Mapping the Spatial Heterogeneity of Watershed Ecosystems and Water Quality in Rainforest Fjordlands.

Abstract

Small coastal watersheds (< 10,000 km²) can play a large role in forming biogeochemical linkages between land and sea, yet the spatial heterogeneity of small watershed ecosystems is poorly understood due to sparse observations in many regions. In this study, we examined the spatial heterogeneity of water quality exported from diverse watersheds in two rainforest fjordland complexes. Samples were collected about monthly for a year from the outlets of 56 watersheds spanning from high mountains to low islands. Many (20) water quality properties varied significantly across six previously established watershed types defined by 12 easily computed geospatial variables. For example, organic matter concentrations ranged from very low in a Glacierized Mountains watershed type (1.2 ± 0.1 mg L^-1 DOC; 28.5 ± 4.6 µg L^-1 DON) to very high (15.1 ± 1.0 mg L^-1 DOC; 215.6 ± 20.4 µg L^-1 DON) in a Rain Lowlands type. Along this gradient, the dominant form of dissolved nitrogen switched from inorganic to organic and the dominant form of phosphorous switched from particulate to dissolved. Watershed type alone explained 67% of the variance in the first principal component of water quality (PC1) representing 20 water properties. Although underlying causes were likely complex, a great deal of spatial variation in water quality (for example, 91% of PC1) was predicted by simple measures of topography and climate (for example, elevation and mean annual precipitation). The physiographic structure of the coastal land mass appears to enable a complex mosaic of watershed ecosystems, which may affect meta-ecosystem function at the coastal margin.

Supplementary information: The online version contains supplementary material available at 10.1007/s10021-025-00964-x.