Search

Didymosphenia geminata is a stalk-forming freshwater diatom which was historically found primarily in oligotrophic lakes and streams, but has recently become a nuisance species in many lotic systems worldwide. In the last 5–8 years, D. geminata has become established in Boulder Creek and South Boulder Creek, two regulated montane streams in the Front Range of the Colorado Rocky Mountains. Factors that may influence the growth of D. geminata were monitored during the summer of 2006. D. geminata abundance decreased in Boulder Creek after an unusual flood event caused by 3 days of sustained rainfall in the headwaters of the watershed. However, within a week, coverage had been restored to pre-flood levels. Variations in D. geminata abundance among sites were found to be negatively correlated with total dissolved phosphorus concentrations and bed movement, as measured by Shields stress. In contrast, D. geminata abundance was not significantly correlated with temperature, conductivity, pH, total suspended solids, or dissolved inorganic nitrogen. Our results suggest that bed movement may be a dominant scouring mechanism that acts to control the growth and distribution of D. geminata. The potential role of total dissolved phosphorus and bed movement in decreasing D. geminata coverage adds to the limited base of knowledge regarding controls on the growth and distribution of this species, and could be investigated by researchers studying D. geminata blooms in other stream ecosystems.

Numerous ephemeral streams flow within the McMurdo Dry Valley Region of Antarctica that transport glacial meltwater to perennially ice-covered, closed-basin lakes during the austral summer. The diurnal behavior for two Taylor Valley streams of different character was examined during the summer of 2010-11. Andersen Creek is a short, 1st-order proglacial stream, whereas Von Guerard Stream is a long, high-order stream with an extensive hyporheic zone that has a substantial cyanobacterial algal mat community in its middle reaches. Both streams display strong daily cycles for temperature, electrical conductivity, dissolved oxygen, and pH. Conductivity varies in concert with flow, with solute dilution occurring during the daily high-flow pulse. Dissolved oxygen co-varies strongly with pH at Andersen Creek but not for Von Guerard Stream. Each stream has a distinct geochemical character that for Andersen Creek is a direct reflection of its glacial source, unmodified by secondary effects, whereas that for Von Guerard Stream is modulated by its resident algal mat community and through extensive hyporheic zone interaction and exchange.

A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration-discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Qslope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM. Une meilleure comprehension de l'influence des processus hydrologiques et biogeochimiques sur les concentrations de carbone organique dissous (COD) et sur la composition des matieres organiques dissoutes (MOD) des cours d'eau d'amont jusqu'aux grands fleuves est necessaire. Nous avons examine un imposant ensemble de donnees sur la MOD du Systeme national d'information sur l'eau de la commission geologique des Etats-Unis (US Geological Survey), qui represente environ 100 000 mesures de concentration de COD et de composition de MOD en de nombreux endroits le long de rivieres a la grandeur des Etats-Unis. L'application de la regression quantile a revele une tendance a l'homogeneisation spatiale et temporelle vers l'aval des concentrations de COD et le passage d'une predominance de MOD aromatiques dans les cours d'eau d'amont a plus de MOD aliphatiques en aval. Les relations concentrations de COD-debit (C-D) a chaque site ont revele une tendance vers une pente nulle vers l'aval. Nous proposons que, malgre les complexites des reseaux hydrographiques qui ont mene a de nombreuses revisions du concept du continuum fluvial, les rivieres tendent vers la chimiostase (pente nulle de la relation C-D) en raison du passage vers l'aval d'une predominance de facteurs hydrologiques qui relient les sources de MOD terrestres aux cours d'eau d'amont, a une predominance de processus biogeochimiques dans le cours d'eau ou a proximite qui se traduisent par des pertes preferentielles de MOD aromatiques et des gains preferentiels de MOD aliphatiques. [Traduit par la Redaction], Introduction In most river ecosystems, the downstream transport of dissolved organic matter (DOM) represents the greatest flux of organic carbon (Schlesinger and Melack 1981; Meybeck 1982). DOM can be defined [...]