Warm Surface Water Research Articles

Simulated upwelling and marine heatwave events promote similar growth rates but differential domoic acid toxicity in Pseudo-nitzschia australis

Along the west coast of the United States, highly toxic Pseudo-nitzschia blooms have been associated with two contrasting regional phenomena: seasonal upwelling and marine heatwaves. While upwelling delivers cool water rich in pCO2 and an abundance of macronutrients to the upper water column, marine heatwaves instead lead to warmer surface waters, low pCO2, and reduced nutrient availability. Understanding Pseudo-nitzschia dynamics under these two conditions is important for bloom forecasting and coastal management, yet the mechanisms driving toxic bloom formation during contrasting upwelling vs. heatwave conditions remain poorly understood. To gain a better understanding of what drives Pseudo-nitzschia australis growth and toxicity during these events, multiple-driver scenario or ‘cluster’ experiments were conducted using temperature, pCO2, and nutrient levels reflecting conditions during upwelling (13 °C, 900 ppm pCO2, replete nutrients) and two intensities of marine heatwaves (19 °C or 20.5 °C, 250 ppm pCO2, reduced macronutrients). While P. australis grew equally well under both heatwave and upwelling conditions, similar to what has been observed in the natural environment, cells were only toxic in the upwelling treatment. We also conducted single-driver experiments to gain a mechanistic understanding of which drivers most impact P. australis growth and toxicity. These experiments indicated that nitrogen concentration and N:P ratio were likely the drivers that most influenced domoic acid production, while the impacts of temperature or pCO2 concentration were less pronounced. Together, these experiments may help to provide both mechanistic and holistic perspectives on toxic P. australis blooms in the dynamic and changing coastal ocean, where cells interact simultaneously with multiple altered environmental variables.

Global Climate Impacts of Greenland and Antarctic Meltwater: A Comparative Study

Abstract Both the Greenland and Antarctic ice sheets have been melting at an accelerating rate over recent decades. Meltwater from Greenland might be expected to initiate a climate response that is distinct, and perhaps different from, that associated with Antarctic meltwater. Which one might elicit a greater climate response, and what mechanisms are involved? To explore these questions, we apply climate response functions (CRFs) to guide a series of meltwater-perturbation experiments using a fully coupled climate model. In both hemispheres, meltwater drives atmospheric cooling, sea ice expansion, and strengthened Hadley and Ferrel cells. Greenland meltwater induces a slowdown of the Atlantic meridional overturning circulation (AMOC) and a cooling of the subsurface ocean in the northern high latitudes. Antarctic meltwater, instead, induces a slowdown of the Antarctic Bottom Water formation and a warming of the subsurface ocean around Antarctica. For melt rates up to 2000 Gt yr−1, the climate response is rather linear. However, as melt rates increase to 5000 Gt yr−1, the climate response becomes nonlinear. Due to a collapsed AMOC, the climate response is superlinear at high Greenland melt rates. Instead, the climate response is sublinear at high Antarctic melt rates, due to the halting of the northward expansion of Antarctic sea ice by warm surface waters. Finally, in the linear limit, we use CRFs and linear convolution theory to make projections of important climate parameters in response to meltwater scenarios, which suggest that Antarctic meltwater will become a major driver of climate change, dominating that of Greenland meltwater, as the current century proceeds. Significance Statement Melting of the Greenland and Antarctic ice sheets is one of the most uncertain potential contributors to future climate change. In this study, we address the comparative role of Greenland and Antarctic meltwater in the climate system and explore the differing mechanisms at work in each hemisphere. We find that the climate response is linear for low melt rates but becomes nonlinear for high melt rates. As the century proceeds, we speculate that Antarctic meltwater will increasingly dominate that of Greenland meltwater, leading to atmospheric cooling, Antarctic sea ice expansion, and contraction and warming of the Antarctic Bottom Water. Greenland meltwater will, instead, affect smaller changes in the Northern Hemisphere.

Foraminiferal assemblages from the Abu Qir coastal area (Alexandria, Egypt): Wave-cut platform versus shallow-bay sediments

Benthic foraminifers living in euphotic coastal and shelf environments commonly occur in association with macrophytes (macroalgae and seagrasses), and many lineages have evolved morphologic adaptations to thrive in such habitats. Species that did not occur in the Mediterranean Sea prior to recent centuries, commonly known as “introduced” or “alien” species, are now widespread, and some are so abundant that they are altering coastal habitats. With accelerating climatic change and warming of Mediterranean surface waters, recognizing how invasive algal and foraminiferal species are affecting sedimentation and microenvironments has become increasingly urgent. This project examined 18 sets of sediment samples collected in the Abu Qir coastal area east of Alexandria for comparison with a previous assessment of live assemblages within the macroalgae. Overall, 68 foraminiferal species were identified, of which sixteen species were categorized as epiphytic. The hyaline epiphytic Elphidium crispum was the most common species found in sediments from the platform, while the porcelaneous free-living Quinqueloculina auberiana dominated sediments from the small bay. Eleven introduced foraminiferal species were recorded, one of which, Amphistegina lobifera, was commonly found, especially in the platform sediment samples. Taxa recorded live (stained) in platform sediments were predominantly (>10% each) Peneroplis, Amphistegina and Elphidium, while bay sediments were dominated by Quinqueloculina, followed in order by Elphidium, Amphistegina and Peneroplis. The assemblage differences from previously reported live assemblages associated with macralgae, compared with live and dead assemblages in sediments, appear to reflect hydrodynamic influence of sediment sorting and resistence of larger taxa to physical degradation.

Effects of the 2018 European heatwave and drought on coastal biogeochemistry in the German Bight

In 2018, Europe experienced an unprecedented heatwave and drought, especially in central and northern Europe, which caused decreased terrestrial production and affected ecosystem health. In this study, the effects of this event on the marine environment are investigate, with a focus on the biogeochemical response in the German Bight of the North Sea. Using time series data from FerryBoxes, research cruises, monitoring programs and remote sensing we compare conditions in 2018 to climatological values. We find that (1) the heatwave caused rapid warming of surface waters, (2) the drought reduced river discharge and nutrient loads to the coast, and (3) these combined effects altered coastal biogeochemistry and productivity. During 2018, both water discharge and nutrient loads from rivers discharging into the German Bight were below the seasonally variable 10th percentile from March onward. Throughout the study domain, water temperature was near or below that threshold in March 2018, but higher than in other years during May 2018, representing not only a heat wave, but also the fastest spring warming on record. This extreme warming period saw concurrent high peaks in chlorophyll a, dissolved oxygen and pH, consistent with the development of a strong spring bloom. It appears that productivity was above 75th percentile of the 21-year record in most of the nearshore region, while offshore it was widely below the 25th percentile in 2018. The drought-related low discharge limited nutrient supply from the rivers, but likely increased water residence time nearshore, where a surge in primary production with efficient nutrient utilization during the spring depleted nutrients available for transport offshore. There, the heatwave-related rapid warming of surface water resulted in the establishment of a stable thermal water column stratification, hindering vertical nutrient supply to the surface layer during the summer.

“Breath holding” as a thermoregulation strategy in the deep-diving scalloped hammerhead shark

Fish moving between different thermal environments experience heat exchange via conduction through the body wall and convection from blood flow across the gills. We report a strategy of preventing convective heat loss at the gills during excursions into deep, cold water by the tropical scalloped hammerhead shark (Sphryna lewini). Adult scalloped hammerhead sharks dive rapidly and repeatedly from warm (~26°C) surface waters to depths exceeding 800 meters with temperatures as low as 5°C. Biologgers attached to adult sharks show that warm muscle temperatures were maintained throughout the deepest portion of each dive. Substantive cooling only occurred during the latter stages of the ascent phase and, once initiated, was rapid. Heat transfer coefficient modeling indicated that convective heat transfer was suspended, probably by suppressing gill function during deep dives. This previously unobserved strategy has broad similarities to marine mammal "breath hold" diving.

Multiproxy analyses of paleoenvironmental and paleoceanographic changes during the Danian-Selandian in East Central Sinai: An integrated stable isotope and planktic foraminiferal data

Forty-three planktic foraminifera samples from the Themed section (East Central Sinai; Egypt) spanning the Zone Parvularugoglobigerina eugubina (Pα) to the Subzone Acarinina subsphaerica (P4b) have been studied. Data from δ13C, δ18O, and planktic foraminifera-based species diversity, depth habitat, preference for warm and cool surface waters, and nutrients (oligotrophic, mesotrophic, and eutrophic conditions) are used to infer paleoenvironmental changes throughout the Danian‒Selandian duration. Based on quantitative multivariate analyses (hierarchical cluster and principal component), three distinct intervals were recognized, Interval 1 (Pα‒P1b), Interval 2 (P1c‒P3a), and Interval 3 (P3a‒P4b). Interval 2 is further subdivided into three subintervals, 2a (part P1c), 2b (part P1c), and 2c (P2‒P3a). Two δ13C events are identified, Dan-C2 and Latest Danian Event (LDE) and elaborated concerning paleoenvironmental changes. During the earliest Danian planktic foraminiferal Pα Zone, moderately shallow and eutrophic conditions prevailed with cool surface waters and a shallow thermocline. Comparable conditions were still prevailing during P1a‒P1b, but with slightly deeper and mesotrophic conditions and a somewhat deeper thermocline and reduced stratification. P1b‒P1c exhibits a major shift from Eoglobigerina to Subbotina‒Parasubbotina with cooler surface waters and moderate mesotrophic conditions. For Subzone P1c (upper part), slightly mesotrophic conditions were inferred, whereas for P2‒P3a (lower part), surface water warming and thermocline shallowing events have inferred with increased oligotrophic conditions. The Latest Danian Event (mid-P3a) is marked by a dramatic negative δ13C excursion, warm waters, increased mesotrophic conditions, and enhanced stratification. The dominance of Morozovella, Acarinina, and Igorina specify warm and oligotrophic conditions for subzones P3b‒P4b.

Patterns and drivers of water quality changes associated with dams in the Tropical Andes

Abstract. The Tropical Andes is a biodiversity hotspot facing pressure from planned and ongoing hydropower development. However, the effects of dams on the region's river ecosystems, as mediated by physicochemical changes in the water quality, are poorly known. Colombia is unique among its peers in South America with respect to managing central public environmental databases, including surface water quality data sets associated with the environmental monitoring of dams. To assess the relationship between hydropower and Colombian river conditions, we analyze monitoring data associated with 15 dams, focusing on oxygen availability, thermal regimes and sediment losses because these properties are influenced directly by river damming and impose fundamental constraints on the structure of downstream aquatic ecosystems. We find that most Colombian dams (7 of 10) seasonally reduce concentrations of total suspended solids by large percentages (50 %–99 %) through sediment trapping. Most dams (8 of 15) also, via the discharge of warm reservoir surface waters, seasonally increase river temperatures by 2 to 4 ∘C with respect to upstream conditions. A subset of four dams generate downstream hypoxia (< 4 mg L−1) and water that is 2 to 5 ∘C colder than inflows, with both processes driven by the turbination and discharge of cold and anoxic hypolimnetic waters during periods of reservoir stratification. Reliance on monitoring data likely leads us to under-detect impacts: many rivers are only sampled once or twice per year, which cannot capture temporal shifts across seasons and days (i.e., in response to hydropeaking). Despite these blind spots, the monitoring data point to some opportunities for planners and hydropower companies to mitigate downstream ecological impacts. These findings highlight the importance of implementing environmental monitoring schemes associated with hydrologic infrastructure in developing countries.

Late Miocene biostratigraphy and paleoenvironment of the Sais Basin (southern Rifian Corridor, Morocco): New insights from the Moulay Yakoub area

During the Late Miocene, the Rifian Corridor was one of the two main corridors allowing exchanges between the Atlantic and the Mediterranean Seas. Sedimentary records accumulated along this corridor and specifically, in the Sais Basin, constitute case-study material for understanding the impact of the South Rifian Corridor on the evolution of the fauna, phytoplankton and environment during the Late Miocene. In order to reconstruct the evolution of marine paleoenvironments in the Moulay Yakoub region and to compare this evolution with other areas of the Sais basin, we conducted an integrated biostratigraphic and palynological study of the Upper Miocene of borehole KA 102. High-resolution biostratigraphy based on planktonic foraminifera revised and defined the chronostratigraphic framework of the Upper Miocene sedimentary series of the Moulay Yakoub region providing an age of 7.8 Ma to 6.52 Ma (Late Tortonian to Early Messinian). In the Late Tortonian, the planktonic foraminiferal assemblages suggest an open and relatively deep marine environment. At the Tortonian/Messinian boundary, planktonic foraminiferal and dinoflagellate associations, as well as distal indices, suggest a relatively deep and neritic distal marine environment with a decrease in diversity and an increase in reworked taxa. Surface waters are warm with a minor cooling trend observed just below this boundary. In comparison to previously studied sections in the Sais Basin, the Lower Messinian is characterized by a deeper and more distal marine environment. However, proximal trends and warm surface water conditions persisted in all studied areas from the basin during this period.

A comprehensive study on the impacts of isolation heat exchangers on the energy performance of surface water heat pump systems

Surface water heat pumps are included as a subset of ground-source heat pumps for the utilization of shallow geothermal energy. For the surface water heat pump applications in which the location of the heat pump plant is remarkably higher than water surface, the pump energy consumption of an indirect system with isolation heat exchangers (IHE) will be lower than that of a direct system without IHE. However, the entering fluid temperature of heat pumps in an indirect system is higher/lower than that in a direct system in cooling/heating modes due to the existence of IHE, which will bring a certain decrease in heat pump performance. This paper presents a comprehensive study on the impacts of IHE on system energy performance. The energy performance models for indirect and direct systems are developed, and a method for energy-saving judgment between indirect and direct systems is proposed. The case study results show that a direct system is more energy efficient for cooling than an indirect system because the elevation difference between water surface and plant is less than the critical value. Under low surface water temperature condition in winter, an indirect system using antifreeze fluid can eliminate the defect that the heating capacity of heat pumps in a direct system decreases much and auxiliary heat is needed. An optimized system configuration that combines the advantages of direct and indirect systems is proposed. The proposed system can operate without auxiliary heat in winter even though the surface water temperature drops to a low level, and can acquire cooling performance close to that of the cooling systems including chillers and cooling towers.

Antarctic Sea Ice Holds the Fate of Antarctic Ice-Shelf Basal Melting in a Warming Climate

Abstract Changes in the Antarctic ice sheet play a critical role in the Southern Ocean and global climates. Although many studies have pointed out that enhanced ocean heat delivery onto the Antarctic continental shelf regions can cause significant changes in Antarctic ice-shelf basal melting, the associated physical mechanisms require further research. Here, we perform numerical experiments using an ocean–sea ice model with an ice-shelf component to simulate future projections in Antarctic ice-shelf basal melting in a warming climate, focusing on the driving mechanism and the physical linkages with the seasonal Antarctic sea ice fields and coastal water masses. The model projects a distinct superlinear response of ice-shelf basal melting to future atmospheric warming, demonstrating that future projections of the Antarctic and Southern Ocean climate bifurcate with the level of global warming. Detailed examinations of sea ice and water masses show that in an extreme warming scenario, a combination of enhanced intrusions of warm deep water and warm summertime surface water can cause the nonlinear response of Antarctic ice-shelf basal melting. A large reduction in Antarctic coastal sea ice and the associated ocean freshening by decreasing coastal sea ice production in winter provide favorable conditions for summertime warm surface water formation and warm deep water intrusions onto some continental shelves. The model results demonstrate that disappearing summertime sea ice along the Antarctic coastal margins in a warming climate heralds the nonlinear increase in Antarctic ice-shelf basal melting, presumably contributing to the negative mass balance of the Antarctic ice sheet and the sea level rise.

Modulation of Upper Ocean Vertical Temperature Structure and Heat Content by a Fast-Moving Tropical Cyclone

Abstract The ocean temperature response to tropical cyclones (TCs) is important for TC development, local air–sea interactions, and the global air–sea heat budget and transport. The modulation of the upper ocean vertical temperature structure after a fast-moving TC was studied at the observation stations in the northern South China Sea, including TCs Kalmaegi (2014), Rammasun (2014), Sarika (2016), and Haima (2016). The upper ocean temperature and heat response to the TCs mainly depended on the combined effect of mixing and vertical advection. Mixing cooled the sea surface and warmed the subsurface, while upwelling (downwelling) reduced (increased) the subsurface warm anomaly and cooled (warmed) the deeper ocean. An ideal parameterization that depends on only the nondimensional mixing depth (HE), nondimensional transition layer thickness (HT), and nondimensional upwelling depth (HU) was able to roughly reproduce sea surface temperature (SST) and upper ocean heat change. After TCs, the subsurface heat anomalies moved into the deeper ocean. The air–sea surface heat flux contributed little to the upper ocean temperature anomaly during the TC forcing stage and did not recover the surface ocean back to pre-TC conditions more than one and a half months after the TC. This work shows how upper ocean temperature and heat content varies by a TC, indicating that TC-induced mixing modulates the warm surface water into the subsurface, and TC-induced advection further modulates the warm water into the deeper ocean and influences the ocean heat budget. Significance Statement Tropical cyclones can cause a strong ocean response that modulates the upper ocean temperature structure and contributes to the local heat budget and transport. This manuscript shows how mixing and vertical advection modulate upper ocean temperature after four fast-moving tropical cyclones, and then gives a parameterization of how sea surface temperature and upper ocean heat change depend on the two mechanisms. The temperature anomalies can propagate into deeper ocean after the tropical cyclones, and sea surface heat flux is not important for upper ocean temperature response during a tropical cyclone. These results show how the upper ocean temperature responses to a tropical cyclone, and influences the local heat budget.

Tracing carbon flow and trophic structure of a coastal Arctic marine food web using highly branched isoprenoids and carbon, nitrogen and sulfur stable isotopes

Climate-driven alterations of the marine environment are most rapid in Arctic and subarctic regions, including Hudson Bay in northern Canada, where declining sea ice, warming surface waters and ocean acidification are occurring at alarming rates. These changes are altering primary production patterns that will ultimately cascade up through the food web. Here, we investigated (i) the vertical trophic structure of the Southampton Island marine ecosystem in northern Hudson Bay, (ii) the contribution of benthic and pelagic-derived prey to the higher trophic level species, and (iii) the relative contribution of ice algae and phytoplankton derived carbon in sustaining this ecosystem. For this purpose, we measured bulk stable carbon, nitrogen and sulfur isotope ratios as well as highly branched isoprenoids in samples belonging to 149 taxa, including invertebrates, fishes, seabirds and marine mammals. We found that the benthic invertebrates occupied 4 trophic levels and that the overall trophic system went up to an average trophic position of 4.8. The average δ34S signature of pelagic organisms indicated that they exploit both benthic and pelagic food sources, suggesting there are many interconnections between these compartments in this coastal area. The relatively high sympagic carbon dependence of Arctic marine mammals (53.3 ± 22.2 %) through their consumption of benthic invertebrate prey, confirms the important role of the benthic subweb for sustaining higher trophic level consumers in the coastal pelagic environment. Therefore, a potential decrease in the productivity of ice algae could lead to a profound alteration of the benthic food web and a cascading effect on this Arctic ecosystem.

Development of phytoplankton of Lake Vesijärvi during recovery from eutrophication

Wastewaters discharged to Lake Vesijärvi ruined its recreational value and demanded mitigation measures. In the mid-1970s, the diversion of wastewaters elsewhere reduced epilimnetic total phosphorus concentration by ~ 40% from > 100 mg P m−3 in 5 years, but this was not enough to eliminate cyanobacterial blooms. In 1979–1984, hypolimnetic oxygenation was applied to decrease internal nutrient loading, but pumping warm surface water to the hypolimnion probably intensified gas ebullition from the sediment, carrying nutrients to the epilimnion and intensifying cyanobacterial blooms. Intensive fish removal in 1989–1994 was more successful. Five years of summer trawling removed over three-quarters of roach and smelt stock, and after the two most intensive fishing years, TP and chlorophyll concentrations abruptly decreased by ~ 35%. During subsequent years fish removal continued at ~ 30% intensity and maintained chlorophyll and total nutrient concentrations at a lower level until the end of the study. At the same time, the frequency of cyanobacterial blooms decreased from annual to twice per decade. Larger-scale oxygenation after 2010 no longer resulted in ebullition, and its effect on phytoplankton was indistinguishable from natural variability. Consequently, it was abandoned. The intensity of fish removal needed to maintain the present status of the lake is still awaiting evaluation.

A metagenomic-based study of two sites from the Barbadian reef system

We study the microbiome of sea water collected from two locations of the Barbadian coral reefs. The two sites differ in several environmental and ecological variables including their endogenous benthic community and their proximity to urban development and runoffs from inland watersheds. The composition of the microbial communities was estimated using whole genome DNA shotgun sequencing with adjuvant measurements of chemical and environmental qualities. Although both sites exhibit a similar degree of richness, the less urbanized site (Maycocks reef at Hangman’s Bay) has a strong concentration of phototrophs whereas the more urbanized location (Bellairs reef at Folkstone) is enriched for copiotrophs, macroalgal symbionts and marine-related disease-bearing organisms from taxa scattered across the tree of life. Our results are concordant with previous profiles of warm ocean surface waters, suggesting our approach captures the state of each coral reef site, setting the stage for longitudinal studies of marine microbiome dynamics in Barbados.

Observations of turbidity currents in a small, slope-confined submarine canyon in the Eastern Mediterranean Sea

Up to date, most in-situ studies and observations of turbidity currents were conducted in submarine canyons that are either, large, shelf incising or adjacent to big perennial rivers, or a combination of these features. Little if any observations have been made in the numerous, small submarine canyons (<20 km) that are confined to the continental slope and located far offshore smaller and often ephemeral streams, which are far more common globally. In this study, measurements were collected from October 2019 until June 2020 in Bat-Galim Canyon, a small (5 km long) submarine canyon located offshore Haifa, Israel, in the Eastern Levantine Basin (southeastern Mediterranean) where only minor, ephemeral streams reach the sea. This was performed using two similar mooring stations, positioned at depths of 350 and 710-meter along the canyon, which carried an array of measuring instruments, set between 2 and 50 meters above the canyon thalweg. Our data shows that the small Bat-Galim canyon is an active conduit for significant sediment transport to the deep-sea, much of it via turbidity currents that flow down the canyon during winter storms. The characteristic values of the turbidity currents in the Bat-Galim canyon were similar to those reported in much bigger submarine canyons. In addition, we observed temperature inversions during these events, wherein the sediment-laden warm surface water plunged and flowed underneath the colder, and otherwise denser, canyon water. This temperature inversion may lead to sediment lofting and upward convection through the water column for hundreds of meters, once sediment settling relieves the warm water of some of its ballast. These unique findings highlight the need to investigate the importance of small submarine canyons on continental slopes worldwide as water and sediment conduits to the deep-sea, even in dryer regions, and the impact of temperature inversions by turbidity currents on various water column processes.

Multidecadal trends in ultraviolet radiation, temperature, and dissolved oxygen have altered vertical habitat availability for Daphnia in temperate Lake Giles, USA

Abstract Long‐term browning has resulted in increases in dissolved organic carbon and reduced water clarity that have altered the vertical physical structure of many lake ecosystems. The primary responses include reduced ultraviolet (UV) penetration, warming surface waters, and decreased deepwater dissolved oxygen concentrations that interactively alter vertical habitat suitability for zooplankton. Over 3 decades, Daphnia populations have decreased in abundance and shallowed in their vertical distribution in temperate Lake Giles (Pennsylvania, USA). Using 3 decades of corresponding long‐term vertical profile data of UV radiation, water temperature, and dissolved oxygen, we modelled both the suitable vertical habitat and thermally optimal habitat for Daphnia to understand the potential role of habitat availability for their population dynamics. The vertical extent of suitable habitat increased over time primarily due to strong decreases in UV penetration. In contrast, thermally optimal habitat decreased due to strong increases in vertical thermal gradients that were especially strong in late summer. The vertical distribution of Daphnia became shallower over this time period and may be a response to lower UV exposure near the surface, but continued warming of surface waters and decreasing deepwater dissolved oxygen concentrations are likely to lead to a vertical habitat squeeze. The biological implications of long‐term browning require more attention due to the complex and important implications for population dynamics, species interactions, and food web structure in lakes.

The influence of temperature on geothermal power plant performance based on geology in Huangshadong geothermal field, China

The geological structure is rather complicated in Guangdong province, in China. The 10 deep fault belts are mainly oriented in North-East. The North-East fault belts are good channels for deep thermal energy upwelling, which can form a geothermal water reservoir zone. The heating of atmospheric precipitation and surface water by deep rocks is the primary formation mechanism for the hydrothermal resources in the Huangshadong geothermal field. The results show that the two-stage conversion system of flash-binary is more reasonable when the geofluid temperature is higher than 130?C. With every 10?C increment of geofluid temperature for flash-binary system, the power output and exergy efficiency in-crease by 21.6-38.7% and 6.0-13.1%, respectively. The power output and exergy efficiency will decrease by about 20-40% when cooling temperature arising from 15?C to 25?C. The research will provide the basic data for the demonstration of geothermal resource exploitation.

Warming of Baltic Sea water masses since 1850

The Baltic Sea is among the fastest-warming seas globally in recent decades affecting biogeochemical conditions such as euxinic areas but also pelagic and benthic marine ecosystems. It is therefore crucial to understand how this heat gain is distributed vertically. We used reconstructed atmospheric forcing fields for 1850–2008 to perform an ocean climate simulation that adequately captures climatogical temperature and salinity profiles. Then, a water mass classification distinguishes three water masses corresponding to the classical view, warm and fresh surface waters, cold and fresh intermediate waters, and cold and salty bottom waters, and two transition water masses. The temperature trends show a similar three layers pattern with fast warming at the surface (~ 0.06 K decade− 1) and bottom (> 0.04 K decade− 1) and slow in the intermediate layers (< 0.04 K decade− 1). The slow warming in the intermediate layer is explained by both weakly warmed water winter convection and the summer surface thermocline isolating the intermediate layers. The warming in the deep layers is related to warm surface inflow from the North Sea and Baltic proper in the southern and northern Baltic Sea respectively. Furthermore, sensitivity experiments show that the warming magnitude is controlled by rising air temperature while the vertical distribution of heat gain is related to surface wind conditions. Finally, the North Atlantic Oscillation and Atlantic Multidecadal Oscillation are well correlated with the temperature minimum and thus modulate the magnitude of warming in the intermediate layers on shorter time scales. This study provides a new picture of the Baltic Sea’s warming and suggests that this complexity is essential for understanding the influence of climate change on marine ecosystems.

A sediment diagenesis model on sediment oxygen demand in managing eutrophication on Taihu, China.

Blue-green algae (CyanoHABs), photosynthetic bacteria that create a harmful aquatic environment, have been a trending issue on Taihu for over a decade. CyanoHABs adapt to varying climatic changes, which explains why the problem on Taihu still thrives. One major drive that keeps the algae is Sediment Oxygen Demand (SOD). In this paper, seasonal and spatial variations of SOD that contribute immensely to nutrient growth in Lake Taihu were done using the Environmental Fluid Dynamics Code (EFDC). The results were analyzed based on Nitrogenous SOD (NSOD) and Total SOD (TSOD). Summer results ranged from - 0.05754 to - 0.0826 (- 0.75658 to - 0.83902) (g/m2/day) and Winter values ranged from - 0.3022 to - 0.40171 (- 1.34486 to - 1.48856) (g/m2/day) indicate a gradual decrease in NSOD (TSOD) values respectively. Relatively higher values in summer are attributed to warmer surface water which sets up thermal stratification to increase the internal loading of nitrogen. Lower winter values are related to inverse stratification, where lower oxygen concentration decreases the SOD to trigger ammonium accumulation in the water column. NSOD (TSOD) values for Autumn results ranged from - 0.1039 to - 0.24786 (- 0.96251 to - 1.39454) (g/m2/day) and Spring values of - 0.43019 to - 0.35959 (- 1.48297 to - 0.54089) (g/m2/day). Transition seasons (i.e., Autumn and Spring) results are impacted by wind mixing that allows dissolved oxygen and nutrients in the whole water column. However, spring values depict a gradual increase in SOD value attributed to spring turnover and gradual stratification, which decrease nutrient concentration. In contrast, decreasing SOD values in autumn are related to mixing, but temperature decreases tend to increase nutrient concentrations. Carbonaceous sediment oxygen demand (CSOD), due to sulfide oxidation, presents high values from the difference between TSOD and NSOD. Based on the high values of CSOD, it is highly recommended that more research on eutrophic Taihu lakes would consider delving into CSOD.

The effect of salt-fingering structure on sound propagation in the Strait of Hormuz

Structures caused by temperature and salinity gradients and their resulting mixing have a significant effect on sound propagation in shallow and deep waters. One of the vertical structures is salt-fingering (SF), which is formed by the vertical gradient of temperature and salinity, when warm and salty water is located above cold and fresh water. Quantification of double diffusion (DD) structures is done by the Turner (Tu) angle theory. Warm surface water from the Oman Sea, evaporation and salty surface water of the Persian Gulf provides the initial conditions for the formation of SF in the Strait of Hormuz. In depth less than 5 m, weak SF (45 < Tu ≤ 67.5) are formed and by dominating water buoyancy, salt fingers gradually grow and strong SF (67.5≤Tu < 90) extend from the surface to 25 m deep. This process causes mixing and changing the depth of the boundary layer. The diffusion structure in the SF plays a significant role in the deflection and energy loss of the sound signal. By passing through the SF structure on the surface, the sound signal is scattered, and its transmission is lost up to 80 dB (TLS), and the propagation of the sound along the channel is weakened. With increasing frequency and propagation angle, the deflection occurs from the location of the SF towards the cold and fresh water. But by passing through the place of the strong SF, the sound signal is propagated with a delay and 10–20 dB increase in TLS.