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Gulf of Alaska ice-marginal lake area change over the Landsat record and potential physical controlsnull (Ed.)Abstract. Lakes in contact with glacier margins can impact glacierevolution as well as the downstream biophysical systems, flood hazard, andwater resources. Recent work suggests positive feedbacks between glacierwastage and ice-marginal lake evolution, although precise physical controlsare not well understood. Here, we quantify ice-marginal lake area change inunderstudied northwestern North America from 1984–2018 and investigateclimatic, topographic, and glaciological influences on lake area change. Wedelineate time series of sampled lake perimeters (n=107 lakes) and findthat regional lake area has increased 58 % in aggregate, with individualproglacial lakes growing by 1.28 km2 (125 %) and ice-dammed lakesshrinking by 0.04 km2 (−15 %) on average. A statisticalinvestigation of climate reanalysis data suggests that changes in summertemperature and winter precipitation exert minimal direct influence on lakearea change. Utilizing existing datasets of observed and modeled glacialcharacteristics, we find that large, wide glaciers with thick lake-adjacentice are associated with the fastest rate of lake area change, particularlywhere they have been undergoing rapid mass loss in recent times. We observe adichotomy in which large, low-elevation coastal proglacial lakes havechanged most in absolute terms, while small, interior lakes at highelevation have changed most in relative terms. Generally, the fastest-changinglakes have not experienced the most dramatic temperature or precipitationchange, nor are they associated with the highest rates of glacier mass loss.Our work suggests that, while climatic and glaciological factors must playsome role in determining lake area change, the influence of a lake'sspecific geometry and topographic setting overrides these external controls.more » « less
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Abstract. Many studies in ecohydrology focusing on hydrologictransport argue that longer residence times across a stream ecosystem shouldconsistently result in higher biological uptake of carbon, nutrients, andoxygen. This consideration does not incorporate the potential forbiologically mediated reactions to be limited by stoichiometric imbalances.Based on the relevance and co-dependences between hydrologic exchange,stoichiometry, and biological uptake and acknowledging the limited amountof field studies available to determine their net effects on the retentionand export of resources, we quantified how microbial respiration iscontrolled by the interactions between and the supply of essential nutrients (C, N, and P)in a headwater stream in Colorado, USA. For this, we conducted two rounds ofnutrient experiments, each consisting of four sets of continuous injectionsof Cl− as a conservative tracer, resazurin as a proxy for aerobicrespiration, and one of the following nutrient treatments: (a) N, (b) N+C,(c) N+P, or (d) C+N+P. Nutrient treatments were considered to be knownsystem modifications that alter metabolism, and statistical tests helpedidentify the relationships between reach-scale hydrologic transport andrespiration metrics. We found that as discharge changed significantlybetween rounds and across stoichiometric treatments, (a) transient storagemainly occurred in pools lateral to the main channel and was proportional todischarge, and (b) microbial respiration remained similar between rounds andacross stoichiometric treatments. Our results contradict the notion thathydrologic transport alone is a dominant control on biogeochemicalprocessing and suggest that complex interactions between hydrology, resourcesupply, and biological community function are responsible for drivingin-stream respiration.more » « less
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Abstract High‐accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017,
https://doi.org/10.1016/j.marchem.2017.10.001 ; Müller & Rehder, 2018,https://doi.org/10.3389/fmars.2018.00177 ), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005 ± 0.016,n = 550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The nonconservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC, that is, per 100‐μmol/kg biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO2invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.
