Scaling Potential Impacts of Ocean Acidification to the Ecosystem Level

Ocean acidification will affect many different marine organisms and also has the potential to reorder entire marine ecosystems. For example, acidification-induced changes in some species’ growth and survival rates could create a new balance among primary producers, prey, and predators. By affecting other aspects of physiology, such as sensory capabilities or metabolic rate, ocean acidification could influence interactions among predators and prey.  Ecological models provide a unique way to scale our understanding of ocean acidification from single-species laboratory studies to projections of whole ecosystem response.

I will present results from scenarios run in Ecopath with Ecosim food web models of Puget Sound and the North Pacific which focus on how ocean acidification could affect ecosystem services, those benefits to humankind provided by natural ecosystems (e.g., seafood, recreation). These ocean acidification scenarios impose changes in productivity on some species groups (direct effects), while still allowing other species groups in the food web to respond to the scenarios in a dynamic way (indirect effects). Sometimes, the direct and indirect effects of ocean acidification countered each other due to interactions between predators and prey within the food web, leading to less change in the food web than expected.  In other cases, direct and indirect effects acted synergistically, causing a greater change in the food web than anticipated from direct effects alone.  Our results were strongly affected by the species on which we imposed direct effects of ocean acidification. This finding emphasizes the importance of knowledge on species susceptibility to ocean acidification for modeling exercises.  

A challenge with identifying susceptible species is that most ocean acidification experiments expose individuals to treatments representative of global average atmospheric CO₂ concentrations. However, few organisms experience global atmospheric CO₂ concentrations for extended periods of time; instead, they experience pCO₂ conditions that vary greatly with time and space. Projecting species response to elevated future CO₂ levels requires a more nuanced understanding of how species respond to the range of pH values that they experience under current conditions. While there is much uncertainty in our predictions of how ocean acidification will affect ecosystems, focusing on the complex interactions among species and between species and their dynamic environment will bring us closer to understanding how marine ecosystem may respond to future changes in ocean chemistry.