Nutrient status of seagrasses cannot be inferred from system-scale distribution of phosphorus in Shark Bay, Western Australia

Abstract

Differences in phosphorus (P) availability can influence the ecology and physiology of seagrass communities; and are usually inferred from changes in the relative P content in seagrass leaves. Shark Bay is a subtropical marine embayment, with decreasing P concentrations in the water column and sediments from north to south across the entire embayment. We examined the P and nitrogen (N) content of seagrass leaves and P content of sediments across the Faure Sill and Wooramel delta region of Shark Bay, to determine whether the leaf content of seagrasses in Shark Bay also decreased from north to south over smaller spatial scales. Nutrient content of Amphibolis antarctica and Halodule uninervis were highly variable and were not strongly correlated with sediment P concentrations. Mean N : P ratios of seagrasses (<33.5) were not indicative of P limitation, as has been previously assumed for Shark Bay. We conclude that availability of P for uptake by seagrasses across Shark Bay may be highly localised and cannot be predicted from system-scale gradients (>100 km) of sedimentary P distributions. We suggest that P availability to seagrasses is more likely a complex function of differing nutrient inputs, rates of delivery to the plants and cycling rates.

Extreme temperatures, foundation species, and abrupt ecosystem change: an example from an iconic seagrass ecosystem

“…High seed abortion rates associated with a marine heat wave have been reported for Posidonia oceanica, a congener of P. australis , in the Mediterranean Sea (Balestri & Cinelli, ). The high abortion of P. australis seeds in Shark Bay during the 2011 heat wave indicates that this species is also at the extreme of its thermal tolerance in this region, and high salinities (42ppt) and low nutrient availability (Fraser et al ., ; Burkholder et al ., ) probably exacerbate the stress of clonal growth combined with sexual reproduction. Sexual reproduction requires a significant investment of resources from maternal plants (Hocking et al ., ), and in Shark Bay's nutrient‐impoverished embayment, abortion soon after the beginning of development appears to be a strategy to avoid wasting resources (fruit growth, seed development, and nutrient reserves).…”

“…Expanding on studies of Shark Bay's seagrasses conducted prior to and immediately following the heat wave (Fraser et al ., ; Burkholder et al ., ), we used in‐water surveys and harvested plants to determine the impact of the heat wave on A. antarctica coverage and biomass and assess the responses of other, less common, seagrasses. Concurrently, we used animal‐borne video footage to quantify changes in A. antarctica condition, a proxy for habitat quality, experienced by seagrass‐associated foragers (sea turtles) after the heat wave compared with a decade earlier.…”

Extreme climate events lower resilience of foundation seagrass at edge of biogeographical range

“…Shark Bay is oligotrophic, with phosphorus concentrations below detectable limits in the water column (Fraser et al . ). Shark Bay contains some of the most extensive seagrass meadows in the world, and contains up to 12 different species of seagrass (Walker, Kendrick & McComb ), making it a global hotspot of seagrass diversity.…”

Heat stress of two tropical seagrass species during low tides – impact on underwater net photosynthesis, dark respiration and diel in situ internal aeration

“…Tissue total N was measured on an CHN analyser-mass spectrometer consisting of a 20/22 mass spectrometer connected to an ANCA-S1 preparation system (Sercon, Crewe, UK) at the Western Australian Biogeochemistry Centre at UWA. For detailed procedures, see Fraser et al (2012).…”