Contraction,fragmentation and expansion dynamics determine nutrient availability in a Mediterranean forest stream     

D. von Schiller  V. Acuña  D. Graeber  E. Martí  M. Ribot  S. Sabater  X. Timoner  K. Tockner 《Aquatic Sciences - Research Across Boundaries》2011,73(4):485-497

Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime, these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus (P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall, changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios.  相似文献   

Photosynthetic pigment changes and adaptations in biofilms in response to flow intermittency     

Xisca Timoner  Teresa Buchaca  Vicenç Acuña  Sergi Sabater 《Aquatic Sciences - Research Across Boundaries》2014,76(4):565-578

Among the environmental factors affecting benthic algae and cyanobacteria in streams, the one often producing the largest effects is flow intermittency. This study aimed to characterize the responses of algal assemblages to flow intermittency in a Mediterranean intermittent stream during the drying, non-flow (112 days), and rewetting phases. Algae growing in the epilithic, epipsammic and hyporheic streambed compartments were analyzed for pigment composition, and for the existence of structural changes in cells. Chlorophyll-a concentrations decreased between 60 to 90 % during the non-flow phase, indicating low resistance of algal assemblages to desiccation. In contrast, fast recoveries of Chlorophyll-a when flow resumed indicated high resilience. Pigment composition revealed that the epilithic algal assemblage was considerably different than the epipsammic and hyporheic ones. These differences were mainly attributed to the physical conditions prevailing on each streambed compartment that allowed the growth of different algal assemblages. During the non-flow phase, the synthesis of protective carotenoids (i.e. echinenone and scytonemin) and the occurrence of cell resistance structures (i.e. enlarged membrane thickness and resistant spores) enhanced resistance of the epilithic biofilm. The resistance observed in the epilithic biofilm might also be related to the tightly adhered growth-form of algae on this substratum. Main results suggest that algal assemblages in the epilithic compartment, which were the most exposed to desiccation, were structurally and functionally better adapted to flow interruption than those colonizing other streambed compartments, and that this compartment plays a crucial role in maintaining ecosystem functions under varying flow periods.  相似文献