Thermal Stratification Of Water Column Research Articles

A sequence of weather‐driven hydrodynamic events stimulates the formation of harmful algal blooms on an oligotrophic lake

AbstractHarmful algal blooms (HABs) are common in many eutrophic lakes and frequently associated with nutrient excesses, warm waters, and calm conditions. While HABs can also occur in oligotrophic waterbodies, bloom‐stimulating factors remain elusive for these ecosystems. Here, using a high‐frequency sensor platform supported by hydrodynamic modeling, we document a clearly linked sequence of three hydrodynamic events that immediately preceded each HAB event on Skaneateles Lake, NY during 2018. HABs were first preceded by interactions between internal waves in the thermal structure of the lake with the northern slope of the lakebed. These interactions disturbed sediments resulting in upwelling of nutrient‐rich sediment and possibly cyanobacterial cells into the lower water column. Next, moderate winds stimulated Langmuir circulation which facilitated the transport of material from the lower water column to well‐lit surface waters. Finally, the sequence culminated with a calm wind period, which is often classically associated with surface HAB formation. While individual steps in this sequence occurred independently and repeatedly throughout the year, the complete sequence occurred only eight times, immediately preceding each of the eight documented HABs. We empirically derive thresholds for and timing of the sequence to independently predict HAB occurrence and elevated cyanobacteria levels in the lake. The discovery of this specific and repeated event sequence and the identified thresholds provides the potential for forecasting of HABs days in advance. Ongoing climate change impacts, including increasing water column thermal stratification and changing wind speeds may alter the frequency, timing, and duration of this sequence and consequent HAB formation.

The impact of weather Condition Changes on Vertical Distribution of Sulfides in Lake Maninjau Based on Observation Data

Sulfide is a crucial parameter in volcanic lakes, as its levels and fluctuations in the lake determine the origin of sulfide and the extent of its impact on the lake ecosystem. In stratified lakes, the sulfide produced tends to be retained beneath the oxic layer. The sulfides rise towards the surface as the oxic layer thins triggered by decreased water column thermal stratification. Meanwhile, the strength or weakness of thermal stratification is greatly influenced by weather conditions. Lake Maninjau is a volcanic lake with a relatively high sulfide content. Its vertical distribution in the water column is highly dependent on the stratification of the water column. When stratification disappears, sulfide rises to the surface (locally known as tubo belerang) and has a negative impact on surface biota. The objective of this study is to examine the distribution of sulfides in the water column of Lake Maninjau under two different weather conditions. We perform two surveys to measure physicochemical parameters and sulfide concentration on 26‒29 November 2022 and 25‒26 August 2023 considering the seasonal pattern. We found that air temperatures and sunshine duration combined with precipitation and wind speed drive the thermal stratification of the water column. The lower air temperature, shorter sunshine duration, higher precipitation, and stronger wind speed in the first survey (west monsoon) compared with the second survey (east monsoon) resulted in lower stratification and triggered the elevated sulfide to the surface. In the middle of the lake, the surface sulfide measured during the first survey was 4.16 µg/L. Meanwhile, in the second survey, it was only observed at 1.16 µg/L. The distribution of sulfides within the water column of Lake Maninjau is regulated by the stratification of the water column, a process directly impacted by weather conditions.

An automatic lake-model application using near-real-time data forcing: development of an operational forecast workflow (COASTLINES) for Lake Erie

Abstract. For enhanced public safety and water resource management, a three-dimensional operational lake hydrodynamic forecasting system, COASTLINES (Canadian cOASTal and Lake forecastINg modEl System), was developed. The modeling system is built upon the three-dimensional Aquatic Ecosystem Model (AEM3D) model, with predictive simulation capabilities developed and tested for a large lake (i.e., Lake Erie). The open-access workflow derives model forcing, code execution, post-processing, and web-based visualization of the model outputs, including water level elevations and temperatures, in near-real time. COASTLINES also generates 240 h predictions using atmospheric forcing from 15 and 25 km horizontal-resolution operational meteorological products from the Environment Canada Global Deterministic Forecast System (GDPS). Simulated water levels were validated against observations from six gauge stations, with model error increasing with forecast horizon. Satellite images and lake buoys were used to validate forecast lake surface temperature and the water column thermal stratification. The forecast lake surface temperature is as accurate as hindcasts, with a root-mean-square deviation <2 ∘C. COASTLINES predicted storm surges and up-/downwelling events that are important for coastal flooding and drinking water/fishery management, respectively. Model forecasts are available in real time at https://coastlines.engineering.queensu.ca/ (last access: January 2022​​​​​​​). This study provides an example of the successful development of an operational forecasting workflow, entirely driven by open-access data, that may be easily adapted to simulate aquatic systems or to drive other computational models, as required for management and public safety.

Biogeochemical causes of sixty-year trends and seasonal variations of river water properties in a large European basin

We evaluated long-term trends and seasonal variations in the major physical–chemical properties of the circum-neutral Slapy reservoir (Vltava, Czech Republic) from 1960 to 2019. Mean annual water temperature increased by 2.1 °C, flow maxima shifted by ~ 13 days from the early April to mid-March, and the onset of thermal stratification of water column and spring algal peaks advanced by 19 and 21 days, respectively, due to climate warming. Concentrations of major ions, phosphorus (P), and chlorophyll increased from the 1960s to the 1990–2000s, then decreased due to changing agricultural practices and legislation, intensified wastewater treatment, and decreasing atmospheric pollution. Concentrations of dissolved organic carbon (DOC) decreased from 1960 to the 1990s due to improved wastewater treatment, then began to increase in response to climate change and reduced acidic deposition. Concentrations of water constituents exhibited varying individual long-term and seasonal patterns due to the differing effects of following major processes on their production/removal in the catchment-river system: (1) applications of synthetic fertilizers, liming and farmland draining (NO3−, SO42−, Cl−, Ca2+, Mg2+, K+, and HCO3−), (2) wastewater production and treatment (DOC, P, N forms), (3) road de-icing (Cl− and Na+), (4) atmospheric pollution (SO42−), (5) climate change (DOC), and (6) the aging of reservoirs (NH4+). The water pH increased until the early 1990s, then decreased and exhibited pronounced seasonal variations, integrating the effects of changing external acidity sources and in-lake H+ sources and sinks (i.e., microbial CO2 production/consumption and availability and transformations of inorganic N), and changes in water buffering capacity. Anthropogenic and climatic effects, reservoir aging, and changes in water eutrophication thus may significantly affect water pH also in circum-neutral systems.

High resolution spatiotemporal patterns of seawater temperatures across the Belize Mesoamerican Barrier Reef

Coral reefs are under increasingly severe threat from climate change and other anthropogenic stressors. Anomalously high seawater temperatures in particular are known to cause coral bleaching (loss of algal symbionts in the family Symbiodiniaceae), which frequently leads to coral mortality. Remote sensing of sea surface temperature (SST) has served as an invaluable tool for monitoring physical conditions that can lead to bleaching events over relatively large scales (e.g. few kms to 100 s of kms). But, it is also well known that seawater temperatures within a site can vary significantly across depths due to the combined influence of solar heating of surface waters, water column thermal stratification, and cooling from internal waves and upwelling. We deployed small autonomous benthic temperature sensors at depths ranging from 0–40 m in fore reef, back reef, and lagoonal reef habitats on the Belize Mesoamerican Barrier Reef System from 2000–2019. These data can be used to calculate depth-specific climatologies across reef depths and sites, and emphasize the dynamic and spatially-variable nature of coral reef physical environments.

Distribution patterns of bacterial communities and their potential link to variable viral lysis in temperate freshwater reservoirs

Man-made reservoirs which receive substantial inputs of terrestrial organic matter are characterized by physiologically diverse and distinct bacterial communities. Here we examined bacterial community structure using Illumina MiSeq sequencing of 16S rRNA genes and evaluated the potential role of viruses in influencing them in two productive freshwater reservoirs namely, Villerest and Grangent (Central France). Two dimensional non-metric multidimensional scaling analyses indicated that bacterial communities in both reservoirs were structurally different in time and space, with Villerest harboring more diverse communities than Grangent reservoir. The bacterial communities in both reservoirs were dominated by hgcI clade (Actinobacteria) and Limnohabitans (Betaproteobacteria) which are known to have adaptive life strategies towards top-down mechanisms and resource utilization. In Villerest, thermal stratification of water column which resulted in temporary anoxia especially during summer promoted the occurrence of anoxygenic phototrophic and methanotrophic bacteria. Overall, low bacterial richness which was linked to viral lytic infection possibly suggests that a relatively small number of highly active bacterial populations sustained high bacterial activity and viral abundances. Weighted UniFrac analysis indicated that a minimum threshold viral infection and virus-to-bacteria ratio (serve as a proxy) of 10% and 10, respectively, is required to exert its impact on phylogenetic structure of bacterial community. Therefore depending on the levels of viral infection we suggest that viruses at times can prevail over other trophic or top-down factors in shaping and structuring bacterial communities in such man-made artificial freshwater systems.

Reduced barnacle larval abundance and settlement in response to large‐scale oceanic disturbances: Temporal patterns, nearshore thermal stratification, and potential mechanisms

AbstractWe assessed the effects of large‐scale oceanic disturbances on the settlement rate of an abundant intertidal barnacle. Daily or weekly settlement, nearshore temperature, currents, and abundance of early‐stage barnacle nauplii and Chthamalus fissus cyprids were measured in La Jolla, California, from the inception of the large‐scale warm‐water anomaly known as the “Blob” in 2014, to 1 yr following the 2015/2016 El Niño. We also measured over 2 yr of weekly settlement rates during the 1997/1998 El Niño in La Jolla and in two Mexican sites. Settlement was low during the Blob and the 2015/2016 El Niño, but increased dramatically after its end, and rates remained high through spring 2017. Barnacle nauplii and C. fissus cyprid abundances were also low during the Blob and El Niño. Nearshore water column thermal stratification and variability of the high‐frequency cross‐shore currents, a measure of internal wave activity, declined during the Blob and El Niño, but increased at the end of El Niño. Similarly, settlement rates in the Mexican sites were low during the 1997/1998 El Niño. The reduction in settlement during the Blob and El Niño may be related to two factors: a decrease in water column stratification, which can influence internal bore larval transport and the cross‐shore distribution of larvae, and also to reduced early‐stage larval abundance, potentially related to lower reproductive output or high nauplii mortality during the disturbances.

A hierarchical modelling framework for assessing physical and biochemical characteristics of a regulated river

Regulated rivers below dams have traditionally been managed using a minimum instream flow to provide adequate aquatic habitat. However, dam management, in conjunction with changes in climate and land use, challenges downstream ecosystem functions that cannot be properly addressed by a simple minimum flow requirement. Depending upon the river system, additional parameters such as water temperature and nutrient loading provide more critical ecological value for organisms than ensuring a constant minimum release. A new modelling methodology, utilizing a cascading hierarchical approach, is proposed and tested on a 614km2 headwater basin in central Idaho, USA. Application of the methodology illustrates that below large dams the river discharge becomes independent of the seasonal hydrology and specifying the discharge alone is insufficient for evaluating ecosystem response. Upstream reservoirs interrupt the watershed continuum and internally modify the thermal, chemical, and biological properties of water prior to release into a downstream river. These water properties depend on the annual hydrologic regime, characteristics of the reservoir and the offtake strategies, offtake structure depth, dam discharge, and the water column thermal stratification. This study describes the use of climatically driven hydrologic forcing and variable dam operations in a coupled reservoir-river system to optimize river ecosystem health by linking physical processes with in situ observations and incorporating multi-trophic species requirements. Such an approach can support real-time decision making on existing reservoir-river systems and provide a virtual means of evaluating ecosystem impacts prior to disturbance from new dam construction or implementation of restoration activities in a watershed.

In situ flume measurements of resuspension in the North Sea

The in situ annular flume, Voyager II, was deployed at three sites in the North Sea in order to investigate resuspension events, to determine the physical characteristics of the seabed, to determine the threshold of resuspension of the bed and to quantify erosion rates and erosion depths. These are the first controlled, in situ flume experiments to study resuspension in the North Sea, and were combined with long-term measurements of waves and currents. Resuspension experiments were undertaken at two muddy, and one sandy site: north of the Dogger Bank (DG: water depths ∼80 m, very fine, poorly sorted, very fine-skewed sediment experiencing seasonal thermal stratification of the water column along with oxygen depletion); the Oyster Grounds (OG: ∼40 m, similar bed properties, year round water column thermal stratification, Atlantic forcing); and in the Sean Gas Field (SGF: ∼20 m, moderately sorted, very coarse-skewed sand, and well mixed water column). The erosion thresholds of the bed were found to be 0.66–1.04 Pa (DG) and 0.91–1.27 Pa (OG), with corresponding erosion depths of 0.1–0.15 mm and 0.02–0.06 mm throughout the experiments. Evaluation of a year of current velocities from 2007 indicated that at OG, resuspension of the consolidated bed was limited to on average ∼8% of the time as a result of tidal forcing alone for short (<30 min) durations, but would potentially increase during the winter as a result of wave influences. At DG, under similar conditions this would increase to 13%, and in the SGF, wave-induced resuspension events occurred throughout the year, with the potential exceedance of the threshold for suspension greater than 50% in January and March. Resuspension of bed material and erosion rates were closely related to applied bed shear stresses, and eroded depths were significantly correlated with the physical properties of the bed. Therefore, while complex variations in biogeophysical factors affected the critical threshold of erosion, once exceeded, erosion rates were related to the nature of the sediment.