What Are Plankton And Why Study Them

The term plankton refers to any small biota (from microns to centimetres) living in the water and drifting at the mercy of currents - ranging from bacteria to jellyfish. This definition is rather loose, as we often include jellyfish and krill (euphausids - and their larval forms) as plankton, yet they are active swimmers and are therefore technically referred to as 'nekton'. Sometimes even good swimmers, such as late-stage fish larvae are incorrectly termed 'planktonic', as they often show up in the plankton net, particularly at night. Another definition of plankton is simply 'that material which is caught in a fine mesh net'!

Phytoplankton, such as diatoms and dinoflagellates, grow in the presence of sunlight and nutrients such as nitrogen and phosphorous. These single celled organisms are the 'grasses of the sea' and form the basis of ocean productivity. Some of these plants - but not all - are in turn grazed by zooplankton, which is dominated by small crustaceans such as copepods, shrimps and their larvae. The amount of phytoplankton in the water column reflects the influence of a number of environmental factors and processes. These competing processes may be summed up as 'bottom-up', such as those caused by nutrients and light, or 'top-down', such as those caused by copepods or other grazers.

The majority of chlorophyll in Australian coastal waters is found in the very smallest of cells - the size of bacteria. Their high surface-area-to-volume ratio allows them to out-compete larger cells in the race for nutrients. Most phytoplankton contain photosynthetically active pigments, such as chlorophyll, which enable them to use energy from sunlight to convert carbon dioxide into complex organic molecules, such as sugar or protein (that is, they are autotrophs). Exceptions abound where some of these 'plants' do not fix their own carbon, but engulf and consume other plant cells (that is, they are heterotrophic). Other phytoplankton may be considered as villains - producing red tides or toxic algae - but there are only a few species responsible. Most phytoplankton are enormously beneficial, such as those used in the aquaculture industry.

In the presence of surplus nutrients, zooplankton grazers may be overwhelmed by rapid exponential growth of some phytoplankton ('bloom') over and above what the ecosystem can assimilate. Nutrients that encourage blooms are discharged from river run-off, sewage discharge, stormwater run-off and from groundwater. The surplus of nutrients in waterways, together with the resultant increase in biomass and altered ecology, is referred to as 'eutrophication'. Eutrophication was described in one report as possibly the greatest single threat facing the coastal environment in Australia.

It is important to remember that many phytoplankton blooms may occur naturally: they may be stimulated during the spring, or by natural events such as rainfall or upwelling. Usually, phytoplankton and zooplankton bloom during the early and late summer period, prompting public concern. And yet springtime blooms of the blue-green phytoplankton and gelatinous salps - as well as red tides of a particular dinoflagellate off eastern Australia -are all examples of natural events (see Box 1.1).

Nutrient assimilation by plankton, and nutrient accountability (is the event natural or induced by humans?), underscore the need for using

BOX 1.1 RED TIDES FORMED BY NOCTILUCA

The major contributor to red tides off the Sydney coast is an unusual single-celled alga - Noctiluca scintillans. Although classified as a dinoflagellate, it has no photosynthetic pigments and feeds at night on other phytoplankton, small zooplankton and their eggs. It contains no toxins, other than a dilute solution of ammonium chloride, which, in large quantities, can irritate the skin and cause localised fish kills. During the final senescent stages of its life, the alga swells up to a comparatively large size of 2 mm diameter and becomes buoyant, thus concentrating at the surface as a reddish, or even bright pink, stain. Its presence year-round off Sydney was never observed before the 1990s, and the frequency of red tides intensified when Sydney's three deep-ocean sewage outfalls were commissioned. Estuaries around the world frequently report Noctiluca blooms in eutrophic waters. Was coastal eutrophication stimulating the growth of phytoplankton prey for Noctiluca, thus increasing the frequency of blooms?

A major clue was the diameter of cells - small cells indicate cell division and increasing abundance, whereas large cells indicate senescence and are prone to advection by wind, transporting them far from the bloom's cause. The incidence of prey inside the cells also highlighted the importance of the East Australian Current and favourable winds, which transported the cells from areas prone to nutrient upwelling well north of Sydney. In only one case was there clear evidence of small well-fed cells near a coastal sewage outfall. So, what had caused the recent year-round abundance and increased reports of red tides? One reason is the more environmentally aware public during the 1990s, which was keen to report any unusual observations. Also, El Niño events and warming of coastal waters, particularly in 1997-1999 enabled Noctiluca's optimal temperature of ~20°C to be achieved off Sydney.

plankton in a study of water quality. In short, we need to examine and monitor the plankton because:

• some phytoplankton produce toxins that become concentrated in filter-feeding animals such as oysters, mussels and even fish

• phytoplankton may assimilate surplus nutrients, which may be grazed by zooplankton and productively pass them up the food chain to fish

• the early life stages of mussels, oysters, prawns and fish all live in the plankton

• some species of phytoplankton or zooplankton can be indicator species of environmental health by, in effect, integrating the conditions of the past few days or weeks

• the chemical attributes of plankton (such as lipids or natural isotopes), and even their shape or health, can indicate if the eutro-phication is natural or human-induced.

These issues will be addressed in the following pages. Water quality is of great concern to the managers of estuaries and coastal waters because unsavoury swimming conditions, poor fishing and bad press translate into reduced spending by tourists and reduced community pride.

Natural Resource Management is a rapidly expanding field, which is increasingly underpinned by better science. In Australia, studies such as the Port Philip Bay Study (Harris et al. 1996), the Huon Estuary Study (CSIRO Huon Estuary Study Team 2000) and the Moreton Bay Study (Dennison and Abal 1999) have provided valuable information to managers and have resulted in better management.

In addition to understanding more about the systems we manage, it is also important to measure the performance or outcomes of management decisions and practices. What is the environmental dollar value for an artificial wetland versus more river bank fencing? This can be achieved by undertaking well-designed, hypothesis-based, monitoring programs.

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