1. Seagrass decline is often related to eutrophication, with sudden and drastic losses attributed to tipping points in nutrient loads. The identification of these threshold loads is an important step in the sustainable management of nutrient discharges.
2. In this study, a novel methodological approach is presented to simulate the spatial and temporal dynamics of coastal nitrogen loads, and its relationship to seagrass loss. The analysis allows the identification of nitrogen thresholds associated with the loss of Posidonia and Amphibolis spp. in Adelaide, South Australia.
3. The identified thresholds varied between 0.8 and 1.1 t N km-2 over 6 months, and explained up to 95% of all seagrass losses in the region.
4. Posidonia spp. was predominantly lost nearshore, indicating a threshold for this species in the higher end of the values reported here. In contrast, Amphibolis spp. was selectively lost further offshore, suggesting a lower threshold for the onset of fragmentation and decline.
5. Nitrogen pressure was concentrated over the wetter months of the year between late autumn and early spring. Pulsed pressure peaking in August likely affected thresholds by reducing resilience in the system.
6. Synthesis and applications. This study derives, for the first time, nitrogen load thresholds for the loss of the seagrasses Posidonia and Amphibolis spp., highlighting the low tolerance of these keystone temperate species to nitrogen pressure. The methodology developed as part of the work provides a tool to inform planning and improved management of nutrient discharges from land to facilitate seagrass conservation.
Daily nitrogen loads from rivers, industry and wastewater treatment plants discharging to the Adelaide coast in 1975, including coordinates. Loads calculated from data compiled during the Adelaide Coastal Waters Study and published by Wilkinson, J., White, N., Smythe, L., Hutson, J., Bestland, E., Simmons, C., Lamontagne, S. & Fallowfield, H. (2005) Volumes of inputs, their concentrations and loads received by Adelaide metropolitan coastal waters. ACWS Technical Report No. 18 prepared for the Adelaide Coastal Waters Study Steering Committee. pp. 83. Flinders Centre for Coastal and Catchment Environments (Flinders University of SA), Adelaide, South Australia.
GIS file showing seagrass cover in Adelaide in 1977 according to Hart, D. (1997) Near-shore seagrass change between 1949 and 1996 mapped using digital aerial orthophotography, metropolitan Adelaide area, Largs Bay-Aldinga, South Australia. A report prepared for the South Australian EPA. pp. 12. Department of Environment and Natural Resources, Adelaide, South Australia.
GIS file showing seagrass loss in Adelaide between 1970 and 1977 according to Hart, D. (1997) Near-shore seagrass change between 1949 and 1996 mapped using digital aerial orthophotography, metropolitan Adelaide area, Largs Bay-Aldinga, South Australia. A report prepared for the South Australian EPA. pp. 12. Department of Environment and Natural Resources, Adelaide, South Australia.
Computer code to calculate area specific loads in coastal areas.