Uptake of inorganic phosphorus by temperate seagrass beds of Posidonia and Amphibolis in Southern Australia.

Seagrasses occupy a narrow band of sandy seabed close to the coast and are therefore vulnerable to anthropogenic influences, particularly meadows near large population centres. Over 5000 ha of seagrasses have been lost from Adelaide coastal waters (South Australia) over the past 70 years and much of this loss has been attributed to nutrient inputs from wastewater, industrial and stormwater discharges. This led to the Adelaide Coastal Waters Study to understand processes along the Adelaide metropolitan coast that led to seagrass loss. This study, a subset of the larger ACWS study, used in situ nutrient spike approach to obtain ecologically relevant estimates of seasonal variability in phosphorus uptake in two species of temperate seagrass common to this coast (Amphibolis antarctica and Posidonia angustifolia). Total uptake of phosphorus by biological components in the seagrass beds, viz., seagrass, epiphytes and phytoplankton, was negligible, never exceeding 0.5% of the total resource. Phosphorus uptake rate varied seasonally with higher rates in winter (1.49 μmol P.g(-1) DW.h(-1)) and lower rates in spring (0.70 μmol P.g(-1) DW.h(-1)) for Amphibolis and highest in winter (2.09 μmol P.g(-1) DW.h(-1)) and least in spring (0.14 μmol P.g(-1) DW.h(-1)) for Posidonia. Low biological uptake rates of inorganic phosphorus could be attributed to carbonate sediments and particulates in the water column binding inorganic phosphorus, limiting its availability for biological uptake. From an environmental perspective, seagrass beds in the Adelaide coastal waters account for the assimilation of only 5.4% (19.53 t yr(-1)) of the total anthropogenic inputs of phosphorus.