Kinetics of nitrogen and phosphorus release in varying food supplies by Daphnia magna

Rates of nitrogen and phosphorus release from individual Daphnia magna were determined by measuring ammonia and soluble reactive phosphorus in successive 10-min incubations in small (0.05 ml) vessels after the animals were removed from their food. Release rates of both nutrients were generally highest initially and decreased with time after removal. The ratio of nitrogen to phosphorus released increased with time after animals were removed from an artificial detritus/bacterial food; ratios were lower and changed with time less for animals fed algae. These data suggest errors may be introduced by assumptions of constant stoichiometry for nutrient release in varying environments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42921/1/10750_2004_Article_BF02185425.pd

Sources of organic carbon in the Torrens River system

http://nrm.sa.gov.au/nrmresearch/display.php?table=research_project&id=313

The loss of aquatic and riparian plant communities: implications for their consumers in a riverine food web

Copyright © 2008 The Authors Journal compilation © 2008 Ecological Society of AustraliaHuman induced alterations to rivers and steams have resulted in significant changes to the structure and diversity of riparian and aquatic plant communities. These changes will impact on the dynamics of riverine carbon cycles and food web structure and function. Here we investigate the principal sources of organic carbon supporting local shredder communities across a gradient in different levels of anthropogenic development along riverine reaches, in South Australia. In forested/wooded reaches with minimum to limited development, semi-emergent macrophytes were the principal sources of organic carbon supporting the local shredder communities. However, in developed reaches, course particulate organic matter and filamentous algae were the principal food sources. The C:N ratios of the food sources in developed reaches were higher than those of their consumers indicating a stoichiometric mismatch. This imbalanced consumer-resource nutrient ratio in those developed reaches is likely to impose constraints on the growth and reproduction of their aquatic shredder communities with probable knock-on effects to higher trophic levels.Brian M. Deegan and George G. Gan

Variations in the buoyancy response of Microcystis aeruginosa to nitrogen, phosphorus and light

Copyright © 2001 Oxford University PressMicrocystis aeruginosa displays a range of variability in buoyancy in response to light which is dependent upon the previous nutrient or light history of the cell. The short-term buoyancy response to light is nested into a longer term response, over a period of days, as cyanobacteria respond to their nutrient and light climate, which may be manifested in their gas vesicle volume, photosynthetic rate and carbohydrate metabolism. Microcystis aeruginosa cultures were subjected to a range of nitrogen (N), phosphorus (P) or light pre-treatments before exposure to saturating irradiance, and these results are used to illustrate that populations can display a range of buoyancy responses dependent upon subtle changes in nutrient and light conditions. N-limited (0 µM) cells suffered a dilution in gas vesicle volume and increased carbohydrate content, which resulted in a loss of buoyancy. Cells in 10 µM N increased their gas vesicle content during light exposure; however, this was insufficient to maintain buoyancy in the majority of cells as carbohydrate increased. Cells in 100 µM N increased their gas vesicle volume, metabolized carbohydrate more efficiently than the N-limited treatments and retained positive buoyancy. During light exposure, there was minimal change in buoyancy in 0 µM P pre-treated cells, although there was a dilution in gas vesicle volume and an increase in carbohydrate. In 0.5 µM P pre-treated cells, the proportion of individuals floating did not change, although gas vesicle volume and carbohydrate increased. There was a significant increase in gas vesicle volume in 10 µM P pre-treated cells during light exposure, and some buoyancy loss due to carbohydrate accumulation. Cells grown in the range of light intensities tested all increased their gas vesicle volume during the 24 h light treatment. The actual rate of gas vesicle production was equivalent for all three light pretreatments; however, the growth-dependent dilution rate was determined by the previous light history, which resulted in different relative gas vesicle volume per cell. There was minimal increase in carbohydrate per cell during the light period, yet all treatments displayed the classical buoyancy loss response.Justin D. Brookes and George G. Gan

A comparison of growth responses between two species of Potamogeton with contrasting canopy architecture

Copyright © 2001 Elsevier Science B.V. All rights reserved.David Cenzato and George Ganfhttp://www.elsevier.com/wps/find/journaldescription.cws_home/503303/description#descriptio

Influence of stomatal conductance on the efficiency of internal pressurisation in Typha domingensis

Since the pores connecting the lacunae (the site of pressurisation in emergent macrophytes) with the atmosphere include the stomatal aperture, it was hypothesised that stomata may influence the effectivity of pressurisation in Typha domingensis. To investigate this, stomatal conductance and effectivity were measured over the diel period. The relative mean pore diameter was predicted using a derivative of the Poiseuille-Hagen equation, and also related to effectivity. Both the stomatal conductance and the relative mean pore diameter were found to be correlated to effectivity (r2=0.95 and 0.93, respectively) suggesting stomatal conductance did influence the ability to pressurise. As the baseline effectivity (e.g., when the stomata were fully open) was 1.4% it is suggested that while the stomatal aperture is the primary tissue involved in the ability to pressurise (contributing up to ca. 30% to the effectivity), other tissues between the lacunae and the atmosphere aid the proces

Water regimes, seedling recruitment and establishment in three wetland plant species

The regeneration niche of three wetland species that co-occur at Bool Lagoon, South Australia, was investigated under nine hydrologic conditions. Typha domingensis grown from seed had the broadest niche requirements; seeds germinated and seedlings were established in all 9 hydrologic regimes, and asexual reproduction occurred in 5 of the 9 regimes, Whether asexual reproduction occurred was dependent upon an interaction between the rate of leaf elongation, the rate of drawdown and whether the leaf was able to broach the water surface. The buoyant nature of seeds and seedlings of Triglochin procerum allowed it to avoid unfavourable regeneration niches. Melaleuca halmaturorum had a narrow regeneration niche that was confined to wet mud flats. The results are consistent with the changes in the floristic composition of the lagoon

Potential for algal blooms in tropical rivers of the Northern Territory, Australia

© CSIRO 2007Rivers in the Northern Territory (NT) of Australia are under pressure from increasing vegetation clearance, land use and nutrient run-off. The literature on algal blooms clearly identifies the predisposing factors but in the NT, these factors are not well researched. We report on the potential for tropical rivers to experience problems related to algal growth. NT rivers were found to have a low nutrient status and a viable inoculum of blue-green, brown and green algal communities. The growth response of these algal groups to nutrient enrichment via bioassays and pulse-amplitude-modulation (PAM) fluorometry measurements varied among rivers and the addition of N, P or N&P. However, the overwhelming findings were that all rivers had the potential to experience algal blooms with enrichment. Back-calculations based on the chlorophyll concentrations recorded in bioassay experiments and stoichiometric ratios of chlorophyll:nutrients suggest there are pools of biologically available organic forms of N and P in addition to inorganic forms. The role of river length in the development of algal blooms was investigated: the longer the river reach, the slower the flow, and the greater the availability of nutrients, the higher potential for algal blooms. Given the strong indications that increased nutrient run-off to tropical rivers will result in algal blooms, prudent land-use and development with nutrient management strategies is essential.George G. Ganf and Naomi Re