Showing papers on "Amphibolis antarctica published in 2013"

Eutrophication offsets increased sea urchin grazing on seagrass caused by ocean warming and acidification

Abstract: The accumulation of atmospheric [CO 2 ] continues to warm and acidify oceans concomitant with local disturbances, such as eutrophication. These changes can modify plant-herbivore grazing interactions by affecting the physiology of grazers and by altering the nutritional value of plants. However, such environmental changes are often studied in isolation, providing little understanding of their combined effects. We tested how ocean warming and acidification affect the per capita grazing by the sea urchin Amblypneustes pallidus on the seagrass Amphibolis antarctica and how such effects may differ between ambient and eutrophic nutrient conditions. Consistent with metabolic theory, grazing increased with warming, but in contrast to our expectations, acidification also increased grazing. While nutrient enrichment reduced grazing, it did not fully counterbalance the increase associated with warming and acidification. Collectively, these results suggest that ocean warming and acidification may combine to strengthen top-down pressure by herbivores. Localised nutrient enrichment could ameliorate some of the increased per capita grazing effect caused by warming and acidification, provided other common negative effects of eutrophication on seagrass, including overgrowth by epiphytes and herbivore aggregation, are not overwhelming. There is value in assessing how global and local environmental change will combine, often in non-intuitive ways, to modify biological interactions that shape habitats.

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 phos- phorus-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 faster- growing 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 ele- mental 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 iso- lated 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 over- simplify our understanding of limiting factors in a system, resulting in management decisions that may have unforeseen effects on different areas within the same ecosystem.