Nutrient enrichment offsets the effects of low light on growth of the kelp Ecklonia radiata

Abstract

AbstractIncreased input of terrigenous matter and pollution into coastal marine ecosystems results in both a decline in benthic irradiance and an influx of nutrients. Though low light conditions have been demonstrated to have adverse effects on benthic primary producers, associated increases in nutrients may alleviate some of this stress by enhancing growth and productivity. We examined the interactive effect of low light and increased nitrogen (N) availability on the growth and physiological response of the kelp Ecklonia radiata using field and mesocosm experiments. Nutrient enrichment had positive effects on the growth of E. radiata in both experiments regardless of light treatment, though increases were c. 50% higher at low light. In contrast, the ability of E. radiata to acclimate to low light and net primary production (NPP) was not affected by nutrient addition, but NPP was over 2 times greater in high light treatments than low light treatments. In low light conditions, E. radiata allocated a higher proportion of assimilated C to growth compared to those in high light conditions and this was accentuated with N‐enrichment. These results indicate that growth, rather than photosynthesis, in E. radiata is limited by N availability and may also indicate a low light coping strategy whereby E. radiata invest more energy into the development of photosynthetically active tissues in order to increase photosynthetic potential. Thus, nutrient enrichment associated with increased turbidity may alleviate some of the stresses associated with living under low light and facilitate the persistence of E. radiata in turbid environments.