Chronic light reduction reduces overall resilience to additional shading stress in the seagrass Halophila ovalis

Seagrasses have substantial capacity to survive long periods of light reduction, but how acclimation to chronic low light environments may influence their ability to cope with additional stress is poorly understood. This study examines the effect of temporal light reduction by adding two levels of shading to Halophila ovalis plants in two meadows with different light histories, one characterized by a low light (turbid) environment and the other by a relatively high light (clear) environment. Additional shading resulted in complete mortality for both shading treatments at the turbid site while the clear site showed a pattern of decreased shoot density and increased photochemical efficiency (F_v/F_m) with increased shading. These contrasting results for the same species in two different locations indicate that acclimation to chronic low light regimes can affect seagrass resilience and highlights the importance of light history in determining the outcome of exposure to further (short-term) stress.     

Seasonal dynamics and exports of elements from a first‐order stream to a large inland lake in Michigan

Headwater streams are critical components of drainage systems, directly connecting terrestrial and downstream aquatic ecosystems. The amount of water in a stream can alter hydrologic connectivity between the stream and surrounding landscape and is ultimately an important driver of what constituents headwater streams transport. There is a shortage of studies that explore concentration–discharge (C‐Q) relationships in headwater systems, especially forested watersheds, where the hydrological and ecological processes that control the processing and export of solutes can be directly investigated. We sought to identify the temporal dynamics and spatial patterns of stream chemistry at three points along a forested headwater stream in Northern Michigan and utilize C‐Q relationships to explore transport dynamics and potential sources of solutes in the stream. Along the stream, surface flow was seasonal in the main stem, and perennial flow was spatially discontinuous for all but the lowest reaches. Spring snowmelt was the dominant hydrological event in the year with peak flows an order of magnitude larger at the mouth and upper reaches than annual mean discharge. All three C‐Q shapes (positive, negative, and flat) were observed at all locations along the stream, with a higher proportion of the analytes showing significant relationships at the mouth than at the mid or upper flumes. At the mouth, positive (flushing) C‐Q shapes were observed for dissolved organic carbon and total suspended solids, whereas negative (dilution) C‐Q shapes were observed for most cations (Na⁺, Mg²⁺, Ca²⁺) and biologically cycled anions (NO₃^?, PO₄^3?, SO₄^2?). Most analytes displayed significant C‐Q relationships at the mouth, indicating that discharge is a significant driving factor controlling stream chemistry. However, the importance of discharge appeared to decrease moving upstream to the headwaters where more localized or temporally dynamic factors may become more important controls on stream solute patterns.     

The Early Lake Ontario barrier beach: evidence for sea level about 12.8–12.5 cal. ka BP beneath western Lake Ontario in eastern North America

An overstepped, concave‐eastward, barrier beach beneath Holocene mud in western Lake Ontario has been delineated by acoustic and seismic reflection profiles and piston cores, and related to Early Lake Ontario (ELO). The average ELO barrier depth below present mean lake level is 77.4 to 80.6 m, or about ?6 to ?2.8 m above present sea level. Trend surface analysis of Champlain Sea (Atlantic Ocean) marine limits defined the contemporaneous marine water surface, and projections of this surface pass ~25 m above the outlet sill of the Lake Ontario basin and extend to the ELO palaeo‐barrier, a unique sand and gravel deposit beneath western Lake Ontario. ELO was connected to the Champlain Sea above the isostatically rising outlet sill for up to three centuries after about 12.8 cal. ka BP, while the glacio‐isostatically depressed St. Lawrence River Valley was inundated by the Atlantic Ocean. During the period of this connection, ELO level was confluent with slowly rising sea level, and the lake constructed a transgressive beach deposit with washover surfaces. ELO remained fresh due to a high flux of meltwater inflow. The marine water level connection stabilized water level in ELO relative to its shore and facilitated shore erosion, sediment supply and barrier construction. Glacio‐isostatic uplift of the outlet sill, faster than sea‐level rise, lifted ELO above the Champlain Sea about 12.5 cal. ka. Shortly after, a hydrological deficit due mainly to a combination of diverted meltwater inflow and dry climate, well known from regional pollen studies, forced the lake into a lowstand. The lowstand stranded the barrier, which remains as evidence of sea level, the farthest inland in eastern North America north of the Gulf of Mexico at the time. The highest palaeo‐washover surface provides a sea‐level index point.     

Stream geochemical response to reductions in acid deposition in headwater streams: Chronic versus episodic acidification recovery

In the decades since dramatic reductions in acid emissions and subsequent deposition in North America and Europe, the vast majority of research on aquatic chemical recovery has focused on trends in acid–base concentrations during baseflow conditions. Missing from such assessments is consideration of higher flow periods, when flow paths and chemical concentrations change and episodic acidification may occur. Stream samples collected weekly and bi‐hourly during high‐flow storm events from 1992 to 2015 at three watersheds representing a gradient in response to acid deposition in the south‐eastern United States were used to evaluate temporal trends in acid anions, base cations, acid neutralizing capacity (ANC), and pH for each of five distinct flow exceedance regimes, as well as trends in concentration–discharge (C–Q) relationships. For the most well‐buffered watershed (mean ANC = 220 μeq L^?1), ANC and pH increased at a similar rate for baseflow and storm flow conditions. Decreases in sulfate controlled ANC trends at higher flows, whereas smaller sulfate decreases in combination with base cation increases resulted in similar ANC improvements for low‐flow periods. For the most acid sensitive watershed (mean ANC = 10 μeq L^?1), no trends in ANC or pH were observed for the lowest flows (>90% flow exceedance), whereas reductions in sulfate resulted in significant increases in ANC during higher flow conditions. At all sites, greater rates of sulfate decline during high‐flow, as compared with low‐flow, conditions are likely a result of a reduced capacity of near surface soils to adsorb and retain sulfur in these non‐glaciated watersheds. Overall, consistent increases in pH (~0.01–0.02 pH units year^?1) during higher flow conditions (<10% flow exceedance) in contrast to the variable trends observed during lower flows (>50% flow exceedance) illustrate that episodic acidification is recovering at an equal or greater rate than chronic acidification in these watersheds.     

The small scatter in BH–host correlations and the case for self-regulated BH growth

Supermassive black holes (BHs) obey tight scaling relations between their mass and host galaxy properties such as total stellar mass, velocity dispersion and potential well depth. This has led to the development of self-regulated models for BH growth, in which feedback from the central BH halts its own growth upon reaching a critical threshold. However, models have also been proposed in which feedback plays no role: so long as a fixed fraction of the host gas supply is accreted, relations like those observed can be reproduced. Here, we argue that the scatter in the observed BH–host correlations presents a demanding constraint on any model for these correlations, and that it favours self-regulated models of BH growth. We show that the scatter in the stellar mass fraction within a radius R in observed ellipticals and spheroids increases strongly at small R . At a fixed total stellar mass (or host velocity dispersion), on very small scales near the BH radius of influence, there is an order-of-magnitude scatter in the amount of gas that must have entered and formed stars. In short, the BH appears to 'know more' about the global host galaxy potential on large scales than the stars and gas supply on small scales. This is predicted in self-regulated models; however, models where there is no feedback would generically predict order-of-magnitude scatter in the BH–host correlations. Likewise, models in which the BH feedback in the 'bright' mode does not regulate the growth of the BH itself, but sets the stellar mass of the galaxy by inducing star formation or blowing out a mass in gas much larger than the galaxy stellar mass, are difficult to reconcile with the scatter on small scales.     

Dissolution of Columbia River Basalt under mildly acidic conditions as a function of temperature: Experimental results relevant to the geological sequestration of carbon dioxide

Increasing attention is being focused on the rapid rise of CO₂ levels in the atmosphere, which many believe to be the major contributing factor to global climate change. Sequestering CO₂ in deep geological formations has been proposed as a long-term solution to help stabilize CO₂ levels. However, before such technology can be developed and implemented, a basic understanding of H₂O–CO₂ systems and the chemical interactions of these fluids with the host formation must be obtained. Important issues concerning mineral stability, reaction rates, and carbonate formation are all controlled or at least significantly impacted by the kinetics of rock–water reactions in mildly acidic, CO₂-saturated solutions. Basalt has recently been identified as a potentially important host formation for geological sequestration. Dissolution kinetics of the Columbia River Basalt (CRB) were measured for a range of temperatures (25–90 °C) under mildly acidic to neutral pH conditions using the single-pass flow-through test method. Under anaerobic conditions, the normalized dissolution rates for CRB decrease with increasing pH (3 ? pH ? 7) with a slope, η, of −0.15 ± 0.01. Activation energy, E_a, has been estimated at 32.0 ± 2.4 kJ mol⁻¹. Dissolution kinetics measurements like these are essential for modeling the rate at which CO₂-saturated fluids react with basalt and ultimately drive conversion rates to carbonate minerals in situ.     

Stable isotope and trace element status of subsistence-hunted bowhead and beluga whales in Alaska and gray whales in Chukotka

Tissues of bowhead, beluga, and gray whales were analyzed for Ag, Cd, Cu, Se, Zn, THg and MeHg (belugas only). Delta15N and delta13C in muscle were used to estimate trophic position and feeding habitat, respectively. Trace element concentrations in tissues were significantly different among whale species. Hepatic Ag was higher in belugas than bowheads and gray whales. Gray whales had lower Cd concentrations in liver and kidney than bowhead and belugas and a sigmoid correlation of Cd with length was noted for all whales. Renal and hepatic Se and THg were higher in belugas than in baleen whales. The hepatic molar ratio of Se:THg exceeded 1:1 in all species and was negatively correlated to body length. Hepatic and renal Zn in subsistence-harvested gray whales was lower than concentrations for stranded whales. Se:THg molar ratios and tissue concentrations of Zn may show promise as potential indicators of immune status and animal health.     

Patterns of inorganic phosphate uptake in Cassiopea xamachana: a bioindicator species

Nutrient levels in the nearshore waters of the Florida Keys have increased over the past few decades concomitant with a decline in the health of Florida's reef system. Phosphorus is a particular concern in the Florida Keys as it may be the limiting nutrient in nearshore waters. We demonstrate that the upside-down jellyfish, Cassiopea xamachana, decreases its rate of phosphate uptake following exposure to elevated levels of dissolved inorganic phosphate. We also show that this subsequent suppression of uptake rates persists for some time following exposure to elevated phosphates. Using these attributes, we experimentally investigated the use of C. xamachana as a bioindicator for dissolved inorganic phosphates in seawater. Our results show that these animals reveal comparative differences in environmental phosphates despite traditional testing methods yielding no detectable phosphates. We propose that C. xamachana is a bioindicator useful for integrating relevant information about phosphate availability in low nutrient environments.