The transitions between the diatoms Aulacoseira spp. (Melosira) and the cyanobacteria Anabaena spp. as dominant phytoplankton species in a turbid-river weir pool are shown to depend directly on the establishment or destruction of persistent thermal stratification. A transition from high to low flow through the pool resulted in the establishment of persistent thermal stratification, causing Aulacoseira to sink out of the euphotic zone at a speed of 0.95 m d-l. Concurrently, the slightly buoyant Anabaena grew within the euphotic zone with a specific growth rate of 0.37 d-l, climaxing after approximately 14 d at a population of 20,000-30,000 cells ml I, at which point its biomass may have been limited by the availability of phosphorus. The stratification thus caused the phytoplankton population to separate into two distinct layers, with Anabaena occupying the illuminated surface layer and Aulacoseiru found only in the lower layer below the euphotic depth. Under stratified conditions, the ratio of the surface layer depth to euphotic depth, z,, : z,,, was approximately 1, whereas for a mixed water column that ratio was >3. Access to light appeared to be the main factor determining the dominant phytoplankton species. of the weir. Thanks also to Wendy Minato and Rhonda Smith, who uncomplainingly counted the algal samples, and to Mark Fink for the chemical analyses.

[1] The Great Barrier Reef (GBR), northern Australia, is the largest coral reef system in the world and provides habitat for highly diverse tropical marine ecosystems. Mixing in the coastal waters of the GBR is an important parameter influencing the health of these ecosystems. We have used the distribution of the four naturally occurring radium isotopes to determine the rate of mixing of nearshore waters of the central part of the GBR lagoon with water from the Coral Sea. The observed radium distribution is modeled using a onedimensional diffusion model. The model improves on previous radium offshore mixing models by incorporating the benthic flux of radium diffusing across the sediment-water interface and offshore changes in water column depth. We find that the inner lagoon diffusivity (<20 km offshore) is best estimated using the short-lived isotopes

No abstract

No abstract

In this paper, the dynamics of primary production in the Daly River in tropical Australia are investigated. We used the diurnal-curve method for both oxygen and pH to calculate photosynthesis and respiration rates as indicators of whole-river productivity. The Daly River has maximum discharges during the summer, monsoonal season. Flow during the dry season is maintained by groundwater discharge via springs. The study investigated how primary production and respiration evolve during the period of low flow in the river (April–November). The relationship between primary production and the availability of light and nutrients enabled the role of these factors to be assessed in a clear, oligotrophic tropical river. The measured rate of photosynthesis was broadly consistent with the estimated mass of chlorophyll associated with the main primary producers in the river (phytoplankton, epibenthic algae, macroalgae, macrophytes). A significant result of the analysis is that during the time that plant biomass re-established after recession of the flows, net primary production proved to be ~4% of the rate of photosynthesis. This result and the observed low-nutrient concentrations in the river suggest a tight coupling between photosynthetic fixation of carbon and the microbial degradation of photosynthetic products comprising plant material and exudates.

This paper presents a biogeochemical model of a coastal lagoon intended to be representative of lagoons occurring along the south-east and south-west coasts of Australia. Many of these lagoons are threatened by increased nutrient loads because of land use change, by alterations to their freshwater inflows and by modification to their tidal flushing regimens. The model simulates the biogeochemical response of the lagoon to nutrient (nitrogen) loading and includes nutrient transformation processes in the sediments, as well as in the water column. The paper focuses on the response of primary producers to increasing and decreasing nutrient loads and how the response is altered by changes in the flushing rate of the lagoon with the sea. In common with lakes, the modelled lagoon exhibits alternative stable states representing macrophyte or phytoplankton dominance depending on nutrient loading and history. A third state representing severe degradation occurs when denitrification shuts down. A characteristic of Australian coastal lagoon systems is that, due to highly sporadic rainfall patterns, nutrient inflows are dominated by intermittent extreme events. The modelling demonstrates that such a loading regimen may be expected to generally increase the vulnerability of the lagoon to increasing nutrient loads. The results of the analysis presented are pertinent to several questions raised by coastal managers, such as what are the expected benefits of improving flushing by dredging and what are the consequences of altering the timing and magnitudes of the loads reaching the lagoons? M F 0 3 0 6 8 C o a s t a l l a g o o n s : p r o c e s s e s a n d m a n a g e m e n t I . T . W e b s t e r a n d G. P . H a r r i s Extra keywords: biogeochemical model, denitrification, episodic loads, flushing time, hysteresis.

Flood events typically enhance primary productivity in estuaries via the increased nutrient inputs from land runoff. This study examined the drivers of phytoplankton biomass accumulation and productivity in a tropical estuary with a distinct wet-dry seasonality, i.e. months of little or no rainfall, and a highly episodic extended wet season. The study found that over two wet seasons, there was little evidence of freshwater inputs increasing nutrient concentrations, and chlorophyll a concentrations and phytoplankton productivity rates decreased in the water column, probably due to low water residence times. The magnitude and duration of freshwater flows in the wet season appeared to affect the scale of reduction of phytoplankton productivity and biomass accumulation. In contrast to many studies, there was also no evidence of post-flooding stimulation of chlorophyll a concentration with net export of nutrients in both the wet and dry seasons. Nitrogen (N) and light appeared to be key limiting factors for phytoplankton growth with estuarine DIN rapidly turned over by phytoplankton, no evidence of N fixation by phytoplankton, and a response to N, but not phosphorus (P), in algal bioassays. Tidal resuspension of sediments was an important physical process that limited light availability for primary productivity. The lack of higher nutrient concentrations as a result of freshwater inputs, and lack of post-flood algal growth stimulation contrasts with the findings of studies in eutrophied systems. in turn, fuel primary production in the months during and post-flooding once flow and turbidity have decreased, and salinity has increased (Mallin et al., 1993; Gillanders and Kingsford, 2002; Murrell et al., 2007). Wet and dry tropical/subtropical river systems account for 68% of Australian estuaries, meaning that they are an important contributor to the productivity of estuarine and coastal systems (Bucher and Saenger, 1994). Indeed studies have shown a correlation between catches of estuarine fish and crustaceans, and measures of freshwater flow which suggests that freshwater inputs may be providing nutrients and carbon to fuel productivity in estuaries (Gillanders and Kingsford, 2002; Robins et al., 2005). Conversely, estuarine productivity appears to be negatively affected if the upstream river is regulated, reducing the magnitude of high flow events (Kenyon et al., 2004; Burford et al., 2011). Eyre (2000) has proposed that estuaries of the wet and dry tropics/subtropics have four regime states throughout a typical year. During floods, freshwater flushes nutrients and sediments out to sea. This is followed by a recovery phase where turbidity begins to decrease, however low light availability prevents phytoplankton growth. In the medium flow phase, increasing light and nutrient availability stimulates phytoplankton biomass. Finally, in the dry season, phytoplankton become nutrient limited again, with a resulting decrease in biomass. This study therefore examined the role of freshwater nutrient inputs in stimulating phytopl...

[1] The variability in the inherent optical properties along an estuary-coast-ocean continuum in tropical Australia has been studied. The study area, the Fitzroy Estuary and Keppel Bay system, is a shallow coastal environment (depth < 30 m) with highly turbid waters in the estuary and blue oceanic waters in the bay and subject to macrotides. Biogeochemical and inherent optical properties (IOPs) were sampled in the near-surface layer spatially and across the tidal phase during the dry season. These determinations included continuous measurements of spectral absorption, scattering and backscattering coefficients, together with discrete measurements of spectral absorption coefficients of phytoplankton, nonalgal particles and colored dissolved organic matter, and concentrations of phytoplankton pigments and suspended matter. Because of a large variability in the characteristics of the water components on short spatial and temporal scales, we observe a large variability in the associated optical properties. From the estuary to the bay, particle scattering and dissolved absorption decreased by 2 orders of magnitude, and nonalgal particle absorption decreased by 3 orders of magnitude. We also observed a strong variability in particle single scattering albedo and backscattering efficiency (by a factor of 6) and in specific IOPs (IOPs normalized by the relevant constituent concentration) such as suspended matter-specific particle scattering and chlorophyll-specific phytoplankton absorption. Superimposed on this strong spatial variability is the effect of the semidiurnal tide, which affects the spatial distribution of all measured properties. These results emphasize the need for spatially and temporally adjusted algorithms for remote sensing in complex coastal systems.