Eugènia Martí

Material Spiraling in Stream Corridors: A Telescoping Ecosystem Model

ABSTRACT Stream ecosystems consist of several subsystems that are spatially distributed concentrically, analogous to the elements of a simple telescope. Subsystems include the central surface stream, vertically and laterally arrayed saturated sediments (hyporheic and parafluvial zones), and the most distal element, the riparian zone. These zones are hydrologically connected; thus water and its dissolved and suspended load move through all of these subsystems as it flows downstream. In any given subsystem, chemical transformations result in a change in the quantity of materials in transport. Processing length is the length of subsystem required to “process” an amount of substrate equal to advective input. Long processing lengths reflect low rates of material cycling. Processing length provides the length dimension of each cylindrical element of the telescope and is specific to subsystem (for example, the surface stream), substrate (for instance, nitrate), and process (denitrification, for example). Disturbance causes processing length to increase. Processing length decreases during succession following disturbance. The whole stream-corridor ecosystem consists of several nested cylindrical elements that extend and retract, much as would a telescope, in response to disturbance regime. This telescoping ecosystem model (TEM) can improve understanding of material retention in running water systems; that is, their “nutrient filtration” capacity. We hypothesize that disturbance by flooding alters this capacity in proportion to both intensity of disturbance and to the relative effect of disturbance on each subsystem. We would expect more distal subsystems (for example, the riparian zone) to show the highest resistance to floods. In contrast, we predict that postflood recovery of functions such as material processing (that is, resilience) will be highest in central elements and decrease laterally. Resistance and resilience of subsystems are thus both inversely correlated and spatially separated. We further hypothesize that cross-linkages between adjacent subsystems will enhance resilience of the system as a whole. Whole-ecosystem retention, transformation, and transport are thus viewed as a function of subsystem extent, lateral and vertical linkage, and disturbance regime.

Pre and post-flood retention efficiency of nitrogen in a Sonoran Desert stream

The objectives of this study were 1) to compare Sonoran Desert streams with other streams in terms of retention efficiency of nitrate; 2) to examine the effects of a flood on nitrate retention and to determine which factors control nitrate retention in the surface stream subsystem in Sycamore Creek, Arizona; and 3) to compare the short-term nutrient addition technique with computations based upon natural nutrient gradients. From June to September 1995, we did 8 short-term nitrate and chloride additions (4 additions before and 4 after a flood) in a 240-m reach to measure nitrate uptake length as an index of surface stream retention efficiency of nitrate. We also calculated nitrate uptake lengths based on a natural downstream decline in nitrate concentration, using data from the addition dates and from previous studies. Nitrate uptake lengths measured in Sycamore Creek were short (<120 m) compared to published values from other streams, indicating a high retention efficiency of nitrate in this nitrogen-limited stream. A midsummer flood caused a 2-fold decrease in retention efficiency of nitrate in the reach (i.e., nitrate uptake length increased from 61 to 124 m); however, this change was within the range of variation measured before the flood. Rapid algal recovery (23 d), the dramatic decrease in discharge, and a large transient storage zone may account for the apparent high resilience of nutrient retention efficiency to disturbance. Most of the temporal variation in nitrate uptake length during the study period was attributed to changes in the algal assemblage. In particular, retention efficiency of nitrate decreased when nitrogen fixers were abundant. Uptake lengths calculated from additions were always shorter than those from natural nitrate declines, supporting our hypothesis that nutrient uptake lengths from short-term nutrient additions reflect gross, rather than net, nutrient uptake. Uptake rates from short-term additions and from natural declines of nitrate over post-flood succession showed a similar temporal pattern, but the ratio between them increased late in succession. This result suggests that, during late successional stages, nutrient release processes became more important than nutrient uptake processes, a prediction that is consistent with the ecosystem succession and nutrient retention hypothesis.

Nutrient Transient Storage by the Invertebrate Assemblage in Streams with Contrasting Nutrient Loads

Headwater streams account for most o f the channe1 1ength in lotic networks, are intimate1y connected with the terrestria1 mi1ieu and are biogeochemica1 hotspots (sensu McCLAIN et al. 2003) at the 1andscape scale. These headwaters can influence larger downstream ecosystems such as rivers, estuaries and even coastal waters through 1ongitudina1 linkage of 1ocal biogeochemica1 processes (ALEXANDER et al. 2000, MEYER & WALLACE 2001). During the transit from up1ands to oceans through 1otic networks, nutrients undergo multip1e cyc1es of uptake, bio1ogica1, chemica1 and physica1 storage and remineralization processes that can formally be described by the nutrient spira1ing concept (WEBSTER & PATTEN 1979, NEWBOLD et al. 1981 ). While recent research on stream nutrient dynamics mostly focused on pristine ecosystems, litt1e is known about human-altered streams. Whi1e we appreciate the ro1e of periphytic algae in nutrient cycling (MuLHOLLAND 1996) and begin to understand the consequences of microbia1 biofi1ms for ecosystem processes (BATTIN et al. 2003a), relative1y litt1e i s known about the immediate effects of invertebrates on whole-stream nutrient retention. Ihe aim o f this conceptual paper i s to p1ace benthic invertebrate consumers within the framework of stream nutrient dynamics in pristine and nutrientenriched situations.