Eutrophication offsets sea urchin grazing on seagrass caused by warming and OA

Amblypneustes pallidus in a Posodonia seagrass meadow. Photo: Owen Burnell

Amblypneustes pallidus in a Posodonia seagrass meadow.
Photo: Owen Burnell

The title to this blog seems a bit counterintuitive, almost like eutrophication is a good thing. Don’t believe that for a second! In a recently published paper, Owen Burnell of the University of Adelaide presents some interesting data on the interactions between eutrophication (an all too common local stressor), ocean acidification and warming (both increasingly alarming stressors of global origin). As I keep discovering, interactions between these stressors never seem to turn out the way we expect:

The accumulation of atmospheric [CO2] 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.

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