Comparative ecology of marine and freshwater phytoplankton

Examination of models of nutrient-limited growth and uptake, nutrient patchiness, resource competition, and rand K-selection with respect to marine and freshwater phytoplankton indicates that organisms from both habitats are ecologically similar. However, there are differences in the physiology and ecology of phytoplankton within a given habitat (either marine or freshwater) during the course of succession. Even though the identity and basic ecology of organisms that are present early and late in successional sequences are generally known, little is known about their physiology. Marked differences in uptake ability, storage capacity, and growth and loss rates will be found for phytoplankton that can be ranked along an rthrough K-selection continuum. This continuum can serve as a unifying concept in phytoplankton ecology. There are numerous similarities between marine and freshwater phytoplankton, but papers and reviews that deal with their ecology often restrict coverage to organisms from one habitat or the other. In this paper we will examine a variety of topics currently of interest to phytoplankton ecologists in order to determine similarities or differences between phytoplankton from each environment. These topics include models of nutrient-limited uptake and growth, nutrient patchiness, resource competition, and rand K-selection. Several books on the ecology of marine (Morris 1980; Platt 198 1; Carpenter and Capone 1983) and freshwater (Reynolds 1984) phytoplankton have appeared recently, and they should be consulted for detailed information. In this review we will also contrast and compare the ecology and physiology of marine and freshwater phytoplankton that can be found along gradients of resource availability (e.g. varying nutrient levels) in either environment. There are substantial differences in the proportions of phytoplankton species representing the major divisions of algae in marine and freshwater environments, and sev’ An international travel award from the National Science Foundation supported travel to the Symposium on the Comparative Ecology of Freshwater and Coastal Marine Ecosystems in Nairobi, Kenya. era1 important groups (classes and orders) are representative of one habitat or the other. The marine phytoplankton is dominated by numerous species of Chrysophyta (diatoms, coccolithophores, and silicoflagellates: Bold and Wynne 1985) and Pyrrhophyta (dinoflagellates). Several other groups of algae are at times either conspicuous or abundant, but they are represented by very few species. These include the Cyanophyta [=cyanobacteria; e.g. very small-celled species of Synechococcus or large bundles of Oscillatoria (Trichodesmium) filaments]. Another characteristic member of the marine phytoplankton is Halosphaera (division Chlorophyta, class Prasinophyceae), which has spherical green cells. Freshwater phytoplankton is well represented by species of most of the major divisions of algae that have a planktonic component. Therefore, a plankton sample from a lake may contain Cyanophyta, Chlorophyta, Chrysophyta, and Pyrrhophyta. Euglenophyta, which can usually be found in small ponds, are not common members of the freshwater phytoplankton (Round 198 1). A large number of species of Chlorophyta and Cyanophyta are found in freshwater, but not in seawater. Coccolithophores, which are characteristic of marine plankton, are rarely observed in freshwaters. Marine and freshwater ecologists generally approach the study of phytoplankton