Tenshi Ayukai

Retention of phytoplankton and planktonic microbes on coral reefs within the Great Barrier Reef, Australia

Concentrations of phytoplankton (coccoid cychobacteria and total chlorophyll) and planktonic microrial communities (heterotropic bacteria, nanoflagellates and ciliates) were lower over leeward reef flats than over open water or reef faces, around Davies Reef and Myrmidon Reef in the central Great Barrier Reef, Australia. Concentrations of cyanobacteria, which accounted for approximately 15–50% of the carbon biomass of phytoplankton in open water, decreased from the reef face towards the leeward reef flat. Concentrations of ciliates were consistently lower at the leeward reef flat than at the reef face. For Davies Reef, the retention rates of phytoplankton and planktonic microbial communities were estimated to reach 253 gC d-1 per 1 m strip of the reef or about 0.09 gC m-2 d-1. This value is virtually equal to estimates of net community production (0.1 gC m-2 d-1). This allocthonous organic subsidy may help maintain spositive carbon balance on both Davies and Myrmidon Reefs on the Great Barrier Reef.

Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L.)

Abstract. The effect of phytoplankton size on feeding rates of planktonic larvae of the crown-of-thorns starfish Acanthaster planci (L.) was evaluated by examining their gut contents under an epifluorescence microscope. Concentrations of coccoid cyanobacteria in natural seawater ranged between 1.73 and 5.33×105 cells ml-1 and were three to four orders of magnitude greater than that of eukaryotes. Under these conditions, A. planci larvae ingested similar or smaller numbers of cyanobacteria than eukaryotes. Consequently, clearance rates of A. planci larvae on cyanobacteria were approximately three orders of magnitude lower than those on eukaryotes. Cyanobacteria and eukaryotes in the gut of A. planci larvae had mean equivalent spherical diameters (ESD) of 1–2 μm and 3.6–4.6 μm, respectively. Thus, the volume of cyanobacteria ingested was less than 10% of the volume of eukaryotes ingested. Acanthaster planci larvae were fed cultured phytoplankton Dunaliella tertiolecta and suspensions of three different sizes of plastic beads with fluorescence labelling. There was no significant difference in clearance rates on 6 and 20 μm plastic beads. Clearance rates on 1 μm plastic beads were, however, much lower than those on 6 and 20 μm plastic beads. Clearance rates of A. planci larvae on D. tertiolecta (ca. 5 μm ESD) were significantly higher than those on 6 and 20 μm plastic beads. Apart from particle size, this result shows that feeding of A. planci larvae is influenced by other properties of potential food particles.

Sources, sinks, and export of organic carbon through a tropical, semi-enclosed delta (Hinchinbrook Channel, Australia)

A mass balance for organic carbon in Hinchinbrook Channel was constructed to identify major sources, sinks, and the magnitude of organic matter available for export to the adjacent coastal zone. Total organic carbon input from the Herbert River and from net production of mangroves, phytoplankton, seagrasses, and benthic microalgae is 8.94 ×109 M Corg yr−1 (moles organic carbon per year). Mangroves and river inputs are the largest carbon sources, accounting for 56% and 27% of the total annual input, respectively. Benthic respiration and burial in sediments are the major sinks, accounting for 46% and 41% respectively of total losses (3.09 ×109 M Corg yr−1). This mangrove‐dominated coastal ecosystem is net autotrophic, with 5.85×109 M Corg yr−1 (65% of total Corg input) available for export to the adjacent nearshore zone. Total export of organic carbon from the region (adding carbon export from Missionary Bay mangroves on the northern end of Hinchinbrook Island) amounts to 82,800 metric tons of organic carbon per year. These results confirm earlier evidence indicating that much of the particulate sediment carbon in the adjacent coastal zone is of mangrove origin. This mass balance, although preliminary, demonstrates the importance of Hinchinbrook Channel as a source of organic matter for the Great Barrier Reef lagoon.

Fluxes of nutrients and dissolved and particulate organic carbon in two mangrove creeks in northeastern Australia

In Coral and Conn Creek, northeastern Australia, the variations in concentrations of nitrate, phosphate, silicate, dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured over tidal cycles on five occasions and along each creek on four occasions. The fluxes of these five properties were then estimated using two methods. The first method is the so‐called Eulerian method, whereby water flow and material concentration are measured at a fixed station near the creek mouth and the net flux is calculated by adding up flux increments over a tidal cycle. The second method first derives the longitudinal eddy diffusion coefficient from the salt mass balance equation and then calculates material fluxes from their observed gradients along the creek. The use of the latter method is permitted only in the absence of freshwater inputs.The Eulerian method was not sensitive enough to examine whether there was any statistically significant difference in fluxes of nutrients, DOC and POC between ebb and flood periods. This casts some doubt over the meaning of individual flux estimates. It is, however, worth mentioning that 17 out of 25 flux estimates were positive (= import) in Coral Creek, whereas only eight positive flux estimates occurred in Conn Creek. In Coral Creek, the average flux values for nitrate, phosphate and DOC were positive, but negative for silicate and POC. In contrast, the average flux values for all properties were negative in Conn Creek. This may be due to the difference in amount of freshwater input between Coral and Conn Creek.The presence of freshwater inputs from upstream sources restricted the use of the salt mass balance equation to the Coral Creek data collected in September, 1996. However, the study of the variability of nutrient, DOC and POC concentrations along the creek could provide valuable insight into their behavior in Coral and Conn Creek. For example, the concentrations of silicate and DOC were consistently higher upstream than downstream and the distance–concentration relationship was statistically significant in seven out of eight measurements. The concentrations of nitrate and POC also decreased from upstream to downstream, but the trend was statistically significant in only 2–3 measurements. The concentration of phosphate was higher downstream than upstream in four measurements and in two of these four measurements, the trend was statistically significant. These results suggest that in Coral and Conn Creek, silicate and DOC are usually exported to adjacent coastal waters, whereas the import and export of nitrate, phosphate and POC are often finely balanced.

Field and model studies of the fate of particulate carbon in mangrove-fringed Hinchinbrook Channel, Australia

A field and model study was undertaken in 1996/1997 of the dynamics of water, fine sediment and particulate carbon in the northern region of the mangrove‐fringed Hinchinbrook Channel, Australia. The currents were primarily tidal and modulated by the wind. Biological detritus acted as a coagulant for the fine cohesive sediment in suspension in the mangrove‐fringed, muddy coastal waters. Plankton and bacteria were the major aggregating agents at neap tides, and mangrove detritus at spring tides. The micro‐aggregates were typically several hundreds of micrometer in diameter and enhanced the settling rate. The fate of fine sediment and particulate carbon was controlled by the dynamics of the coastal boundary layer, a turbid shallow coastal water zone along the mangrove‐fringed coast. A tidally‐modulated, turbidity maximum zone was found in this layer. Wind stirring increased the turbidity by a factor of five.The channel behaves as a sink trapping fine sediment and particulate carbon. However, the sink was ‘leaky’ because the dynamics of the coastal boundary layer generated a net outflow of fine sediment out of the channel along the western coast. The biologically enhanced settling of cohesive sediment limited the offshore extent of the muddy suspension to within a few hundreds of meters from the coast.At spring flood tides, some of this particulate carbon was advected into the mangrove forest where it would remain trapped. On a yearly basis about six times as much particulate carbon was exported out of Hinchinbrook Channel through the coastal boundary layer than was trapped in the fringing mangroves.

Phytoplankton biomass, production and grazing mortality in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia

Abstract Phytoplankton biomass, production and grazing mortality were measured in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia. In the Gulf, chlorophyll a concentrations were typically 0.2–0.3 mg m −3 and phytoplankton production rates were mostly below 25 mg of C m −3 d −1 . The low phytoplankton biomass and production in the Gulf are seemingly related to the aridity of the region and hence the small terrestrial runoff of nutrients. In dilution experiments, which complemented 14 C incorporation experiments, the proportion of potential primary production grazed ranged from 79 to 155% in the fluorometric analysis of chlorophyll a and from 73 to 191% in the HPLC analysis of chlorophyll a . There may be some excess phytoplankton production on the relatively well flushed, western side of the Gulf. On the eastern side of the Gulf, however, the estimated grazer biomass ranged between 4.6–8.8 mg of C m −3 , not much less than the estimated phytoplankton biomass (6–15 mg of C m −3 ), and this grazer population appeared to consume more organic matter than the phytoplankton population could produce. The disproportionally large grazer biomass and the relatively high grazing mortality of phytoplankton may be due to the supply of additional organic matter from benthic macroalgal communities and/or mangrove and salt flat systems present on the eastern side of the Gulf.

Possible limitation of the dilution technique for estimating growth and grazing mortality rates of picoplanktonic cyanobacteria in oligotrophic tropical waters

Abstract In total, ten dilution experiments were conducted to determine growth and grazing mortality rates of picoplanktonic cyanobacteria. Concentrations of nitrogenous nutrients (NO 3 + NO 2 + NH 4 ) and phosphate (PO 4 ) were relatively low, being in the range between 0.09–0.82 μM and between 0.05–0.11 μM, respectively. Specific growth and grazing mortality rates of cyanobacteria were estimated as 0.0101–0.0424 h −1 and −1 , respectively. The reliability of these estimates were, however, rather questionable. One of the critical assumptions in the dilution technique is that phytoplankton growth rates are identical in diluted and undiluted water. This assumption was not always valid for cyanobacterial populations sampled. In seven of ten experiments, the day-time peak of the frequency of dividing cells (FDC) of cyanobacteria was lower in diluted water than in undiluted water, with the difference in mean value being in the range between 1.3–3.9%. No such difference was, however, observed in nutrient enriched treatments. This suggests the likelihood of nutrient limitation and the need for including nutrient enriched treatments in the experimental design. Incorporating FDC measurements into the experimental design is also a worthwhile practice as an independent check of the reliability of growth and grazing mortality rate estimates, particularly those for cyanobacterial populations in oligotrophic tropical waters.

Ingestion of Ultraplankton by the Planktonic Larvae of the Crown-of-Thorns Starfish, Acanthaster planci

There has been a debate over whether the growth and development of the larvae of the crown-of-thorns starfish Acanthaster planci are severely food-limited. This debate has raised a range of questions, including the one relating to the role of heterotrophic bacteria in the nutrition of larvae. In this study, the feeding rate of larvae on bacteria as well as on other ultraplankton (<5 {mu}m) was determined by counting the number of the fluorescence-labeled cells (FLC) in the gut after short incubation. Preliminary experiments showed no detrimental effect of the fluorescence dye (5-(4,6-dichlorotriazin-2-yl) aminofluorescein) on the development of larvae and demonstrated the usefulness of FLC in feeding experiments as food particles analogous to living cells of ultraplankton. There was no evidence that larvae ingested bacteria. Larvae did ingest two strains of photosynthetic cyanobacteria, which had equivalent spherical diameters (ESD) of 1 and 1.8 {mu}m, but these tiny cells were cleared more than 10 times slower than the larger algae Phaeodactylum tricornutum (4.7 {mu}m ESD) and Dunaliella tertiolecta (5.1 {mu}m ESD). Regardless of the size of FLC used, the clearance rate (volume of water cleared per animal per unit time) increased by 50-120%, as larvae developed from the late bipinnaria stage to the late brachiolaria stage. These results show that larvae may derive a sizable proportion of their nutrition from ultraplankton, but not from bacteria.

Patchiness in the Fly River plume in Torres Strait

Abstract Oceanographic studies were carried out from August 1994 to March 1995 on the intrusion of the Fly River plume in Torres Strait. Measurements at offshore coral reefs revealed an event of decreased salinity (≈24) while salinity of the water over the reefs fluctuated between 30–34 the rest of the time. Modelling suggests that this event resulted from the reversal of longshore currents advecting old river plume water back past the river mouth. There the new river water mixed with the old river plume water generating a patch of low-salinity water. While such events may be infrequent, they have the potential to leave a terrestrial signature on offshore coral reefs, in terms of (1) an input of terrigenous sediment and (2) the possible incorporation of riverine particulate metal into the food chain. The impact during an intrusion event may be significant. In the long term the riverine material is diluted in calcareous sediment produced throughout the year by bio-erosion of coral reefs.

Concentration and molecular weight distribution of dissolved organic carbon in a mangrove creek in the Hinchinbrook area, Australia

The concentration and molecular weight distribution of dissolved organic carbon (DOC) are reported for a mangrove creek in the Hinchinbrook area, Australia. DOC concentration ranged from 1.0 mg C l−1 near the creek mouth to 2.2 mg C l−1 at the innermost part of the creek. There was no apparent spatial trend in molecular weight distribution of DOC, with a >300 g mol−1 fraction accounting for about 70% of the total in all samples. DOC concentration fluctuated between 0.5 and 1.2 mg C l−1 over one tidal cycle. The samples collected at low tide and during the rising tide were dominated by DOC of >300 g mol−1 and <300 g mol−1, respectively. This suggested the export of high molecular weight DOC, probably originating from litter leachates, and the import of low molecular weight DOC to the creek.