Pelagic Food Chains

Most production in pelagic systems is generated in the 'photic zone' where phytoplankton photosynthe-sise and produce the sugars necessary for life. About 70% of all carbon fixed by primary producers on the GBR originates from phytoplankton production and two thirds of this originates from organisms <2 ^m (picoplankton) in size. Phytoplankton account for about 50% of global primary production and, therefore, have a major role in cycling atmospheric CO2. In tropical waters the photic zone may reach a depth of about 150 m due to the clarity of the water column. Below this depth, phytoplankton respiration will exceed the energy derived from the generation of sugars (Fig. 14.5). The pelagic environment can be a 'bottom-up system' where the biomass of plankton and in turn that of higher trophic levels (e.g. fishes, squid and whales) depends on concentrations of nutrients (especially nitrates, nitrites and phosphates) and trace elements (e.g. iron). At other locations and times it can be a 'top-down system' controlled by herbivores and predators. For example, predators remove zooplankton grazers, relieving grazing pressure on phyto-plankton and resulting in an increase in phytoplankton biomass.

Nutrient concentrations alter according to recycling through producers and consumers (i.e. excretion) and variation in the input of new nutrients from upwelling and riverine runoff. Production cycles vary greatly by latitude. High latitude ecosystems have great variation in production and biomass of plankton from seasonal changes in day length, temperature, storms and upwelling. In contrast, tropical systems generally have low variation in productivity and biomass, and pulses in production are event driven (e.g. floods, cyclones, upwelling intrusions). Tropical systems are generally considered high turnover, low biomass systems and the waters are generally oligo-trophic (i.e. low in nutrients) and clear. Some apparent seasonality can occur in tropical waters because of increased frequency of events such as seasonal rains, which are typical of monsoonal/wet season locations. In the 'dry tropics' (e.g. the central and southern GBR) the rains are not predictable, but significant input of freshwater results during cyclone/storm events and these have a great impact on physical attributes of the pelagic environment and on planktonic assemblages and processes.

Upwelling of cold, nutrient rich, deep ocean water has a great influence on pelagic systems and it is the reason that temperate regions off the coast of Peru, the west coast of North America, and South Africa have green waters from phytoplankton growth and a wealth of consumers from copepods and krill to whales. As a result, these are the sites of some of the great fisheries of the world (e.g. Peruvian anchovy). The upwelling of nutrient rich waters into the photic zone is determined by currents, wind and topography.

affected by flood plumes

Upwelling brings nutrients

* Examples of trophic linkages B w > Connectivity - eggs and larvae dispersed between reefs

Figure 14.5 Tropical pelagic food chain showing plankton as an important source of food for pelagic nekton and reef associated species such as planktivorous invertebrates and fishes. Demersal zooplankton migrate from reefs and other substratum into the water column at night. Input of freshwater, upwelling and cyclones are factors that influence nutrient availability and growth of phytoplankton. (Image: GBRMPA.)

Tropical waters are generally considered oligotrophic and Charles Darwin considered this a great paradox given the wealth and diversity of life in tropical waters. There is regionalised and sporadic upwelling along the shelf of the GBR, but this is poorly understood. It is clearly not as great as in some temperate regions, but is of great potential importance to the tro-pho-dynamics of reef and inter-reefal habitats. Cool upwelled waters, and therefore high density waters, often hug the bottom rather than emerging at the surface. They immerse deep inter-reefal assemblages in nutrient rich waters. Phytoplankton thrive in these conditions and ultimately disperse throughout the water column to support a diversity of consumers. On some occasions, upwelling intrusions, probably driven by winds, can almost extend across the entire shelf of the GBR.

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