Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 97

Report describing the potential effects of changes in hydromorphometry on in-stream nutrient retention

The proportion of water flow that moves at slower velocity than the average velocity is generically referred to as transient water storage (As, Bencala and Walters 1983). Surface-to-subsurface water exchange is widely thought to be a major contributor of the As; and thus As is usually associated to the hyporheic zone. However, surface structures determining the physiographic nature of the channel morphology can also contribute to enlarge As. For instance, pools, eddies, backwaters and debris dams, but also leaf packs and woody debris, contribute to increase water residence time along a stream reach. This shapes the habitat diversity of fluvial ecosystems. Quantification of As has become a critical issue in biogeochemical studies of stream ecosystems addressed to examine spatial or temporal patterns of nutrient retention. It is expected that higher As may enhance interaction between nutrients and microbial communities developed in the different habitats and thus, increase nutrient retention. However, results from existing literature on this issue are controversial. While some studies have shown significant relationships between nutrient retention and As (Ensign and Doyle 2005, Valett et al. 1996), others have indicated a lack of consistent pattern.

We hypothesize that besides of the quantity of As, the quality or nature of it can also be a key factor controlling nutrient retention which may explain these contrasting findings. For example, retention processes associated with pools may likely differ to those associated with the hyporheic zone, basically because communities developed in these two habitats are distinct.

In this study we tested our hypothesis by conducting manipulation experiments in 4 reaches of a semi-natural canal. In 3 reaches we introduced 3 different types of naturally colonized substrata packs (mud, sand and cobbles) to increase Ase. The fourth reach was left without substrata to serve as a control. We performed a constant rate addition of ammonium, phosphate, and chloride on each reach on every sampling date. A total of 4 sampling dates were performed during November 2006. We used the OTIS model to estimate transient water storage area and exchange coefficient. We estimated the nutrient uptake rate and the uptake coefficient as the metrics to assess nutrient retention. At the end of the experiment, and to understand which type of substrata pack contributed more to nitrogen retention, we performed a 15NH4 addition and we analyzed the amount of 15N in each type of substrata pack after 14 h of release.

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