? Probably not, after all there are many people who have successful reef tanks without ever having added phytoplankton to the aquarium. But if your question is really “will it benefit some animals in my tank?” rather than “do I need it?” then the answer is pretty likely to be yes, it will almost certainly benefit some animals in the average reef aquarium. There will likely always remain a group of people who point out that there are coral tanks to which no food has ever been added intentionally, and the corals are still alive, but I think most people recognize that there can be a big difference between “alive” and “thriving.” So let’s take a look at the role of phytoplankton in the food chain of coral reef animals and see if I can give you a better idea of exactly what phytoplankton is supposed to do that may help you to decide whether or not you want to add some to your tank.
Many coral reef animals feed directly on phytoplankton, and as I just said, even those that do not, rely ultimately on the nutrition gained from phytoplankton and other marine algae eaten by other organisms. Some essential nutrients provided by marine algae, phytoplankton in particular, cannot be synthesized by any animals, and are therefore extremely important components of a healthy diet. The most important of these nutrients are the class of lipids known as long chain Omega-3 fatty acids (yep, these are the same ones you hear about in regards to human health and nutrition as well). The two Omega-3 fatty acids of primary interest in this case are the highly unsaturated fatty acids (HUFA for short) DHA (docosahaxaenoic acid) & EPA (eicosapentaenoic acid) which come almost exclusively from marine algae. One of the major breakthroughs in the aquaculture of marine animals was the discovery that these fatty acids were an essential part of the diet, and prior to the practice of actively supplementing these fatty acids, nutritional deficiencies or arrested development were common problems. For example, newly hatched brine shrimp (Artemia) are a simple and easily cultured food for the larvae (and/or juveniles) of many marine organisms, but because these shrimp lack sufficient quantities of EPA & DHA, many species fed exclusively on baby brine fail to complete development, or experience very high mortality rates. The widespread success of culturing and breeding many marine animals has come only since the discovery of the importance of including these fatty acids in the diet. DHA has been shown to be important in the normal growth and development of the central nervous system, and in particular the brain, eyes and reproductive organs, while EPA is important to cardiovascular health and plays an essential role in certain immune responses. Among the common symptoms of EPA/DHA deficiency in marine animals are1) Sudden fright syndrome - shock, convulsion or even death when the animals are frightened; 2) poor vision, and reduced ability to locate prey; 3) worn or mysteriously eroding fins; 4) poor growth rates or sudden massive die offs during early development; 5) low egg viability or infertility; and 6) high mortality and disease rates, particularly when under stress (e.g., shipping or acclimation). By "enriching" food items such as Artemia with phytoplankton prior to feeding them to the marine animals being raised, the amount of EPA & DHA is often increased to the point that die-offs and developmental problems previously encountered are completely avoided. You don’t need to feed phytoplankton to provide these nutrients, however, and many people have gotten into the habit of soaking any food provided to their marine fish in a HUFA enrichment product (of which there are several on the market) to ensure that their animals get sufficint amounts of these essential fatty acids.
However, many coral reef animals feed on particles that are far smaller than those provided in the typical fish foods. Virtually everyone realizes that to keep fish healthy in an aquarium, they need to be fed, but many do not realize that the invertebrate animals in their reef aquarium need feeding just like their fish. In an effort to prevent nutrient accumulation in a reef aquarium, many people try to keep feeding to the absolute minimum necessary to prevent their fish from starving, and would not consider feeding their corals, clams, feather dusters, etc. for fear of fueling algal growth. The simple fact is that animals (even those with algal symbionts that provide substantial nutrition via photosynthesis) need some food, and most of the animals on a coral reef are very effective at capturing plankton from the water that passes over them. Many of these animals can (and do) capture phytoplankton directly, and the ones that cannot (such as the fishes) often still take advantage of the nutritional quality of these tiny algae by preying on small animals that do eat them (such as copepods, rotifers and mysids).
Despite the fact that phytoplankton form the basis of marine food webs in general, and are an essential component of the diet for many reef creatures commonly kept in aquaria (such as fan worms, sea apples, giant clams, gorgonians and tunicates), they are probably the least common element included in feeding a reef aquarium. It is easy to feed flake, freeze-dried or frozen prepared foods to the fishes and large invertebrates in the aquarium, and virtually every aquarist does so. It is a little more difficult to feed live zooplankton to the animals in the aquarium, although newly hatched or live adult Artemia are becoming more widely used all the time. Unfortunately, although these are the most widely available zooplankton, as describe above, brine shrimp are not nutritionally complete as food items without being enriched; after direct (e.g., a HUFA enrichment additive) or indirect (e.g., phytoplankton) enrichment, however, live brine shrimp make an excellent source of nutrition for most reef animals, both fish and invertebrate.
I have mentioned enrichment a couple of times, but I do not want to give the impression that phytoplankton are important only as a nutritional supplement, of course. As I mentioned above, many reef-dwelling animals for sale in pet shops are filter feeders which specialize on eating these floating algae, and the majority of them will slowly starve to death unless provided with these tiny particles on which to feed. Until recently, phytoplankton was only available if you were willing to culture your own, and although it is not difficult or expensive to do so, it is time consuming and, frankly, a royal pain. I have to raise phytoplankton for my research, and have even written articles explaining in detail how to grow your own greenwater at home in the past, but even with my experience and easy access to the equipment, I must admit that I use a commercial phytoplankton to feed my reef tank rather than grow it myself!
I think that the poor record of survival in aquaria for many obligate suspension feeders (such as Christmas Tree Worms, Sea Apples, Carnation Corals, and the like) is most likely a direct consequence of their starvation without phytoplankton being introduced regularly at sufficient concentration to allow these animals to feed in the aquarium. I think it ought to be beneficial to provide phytoplankton to most well-stocked reef tanks, because even animals which contain symbiotic zooxanthellae, such as corals and giant clams, cannot gain 100% of their nutritional requirements from light alone. For example, researchers on the Great Barrier Reef in Australia showed that 75% of the phytoplankton (2-50 Φm) passing over the reef was captured and eaten by giant clams (T. gigas); furthermore, juvenile clams were found to obtain 65% of their energy from filtering phytoplankton rather than photosynthesis, and that percentage reversed as the clams grew to adulthood – an obvious potential explanation for the widespread observation that juvenile Tridachnid clams have much lower survival rates than the adults is that the relative requirement for feeding is much higher in juvenile than adult clams. That is not to say that providing phytoplankton to an aquarium with juvenile clams will ensure survival, but it is probably going to be a real benefit to the animals.
The benefits of phytoplankton to most reef creatures is great, and with the recent availability of a number of phytoplankton products ranging from live cultures to frozen algal paste concentrates, to spray-dried powders, you can now give your reef tank every opportunity to succeed by providing this important food source to suspension-feeding animals in your reef aquarium.