Quantifying phosphorus and light effects in stream algae

Abstract

Simultaneous gradients of phosphorus and light were applied in experimental streams to develop quantitative relationships between these two important abiotic variables and the growth and composition of benthic microalgae. Algal biovolume and whole-stream metabolism responded hyperbolically to phosphorus enrichment, increasing approximately two-fold over the 5-300 mg L 21 range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 mg L 21 of soluble reactive phosphorus (SRP). Light effects were much stronger than those of phosphorus, resulting in a nearly ten-fold increase in algal biovolume over the 10-400 mmol photons m 22 s 21 range of experimental irradiances. Biovolume accrual was light-saturated at 100 mmol photons m 22 s 21 (5 mol photons m 22 d 21 ). Light effects were diminished by low phosphorus concentrations, and phosphorus effects were diminished by low irradiances, but evidence of simultaneous limitation by both phosphorus and light at subsaturating irradiances was weak. Contrary to the light : nutrient hypothesis, algal phosphorus content was not significantly affected by light, even in the lowest SRP treatments. However, algal nitrogen content increased substantially at lower irradiances, and it was very highly correlated with algal chlorophyll a content. Phosphorus enrichment in streams is likely to have its largest effect at concentrations ,25 mg L 21 SRP, but the effect of enrichment is probably minimized when streambed irradiances are kept below 2 mol photons m 22 d 21 by riparian shading or turbidity.

Comparison of the utility of a frequently used diatom index (IPS) and the diatom ecological guilds in the ecological status assessment of large rivers

“…Besides he recognized that light levels had the strongest effect on community composition. Hill et al (2009) also observed in experiments that light affected algal community structure much more than the phosphorus enrichment utilized. In addition, although high profile taxa are able to form colonies, which give advantageous for exploiting resources (Berthon et al 2011), these colonies may be tumbled by gross amount of suspended matters in the river.…”

“…This allows us to assume that it is not the nutrients inevitably which had the greatest influence on guilds proportions. Some cases can occur when such factors can have importantly roles like types of substrates or light intensity (Hill et al 2009, Lange et al 2011. For instance in some samples no diatoms were detected, or only just a few valves.…”

Nutrient and light limitation of periphyton in the River Thames: Implications for catchment management

“…However, in this study (as in a number of other studies e.g. Hill et al, 2009;Rosemond et al, 2000) a large range in P concentration was used, and it suggests that the diatom community is resistant to major changes in water phosphorus concentrations, at least over the short time scales investigated. Furthermore, it is likely that the species pool within the River Thames is dominated by taxa adapted to high nutrient concentrations.…”

“…These numbers of mesh layers were then placed over the downstream sections of the flumes, to recreate the light intensities equivalent to these different levels of tree shading. Previous studies have successfully used similar approaches to investigate light limitation of periphyton (Hill et al, 2009). Each set of three flumes contained an unshaded, intermediate shaded (equivalent to dappled tree shade) and fully shaded (equivalent to full tree canopy cover) flume.…”

“…However, the response of algal communities to light and nutrients is often inter-dependent (Hill et al, 2009;Rosemond et al, 2000), and manipulating both factors simultaneously within the River Thames could potentially lead to still further decreases in algal biomass, and this requires further investigation.…”