The nutrient-color paradigm : A test based on a comperative analysis of lakes in four ecoregions in Sweden

Sammanfattning: The nutrient-color paradigm explains lake productivity as a function of the concentrations of nutrients and colored dissolved organic matter (CDOM) in the water column.  Specifically, the paradigm predicts that primary production decreases with increasing concentrations of CDOM due to reduced light penetration, and increases with nitrogen or phosphorus concentration because these are typically the growth limiting elements for phytoplankton. A key assumption of the nutrient-color paradigm is that these factors are independent, forming orthogonal axes of a habitat template. This thesis presents an empirical test of this assumption based on water chemistry data collected from Swedish lakes in four ecoregions with different land cover and land use characteristics. Lakes in southern Sweden have weak correlations between metrics of nutrient concentration and CDOM. In contrast, there were strong correlations between nutrient and CDOM concentrations in northern Sweden. These results indicate that the orthogonality assumption of nutrient-color paradigm only holds in some ecoregions, specifically those where there are substantial inorganic nutrient sources available like in southern Sweden due to agriculture and urbanization. I describe how this observation reconciles apparently contrasting patterns of primary production observed when comparing lakes in northern Sweden compared to those reported in the temperate ecoregions.