Phytoplankton are minute free-floating aquatic plants In addition to the marked changes in abundance observed in phytoplankton over the course of a year, there is also a marked change in species composition. This change in the dominant species from season to season is called seasonal succession, and it occurs m a wide variety of locations. Under seasonal succession, one or more species dominate the phytoplankton for a shorter or longer period of time and then are replaced by another set of species. This pattern is repeated yearly. This succession is different from typical terrestrial ecological succession in which various plants replace one another until finally a so-called climax community develops, which persists for many years.

What are the factors causing this phenomenon? Considering that seasonal succession is most often and clearly seen in temperate seas, which have a marked change in temperature during a year, temperature has been suggested as a cause. This may be one of the factors, but it is unlikely to be the sole cause because there are species that become dominant species at various temperatures. Furthermore, temperature changes rather slowly in seawater, and the replacement of dominant species often is much more rapid.

Another suggested reason is the change in nutrient level over the year, with differing concentrations favoring different phytoplanklon species. While this factor may also contribute, observations suggest that phytoplankton populations rise and fall much more quickly than nutrient concentrations change.

Yet another explanation is that species succession is a consequence of changes in seawater brought about by the phytoplankton living in it. Each species of phytoplankton secretes or excretes organic molecules into the seawater. These metabolites can have an effect on the organisms living in the seawater, either inhibiting or promoting their growth. For any individual organism, the amount of metabolite secreted is small. But the effect of secretions by all the individuals of the dominant species can be significant both for themselves and for other species.

These organic metabolites could, and probably do, include a number of different classes of organic compounds. Some are likely toxins, such as those released by the dinoflagellates (a species of plankton) during red tides, which inhibit growth of other photosynthetic organisms. In such cases, the population explosion of dinoflagellates is so great that the water becomes brownish red in color from the billions of dinoflagellate cells. Although each cell secretes a minute amount of toxin, the massive dinoflagellate numbers cause the toxin to reach concentrations that kill many creatures. This toxin can be concentrated in such filter-feeding organisms as clams and mussels, rendering them toxic to humans.

Another class of metabolite is the vitamins. It is now known that certain phytoplankton species have requirements for certain vitamins, and that there are considerable differences among species as to requirements. The B vitamins, especially vitamin B12, thiamine and biotin, seem to be the most generally required Some species may be unable to thrive until a particular vitamin, or group of vitamins, is present in the water. These vitamins are produced only by another species: hence, a succession of species could occur whereby first the vitamin-producing species is present and then the vitamin-requiring species follows.

Other organic compounds that may inhibit or promote various species include amino acids, carbohydrates, and fatty acids. Although it is suspected that these organic metabolites may have an important role in species succession and it has been demonstrated in the laboratory that phytoplankton species vary both in their ability to produce necessary vitamins and in their requirements for such in order to grow, evidence is still inadequate as to their real role in the sea.

There is also evidence to suggest that grazers (animals that feed on plants or stationary animals), particularly selective grazers, can influence the phytoplankton species composition. Many copepods (small, herbivorous crustaceans) and invertebrate larvae pick out selected phytoplankton species from mixed groups, changing the species composition.

A growing body of evidence now suggests that all of the factors considered here are operating simultaneously to produce species succession. The importance of any factor will vary with the particular phytoplankton species and the environmental conditions.

游生物是一种微小的、自由漂浮的水生植物。除了每年里浮游生物数量的明显变化，它们的种类组成也有显著改变。这种季节性的优势物种的变化被称为季节性更替，这种更替会在很多地点发生。在季节性更替下，一种或者多种浮游生物会主宰整个浮游生物群或长或短的时间，然后被另一些种类的浮游生物所取代。这种变化模式是年年重复的。

那么是什么原因导致了这个现象呢？如果我们认为季节性更替在温带海洋（在这里气候全年里有显著变化）里更可能出现、更明显地可以被观察到的话，那么温度就应该是原因之一。温度可能是原因之一，但是不可能是唯一一个原因。因为存在某些物种，它们可以在不同的温度下成为优势物种。

另一种原因是全年里海水所含营养的水平导致了季节性浮游生物的更替，因为不同种类的营养物质的浓度升高会促进不同种类的浮游生物的生长。这个因素可能也仅仅是原因之一，观察到的现象告诉我们，不同浮游生物数量的变化远远快于海水里营养物质浓度的变化。

然而，另一个解释是海水里原有的浮游生物的变化导致了其自身种类的更替。每一种浮游生物会向海水中分泌有机物质。这些新陈代谢物可以影响生存在海水里的生物，抑制或者促进它们的生长。对于任意一个单独的生物体，其分泌的物质是很少的。但是如果优势物种的大量个体一起分泌的话，对于其自身和其他物种的影响会是巨大的。

这些有机新陈代谢物能够、并且很可能确实包括了大量的不同种类的有机化合物。一些是类似于毒物，比如由腰鞭毛虫在赤潮中释放的物质。这种物质会抑制其他光合作用的有机物。在这些例子里，腰鞭毛虫数量剧增导致海水变成了同其细胞一样的棕红色。尽管每个细胞分泌的毒物的量只有一点点，大量的腰鞭毛虫会导致毒物积累到足以杀死很多生物的浓度。这种毒物可以在蛤和贻贝这种的滤食性生物体内堆积，最终使得他们对于以此为食的人类有毒。

另一种种类的新陈代谢物是维生素。我们知道，特定种类的浮游生物会对于特定种类的维生素有需求，不同种类的浮游生物对于维生素的需求有很大的差异。维生素B，特别是维生素B12，硫胺素和生物素应该算是被最广泛需求的维生素种类。一些物种可能在缺少某种特定维生素、或者在缺少某族维生素的时候无法生存。其他可以抑制或者促进各种物种的生存的有机化合物包括氨基酸和碳水化合物，还有脂肪酸。尽管我们怀疑这些新陈代谢有机物可能在物种更替中起到重要作用，而且实验也证明不同种类的浮游生物在产生必须维他命的能力和对维他命的需求上是有差异的。它们在海里所起的真正作用还需要更多证据来确定。

我们也还有证据来证明，食草动物（吃植物或者不动的动物的动物），尤其是挑剔的捕食者，也可以影响浮游植物的种类组成。很多桡足动物（小的、甲克纲的食草动物）和无脊椎动物的幼体会从复杂的种群中挑出特定的浮游生物种类，这就改变了环境的状况。

越来越多的证据表明，这里所考虑到的所有因素都在同时作用，导致物种的更替。任何一种因素的重要性会随着特定浮游生物种类不同和环境状况不同的改变而改变。