Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 82

Report on variability in stream nutrient retention response comparing temporal variability within a stream and spatial variability across streams located along an altitudinal gradient

Stream nutrient dynamics are influenced by both biotic and abiotic processes occurring in the watershed as well as within the stream. While factors operating at the watershed scale may determine stream nutrient availability and its regime, factors operating within the stream influence the coupling between nutrient transport and in-stream processing and retention. In turn, in-stream biogeochemical processes can be highly influenced by factors operating at both the catchment (e.g., climate, geology and vegetation) and the stream (e.g., morphology and nutrient loading) scales. Many studies evaluating stream biogeochemical responses have focused on headwater streams given their importance in processing and retaining nutrients and their metabolic relevance, especially in organic matter processing. The influence of single or multiple environmental factors on nutrient retention and stream metabolism has commonly been assessed by examining relationships between the variability in factors and processes measured among streams or within streams over time. Based on this approach, discharge or specific discharge (velocity x depth) has been shown to account for a large fraction of the variation in nutrient retention efficiency among and within streams. Stream nutrient retention efficiency decreases as discharge increases. Stream geomorphology and hydraulics may also influence nutrient uptake through the influence of these characteristics on water and solute transient storage, which increases residence time and the interaction between solutes and microorganisms. However, published results on this topic are less consistent than those for the effect of discharge. On the other hand, biological activity (i.e., metabolism) and its potential capacity for nutrient uptake can be influenced by light availability, which is modulated by the existence of riparian canopy or sediment loads and turbidity of stream water. In addition, other studies have found significant relationships between stream metabolism and catchment vegetation and a great number of studies evidence the relevance of temperature on ecosystem respiration.

Variation of these key factors among streams is mostly imposed by their climatic and physiographic setting; and thus, it is expected that some spatial patterns on biogeochemical responses emerge when examining their variation among bioclimatic distinct regions. For instance, several authors have compared metabolism between tropical and temperate streams. Other studies have used an inter-biome perspective as an approach to determine fundamental controls on ecosystem processes like ammonium uptake and steam metabolism. This information is relevant within the context of current climate change to help forecast the sensibility of stream biogeochemical responses to future changes and the potential direction of the change. However, current knowledge is still restricted to few studies mostly done at large spatial scales that difficult the realistic extrapolation of space-for-time substitution.

This study aims to examine patterns in stream biogeochemical responses along an altitudinal gradient; and thus, associated to differences in microclimatic conditions. Among-stream changes in environmental factors like temperature, precipitation regime, light, riparian vegetation, and geomorphology are associated to the altitudinal position of each stream along the gradient. The study was done in the Central region of the Pyrenees (N of Spain), which exhibits marked altitudinal gradients. The region is subjected to a Mediterranean-continental climate, but the altitudinal gradient determines dramatic changes in microclimatic conditions over relatively short geographical distances. Gradual changes along the gradient within the same climatic region may be more representative of shifts induced by future climate change conditions. The Mediterranean regions are often rugged with a marked change in elevation and, therefore, also in climatic conditions along relatively short horizontal distances. Consequently, headwaters of some streams may be in high elevation areas where the climate is too wet and cold to be Mediterranean.

Most of the multifactorial studies addressed to stream biogeochemical responses tend to focus on a single functional parameter. We haven’t found in the literature any study that performed a study of variability of multiple functional responses. Nutrient retention in lotic ecosystems is a function of synergistic effects resulting from the interaction of hydrologic, chemical, and biological properties, but also can be influenced by the relative availability of different solutes. Biotic components of stream ecosystems influence retention by generating, immobilizing, transforming, or removing biologically active solutes through ecosystem metabolism. Therefore, some authors have focused on the analysis of the interaction between nutrient retention and ecosystem metabolism. Finally, while most of the studies have focused on retention efficiency of inorganic nutrients, very few studies have examined retention efficiency of organic forms of nutrients; despite the marked heterotrophic character of most headwater streams.

In this study we evaluated the variation in retention of multiple nutrients (acetate, glycine, nitrate, ammonium and phosphate) and stream metabolism among 14 headwater streams located along an altitudinal gradient ranging from 700 to 2100 m a.s.l. in the Central Pyrenees (North of Spain). We also examined how changes in physical, chemical and ecological factors associated to the altitudinal gradient influenced spatial variability in the biogeochemical responses (nutrient retention and metabolism) among the streams by using a multiparametric approach.

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