The response of two coupled one-dimensional mixed layer/planktonic ecosystem models to climate change in the NE subarctic Pacific Ocean

In this paper, we report on simulations of ecosystem responses to climate change with two planktonic ecosystem models, both coupled to a one-dimensional mixed-layer model run with annual wind and solar heating from Ocean Station P (50°N, 145°W) in the NE subarctic Pacific. The first ecosystem model is a four-component model previously tested with extensive observations from OSP (Deep-Sea Res. II 46 (1999) 2877). The second ecosystem model is more complex, including phytoplankton partitioned into two size classes, and imposed grazing by mesozooplankton, which varies in time according to long-term observations from OSP. Both models include temperature dependence of physiological rates. Two possible climate change scenarios are considered: (i) increasing ocean temperatures by 2°C (and 5°C) applied only to the ecological component, and (ii) changing the availability of iron to phytoplankton in the subarctic Pacific. Responses of the two models are similar, indicating that they are not primarily model-dependent. In the warming cases, annual behavior and average standing stocks decrease marginally (⩽10% for T=2°C, and ⩽22% for T=5°C, second model only), ecosystem recycling increases with warming, and losses of organic particles to the ocean interior decrease (∼10%) in the simpler model or increase slightly (<10%) in the complex model. Removal of any limitation by iron on phytoplankton growth changes phytoplankon standing stocks by 12% or less, but increases standing stocks of microzooplankton by 150% in the simple model and 225% in the complex model. The loss or export of organic particles to the ocean interior, indicative of the rate at which the ecosystem can sequester carbon, increases ∼20% in the first model and 37% in the second model, all of the increase in the second model via grazing by the mesozooplankton. The winter-to-summer drawdown of surface layer nitrate increases in all the climate change simulations. Sensitivity of the second model for a warming of T=2°C to changes in the strength of temperature dependence of the physiological rates was generally small, except for changes in maximum microzooplankton biomass with increased dependence of their physiological rates. Increasing the temperature dependences of all physiological rates accentuated the vertical gradient in physiological rates resulting from the vertical temperature gradient, similar to what might be expected with increased thermal stratification.