Seasonal variation in light response of polar phytoplankton

The seasonality of light response curves was observed in phytoplankton samples taken in the Eastern Bering Sea (EBS) shortly before and during the spring bloom. Under-ice samples were found to have lower values of both the maximum growth rate (μ0) and the initial slope (α) of the photosynthesis-irradiance (PE) curve. This trend in α was also noted in a literature review of photoacclimation studies that looked at acclimation periods of 30 days or more. A trade-off is proposed between α and maintenance respiration such that below the compensation intensity EC it becomes advantageous to decrease α to mitigate the costs of respiration. An existing NPZD model of the EBS was then extended to reflect this trade-off with a seasonal transition from low to high α, and likewise μ0, at the point where available light is greater than EC. A parameter analysis found that with this seasonal plasticity the model could accurately reproduce the timing and magnitude of the 2009 spring bloom using parameter combinations within realistic ranges. Without this seasonality, no parameter set could be found that reasonably reproduced the observations. This strongly suggests that ecosystem models of phytoplankton should consider the effects of seasonality within parameters, including α which may be lower in over-wintering populations