On the influence of stratification and lake size on pelagic and benthic algal dynamics: A modelling study using 'Lake2D'

This study investigates the influence of mixed layer depth and lake size on the dynamics of pelagic and benthic algae in stratified lakes using a two-dimensional, spatially explicit modeling approach. To this end, we modified the process-based, two-dimensional model Lake2D to account for stratification in lakes by simulating a shallow layer with fast turbulent mixing, and a deep layer with slow turbulent mixing, separated by a sharp boundary. Using this modified version of Lake2D we comprehensively explored pelagic and benthic algal dynamics across a wide range of ecological conditions by varying lake mean depth, total nutrient content, lake size, and mixing depth in a full-factorial manner. Our results reveal distinct regimes of whole-lake algal dynamics: In shallow, nutrient-poor lakes, pelagic algae are nutrient-limited and sparse, while benthic algae thrive and constitute the majority of the total biomass, with no notable influence of mixed layer depth on algal dynamics. As lakes become deeper or more nutrient-rich, pelagic biomass increases and can become the dominant constituent of total biomass. For deep lakes, the model shows a characteristic L-shaped pattern in algal biomass for varying lake size and mixing depth. Pelagic algae exhibit low biomass in small lakes with a deep mixed layer, and high biomass otherwise. Benthic algae show the opposite pattern, exhibiting high biomass in small, deep lakes with a deep mixed layer, and significantly lower biomass otherwise. The study also uncovers a novel phenomenon, a benthic deep chlorophyll maximum, where the benthic algal biomass peaks at an intermediate depth along the lake bottom. This phenomenon arises in lakes of moderate depth and nutrient content, where a shallow mixed layer enables opposing vertical gradients of light and nutrients. These findings highlight the role of mixed layer depth and horizontal transport in structuring the spatial distribution and overall composition of algal biomass in lakes, underscoring the importance of vertical mixing and lake size for understanding lake primary production.