Miles Furnas

Spatial and temporal patterns of near-surface chlorophyll a in the Great Barrier Reef lagoon

Surface chlorophyll a concentrations in the Great Barrier Reef (GBR) lagoon were monitored at individual stations for periods of 6 to 12 years. The monitoring program was established to detect spatial and temporal changes in water quality resulting from increased loads of nutrients exported from the catchments adjoining the GBR. Sampling occurred monthly at up to 86 sites that were located in transects across the width of the continental shelf. In the central and southern GBR (16–21°S), there was a persistent cross-shelf chlorophyll a gradient, with higher concentrations near the coast. No cross-shelf gradient was observed in the far northern GBR (12–15°S). Mean chlorophyll a concentrations in the far northern GBR (0.23 µg L–1) were less than half those in the south and central GBR (0.54 µg L–1). Chlorophyll a varied seasonally within regions, with mean summer-wet season (December–April) concentrations ~50% greater than those in the winter-dry season (May–November). Sub-annual, inter-annual and event-related variations in chlorophyll a concentrations were observed in several zones. Multi-year patterns in concentrations suggest that relatively short (5–8 years) time series may give spurious estimates of secular trends. Higher chlorophyll a concentrations in inshore waters south of 16°S were most likely related to the levels of river nutrient delivery associated with agricultural development on adjacent catchments.

Phytoplankton biomass and primary production in semi-enclosed reef lagoons of the central Great Barrier Reef, Australia

Phytoplankton biomass and primary production rates within semi-enclosed reef lagoons of the central Great Barrier Reef were compared with adjacent shelf waters. Chlorophyll concentrations and surface primary production rates were usually higher in lagoons although seasonal differences were only significant during the summer. Nitrate concentrations were higher in lagoons than in shelf waters year-round. Nano- (<20 μm size fraction) or pico-phytoplankton (<2 μm size fraction) dominated phytoplankton biomass and production within reef lagoons throughout the year. Net phytoplankton (>10–20 μm size fraction), however, were relatively more important in both reef lagoons and open shelf waters during the summer. Biomass-specific production within lagoons (range 41–90 mg C mg chl−1 day−1) was high, regardless of season. Lagoonal phytoplankton production (range 0.2–1.6 g C m−2 day−1) was directly correlated with standing crop and inversely related to lagoon flushing rates. Phytoplankton blooms develop within GBR reef lagoons during intermittent calm periods when water residence times exceed phytoplankton generation times.

Phytoplankton dynamics in the central Great Barrier Reef. I. Seasonal changes in biomass and community structure and their relation to intrusive activity

Phytoplankton dynamics in the central Great Barrier Reef and the relation of these dynamics to the seasonal upwelling of nutrient-enriched Coral Sea water onto the outer continental shelf were followed over an annual cycle. Event scale changes were sampled by frequent (2-day to 2-week) re-occupations of a cross-shelf transect during the latter half of the summer. Intrusive activity between November and April episodically injects nitrate-N enriched water into oligotrophic shelf waters. Persistent mid-shelf and intermittent outer shelf mid-water phytoplankton accumulations were observed from late-January to March. The 10, 10to2and<2μm phytoplankton size fractions were, respectively, dominated by diatoms, microflagellates + unarmored dinoflagellates and unicellular cyanobacteria + coccoid eukaryotes + very small flagellates. In the absence of intrusive activity, nanoplankton (10 to 2 μm) and picoplankton (<2μm) dominated biomass. These two size fractions dominated biomass on the outer shelf and in the Coral Sea throughout the year. Picoplankton frequently made up50% of the chlorophyll standing crop. Mid-shelf blooms, in which the highest chlorophyll levels (2μg 1−1) were found, were largely due to increases of the 10μm fraction. In contrast to marked summer fluctuations of mid-shelf diatom populations, nano- and picoplankton standing crop varied little with time, cross-shelf location or depth. Low water column DIN levels and DIN:Chla ratios suggest phytoplankton populations were nitrogen-limited. In open waters of the central GBR, phytoplankton apparently convert most of the intruded nitrogen into particulate form.

Subsurface intrusions of Coral Sea water into the central Great Barrier Reef—I. Structures and shelf-scale dynamics

Abstract Upwelling-related intrusions of Coral Sea water into the shelf sea of the central Great Barrier Reef were monitored for 6 months to establish their horizontal extent and coherence over a 350 km array of current meters and temperature sensors. A concurrent time series of cross-shelf hydrographic sections was obtained at the centre of the instrument array to map vertical structure during the progress of individual events. While intrusive activity occurs throughout the year at some level, large events are most frequently observed during summer (October to May) when bottom-trapped intrusions episodically create a thermocline on the shelf in response to relaxations or reversals of the longshore, equatorward tradewinds. Intrusion-related thermal signals in the reef zone on the outer half of the 120 km wide shelf are coherent throughout the full latitudinal extent of the array (17 to 20°S) and propagate shorewards at speeds up to 60 cm s−1. Longshore currents lag the wind in the 10 to 30 day forcing band by 15 h inshore and 22 h at the shelf break, with no apparent longshore propagation. A seasonally varying poleward flow on the shelf is modulated by the wind-forced interior surges, with frictionally attenuated near-bottom flow on the order of 15 cm s−1 rotated 20° clockwise of the interior flow. Intrusions are advected onshore in this frictional Ekman layer with penetration speeds observed in two large events of 14 and 21 cm s−1. Hydrographic sections also reveal that individual events begin as boundary layers ∼5m> thick that can grow vertically to occupy half of the water column. Vertical thermohaline contrasts can reach 8°C and 0.5‰ near the shelf break, but are typically half these values. Scaling considerations together with the apparent longshore homogeneity of the fluctuations in temperatures and in longshore currents indicate that to first order, the longshore currents are barotropic. The two dimensional (homogeneous longshore) interior response may be decoupled from fluctuations on the slope, and density enters the problem only as a passive scalar advected in a frictional bottom boundary layer.

Diel variations in iron speciation in northern Australian shelf waters

Iron in northern Australian shelf waters was found predominantly in fine (< 0.2 μm) 8-hydroxyquinoline-reactive form or in larger (1.0 μm) particulate form. Partitioning into the larger size fraction dominated during a period of high turbidity following a cyclonic resuspension event. Only a small component of the iron pool reacts rapidly with the strong ferrous binding agent ferrozine. The concentration of the ferrozine-reactive component is strongly dependent on light intensity with maximum concentration observed at peak light intensity. The close correlation between ferrozine-reactive iron concentration and light intensity suggests a fine balance between Fe(III) reduction and Fe(II) oxidation with the steady state concentration observed being strongly influenced by light induced changes in redox kinetics. An observed lack of association between particulate iron concentrations and the concentration of ferrozine-reactive iron suggests that the soluble rather than the particulate iron pool is most influenced by light. The chromophore may be an Fe(III)-organic complex with the strong iron binding ligand that is now recognised to be present in seawater.

Rapid changes in shelf waters and pelagic communities on the southern Northwest Shelf, Australia, following a tropical cyclone

Abstract A pronounced shift in water column characteristics and in the composition of plankton communities was observed following the passage of Tropical Cyclone Tiffany along the margin of the southern Northwest Shelf, Australia in January 1998. Satellite-derived images of sea surface temperature, meteorological and hydrographic data indicate a southward movement of shelf waters into the study area near North West Cape (21°46′S). Changes in water mass temperature and salinity characteristics also occurred as a result of local heating and evaporation. Local in situ growth was likely to have caused increases in micro-phytoplankton abundance, biomass and primary production on the shelf. A diverse, copepod-dominated shelf mesozooplankton community changed to a less diverse assemblage dominated by copepods usually found in shallow nearshore habitats. Post-cyclone larval fish catches included families absent or rare in pre-cyclone samples. In the case of copepods and larval fish, southward transport of water masses along the shelf was most likely to have caused the observed changes. Long-shore water transport forced by cyclonic winds may be a recurrent, but episodic mechanism of planktonic dispersal on the North West Shelf.

Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.

Relationships between land use and nutrient concentrations in streams draining a 'wet-tropics' catchment in northern Australia

Differences in stream nutrient concentrations typically reflect upstream differences in land use. In particular, nitrate concentrations are greatly increased by losses from nitrogen (N) fertiliser applied to areas of intensive cropping. In the present study, a relationship between the area of such land use and the nitrate concentrations in the receiving streams was predicted. This relationship was tested using several data sets from the Tully basin, in the wet-tropics bioregion of north Queensland, Australia. The proportions of fertiliser-additive land use (FALU), mostly sugarcane and bananas, were correlated with the concentrations of nutrients in streams that drain these land uses. The data compared included two long-term sampling studies in the Tully River catchment and more recent, broader catchment sampling and plot-scale studies in this region. A strong relationship was shown for nitrate, but weaker relationships were observed for other N-nutrient and P-nutrient forms. Comparisons were made with contemporary and historical land-use changes in the Tully basin. The strong relationship of FALU with nitrate provides evidence that the nitrate exports from this catchment are largely derived from fertiliser use. This relationship can be used to derive nitrate run-off coefficients for fertilised land use in catchment models or to monitor changes following management to reduce fertiliser usage.

Phytoplankton dynamics in the central Great Barrier Reef—II. Primary production

Water column primary production was measured 43 times over a 3-year period (1983–1985) on the outer shelf of the central Great Barrier Reef province and in the adjacent East Australian Current. No seasonal production cycle could be established. Estimated daily production rates in shelf and offshore waters ranged between 0.1 and 1.5 g C m−2, with most between 0.2 and 0.7 g C m−2. Annual primary production during 1983 in mid-shelf, shelf break and offshore zones was estimated to be 183, 167 and 211 g C m−2, respectively. Annual production by the<10μm size fraction at the same sites was 114, 94 and 127 g C m−2, 62, 56 and 60% of total production, respectively. Corresponding proportions of chlorophyll standing crop in the<10μm fraction were 65, 85 and 79%. In a subset of the above experiments, picoplankton (<2μm size fraction) accounted for 37–99% of total water column production and 32–85% of the chlorophyll standing crop. At first order, annual phytoplankton plus coral reef primary production on the outer shelf in the central GBR is estimated to be 404 g C m−2, with phytoplankton contributing 37%.

Nutrient inputs into the central Great Barrier Reef (Australia) from subsurface intrusions of Coral Sea waters: a two-dimensional displacement model

Abstract In the central Great Barrier Reef (17°–20°S), near-bottom intrusions of cool, nutrient-rich water upwell episodically onto the outer shelf several times each summer (October–April). Near-bottom water temperature at the shelf break was found to be a useful but conservative predictor of the volume of intruded water and inorganic nutrient (N, P) inputs in cross-shelf sections. Large upwelling events that occur at least once per summer may displace up to 1 3 of the water on the outer shelf and import phosphate and nitrate stocks 2–6 times those normally present in outer-shelf waters. The concurrently displaced outer-shelf waters export substantial amounts of organic N and P. Net N and P imports are on the order of 75% and 30% of gross N and P imports, respectively. Interannual variations in nutrient inputs from the Coral Sea are related to the number and intensity of intrusive upwelling events occurring each summer. Comparison with other nutrient fluxes suggests upwelling is a substantial source of “new” N and P to the central Great Barrier Reef.