A prevailing view exists in the literature which suggests that macroalgae growing in slow-flow environments (<4 cm s⁻�) are less productive because of "mass-transfer" limitation compared to fast-flow environments. Macroalgae in slow-flow environments are thought to have thicker diffusion boundary-layers which limit the flux of essential molecules to and from the algal thallus. However nutrient demand of a macroalga can also influence nutrient flux. The main objective of this research was to determine the relative importance of physical (mainstream velocity) and physiological (nutrient demand) factors influencing the growth rate of Adamsiella chauvinii, a small (<20 cm) red algal species, that grows within the benthic boundary-layer in a soft sediment habitat. To establish the influence of water velocity, the growth rate of A. chauvinii was measured in situ each month (March 2003 to March 2004) at three sites with varying degrees of water velocity (slow, intermediate and fast) at which all other environmental parameters (photon flux density, seawater temperature and nutrients) were similar. To determine the metabolic demand and nutrient uptake rate of A. chauvinii, the internal nutrient status (C:N, soluble tissue nitrate, ammonium and phosphate), uptake kinetics (V[max] and K[s]) and nutrient uptake rate at a range of mainstream velocities were also determined on a seasonal basis. The hydrodynamic environment around A. chauvinii canopies was characterised in situ and compared with controlled laboratory experiments.
Growth rates of Adamsiella chauvinii thalli at the slow-flow site were significantly lower in winter (June) to summer (February) than the intermediate- and fast-flow sites, while in autumn growth rates were similar between sites. However, A. chauvinii at the slow-flow site had similar or higher tissue N content compared to thalli at the other two sites during winter, spring and summer suggesting that growth rates of A. chauvinii were not mass-transfer limited. Nitrogen uptake rates of A. chauvinii were similar between sites in summer and winter, however uptake rates were lower in summer compared to winter even though thalli were nitrogen limited in summer. Water velocity had no effect on nitrate uptake in either summer or winter and uptake of ammonium increased with increasing water velocity during summer only. Two hydrodynamically different environments were distinguished over a canopy of A. chauvinii, with both the laboratory and field velocity profiles in good agreement with each other. In the top half of the canopy, the Turbulent Kinetic Energy (TKE) and Reynolds stresses were greatest while in the bottom half of the canopy flow rates were less than 90 % of mainstream velocity (< 1 cm s⁻�).
When considered together, the influence of water velocity on the growth rates of A. chauvinii was not completely clear. Results suggest that mainstream velocity had little influence on nutrient availability to A. chauvinii because of the unique hydrodynamic environment created by the canopy. Nutrients, especially ammonium and phosphate, derived from the sediment and invertebrates, may provide enough nitrogen and phosphate to saturate the metabolic demand of Adamsiella chauvinii, consequently, A. chauvinii is well adapted to this soft-sediment environment.
| Identifer | oai:union.ndltd.org:ADTP/266197 |
| Date | January 2007 |
| Creators | Kregting, Louise Theodora, n/a |
| Publisher | University of Otago. Department of Botany |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Louise Theodora Kregting |
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