Eric I. Paling

Nitrogen uptake and allocation in the seagrass Amphibolis antarctica

Abstract Ammonium acquisition and internal allocation of nitrogen in the seagrass Amphibolis antarctica ((Labill.) Sonder ex. Aschers.) were studied on freshly collected plants in laboratory experiments. The uptake kinetics were studied from the depletion of ammonium in split chamber experiments, while N uptake by entire plants and internal allocation patterns were studied using 15 N techniques on culture plants. The uptake of ammonium was concentration dependent and followed Michaelis-Menten kinetics. Maximum uptake rates for leaves were 5–38-fold higher than for the root-rhizome complex and the ammonium uptake by leaves was transiently enhanced when plants were suddenly exposed to ammonium. Transiently elevated uptake rates were relatively short-lived and only significant at very high substrate concentrations. Amphibolis rarely, or never, experience nitrogen concentrations that high, and so, surge uptake has only little ecological relevance. The uptake of ammonium at low and ecologically relevant substrate concentrations could only supply about 70% of the nitrogen demand of rapid Amphibolis growth during summer and the remaining 30% had to be met from internal sources. The 15 N experiments showed that both young and old leaves took up nitrogen but most of the nitrogen taken up by old leaves was immediately exported to young actively growing plant parts. Also, nitrogen was re-mobilized and subsequently exported from old to young plant parts. This export of re-mobilized nitrogen could supply about 36% of the nitrogen incorporated into young actively growing tissues, thus lowering the demand for external nitrogen by an equivalent amount. Re-mobilization and subsequent allocation of nitrogen from old plant tissues seem to be an important way to reduce the demand for external nitrogen in Amphibolis and, therefore, this seagrass seems well adapted to sustain rapid growth in nutrient-poor environments.

Autumn biomass, below-ground productivity, rhizome growth at bed edge and nitrogen content in seagrasses from Western Australia

Below-ground biomass and productivity were determined on one occasion in autumn for four local meadow-forming species of seagrass from the genera Posidonia and Amphibolis (P. sinuosa Cambridge and Kuo, P. australis Hook. f., A. antarctica (Labill.) Sonder and Aschers and A. griffithii (Black) den Hartog. In P. australis and P. sinuosa from Cockbum Sound, 59 and 48%, respectively, of living material was below-ground. Detritus within the sediment was of the same magnitude or exceeded the living material. In Amphibolis species, there was a greater proportion of living material above- ground than below (A. antarctica 82% and A. griffithii 72%). Tissue nitrogen concentrations in leaves, roots and rhizomes were 5-17 mg N g-1 dry weight, and were higher in leaves of Posidonia than Amphibolis. The total nitrogen content, on a m-2 basis, was dominated by leaves in all species. The percentage of below-ground to above-ground autumn productivity varied from 15% in Amphibolis to 25% in Posidonia. Rhizome growth at the edge of the meadow was predicted to be 796±79 mm per year for P. australis, 576±42 mm per year for P. sinuosa, 365±49 mm per year for A. antarctica and 146±24 mm per year for A. griffithii, the first record of rhizome growth rates of these species in Western Australia.

Improving mechanical seagrass transplantation

Until recently seagrass transplantation efforts have met with limited success in areas with high wave energies. Survival in Western Australia has been markedly improved by the deployment of large, mechanically transplanted units which provide sufficient anchorage to overcome water motion. ECOSUB1 was an underwater seagrass harvesting and planting machine designed to extract and plant large seagrass units with minimal disturbance. Over 2000 sods have been planted, with an average survival of approximately 70% over 3 years. New machines (ECOSUB2) have now been constructed to improve efficiency; these are located semi-permanently on the seafloor and allow for concurrent seagrass harvesting and planting.

Nitrogen and phosphorus uptake in seedlings of the seagrass Amphibolis antarctica in Western Australia

The uptake of nitrate, ammonium and phosphate was examined in vitro in seedlings of the seagrass Amphibolis antarctica ((Labill.) Sonder ex Aschers.). Uptake of all three nutrients was significantly correlated with external concentration up to 800 µ g l−1. The uptake of nitrate (0–200 µ g NO3-N g dry wt−1 h−1) was significantly lower than the uptake of ammonium (0–500 µ g NH4-N g dry wt−1 h−1), suggesting that the seedlings have a higher affinity for this form of nitrogen in the water column.Data were in general agreement with uptake rates recorded for other seagrasses, notably Zostera marina. In comparison to the dominant macroalgae for the same region, seedlings had either similar or higher uptake rates in relation to external concentration, lending support to the hypothesis that seedlings, which do not possess roots, behave like macroalgae in terms of nutrient acquisition from the water column.A comparison with literature data on adult seagrass suggests, however, that seagrasses show lower uptake rates than macroalgae suggesting that the macroalgae, which are totally reliant on the water column for nutrients, are more efficient at uptake than seagrasses which may potentially use the sediment for a nutrient source.