Spatial pattern in seagrass stoichiometry indicates both N-limited and P-limited regions of an iconic P-limited subtropical bay

Abstract

We investigated seagrass species distribution and nutrient content in the iconic phosphorus-limited Shark Bay, Western Australia. We found the slower-growing, temperate species Amphibolis antarctica and Posidonia spp. had lower N and P content compared to the fastergrowing tropical species Halodule uninervis, Syringodium isoetifolium, Cymodocea angustata, Halophila ovalis and Halophila spinulosa. Further, by comparing elemental content of different seagrass species at sites where species co-occurred, we were able to standardize seagrass elemental content across sites with different species composition. This standardization allowed us to make ecosystem-scale inferences about resource availability despite taxon-specific distributions and elemental content. We found a marked spatial pattern in N:P of seagrasses across the system, indicating that P limitation occurred, despite calcium carbonate sediments, only in the most isolated portions of the bay. Large areas closer to the mouth of the bay were either N limited or were not limited by N or P availability. Our results suggest that large-scale nutrient budgets may oversimplify our understanding of limiting factors in a system, resulting in management decisions that may have unforeseen effects on different areas within the same ecosystem.