Estuaries are dynamic systems which can support diverse benthic communities. Benthic macrophytes can play an important role in nutrient cycling and may act as substantial sources or sinks for nutrients within these systems. To assess nutrient processes within an estuary it is important to understand nutrient cycling between these benthic communities and the estuary. Nutrient cycling by the seagrass Halophila ovalis (R.Br.) Hook f. in the Swan Canning estuary was investigated as were aspects of their physiology that enable these plants to survive in these unique systems.

To assess the role of H. ovalis meadows in nutrient cycling, seasonal variations in biomass within the seagrass meadows were established and related to physical changes in the estuary. H. ovalis biomass followed a seasonal pattern with low biomass in winter when salinity, temperature and light were limiting and high biomass in summer. Of the physical parameters measured, salinity was the factor that best described variations in H. ovalis biomass.

Concentrations of inorganic nitrogen and phosphorus in the water column and sediment porewater were assessed over an annual cycle, as were tissue nutrients of H. ovalis. Relationships between external and internal nutrient concentrations were also examined. Nitrogen and phosphorus concentrations in the estuary followed different seasonal patterns. Phosphorus concentrations in the water column and sediment porewater were constant over the study period, whereas nitrogen concentrations varied seasonally. Concentrations of nitrogen in the estuary increased in winter with the onset of rains, and decreased rapidly in spring and summer. H. ovalis tissue nutrients also showed a seasonal pattern, and concentrations were higher in winter than over summer. Results suggested H. ovalis may take up and store nutrients in spring in preparation for growth in summer when nitrogen in the water column becomes severely depleted.

H. ovalis tissue nutrient concentrations were used to assess if external nutrient concentrations could potentially limit growth in summer. Tissue nitrogen concentrations were well below quoted literature values, suggesting that in summer when biomass was high, plant growth may be limited by nitrogen. Ammonium uptake experiments were conducted to elucidate whether H. ovalis could meet the nitrogen requirements for plant growth by root uptake, when nitrogen concentrations in the water column were depleted. H. ovalis could take up ammonium from both water column and sediment porewater. Root uptake was independent of shoot uptake suggesting H. ovalis may rely primarily on sediment nitrogen for growth. In summer when nitrogen in the water column is severely depleted, H. ovalis can utilise sediment nitrogen pools for plant nitrogen requirements.

To understand the potential impact of oxygen loss from H. ovalis roots on sediment nutrient dynamics, experiments were set up to investigate spatial patterns of oxygen loss along roots. Radial oxygen loss (ROL) was measured as a function of distance behind the root apex. Results showed that oxygen lost radially from the roots was photosynthetically derived and H. ovalis contained a barrier to ROL in the more basal regions of the root. These were the first experiments to elucidate spatial patterns of ROL along seagrass roots, a feature shown to be an important adaptation of roots of emergent wetland plants to growth in anaerobic sediments. Oxygen loss from seagrass roots into the sediment may increase nitrogen transformations directly by increasing nitrification and nitrogen fixation and indirectly by increasing decomposition and thus increasing ammonification. Oxygen loss was localised around the root tip and the distance to which this influence extends is unknown. Although oxygen loss was concentrated around the root tip, this increase may increase nitrogen transformations in the sediment. In turn, this may increase the availability of nitrogen in sediment porewater, thereby increasing the availability of nitrogen for plant uptake.

In conclusion, the features of H. ovalis enable the plant to take up and exploit nutrients from the system when external nutrient resources are in abundance. H. ovalis has the ability to store these nutrients and, when external nutrients in the water column are insufficient to support growth, translocate and utilise these internal resources. At these times of year H. ovalis meadows act as a substantial sink for nutrients in the estuary. In contrast, in winter when there are large losses in plant biomass, H. ovalis meadows become a source for nutrients in the estuary. This study has illustrated features of this plant that allow H. ovalis to succeed in an environment where nutrient availability is transient. Halophila ovalis acts as both a sink and a source of nutrients in the Swan Canning estuary, demonstrating its importance in nutrient cycling on a system scale.