Phytoplankton (multiple phyla)
The dramatic appearance of a "red-tide" algal bloom caused by Noctiluca scintillans (inset) near Cape Rodney in the Hauraki Gulf. Photographer Miriam Godfrey.
Hoe Chang
General Description
Phytoplankton are free-floating, single-cell, microscopic organisms that are predominantly engaged in photosynthetic activities, though some have lost their ability to photosynthesise and instead can ingest food particles. Although they drift passively in the sea, these tiny organisms are mostly confined to the sunlit upper 100 m of the ocean. There are, however, also a smaller number of species that live either at the bottom of shallow seas or estuaries or on large seaweeds. From time to time some of these species might become "planktonic" because they have been removed from their normal environment by turbulence from wind and tide action. Phytoplankton is not a coherent group, in a classification sense, and includes taxa from ten different classes belonging to four different kingdoms of organisms.
The size of the free-living, pelagic species ranges from less than 2/1000 mm (mm) to approximately 1 mm. The shape of these tiny algae varies from a simple pillbox shape to very complex structures with hair-like swimming organs or with complicated processes extending from the cell wall.
Phytoplankton absorb sunlight for photosynthesis through pigments such as chlorophyll. Colour differences in these tiny algae are due to the nature and quantities of other pigments such as carotenes/xanthophylls present that mask the green of the chlorophyll in the cells. Therefore, many members of phytoplankton have chloroplasts which are brown, golden-brown, yellow-green, or blue-green. Thus, groups of these organisms can be characterised by their pigmentation. The best known are golden-brown algae such as the silicate-walled diatoms (Bacillariophyta), dinoflagellates (Dinophyta) and several groups of flagellates (Chrysophyta, Haptophyta), groups of other yellow-green flagellates (Xanthophyta, Cryptophyta, Eustigmatophyta), blue-green (Cyanophyta), green (Chlorophyta, Euglenophyta), and red (Rhodophyta) algae.
In studies of how ecosystems function, we tend to think in terms of broad groupings of organisms according to their function in the pelagic system. That is, the existence of the whole system is underpinned by the primary production of chlorophyll-containing phytoplankton. They do this by turning sunlight energy, CO2 (mostly in the form of bicarbonate), and inorganic nutrients into new organic matter. Phytoplankton are consumed by zooplankton (ranging from protozoa to complex animals - see Chapter 31) while decomposers (bacteria and fungi, which are not dealt with in this report) mop up non-living organic matter, recycle inorganic nutrients, and are themselves also food for zooplankton.
Status / Significance
Phytoplankton are the primary producers of organic matter in the ocean. Collectively, they directly or indirectly support the entire animal population, apart from those dependent on chemosynthetic bacteria around submarine vents, and thus form the basis of most marine food webs. When the mixed layer becomes shallower in spring, phytoplankton are exposed to higher light intensity in the upper sunlit ocean. After nutrient enrichment caused by turbulent mixing in winter of deep water into surface waters, phytoplankton can grow rapidly and build up to very high concentrations (tens of million cells per litre) in a matter of days. The massive build-up of phytoplankton in spring directly contributes new organic carbon to support the zooplankton, which, in turn, benefits larger marine animals including fish, squid, birds and marine mammals in the ocean.
The massive build-up of microalgal species to bloom proportion sometimes discolours surface waters and is known as a "red-tide'. The build up of some toxic species however, can damage marine ecosystems. Toxins produced by these species sometimes kill a wide range of marine life and can lead to illness and death in humans. Filter-feeding bivalve shellfish contaminated by harmful species cannot be harvested, which can have negative consequences for the aquaculture industry. Six different types of toxins have been detected in New Zealand: fish-killing toxins (ichthyotoxins), seafood poisons (eg, maitotoxins), and four other major algal toxins that caused paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP), and diarrhoetic shellfish poisoning (DSP). In New Zealand, dinoflagellates as a group has been thought to be responsible for three out of four of these major shellfish toxins (NSP, PSP, DSP), while diatoms are responsible for one (ASP).
Key Locations
Phytoplankton are ubiquitous throughout the sea and all kinds of aquatic environments. Typically, both in coastal water and in the open ocean diatoms are the first group to take advantage of the change from winter to spring conditions and form blooms as a result of nutrient enrichment in winter. Diatoms also do well in coastal waters enriched by upwelling and deep mixing. Dinoflagellates, however, prevail in inshore waters in late spring/early summer and autumn, and particularly in areas where there is freshwater input from rivers, which seem to contain substances that enhance the growth of dinoflagellates. Because of their small size (with relatively large surface area), flagellates of various groups tend to do well in regions where nutrients are either low or have been depleted after major diatom/dinoflagellate blooms.
Summary of Threats
There does not appear to be any direct threat to phytoplankton in the ocean. But "global warming" and severe climatic changes might have impacts on the growth and size composition of phytoplankton in the ocean. Prolonged warming would cause increased thermal stratification at the surface and thus reduce the transfer of nutrient-rich water from the deep to the upper water column. This, in turn, would promote depletion of nutrients in the surface waters and the dominance both of very small phytoplankton species and of non-photosynthetic species (eg, bacteria-size blue-green algae and small colourless flagellates that need to ingest food) specialised for these conditions.
Typical Habitats
Phytoplankton occur everywhere in the water column, especially the upper well-lit photic zone though non-photosynthesising forms especially, may at times be found much deeper. Inshore waters or areas of strong upwelling that are nutrient rich typically have dense populations of phytoplankton that discolour the water according to the dominant phytoplankton species. Clear offshore waters that are typically nutrient poor have much lower abundances of phytoplankton.
State of Information
In the past decades new phytoplankton taxa have been described from New Zealand. In the last twenty years alone six toxic species were described. New Zealand coastal waters appear to support a disproportionately large number of undescribed species, including toxic species. It is clear that, so far, only the more common and widespread species have been recorded in New Zealand and many more species are still out there waiting to be found. Moreover, studies of cysts, produced by many dinoflagellate species, in ports and harbours of New Zealand have just got underway. It is expected that cysts of either new or invasive species will be discovered in the near future. Recently, both molecular genetic and immunofluorescence techniques have been applied to aid species identification. This certainly helps to improve our ability to detect more new species, including cryptic species, those species unable to be distinguished morphologically from their near relatives.
Significance to Maori
No specific use or significance of phytoplankton is known, though it is likely that the colour and transparency of the different water masses due to phytoplankton was used by Maori as an aid to locating appropriate fishing grounds and navigation generally.
Key References
Caldeira, K and Wickett, M E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.
Chang, F H. 2004. Marine harmful microalgae of the South Pacific with special emphasis on bloom-forming species in Australasia. The Japanese Journal of Phycology 52 (suppl.): 49-56.
Gordon, D P. (ed) In press: The New Zealand Inventory of Biodiversity. Volume 3. Kingdoms Bacteria, Protozoa, Chromista, Plantae, and Fungi. Canterbury University Press, Christchurch (and references therein).
Hardy, A. 1956. The Open Sea, its Natural History: Part 1, The World of Plankton. London, Collins, 335pp.
Table 31: Numbers of Phytoplankton species in New Zealand marine waters
| Higher TaxonPhylum | Class | Order | Endemic species | Other species | Total species |
|---|---|---|---|---|---|
| PROKARYOTA | |||||
| Kingdom Bacteria | |||||
| Cyanobacteria | Cyanophyta | 4 | 0 | 30 | 30 |
| 4 | 0 | 30 | 30 | ||
| EUKARYOTA | |||||
| Kingdom Protozoa | |||||
| Euglenozoa | Euglenoidea | 4 | 1 | 5 | 6 |
| Myzozoa | Dinophyceae | 8 | 0 | 234 | 234 |
| 12 | 1 | 239 | 240 | ||
| Kingdom Chromista | |||||
| Cryptista | Cryptophyceae | 2 | 1 | 9 | 10 |
| Ochrophyta | Bacillariophyceae | 16 | 0 | 572 | 572 |
| Raphidophyceae | 1 | 0 | 4 | 4 | |
| Chrysophyceae | 3 | 0 | 15 | 15 | |
| Dictyochophyceae | 2 | 0 | 6 | 6 | |
| Eustigmatophyceae | 1 | 0 | 1 | 1 | |
| Haptophyta | Prymnesiophyceae | 6 | 0 | 1 | 74 |
| 31 | 1 | 608 | 682 | ||
| Kingdom Plantae | |||||
| Chlorophyta | Chlorophyceae | 3 | 0 | 11 | 11 |
| 3 | 0 | 11 | 11 | ||
| Grand Total | 50 | 2 | 888 | 963 |