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[1] Storm events have major implications for biogeochemical cycles at local and regional scales and they provide an excellent opportunity to study the hydro-biogeochemical functioning of catchments. However, concentration-discharge (C-Q) responses have only been studied in detail for short periods or a few selected events. In consequence, it is difficult to quantify the diversity of C-Q responses in a hydrological system and impossible to assess whether the succession of forms of C-Q responses follows a predictable sequence or not. Bearing in mind these shortfalls, the variability of dissolved organic carbon (DOC) and nitrate (NO 3 ) pulses during storms is analyzed in a detailed 4-year series from an intermittent Mediterranean stream. In this study, each DOC and NO 3 -Q response is synthesized by two descriptors that summarize its trend (DC; dilution/ flushing/no change) and shape (DR; linear/nonlinear response). We observe that C-Q responses are widely distributed along the two-dimensional DR versus DC continuum. Furthermore, the temporal succession of forms of DOC and NO 3 -Q responses follow a random pattern, and only the dynamics of the DR (NO3) descriptor show periodicity. The long-term data set reveals that it is impossible to predict with reasonable precision the full properties of DOC and NO 3 -Q responses. Thus, a ''typical'' C-Q response does not really exist at our study site, and this apparent diversity of responses has to be handled with a probabilistic approach that allows synthesis of the complexity of the hydrobiogeochemical functioning of a specific catchment.Citation: Butturini, A., M. Alvarez, S. Bernal, E. Vazquez, and F. Sabater (2008), Diversity and temporal sequences of forms of DOC and NO 3 -discharge responses in an intermittent stream: Predictable or random succession?,

River networks modify material transfer from land to ocean. Understanding the factors regulating this function for different gaseous, dissolved, and particulate constituents is critical to quantify the local and global effects of climate and land use change. We propose the River Network Saturation (RNS) concept as a generalization of how river network regulation of material fluxes declines with increasing flows due to imbalances between supply and demand at network scales. River networks have a tendency to become saturated (supply) demand) under higher flow conditions because supplies increase faster than sink processes. However, the flow thresholds under which saturation occurs depends on a variety of factors, including the inherent process rate for a given constituent and the abundance of lentic waters such as lakes, ponds, reservoirs, and fluvial wetlands within the river network. As supply increases, saturation at network scales is initially limited by previously unmet demand in downstream aquatic ecosystems. The RNS concept describes a general tendency of river network function that can be

Climate exerts a powerful influence on biological processes, but the effects of climate change on ecosystem nutrient flux and cycling are poorly resolved. Although rare, long-term records offer a unique opportunity to disentangle effects of climate from other anthropogenic influences. Here, we examine the longest and most complete record of watershed nutrient and climate dynamics available worldwide, which was collected at the Hubbard Brook Experimental Forest in the northeastern United States. We used empirical analyses and model calculations to distinguish between effects of climate change and past perturbations on the forest nitrogen (N) cycle. We find that climate alone cannot explain the occurrence of a dramatic >90% drop in watershed nitrate export over the past 46 y, despite longer growing seasons and higher soil temperatures. The strongest climate influence was an increase in soil temperature accompanied by a shift in paths of soil water flow within the watershed, but this effect explained, at best, only ∼40% of the nitrate decline. In contrast, at least 50-60% of the observed change in the N export could be explained by the long-lasting effect of forest cutting in the early 1900s on the N cycle of the soil and vegetation pools. Our analysis shows that historic events can obscure the influence of modern day stresses on the N cycle, even when analyses have the advantage of being informed by 0.5-century-long datasets. These findings raise fundamental questions about interpretations of long-term trends as a baseline for understanding how climate change influences complex ecosystems.forest ecosystems | long-term monitoring | streamwater chemistry | precipitation chemistry | nutrient cycles O ur understanding of how climate change impacts complex ecological systems depends on our conception of a baseline against which change can be judged and knowledge of how this baseline has been shaped by historical conditions. At the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, for example, we know that current concentrations of nitrate in watershed streams are the lowest in 46 y of measurement and that ecosystem nitrate losses have decreased by >90% over this time (Fig. 1A). If we were to take the early high nitrate period (1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976) as the historical reference, we would estimate that nitrate export has dropped by a total of ∼125 kg nitrogen (N) ha −1 during the 30 y of the decline (Fig. 1A). Such a large drop in N export is ecologically relevant and constitutes a dramatic shift in the ecosystem N cycle: from a leaky cycle that retained only ∼30% of external inputs in the high stream water nitrate period to a highly retentive cycle that currently captures ∼90% of atmospheric inputs (Methods).We adopt a watershed mass balance approach (1, 2) to examine the factor(s) responsible for this dramatic change in the forest N cycle (Fig. 2). Because climate is an overriding and powerful driver of biological process, we pay particular attention to whether the observed changes i...