Water column changes under ice during different winters in a mid-latitude Mediterranean high mountain lake

Authors

Ignacio Granados, Manuel Toro, Santiago Giralt, Antonio Camacho & Carlos Montes

Winter limnology has been developed mainly in northern-temperate or subarctic lakes, while mid-latitude Mediterranean high mountain lakes have been less studied during the ice-covered season. We present a five-year study in one of these lower latitude high mountain lakes, conducted with multiprobes located at two different depths recording high-frequency data. We have studied highly contrasted winters regarding temperature and snowfall. The results of a principal component analysis and a subsequent hierarchical clustering show that bottom oxygen depletion under ice is strongly influenced by winter cover thickness and transparency. A thick and opaque winter cover (dark mode) leads to an intense oxygen depletion until anoxia, while a thin and clear winter cover (clear mode) allows higher dissolved oxygen saturation due to oxygen production by photosynthesis. Under unusual conditions of cold and dry climate, only a weak decrease in bottom dissolved oxygen was observed. The extent of dissolved oxygen depletion and its relationship with meteorological parameters and climate patterns are further investigated with generalized additive models. Winter cover thickness and phenology are driven by the joint effect of the North Atlantic Oscillation and East Atlantic climatic patterns. Ice phenology could also influence the following ice-free period given the correlation of lake heat content with the ice-off date and the ice cover length. We propose some features jointly occurring in Lake Cimera as a model for mid-latitude Mediterranean high mountain lakes, including (yet not exclusive) the occurrence of very thick winter covers, the prevalence of dark mode, the relatively high and similar through the years oxygen depletion rates, and high water temperatures during the ice-free periods. It is unclear how the ongoing climate change will affect the prevalence of dark over clear modes due to the expected antagonistic effects of warmer temperatures and lower snowfalls.

Vertical mixing in a shallow tropical reservoir

Authors

Peipei Yang, Derek A. Fong, Edmond Yat-Man Lo, Stephen G. Monismith

This paper presents observations of diurnal cycles of stratification and vertical mixing in Kranji Reservoir, a shallow tropical reservoir with an average depth of 6.7 m located in the northwest corner of Singapore, via field measurements, focusing on a series of three 24-h self-contained autonomous microprofiler (SCAMP) measurements. This data, representing one of the most complete data sets, shows vertical mixing parameters for a tropical shallow reservoir over a diurnal cycle. Responding to diurnal cycles, the observations indicate that the thermal and flow structures are delineated by two distinct thermoclines: a three-layer structure during the daytime and a two-layer structure during the night-time when the two thermoclines merged into one. The daytime structure consisted of a thin surface mixed layer (SML) above the diurnal thermocline, an intermediate hypolimnion layer with the two thermoclines as its boundaries, and an underflow layer below the secondary thermocline. This underflow is shown to primarily arise from a horizontal temperature gradient generated by the daytime wind and stratification, and maintained by night-time differential cooling. The nocturnal structure is comprised of a SML deepened by penetrative cooling and the underflow layer. Analysis of relevant dimensionless numbers, the turbulent kinetic energy (TKE) budget, and Fr_T–Re_T turbulent phase diagram provide a consistent picture of the different mixing mechanisms within each layer. This short time scale variability of vertical mixing as observed in Kranji Reservoir would be a factor in favouring primary production for such shallow tropical systems.

The impacts of changing nutrient load and climate on a deep, eutrophic, monomictic lake

Authors

Alan D. Radbourne, J. Alex Elliott, Stephen C. Maberly, David B. Ryves, Nicholas John Anderson

1. Nutrient availability and climate have substantial effects on the structure and function of lakes. Predicted changes to climate (particularly temperature) over the 21st century are expected to adjust physical lake functions, changing thermal and nutrient use processes. Both increasing anthropogenic nutrient inputs and net reductions following remediation will also drive ecological change. Therefore, there is an increasing necessity to disentangle the effects of nutrient and temperature change on lakes to understand how they might act in additive and antagonistic ways.
2. This study quantified internal and external nutrient loads at Rostherne Mere, U.K., a deep (z_max = 30 m), monomictic eutrophic lake (average annual total phosphorus >100 μg/L) that has a long, stable period of stratification (c. 8.5 months). A lake biophysical model (PROTECH) was used to assess the effect of changes in these loads and climate change on lake productivity in a factorial modelling experiment.
3. During the summer, phosphorus released from the sediment is largely restricted to the hypolimnion and phytoplankton production is supported by the external load. On overturn, phosphorus at depth is distributed throughout the water column with the elevated concentration persisting to support algal productivity in the following spring.
4. Consequently, the model showed that internal nutrient loading was the main driver of current and future changes in the concentration of phosphorus (responsible for up to 86% P reduction), phytoplankton chlorophyll a and cyanobacterial blooms. However, although the external phosphorus load had a relatively small influence on annual mean phosphorus concentration, it had a statistically significant effect on chlorophyll a concentration, because it supported algal production during summer stratification.
5. Climate had minimal direct impact, but a substantial indirect impact by altering the timing, depth and length of lake stratification (c. 14 days longer by 2100), and therefore altered nutrient cycling and phosphorus availability.
6. In summary, the recovery trajectory at Rostherne Mere is limited by the annual internal soluble reactive phosphorus load replenishment that realistically is unlikely to change greatly on a shorter time‐scale. Therefore, the external soluble reactive phosphorus load has the potential to play an important role as it can be managed further, but is complicated by the indirect impact of climate changing stratification and flushing patterns.

PCLake+: A process-based ecological model to assess the trophic state of stratified and non-stratified freshwater lakes worldwide

Authors

Annette B.G. Janssen, Sven Teurlincx, Arthur H.W. Beusen, Mark A.J. Huijbregts, Jasmijn Rost, Aafke M. Schipper, Laura M.S. Seelen, Wolf M. Mooij, Jan H. Janse

The lake ecosystem model PCLake is a process-based model that was developed to simulate water quality based on ecological interactions in shallow, non-stratifying lakes in the temperate climate zone. Here we present PCLake+, which extends the PCLake model to cover a wide range of freshwater lakes that differ in stratification regime and climate-related processes. To this end, the model was extended with a hypolimnion layer that can be invoked and configured by forcing functions or by simple built-in empirical relationships that impose stratification. Further adjustments to the original PCLake model have been made with respect to the calculation of 1) light irradiation in the water column, 2) evaporation processes and 3) phenology of macrophytes. The simulation output of PCLake+ for different types of lakes complies well with generally accepted limnological knowledge, thus holding promise for future contributions to ecological theory and application to lakes around the globe.

Investigating Abiotic Drivers for Vertical and Temporal Heterogeneities of Cyanobacteria Concentrations in Lakes Using a Seasonal In‐situ Monitoring Station

Authors

A. A. Wilkinson, M. Hondzo, M. Guala

Harmful Algal Blooms (HAB) are ubiquitous ecological and public health hazards because they are comprised of potentially toxic freshwater microorganisms, called cyanobacteria. The abiotic drivers for toxic HAB are investigated using a research station deployed in a eutrophic lake in Minnesota in 2016. This research station provides full depth water quality (hourly) and meteorological conditions monitoring (5 minutes) at the sampling site. Water quality monitoring provides chemical, physical and biological measurements; i.e. phycocyanin concentration, a photosynthetic pigment distinct to cyanobacteria. The high cyanobacteria biovolume (BV) in the epilimnion observed in mid‐July, persisted until late September when it was distributed uniformly throughout the water column. A scaling relationship was developed among BV heterogeneity, thermal stratification stability, and surface water temperature. This relationship was verified in a dimictic lake the following year. The proposed scaling relationship is relevant to sampling protocols of HAB as it informs if the sample depth is representative of the entire water column. During the strongly stratified period, BV accumulated above the thermocline and in the photic zone, with distinct peaks forming occasionally both near the water surface and at locations with photosynthetic active radiation approximately equal to 10 μE/m²s. Our observations suggest that the temporal/vertical variability of cyanobacteria BV is strongly influenced by lake dynamics, thermal structure, seasonal temperature variation, and light availability. These observations demonstrate that cyanobacteria tend to move and accumulate in specific warm water layers, confined by the thermocline and determined by well‐defined light conditions.

Water temperature dynamics and the prevalence of daytime stratification in small temperate shallow lakes

Authors

Kenneth Thorø Martinsen, Mikkel René Andersen, Kaj Sand-Jensen

Small lakes are understudied compared to medium- and large-sized lakes, but have recently received increased attention due to their abundance and importance for global scale biogeochemical cycles. They have close terrestrial contact, extensive environmental variability, and support high biodiversity among them. Temporal and spatial variability of water temperature, oxygen, and stratification–mixing dynamics were examined during a year in nine small Danish lakes. We found that diel mean surface water temperatures were similar among lakes while the diel range decreased with increasing water depth. Vertical temperature stratification occurred on 47% of the days during the entire year and 64% of summer days, usually with daytime stratification and nocturnal convective mixing. The probability of daytime stratification increased with higher incident irradiance, higher air temperature, and lower wind speed. During spring, daytime stratification caused differences in oxygen saturation between surface and bottom waters. These findings offer new insights on the high variability of water temperature and oxygen in time and space in small temperate shallow lakes. The variable water temperature and the regular stratification–mixing processes will have a pronounced influence on biogeochemical cycles. Also, these features are expected to affect the performance and evolutionary process of organisms associated with small lakes.

Metalimnetic oxygen minimum and the presence of Planktothrix rubescens in a low-nutrient drinking water reservoir

Authors

Valerie C. Wentzky, Marieke A. Frassl, Karsten Rinke, Bertram Boehrer

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

Oxycline oscillations induced by internal waves in deep Lake Iseo

Authors

Giulia Valerio, Marco Pilotti, Maximilian Peter Lau, and Michael Hupfer

Lake Iseo is undergoing a dramatic de-oxygenation of the hypolimnion, representing an emblematic example among the deep lakes of the prealpine area that are, to a different extent, suffering from reduced deep water mixing. In the anoxic deep waters, the release and accumulation of reduced substances and phosphorus from the sediments is a major concern. Since the hydrodynamics of this lake was shown to be dominated by internal waves, in this study we investigate for the first time the role of these oscillatory motions on the vertical fluctuations of the oxycline, currently situated at a depth of around 95m, where a permanent chemocline inhibits deep mixing by convection. Temperature and dissolved oxygen data measured at moored stations show large and periodic oscillations of the oxycline, with amplitude up to 20m and periods ranging from 1 to 4 days. A deep dynamics characterized by larger amplitudes at lower frequencies is shown to be favoured by the excitation of second vertical modes in strongly thermally stratified periods and of first vertical modes in weakly thermally stratified periods, when the deep chemical gradient can support baroclinicity anyhow. These basin-scale internal waves cause in the water layer between 85 and 105m depth a fluctuation of the oxygen concentration between 0 and 3mgL⁻¹ that, due to the bathymetry of the lake, changes the redox condition at the sediment surface. This forcing, involving about 3% of the lake’s sediment area, can have major implications for the biogeochemical processes at the sediment water interface and for the internal matter cycle.

Annual stratification patterns in tropical mountain lakes reflect altered thermal regimes in response to climate change

Authors

Labaj, Andrew L.; Michelutti, Neal; Smol, John P.

Many lakes in the tropical Andes have historically been described as polymictic, experiencing only brief periods of thermal stratification. Recent work in Cajas National Park, in the Southern Sierra of Ecuador, however, shows that lakes presently undergo extended periods of thermal stratification. Nevertheless, questions remain about annual patterns of stratification and the main drivers that influence these new thermal regimes. Here, we present two years of lake temperature profiles from four Cajas-area lakes, which span more than 1,000 m elevation. All lakes experienced prolonged periods of thermal stratification, notably from November to May, when air temperatures were highest and wind speeds were lowest. In the three lakes at lowest elevations, the strength of thermal stratification was significantly positively correlated with air temperature and negatively correlated with wind speed. From May to October, when air temperatures were lowest and wind speeds were highest, each of the lakes experienced isothermal periods. The pattern of thermal stratification in each of the lakes was similar between the two years. Our high-resolution data show that stratification does not breakdown overnight, as has previously been hypothesized. As climate models predict future warming to be greatest with increasing altitude, the strength and duration of stratification in tropical mountain lakes will likely increase over time, with widespread implications for chemical and biological lake processes.

Estimation of water quality dynamics under long-term anoxic state in organically polluted reservoir by field observations and improved ecosystem model

Authors

Tran Tuan Thach, Masayoshi Harada, Kazuaki Hiramatsu, Toshinori Tabata

In closed water bodies with significant organic pollution, anoxification due to thermal stratification leads to the elution of nitrogen and phosphorus from the bottom sediment and the generation of sulfide, resulting in further degradation of the water environment. This study focuses on the water quality dynamics in an organically polluted reservoir exhibiting long-term anoxification using two approaches: (1) field observations of seasonal changes in vertical profiles of dissolved oxygen, nitrogen, phosphorus, and sulfide and (2) construction of a water quality prediction model based on an ecosystem model incorporated with anaerobic biochemical processes. Iron and sulfate reduction occurred simultaneously because nitrate–nitrogen was reduced by denitrifying bacteria after the anoxification, and iron reduction became the main factor of the increase in ammonium–nitrogen and phosphate–phosphorus. The redox state of the bottom sediment surface, when anoxification began to occur, greatly affected the water quality dynamics caused by gradual reductive reactions under anaerobic conditions. Furthermore, the calculation accuracy of ammonium–nitrogen, phosphate–phosphorus, and sulfide was highly improved by modifying the conventional model based on the field observations. The characteristics of water quality under anaerobic conditions were sufficiently reflected in the upgraded ecosystem model. The proposed water quality prediction model could be used to quantitatively estimate the water environment dynamics in organically polluted water bodies. The model could be developed further in the future to solve the problems caused by long-term anoxification.