Abstract
The dominant seagrass in Port Phillip Bay (PPB), Australia, Zostera nigricaulis, declined between 2000 and 2011, coinciding with the ‘Millennium drought’ that ended in 2009. These seagrasses are nitrogen-limited, underpinning the need to develop nitrogen budgets for better ecosystem management. Environmentally realistic measurements of specific uptake rates and resource allocation were undertaken to develop nitrogen budgets and test the hypothesis that the above-ground and below-ground compartments are able to re-mobilise ammonium and nitrate through uptake, translocation and assimilation to adapt to varying levels of nitrogen in the ecosystem. Uptake of 15N labelled ammonium and nitrate by above- and below-ground seagrass biomass, epiphytes and phytoplankton was quantified in chambers in situ. Preferential uptake of ammonium over nitrate was observed, where the uptake rate for nitrate was about one sixth of that for ammonium. Epiphytes and phytoplankton also registered an increased affinity for ammonium over nitrate. Translocation experiments demonstrated the uptake by both the above-ground and below-ground biomass, respectively from the water column and pore water, and subsequent translocation to the opposite compartment. Acropetal translocation (below- to above-ground biomass) was more prevalent than basipetal translocation. This is a unique outcome given basipetal translocation has been widely reported for Zostera by other researchers.
Highlights
Background samplesCored seagrass samples for biomass estimation was rinsed in clean, filtered seawater, and cleaned of drift algae, epibionts, dead leaves and sediments
This study has demonstrated the uptake of nutrients by the above-ground and below-ground seagrass biomass from the water column and pore water, and the subsequent translocation to the opposite compartment
Zostera nigricaulis in Phillip Bay (PPB) demonstrated a clear preference for ammonium over nitrate as the source of nitrogen
Summary
Cored seagrass samples for biomass estimation was rinsed in clean, filtered seawater, and cleaned of drift algae, epibionts, dead leaves and sediments. Wet weight measurements of the above-ground biomass, below-ground biomass and epiphytic biomass were made. Moisture content of the sub-samples of the above- and below-ground biomass and epiphytes was assessed gravimetrically after freeze-drying the samples. The above- and below-ground biomass were expressed on a dry weight basis. Epiphytes were scraped off the above-ground biomass. Epiphyte loading was deducted from the above-ground biomass to obtain the corrected above-ground biomass values on a dry weight basis; these values were used for subsequent calculations
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