The canopy effect in filamentous algae: Improved modeling of Cladophora growth via a mechanistic representation of self-shading

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Great Lakes Research Center


For decades, nuisance algal growth has wreaked havoc in systems across the world. It has been particularly problematic in the Laurentian Great Lakes. Although managing nutrient loads has resulted in some mitigation, ecosystem perturbations in the last two decades have resulted in favorable conditions for a Cladophora resurgence.

This paper reports on improvements to the Great Lakes Cladophora Model, which has been used to inform management of Great Lakes nuisance algal growth since it was first developed over 30 years ago (Auer et al., 1982; Auer and Canale, 1982; Canale and Auer, 1982a,b). Like earlier versions, this recent configuration of the model, GLCM v3, simulates algal biomass density (g dry mass m−2) and stored (or cellular) phosphorus content (P as % dry mass) over the spring, summer and fall growth cycle. Two major advances over previous versions of the model are presented: a) an improved characterization of the light and temperature response surfaces driving gross growth and respiration and b) the development and implementation of a segmented-system (canopy) approach for simulating the impact of self-shading (carrying capacity) on growth. Prior versions of the GLCM treated the algal mat as a lumped system, utilizing the logistic model to simulate the carrying capacity effect. In that approach, a carrying capacity coefficient (Xmax, maximum biomass density) placed a ceiling on biomass accrual. However, that approach was not mechanistic (i.e., physiologically supported) and empirical specification of the coefficient is undermined by significant intra- and inter-site variability. This uncertainty places too much emphasis on Xmax as a tuning parameter. The signal contribution of this work to the development of the GLCM v3 is the replacement of the lumped system approach with a vertically segmented, mechanistic treatment of self-shading: the “canopy effect.” Here, biomass accrual is mechanistically governed by light attenuation through the canopy as quantified by kalg, the vertical extinction coefficient for light passing through the mat. This coefficient may be determined by direct, in situ measurement and offers much less freedom for use as a tuning parameter. The advances in Cladophora growth modeling provided here and embodied in the GLCM v3 offer a more mechanistic and robust tool than previous versions, strengthening its credibility for the management of nuisance algal growth such as that mandated by the Great Lakes Water Quality Agreement of 2012. Further, the vertical determination of net growth provides a more mechanistic basis for future modeling of other key processes such as sloughing (detachment) in both the Great Lakes and other natural waterbodies.

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Ecological Modelling