McKinnon et al., 2007:

Biodiversity

"Phytoplankton communities in the GBR ecosystem are diverse and cosmopolitan in character, comprising a mixture of oceanic forms with global pan-tropical distributions, and assemblages of diatoms and dinoflagellates¹⁰⁹ found in coastal and upwelling regions worldwide. There are no known phytoplankton species endemic to the GBR. A three-year survey of the microphytoplankton in the 1970s produced a species list of 220 diatoms and 176 dinoflagellates⁹³. The colonial nitrogen-fixing cyanobacterium Trichodesmium episodically accounted for a significant proportion of the microphytoplankton in lagoon samples, with abundances inversely correlated with those of diatoms⁹²."

Distribution

"Phytoplankton studies spanning the width of the GBR ecosystem have demonstrated a strong onshore–offshore gradient. Communities in nearshore waters are more frequently dominated by diatoms⁹³ because of more consistent nutrient inputs and greater nutrient availability from adjacent terrestrial sources and shallow sediments. Diatom-dominated assemblages within GBR waters are therefore diagnostic of enhanced or persistent nutrient inputs into a region. Diatoms achieve dominance after disturbances, for brief periods at least, because they have faster intrinsic growth rates³² than picoplanktonic cyanobacteria³⁴. The difference in response times between the flagellate grazers of picoplankton (days) and the metazoan grazers of microplankton diatoms (weeks) also contributes to the persistence of diatom blooms. By contrast, communities in oligotrophic (low nutrient) outer-shelf and oceanic waters are dominated by picoplankton-sized unicellular cyanobacteria (Synechococcus) and prochlorophytes (Prochlorococcus), together with nitrogen-fixing cyanobacterial rafts of Trichodesmium and characteristic assemblages of open-ocean dinoflagellates^{92, 93, 20, 21}.

Mesozooplankton communities also show cross-shelf patterns, with distinct inshore and offshore assemblages^103,115,80. Inshore and estuarine zooplankton communities, where temperatures can seasonally exceed 30°C, are dominated by small copepods⁸¹. Most (62%) of the mesozooplankton biomass is comprised of organisms less than 350 micrometres in size, and regional differences in zooplankton community composition are very small, at least within the inshore community⁸¹.

These marked cross-shelf changes in plankton composition are a result of gradients in the physicochemical properties of water. These gradients are determined by the dynamic balance between terrestrial inputs of nutrients, water movements alongshore, and oceanic exchanges. The cross-shelf extent of terrestrial influence is governed by bathymetry, the limited cross-shelf extension of river plumes, and the magnitude of a northward-flowing, wind-driven coastal current. As a result, nearshore waters are insulated to some degree from mixing with inter-reef waters on the outer shelf^64,73. A variety of indicators show that the direct effects of runoff from the land are restricted to the nearshore zone 10 to 20 km in width^41,68,104. Conversely, at the seaward end of the gradient, upwelled intrusions of the Coral Sea thermocline episodically inject nutrient-rich water onto the outer shelf^5,38. On occasion, large intrusions of Coral Sea water can extend almost the full width of the GBR lagoon³⁸."

Role in the GBR

"Phytoplankton account for approximately half the global primary production, and consequently play a major role in cycling of atmospheric carbon dioxide (CO2). They are also the major primary producers in the GBR ecosystem³⁷. Approximately 70 percent of the estimated 2.2 x 105 tonnes of carbon (C) fixed daily by primary producers in the GBR shelf ecosystem originates from phytoplankton production (58 x107 tonnes C per year) and, of this, two-thirds is fixed by picoplankton³⁸.

Micro- and mesozooplankton are the basis of food webs supporting oceanic and many coastal fisheries. Plankton and suspended non-living organic particles directly support a wide variety of suspension-feeding organisms and planktivorous fish on coral reefs. In addition, most benthic macroalgae, invertebrates and fish have a planktonic life stage that is dispersed by currents. Plankton inhabit and dominate (both numerically and by mass) the largest habitat within the GBR, the pelagic ecosystem. Within the GBR, which has an area-weighted average water depth of 36 metres, this ecosystem has a total water volume of over 7200 km³. By contrast, coral reefs comprise about 6 percent of the area within the GBR Marine Park⁶⁹."

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Citation and/or URL

McKinnon, A.D., Richardson, A.J., Burford, M.A. and Furnas, M.J. 2007. Chapter 6: 'Vulnerability of Great Barrier Reef plankton to climate change'. In Climate Change and the Great Barrier Reef: A Vulnerability Assessment, eds. Johnson, J.E. and Marshall, P.A., Great Barrier Reef Marine Park Authority and Australian Greenhouse Office, p. 121-152

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Review paper

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Review paper

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