Department of the Environment

About us | Contact us | Publications

Header imagesHeader imagesHeader images

Publications

Disclaimer

Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.

State of the Marine Environment Report for Australia: The Marine Environment - Technical Annex: 1

Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland

Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995

ISBN 0 642 17399 0

Land-sea interactions and oceanographic processes affecting the nutrient dynamics and productivity of Australian marine ecosystems

Miles J. Furnas

Australian Institute of Marine Science
P.M.B. 3, Townsville, Qld, 4810
Sydney NSW 2033

Introduction

Australia is an island continent (Figure 1). The coastal, continental shelf and oceanic ecosystems within Australian territorial waters and the Australian Exclusive Economic Zone span a 33-degree latitudinal range between the tropics (Cape York, c. 10S) and temperate zone (Hobart, 43S); a 58-degree span if Antarctica and the subantarctic islands are included. This large latitudinal range encompasses a diverse range of pelagic and coastal ecosystems. The biological productivity of these ecosystems is dependent upon nutrients delivered, removed and recycled by geochemical and biological processes which are, in turn, coupled to the physical oceanographic environment. This report briefly describes key processes relevant to understanding oceanographic controls on nutrient inputs, nutrient cycling and the productivity of Australian seas.

The mixing of seawater is determined by its density which is related to its temperature and salinity characteristics. In the absence of vertical mixing, water masses in the ocean form a layered structure. These layers are generally characterised by warm or low-salinity water of low density and usually low nutrient content at the surface overlying progressively cooler and/or saltier layers of increasing density. Away from freshwater sources, the specific gravity (density) of seawater falls within a fairly narrow range (c. 1.020-1.028 g/cm3). Within this narrow range, small changes in seawater density control the amount and extent of vertical mixing. Figure 2 shows representative profiles of salinity, temperature and density measured in the upper 300 m of the Coral Sea (19S) - where clear vertical changes in temperature and density are evident - and the Southern Ocean (54S) - where the intensity of vertical mixing is much greater and there is little variation in density within the upper water column. Horizontal turbulent mixing is driven by wind stress and current shear as water masses move past each other. The extent of vertical turbulent mixing of water is governed by the magnitude of the vertical density gradient and fluxes of thermal and mechanical energy relative to the gravitational potential energy stored in the vertical density gradient. On a regional scale, rates of horizontal turbulent mixing are approximately one million (106) times greater than vertical turbulent mixing rates.

Much of Australia's continental shelf and coastal marine environment lies within the tropics. At low latitudes, intense, year-round solar heating produces a warm surface layer of sufficiently low density that turbulent mixing of water and nutrients across the thermocline is strongly inhibited. Below the thermocline, dissolved nutrient concentrations increase with depth and decreasing temperature. The productivity of surface waters is closely related to the rate at which water and its associated nutrients below the thermocline are transported or mixed into the euphotic zone (Eppley & Peterson 1979). In open ocean and continental shelf systems, regional wind stress (eg Clementson et al. 1989, Harris et al. 1991) and storm events (McGowan & Hayward 1978) directly affect vertical mixing of nutrients into the surface layer, regulating the level of primary production. Localised upwelling of nutrient enriched waters can occur at current divergences or in the vicinity of headlands, reefs and islands (eg Wolanski, Imberger & Heron 1984). At the same time, nutrient materials (eg nitrogen, phosphorus, silicon, iron) are continually removed from the surface layer by a rain of particulate material comprised of living plankton, detritus and faecal materials produced by pelagic grazers. Below the thermocline, organic materials are mineralised back into inorganic nutrients by microorganisms.

Figure 1: Regions in the Australian marine environment affected by major current systems, cyclones and oceanographic mixing regimes.

Figure 1

At higher latitudes, water temperature, levels of wind mixing and inputs of light necessary for photosynthesis go through pronounced seasonal cycles (Rochford 1984, Harris et al. 1987, Clementson et al. 1989, Harris et al. 1991). During the winter, strong winds and cool surface water temperatures enhance vertical mixing processes, breaking down vertical density gradients and allowing nutrient-rich waters to mix into the surface layer. Winter production by planktonic algae is limited by seasonally low light levels and deep vertical mixing which keeps planktonic algal populations out of the euphotic zone for extended periods. In the spring, increasing solar energy inputs lead to a warming of surface waters, thus re-establishing a shallow thermocline. Phytoplankton above the thermocline remains exposed to higher light levels and, briefly, to high nutrient levels. These lead to the development of a spring bloom (eg Harris et al. 1987, Harris et al.1991). Temperature, per se, has relatively little effect on the initiation, magnitude and duration of spring blooms: the important factors are the thickness of the surface mixed layer, pre-existing nutrient concentrations and subsequent rates of nutrient input or recycling. The life cycles and population dynamics of many species inhabiting temperate and polar waters are directly or indirectly linked to the annual flowering of planktonic algae in the spring bloom (eg Thresher et al. 1989). Phytoplankton and organic matter which are not consumed during the spring bloom ultimately falls to the seabed to support benthic food chains and demersal fish stocks.

Because of the annual winter replenishment of nutrients, temperate oceanic and continental shelf regions tend to support larger seasonal blooms of algae and greater production at higher trophic levels than do (non-upwelling) tropical ocean and shelf systems. Lacking a well defined seasonal cycle, biological variability in tropical ecosystems is more closely related to disturbance events and oceanographic processes such as upwelling (Furnas & Mitchell 1986), floods (Brodie & Mitchell 1991), tidal mixing (Holloway et al. 1985) and cyclones (Furnas 1989). The overall productivity of temperate Australian continental shelf waters is restrained by the poleward transport of low-nutrient tropical waters along the continent's eastern and western margins by the East Australian Current (Nilsson & Cresswell 1980) and Leeuwin Current (Godfrey & Ridgeway 1985). There are no large seasonal blooms producing surpluses of organic matter. As a result, Australia lacks the large demersal fisheries that characterise northern hemisphere continental shelf systems.

Figure 2: Representative vertical profiles of water temperature, salinity and density in the Coral Sea (19S, left) and Southern Ocean (54S, right).

Figure 2

Figure 3: A two-year record of instantaneous discharge (m3 sec-1)in the South Johnstone River, north Queensland and the concurrent cumulative export of total water-borne nitrogen and phosphorus (tonnes).

Figure 3

Figure 4: short-term variability concentrations of dissolved and particulate forms of nitrogen over a two year period in the South Johnstone River, North Queensland. The instantaneous discharge pattern is shown in Figure 3.

Figure 4

The amount of biomass an ecosystem will support ultimately depends upon the stocks of nutrients available. There remains some debate as to whether nitrogen (N) or phosphorus (P) is the predominant limiting nutrient in marine systems (Smith 1984). Examination of nutrient stocks in many systems has generally led plankton biologists to favour N limitation, while geochemists favour P limitation as there is no significant atmospheric source of P. This is in contrast to N which can be fixed from atmospheric N2 by specialised bacteria (Carpenter, Capone & Reuter 1992). An emerging view suggests that in some open ocean systems other elements such as iron (Fe) may the be limiting nutrient (Chisholm & Morel 1991), although this suggestion has never been tested in Australian waters. Even in systems with low nutrient levels, rapid rates of in situ N and P mineralisation (eg Harrison 1978, Smith, & Harris 1985) ensure that populations of planktonic algae and microorganisms have continual access to at least small amounts of these elements (and likely others) to sustain their growth.

River runoff and nutrient inputs

Most of the nutrients and sediments washed from the land into coastal waters are transported by rivers. Locally, submarine discharges of groundwater can also occasionally be significant (Johannes 1980). Reliable, year-round rainfall is largely restricted to narrow zones along the eastern and southern margins of the dry Australian continent. In terms of discharge, Australia has no world class rivers. With the exception of the Murray-Darling river system, Australian rivers are short. Because of the small size of most catchments, flood events are usually of short duration, often lasting only a few days (Figure 3). These high flow periods are closely coupled to local rainfall patterns. In the monsoonal climate of northern Australia, significant flow into and nutrient discharge from rivers is restricted to the summer wet season and is highly variable between and within years (Isdale 1984). Major flood events are related to the activity of monsoonal depressions or tropical cyclones. Relative to overall size of Australia's continental shelf environment, river runoff is a small and at most, a regional contributor to shelf nutrient processes and fluxes.

The high temporal variability of flow within most Australian river systems presents significant problems for sampling and the estimation of nutrient, pollutant and sediment delivery rates to coastal ecosystems. Fortunately, measurements of water levels and discharges are made in most significant river systems. Concentrations of suspended sediments and dissolved materials in river waters vary greatly in response to varying flow rates and flood events occurring in the annual cycle (Figure 4). Peaks in concentrations of suspended sediments, nutrient materials and other chemical compounds coincide with peaks in flow rates as soil and associated soluble materials are washed off the land and into the rivers. Dilution of specific nutrient or ion concentrations in river waters can occur when watershed stocks are limited or have been washed out in floods earlier in the wet season. The relative importance of dissolved and particulate-associated nutrient materials varies between individual nutrient elements, between rivers and between flow events within seasons. After prolonged dry periods, the first flood event of the season frequently contains high concentrations of nutrient materials which have built up within the watershed or are stored with sediments in upper reaches of individual streams. To resolve such variability, rivers must be sampled at frequent intervals (at least once and perhaps several times daily) during periods of high flow variability (eg Cosser 1989).

Estuaries

Estuaries are the zones where fresh and salt waters mix (Figure 5). Because nutrient delivery is spatially concentrated within the estuarine mixing zone, estuaries usually have a higher biological productivity than adjoining coastal and continental shelf waters (Nixon et al. 1986, Nixon 1988). Many regionally important fisheries are either based within estuaries or are dependent upon estuarine productivity at some time in the target species' life cycle (eg Rothlisberg, Hill & Staples 1985). As estuaries often provide convenient, sheltered locations to develop ports, virtually all of Australia's original settlements were located adjacent to estuaries. As these settlements have grown to towns and cities, the estuaries on which they were established have become the recipients of increasing volumes of nutrients and other human-derived materials and are now a focus for pollution.

Mixing between fresh and saline waters within estuaries is related to the relative magnitudes of wind stress, tidal energy and the density difference between freshwater and seawater. Where estuaries occur within enclosed coastal embayments or drowned river valleys, the freshwater floats upon a wedge of saltwater which intrudes landward along the bottom of the estuary. Current shear between the two layers and turbulence caused by winds and tides cause the layers to mix. The mixing in most cases produces a gradient of salinities at the surface which range from pure seawater outside of the estuary mouth to pure freshwater at the head of the estuary. The rapidity of the transition between fresh water and salt water depends on the freshwater discharge rate, the size and depth profile of the estuary, the tidal range and wind stress. Because river flow rates, tides and winds are variable, distributions of salinity, nutrients and plankton populations within estuaries are rarely stable. Benthic communities are located relative to longer term average distributions of salinity and tidal range and are more predictable. Under flood conditions, the discharge of freshwater may be so high that seawater cannot intrude into the estuary. On these occasions, mixing and other processes which normally occur inside the estuary take place within or on the boundaries of the flood plume on the adjacent continental shelf (eg Wolanski & van Senden 1983, Brodie & Mitchell 1991).

Figure 5: Time averaged water flows, mixing patterns and major nutrient fluxes in a partially mixed salt wedge estuarine system.

Figure 5

Because discharge from many Australian rivers is low for much of the year, the extent of vertical stratification is generally restricted in many estuaries. More commonly, tidal and wind mixing control the distribution of materials in the estuary, leading to the development of longitudinal gradients of salinity and other properties (eg Hodgkin et al. 1980) rather than layered saltwedge structures.

On arid coastlines, high evaporation rates from shallow embayments without significant freshwater inflows and restricted tidal exchange can create environments where salinities exceed those in normal seawater ('inverse estuaries'). These inverse estuaries may be permanent (eg Shark Bay: Smith & Atkinson 1983, Spencer Gulf: Nunes & Lennon 1986) or seasonal in monsoonal regions (eg Wolanski 1986). The productivity of inverse estuaries is related to tide and wind-driven exchanges of materials with the neighbouring ocean, nitrogen fixation by benthic bacteria, sedimentation, and recycling within the ecosystem (Smith & Atkinson 1983; Smith & Veeh 1989).

Figure 6: Schematic depiction of solute:salinity relationships in estuaries for materials not effected by estuarine processes, materials removed or solubilised within the estuary.

Figure 6

Supra-tidal mud flats are found along a number of arid and dry-tropical coastlines, such as the southern Gulf of Carpentaria. These flats concentrate salt and nutrients for extended periods following tidal inundations, sheet runoff, rainfall and groundwater intrusions, then release salty, nutrient-laden water into the coastal zone following dissolution during spring tidal immersions (Ridd, Sandstrom & Wolanski 1988). The quantitative contribution of these release events to the coastal zone are not well known.

A wide variety of inter-connected geochemical and biological processes operate in the estuarine environment to alter the concentration and speciation of biologically important nutrient materials, metals and pollutants. Individual elements (eg sodium, chloride) may be unreactive within the estuary. Their concentration reflects the relative dilution of the fresh or saltwater end members (Kaul & Froelich 1984). Other materials are actively taken up or released into solution (Figure 6). The transition from freshwater (low ionic strength) to saltwater (high ionic strength) directly affects the concentration and speciation of many elements and ions carried in river waters. For example, iron, a biologically essential element common in terrestrial soils and rocks, is relatively soluble in freshwater but is highly insoluble in oxygenated seawater. As a result, most of the soluble iron transported by rivers is precipitated within or near estuaries (Boyle, Edmond & Sholkovitz 1977). In contrast, copper, which is also an essential trace element but highly toxic to marine life at only slightly higher concentrations, is less affected in the transition from fresh to seawater (Sholkovitz & Copland 1981). The solubility of copper and many other trace metals is strongly dependent upon the presence of organic matter in fresh and saltwaters to bind the metal ions and keep them in solution.

Much of the phosphorus transported by river systems is bound to soil particles (Meybeck 1982,). Some, though not all of this phosphorus is solubilised to phosphate (PO4) within the estuarine zone by equilibrium desorption processes (Fox, Sager & Wofsy 1986, Froelich 1988). Thereafter, it is available for uptake by planktonic algae.

Nitrogen undergoes a number of biologically mediated transformations in estuarine systems. Inorganic species such as ammonium (NH4) and nitrate (NO3) are both rapidly taken up and mineralised by plankton (McCarthy, Taylor & Taft 1977, McCarthy, Kaplan & Nevins 1984). Organic matter is actively mineralised in both the water column and benthos. Importantly, as much as half of the organic nitrogen which falls to the seabed in estuaries may be removed from the ecosystem by denitrifying bacteria (Seitzinger 1988). Rates of nitrification and denitrification are in turn directly coupled to the degree of nutrient loading (McCarthy, Kaplan & Nevins 1984, Seitzinger & Nixon 1985). All such processes need to be taken into account when estimating net fluxes of nutrients, pollutants or other compounds to coastal marine ecosystems.

The productivity of plankton communities in estuaries is dependent upon the rate of nutrient supply (usually through freshwater inputs: Nixon 1980), the amount of physical mixing within the estuary (Nixon 1988) and the residence time of water within the estuary (Ketchum 1954). Where nutrient input rates are high and water residence times are long relative to the generation times of algae, pronounced algal blooms can develop (eg the Peel-Harvey estuary in Western Australia: Hodgkin et al. 1980, Hillman, Lukatelich & McComb 1990). Residence times for water and plankton are determined by a number of factors, including rates of freshwater input, the shape of the estuary (eg long and narrow versus short and wide), the presence of adjoining wetlands (Wolanski & Ridd 1986a) and the volume of the estuary relative to the tidally exchanged volume. Where the estuary is relatively open to the adjoining shelf, wind-driven currents may be important in flushing it. In shallow estuaries and bays, exchanges of nutrients between the water column, the benthos and communities of benthic filter feeders exert a significant effect on water chemistry (Smith & Atkinson 1983) and system productivity (Nixon et al. 1986). However, for bays and continental shelf systems of even moderate water depths (c. 10 m), microbial mineralisation processes within the water column dominate short-term nutrient availability (Harrison et al. 1983, Furnas, Smayda & Deason 1986).

Once past the estuary, freshwaters and nutrient materials do not disperse haphazardly into the adjoining ocean. Buoyant plumes of river water generally flow along the coastline (eg Wolanski & van Senden 1983). As a result, terrestrial sediments, nutrient materials and contaminants are usually deposited close to the coast (eg Gagan, Sandstrom & Chivas 1987). Interactions between buoyancy-generated coastal currents, bottom friction and wind stress frequently lead to the formation of coastal boundary zones (King & Wolanski 1991). These dynamic features trap water and terrestrial materials near the coast enhancing nearshore nutrient concentrations and slowing dispersion into deeper shelf waters. In the Gulf of Carpentaria, annual prawn catches are directly correlated with the level of terrestrial runoff into the Gulf (Staples 1985). Much of this material can be trapped within the shallow nearshore zone (Wolanski & Ridd 1986b) for extended periods.

Cyclones

Tropical cyclones are common seasonal events in many parts of northern Australia (Figure 1). Cyclones, particularly those that cross the coast, exert pronounced effects on continental shelf and coastal marine ecosystems. In parts of northern Australia (eg the Kimberley region and north Queensland), the rainfall that accompanies cyclonic weather systems can be a major source of freshwater to the region, causing widespread, though episodic, flooding.

The high winds and associated waves generated in tropical cyclones induce mixing of surface waters in the deep ocean to a depth of approximately 100 m (Price 1981). This mixing often extends into the oceanic thermocline, bringing nutrient-enriched water to the surface along the storm track. As tropical cyclones move onto and over the continental shelf, cyclonic mixing (Figure 7) reaches to the bottom (Hearn & Holloway 1990) and is sufficiently powerful to re-suspend sediments over wide areas as the cyclone passes across the continental shelf (Gagan, Chivas & Johnson 1989). Dissolved and particulate nutrients within the sediments are re-suspended into the water column. There they are mineralised by bacteria, in turn triggering regional phytoplankton blooms. In conjunction with nutrients in rainfall and floodwaters near the coast, regional plankton biomass and primary productivity can increase 5-10-fold within a matter of days following a cyclone (Furnas 1989).

Figure 7: Mixing dynamics and significant nutrient fluxes associated with cyclonic storms moving over the continental shelf.

Figure 7

Upwelling

Ocean current and wind systems along the eastern and western coasts of Australia inhibit the development of the large, highly productive, Ekman-forced upwelling systems like those which occur along the western margins of North America, South America and Africa. This does not mean that upwelling processes are not important in Australia's marine environment. Rather, their spatial extent and trophic significance is more constrained. Locally significant upwelling of nutrient-enriched waters is known to occur along much of Australia's eastern seaboard (Andrews & Gentian 1982, Rochford 1984) and at sites along the southern coast of Victoria and South Australia (eg Schahinger 1987). At higher latitudes - near Tasmania - seasonal storm events accelerate the mixing of nutrients onto the shelf (Harris et al. 1991). On a local scale, upwelling frequently occurs in the lee of headlands and islands (eg Wolanski, Imberger & Heron 1984).

Figure 8: Temperature section through an intrusive upwelling event.

Figure 8

Along much of the eastern coast of Australia, the depth of the thermocline and associated nutricline is similar to the depth of the shelf break. Episodic oceanographic forcing leading to vertical excursions of the thermocline (eg Rochford 1984, Tranter, Leech & Airy 1982, Nof & Middleton 1989) results in the intrusion of cool, nutrient-enriched waters onto the continental shelf (Figure 8). These intrusion events lead to seasonal and episodic changes in the productivity of outer shelf waters (Holloway et al. 1985, Furnas & Mitchell 1986). For example, in the central Great Barrier Reef (GBR), one to several significant intrusion events may occur within a season (Andrews & Furnas 1986). Smaller mini-intrusions occur more frequently near the shelf break due to internal tides, shelf waves and local topographic effects (Wolanski & Pickard 1983, Griffin, Middleton & Bode 1987). The magnitude and duration of individual intrusion events is related to fluctuations in the direction and magnitude of both large scale and along-shore wind stress, current meanders, fluctuations in current strength and internal tides within the thermocline. Large intrusion events can transport substantial volumes of water onto the shelf. During large intrusion events in the central GBR, as much as 40% of the water volume over a section of the continental shelf can be intruded sub-thermocline water. In such cases, inputs of nutrients are proportional to the volume of water intruded. The importance of intrusive activity can only be resolved through the acquisition of long-term oceanographic and meteorological data sets (eg Rochford 1984, Harris et al. 1987, Harris et al. 1991) - particularly those with temporal resolution capable of resolving individual events and the environmental parameters accompanying them.

Tidally mixed systems (the Great Barrier Reef, Torres Strait, North West Shelf)

Tidally induced mixing is a major contributor to the nutrient dynamics of a number of Australian marine ecosystems. In these systems, bottom friction acts in a manner analogous to wind stress on the surface to mix the water column.

Figure 9: Water movements and mixing processes within a frontal zone.

Figure 9

In the complex reef matrix of the GBR, 2- and 3-dimensional wakes (Wolanski & Hamner 1988) are formed in the lee of individual reefs or behind gaps between reefs (Wolanski 1992, Liston et al. 1992). This constant mixing ensures that GBR shelf waters are largely homogenous in character, with nutrients being rapidly distributed throughout the euphotic zone. Topographically accentuated upwelling and mixing of tidal currents along the seaward margin of the GBR (eg Nof & Middleton 1989, Wolanski et al. 1988a) contributes to localised zones of high productivity along the seaward margin of the reef (Astley-Boden 1985). The influence of topographic stirring is greatest in the Torres Strait and far northern GBR where oscillating tidal currents through a dense reef and island matrix (Wolanski, Ridd & Inoue 1988) cause complete homogenisation of the water column and active resuspension of sediments. Phytoplankton populations in the Torres Strait and the far northern GBR exhibit high rates of productivity in surface waters (Furnas, unpub.). However, regional productivity is constrained by turbidity and the shallowness of the water column.

The continental shelf system of north-western Australia is also characterised by a highly energetic tidal regime. The tidal energy is dissipated through the breaking of internal tidal waves and the formation of benthic frictional boundary layers (Holloway 1983). Relatively little is known about the productivity and nutrient dynamics of this region. Holloway et al. (1985) proposed that a variety of mechanisms (shelf break upwelling, internal tidal activity and cyclonic disruptions) collectively contribute to nutrient inputs that support a productive shelf ecosystem. Tranter and Leech (1987) demonstrated that onshore propagation of the pycnocline and a subsurface chlorophyll maximum during the seasonal relaxation of the Leeuwin Current can bring phytoplankton biomass and nutrients onto the North West Shelf.

Frontal systems

In both

Department of Environment, Sport and Territories logo