Stream lighting in five regions of North Island, New Zealand: control by channel size and riparian vegetation

Lighting of streams profoundly influences their ecology, particularly through primary production and thermal behaviour. We used paired canopy analysers, instruments with fish-eye lens imaging, to measure sunlight exposure of streams in five regions of North Island, New Zealand. Reach-averaged stream lighting, at both water and bank level, was strongly influenced by riparian vegetation type. Pasture streams had comparatively high light exposure (median water level lighting = 45% of ambient), with most shading contributed by banks and overhanging herbs. Lighting was low in small forest streams (median = 1.3% for native forest, 1.2% for pine plantations), but increased sharply as the gap in the canopy widened with increase in channel width above c. 3.5 m. The understorey in pine plantations contributed more shade than the pines themselves: damage to this understorey (e.g., by goat browsing or floods) increased lighting markedly. Harvesting of pine plantations exposed streams to high light levels except where a riparian buffer was maintained. Periphyton biomass, varying over more than four orders of magnitude in the study streams, correlated broadly with lighting.