Time‐varying responses of lake metabolism to light and temperature

Abstract

AbstractLight is a primary driver of lake ecosystem metabolism, and the dependence of primary production on light is often quantified as a photosynthesis‐irradiance or “P‐I” curve. The parameters of the P‐I curve (e.g., the maximum primary production when light is in excess) can change through time due to a variety of biological factors (e.g., changes in biomass or community composition), which themselves are subject to external drivers (e.g., herbivory or nutrient availability). However, the relative contribution of variation in the P‐I curve to overall ecosystem metabolism is largely unknown. I developed a statistical model of ecosystem metabolism with time‐varying parameters governing the P‐I curve, while also accounting for the influence of temperature. I parameterized the model with dissolved oxygen time series spanning six summers from Lake Mývatn, a shallow eutrophic lake in northern Iceland with large temporal variability in ecosystem metabolism. All of the estimated parameters of the P‐I curve varied substantially through time. The sensitivity of primary production to light under light‐limiting conditions was particularly variable (>15‐fold) and had a compensatory relationship with ambient light levels. However, the 3.5‐fold variation in the maximum potential primary production made the largest contribution to variation in ecosystem metabolism, accounting for around 90% of the variance in net ecosystem production. Much of the variation in maximum primary production was attributable to cyanobacterial blooms, which occur in some but not all years in Mývatn. Overall, these results illustrate how changes in the P‐I curve contribute substantially to temporal variation in lake ecosystem metabolism.