Original paper

Differential effects of nutrients and light on the primary production of stream algae and mosses

Ylla, Irene; Romaní, Anna M.; Sabater, Sergi

Abstract

Benthic algae and mosses are the main primary producers in headwater streams. In these ecosystems, primary production is mainly determined by the availability of light and nutrients. The present study analyses the different effects of nutrient enrichment (inorganic phosphorus and nitrogen) and light availability on the primary production of both algal and moss communities from a headwater forested stream using laboratory experiments. These laboratory experiments consider four treatments (low light-unenriched, high light-unenriched, low lightenriched and high light-enriched). Photosynthesis-irradiance curves of both communities were also performed. Respiration and net primary production (NPP) of algae and mosses were measured using incubation chambers and oxygen balance measurements. The moss community was dominated by Fontinalis antipyretica and diatom epiphytes, while the algal community consisted of Cladophora glomerata and attached diatoms. Under all treatments, NPP was significantly higher for algae (3-18 mg O2 mg chl−1 h−1) than for mosses (0.4 and 1.8 mg O2 mg chl−1 h−1). Light (as a single factor) significantly affected GPP (gross primary production) and NPP of algae and mosses. Noninhibitory effects of GPP were observed at saturating light conditions, showing the high plasticity of the stream algal and moss communities. Nutrients produced a signifi cant enhancement of NPP only under high light availability. The greater response of algae to nutrient enrichment was also shown by the highest phosphate uptake rate of this community. Extrapolation of laboratory primary production measurements to the field scale showed the occurrence of significant autotrophic production pulses during periods of light availability (e.g.before leaf emergence and after leaf fall in Mediterranean forested streams), in contrast to the major heterotrophic behaviour, especially in autumn, of these ecosystems. These production pulses in heterotrophic streams could provide high-quality resources to downstream reach, which might be essential for consumers.

Keywords

primary productionnutrient-light interactionsincubation chambersmediterranean streamsmossesalgae