Microbial control of ocean carbon and biogeochemical cycling in a changing polar ocean: Insights from ecosystem modeling of the West Antarctic Peninsula

The West Antarctic Peninsula (WAP) is one of the most rapidly warming regions in the Southern Hemisphere, with atmospheric and surface ocean warming evident for the last 50 years. Multi-decadal, routine monitoring efforts through the Palmer Long-Term Ecological Research (PAL-LTER; since 1991) have revealed varying responses of marine ecological and biogeochemical processes to changing environments. The wealth of PAL-LTER observations of food-web components also provides an important source for ecological modeling. To further study the WAP ecosystem response to changing climates, we have developed a one-dimensional data assimilation ecosystem model based on a variational adjoint scheme. By assimilating eleven types of PAL-LTER observations and optimizing model parameters, the model generates dynamic simulations of carbon stocks and flows mediated via complex marine food-web interactions. We optimized the models specific to high and low sea-ice years and conducted temperature perturbation experiments to understand microbial processes in warming oceans. Both ice years were characterized by recycling favorable microbial food-webs with similar microbial loop activity. In the high sea-ice year, annual mean depth-integrated net primary production (NPP) and bacterial production were substantially higher, while depth-integrated net community production (NCP) and carbon export were significantly lower due to high heterotrophic respiration and a combination of low NCP and a slow optimized detrital particle sinking speed. The temperature perturbation experiments showed that NPP, carbon export, and bacterial activity all increased as a result of warming temperatures. Our model results partly challenge the conventional views learned from observational studies on the WAP dynamics by revealing mechanisms involved in key microbial flows and ecological interactions.