Summer Stratification Period Research Articles

Habitat selection of a migratory freshwater fish in response to seasonal hypoxia as revealed by acoustic telemetry

Adaptive efforts to achieve water quality objectives by modifying nutrient loading can have attendant impacts on fish habitats and fisheries. Thus, coordinating fishery and water quality management depends on knowledge of fish behavioral responses to habitat change. This study combined acoustic telemetry of fish with water quality modelingto understand how water quality management might impact fishery management. We examined habitat use of a native demersal fish, lake whitefishCoregonus clupeaformis, in Lake Erie. We focused on the summer stratified period when habitat was expected to be most limiting and used a forecast model to predict temperature and oxygen in the hypolimnion when fish were detected. As hypothesized, lake whitefish occupied a subset of available conditions with occupied habitats characterized by a cool, normoxic, hypolimnion. On some occasions fish were detected when the hypolimnion was predicted to be hypoxic, suggesting that fish were either displaced vertically or horizontally into marginal habitats or uncertainty in model predictions was high. Still, when hypolimnetic conditions were hypoxic, fish tended to move toward normoxia as expected, but when initial conditions were cold with high dissolved oxygen, fish movements were toward lower oxygen (but still normoxic) conditions. We also observed a high affinity for fish to remain near the southern shore in eastern Ohio, Pennsylvania, and New York. If current nutrient reduction objectives are achieved and the extent and severity of hypoxia is reduced, an expansion of lake whitefish habitat and distribution may have significance to the spatial regulation of fishing effort in Lake Erie.

Increasing air temperature relative to water temperature makes the mixed layer shallower, reducing phytoplankton biomass in a stratified lake

Abstract The depth of the mixed layer is a major determinant of nutrient and light availability for phytoplankton in stratified waterbodies. Ongoing climate change influences surface waters through meteorological forcing, which modifies the physical structure of fresh waters including the mixed layer, but effects on phytoplankton biomass are poorly known. To determine the responses of phytoplankton biomass to the depth of the mixed layer, light availability and associated meteorological forcing, we followed daily changes in weather and water column properties in a boreal lake over the first half of a summer stratification period. Phytoplankton biomass increased with the deepening of the mixed layer associated with high wind speeds and low air temperature relative to the temperature of the mixed layer (Tair−Tmix < 0), whereas heatwave conditions—shallow mixed layer driven by high Tair−Tmix value and low wind speed—reduced the biomass. Improving light availability from low to moderate light conditions increased the phytoplankton biomass, while the highest light availability was associated with low phytoplankton biomass. Our study demonstrates that the climatic impact‐drivers wind speed and Tair−Tmix are major drivers of mixed layer depth, which controlled phytoplankton biomass during the early summer stratification period. Our study suggests that increasing air temperature relative to water temperature and declining wind speeds have potential to lead to reduced phytoplankton biomass due to a shallower mixed layer during the first half of the stratification period in non‐eutrophic lakes with sufficient light availability.

Documenting the impacts of increasing salinity in freshwater and coastal ecosystems: Introduction to the special issue

Documenting the impacts of increasing salinity in freshwater and coastal ecosystems: Introduction to the special issue

Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

Abstract Carbon isotope analysis (δ13C) of Daphnia resting‐eggs (ephippia) in sedimentary records can be used to reconstruct past carbon transfers in pelagic food webs in lakes. However, there may be seasonal variability of cladoceran δ13C and ephippia production that could affect their use as palaeoecological indicators of pelagic carbon transfers. This is particularly likely in stratified lakes where availability of different pelagic carbon sources is seasonal. In addition, there are currently no studies on spatial variability of ephippia δ13C in deep areas of lakes and its implications for sampling strategies in paleolimnological studies. Four French lakes were sampled for over a year to evaluate seasonal variation of the carbon sources consumed by Daphnia using analyses of the Daphnia carbon isotope signature (δ13CDaphnia) and suspended organic matter signature (δ13Cseston). Daphnia ephippia were also collected from surface sediments at maximum depth in the four lakes. Ephippia signatures (δ13Cephippia) were compared to the δ13CDaphnia to evaluate ephippia production periods and the implications for the use of δ13Cephippia in trophic functioning studies. In addition, spatial variability of δ13CDaphnia among the four lakes and the variability of δ13Cephippia in the deep area of one lake were assessed to determine the relevance of a single sampling point in the context of paleolimnological studies. A similar pattern was observed in all four lakes: δ13CDaphnia was close to δ13Cseston during the summer stratification period, but δ13CDaphnia became much lower than δ13Cseston after the autumnal turnover. Ephippia seem to be produced either during the summer stratification period or after autumnal turnover depending on the lake. No spatial variability was observed among the sampling points in δ13CDaphnia in the four lakes, and δ13Cephippia seems to be spatially randomly distributed in the deep zone of the studied lake. In the four lakes, summer δ13CDaphnia values were never below −40‰, reflecting mainly the consumption of phytoplankton, which in turn uses varying amounts of CO2 from respiration (depending on the lake). After autumnal turnover, the δ13CDaphnia values reached values far below −40‰, reflecting the transfer of a non‐negligible part of C‐CH4 for three of the lakes. Seasonal stratification and turnover mechanisms seem to influence the availability of carbon sources in the pelagic compartment of the four lakes. This study shows that the timing of ephippia production affects the information provided by δ13Cephippia due to seasonal differences in the mechanisms (stratification, autumnal turnover) that determine which carbon sources are available in the pelagic compartment. Therefore, using δ13Cephippia to study past pelagic transfers of carbon in stratified lakes may entail uncertainty if used alone and requires multi‐proxy studies. Finally, δ13Cephippia appear to be homogeneously distributed in the deepest area of a medium‐sized, single basin. A single core retrieved from the deepest part of the lake should therefore provide a representative sample of the ephippia produced in a lake.

Integrating Deep Learning and Hydrodynamic Modeling to Improve the Great Lakes Forecast

The Laurentian Great Lakes, one of the world’s largest surface freshwater systems, pose a modeling challenge in seasonal forecast and climate projection. While physics-based hydrodynamic modeling is a fundamental approach, improving the forecast accuracy remains critical. In recent years, machine learning (ML) has quickly emerged in geoscience applications, but its application to the Great Lakes hydrodynamic prediction is still in its early stages. This work is the first one to explore a deep learning approach to predicting spatiotemporal distributions of the lake surface temperature (LST) in the Great Lakes. Our study shows that the Long Short-Term Memory (LSTM) neural network, trained with the limited data from hypothetical monitoring networks, can provide consistent and robust performance. The LSTM prediction captured the LST spatiotemporal variabilities across the five Great Lakes well, suggesting an effective and efficient way for monitoring network design in assisting the ML-based forecast. Furthermore, we employed an explainable artificial intelligence (XAI) technique named SHapley Additive exPlanations (SHAP) to uncover how the features impact the LSTM prediction. Our XAI analysis shows air temperature is the most influential feature for predicting LST in the trained LSTM. The relatively large bias in the LSTM prediction during the spring and fall was associated with substantial heterogeneity of air temperature during the two seasons. In contrast, the physics-based hydrodynamic model performed better in spring and fall yet exhibited relatively large biases during the summer stratification period. Finally, we developed a statistical integration of the hydrodynamic modeling and deep learning results based on the Best Linear Unbiased Estimator (BLUE). The integration further enhanced prediction accuracy, suggesting its potential for next-generation Great Lakes forecast systems.

Annual water residence time effects on thermal structure: A potential lake restoration measure?

Innovative methods to combat internal loading issues in eutrophic lakes are urgently needed to speed recovery and restore systems within legislative deadlines. In stratifying lakes, internal phosphorus loading is particularly problematic during the summer stratified period when anoxia persists in the hypolimnion, promoting phosphorus release from the sediment. A novel method to inhibit stratification by reducing residence times is proposed as a way of controlling the length of the hypolimnetic anoxic period, thus reducing the loading of nutrients from the sediments into the water column. However, residence time effects on stratification length in natural lakes are not well understood. We used a systematic modelling approach to investigate the viability of changes to annual water residence time in affecting lake stratification and thermal dynamics in Elterwater, a small stratifying eutrophic lake in the northwest of England. We found that reducing annual water residence times shortened and weakened summer stratification. Based on finer-scale dynamics of lake heat fluxes and water column stability we propose seasonal or sub-seasonal management of water residence time is needed for the method to be most effective at reducing stratification as a means of controlling internal nutrient loading.

Simulating thermal dynamics of the largest lake in the Caucasus region: The mountain Lake Sevan

Lake Sevan is the largest freshwater body in the Caucasus region, situated at an altitude of 1,900 m asl. While it is a major water resource in the whole region, Lake Sevan has received little attention in international limnological literature. Although recent studies pointed to algal blooms and negative impacts of climate change and eutrophication, the physical controls on thermal dynamics have not been characterized and model-based assessments of climate change impacts are lacking. We compiled a decade of historical data for meteorological conditions and temperature dynamics in Lake Sevan and used a one-dimensional hydrodynamic model (GLM 3.1) in order to study thermal structure, the stratification phenology and their meteorological drivers in this large mountain lake. We then evaluated the representativeness of meteorological data products covering almost 4 decades (EWEMBI-dataset: 1979-2016) for driving the model and found that these data are well suited to restore long term thermal dynamics in Lake Sevan. This established model setting allowed us to identify major changes in Lake Sevan’s stratification in response to changing meteorological conditions as expected from ongoing climate change. Our results point to a changing mixing type from dimictic to monomictic as Lake Sevan will experience prolonged summer stratification periods and more stable stratification. These projected changes in stratification must be included in long-term management perspectives as they will intensify water quality deteriorations like surface algal blooms or deep water anoxia.

Minor Contribution by Biomineralizing Phytoplankton to Surface Ocean Biomineral Pools in the Late Stratified Period

Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Coccolithophores were generally more numerous in the surface mixed layer, whilst PIC concentrations were more homogenous with depth throughout the euphotic zone. Diatoms were notably more abundant and more diverse in the lower euphotic zone beneath the mixed layer in association with subsurface maxima in chlorophyll-a, bSi and oxygen concentrations. The four dominant coccolithophore species (Emiliania huxleyi, Gephyrocapsa muellerae, Syracosphera spp., and Rhabdosphaera clavigera) represented 78 ± 20% (range 31–100%) of the observed community across all sampled depths yet simultaneously contributed an average of only 13% to measured PIC pools. The diatom community, which was dominated by Pseudo-nitzschia spp. and by a species tentatively identified as Nanoneis longta, represented only ~1% of the bSi pool on average, with contributions increasing within the chlorophyll maximum. Despite a slow gradual deepening of the surface mixed layer in the period prior to observation, and adequate nutrient availability beneath the mixed layer, biomineral pools at this time consisted largely of detrital rather than cellular material.

Predicting arctic-alpine lake dissolved oxygen responses to future tree line advance at the Swedish forest-tundra transition zone.

High latitude regions of the globe are experiencing higher warming rates than many other areas and are expected to undergo dramatic change with future warming (Carpenter et al., 2011; Kuhn et al., 2021). One such change is the poleward advance of tree line into prior tundra environments. While nominally a terrestrial ecosystem change, this advance may dramatically alter aquatic ecosystems, with lakes in particular being highly responsive to events that occur in the surrounding landscape. Klaus et al. (2021) address this important issue by using a space-for-time substitution across 40 Swedish arctic-alpine lakes to understand how a transition from tundra to forested watersheds will influence lake dissolved oxygen (DO).

Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.

Seasonality of phytoplankton cell size and the relation between photosynthesis and respiration in the Ría de Vigo (NW Spain)

The Ría de Vigo is a dynamic and productive upwelling ecosystem. We measured 14C incorporation (TO14CP) and gross primary production (GPP), community respiration (DCR), net production (NCP) and size-fractioned chlorophyll a (chl a) fortnightly from May 2012 to May 2013 in the euphotic layer of the Ría. Our aim was to improve the depiction of plankton metabolism in the Ría and to test the general hypothesis that community structure determines the degree of heterotrophy in planktonic ecosystems. Higher primary production was measured after upwelling episodes and during the spring bloom, when the community was dominated by microphytoplankton (>70% chl a>20μm). Lower primary production was observed during summer stratification periods (~65% chl a>20μm), and during the pico- and nanophytoplankton-dominated winter (~25% chl a>20μm). Coupling between phytoplankton photosynthesis and biomass varied seasonally, mainly driven by environmental conditions. DCR was 3 times lower and 8 times less variable than GPP, and its variability was mainly driven by the changes in chl a. The integrated metabolic balance was autotrophic most of the year, despite the negative NCP rates at depth. There was an inverse relationship between the DCR:GPP ratio and the percentage of microphytoplankton (% chl a>20μm) only in the summer. However, DCR:GPP and DCR:chl a ratios were similar in winter and spring, despite the seasonal differences in primary production and size structure. The similar TO14CP:NCP and chl a:DCR relations in spring (>70% chl a>20μm) and winter (~25% chl a>20μm), and the differences in summer (>70% chl a>20μm) confirm cell size independence in trophic functioning. We conclude that respiration variability is relevant for the metabolic balance in the Ría, and that the degree of heterotrophy is not systematically related to phytoplankton size over the scales of our study.

Speed of sound gradients due to summer thermal stratification can reduce the detection range of acoustic fish tags: results from a field study in Hamilton Harbour, Ontario

Understanding detection range is a key factor for the use of acoustic telemetry in fisheries research. Lakes have strong seasonal changes in thermal stratification, as well as short-term changes due to internal seiches. These thermal gradients in lakes imply strong sound-speed gradients that can refract and diverge acoustic signals, leading to acoustic attenuation and smaller detection range. Using field-based range testing and the Bellhop acoustic model, we investigated how changes in stratification lead to changes in detection range within Hamilton Harbour, Ontario, Canada. During the summer stratified period, the detection range was less than 350 m, whereas in the isothermal fall, range was up to 500 m. Range test data from three separate field observations showed a good correlation with Bellhop predictions. Owing to the intense internal seiches in Hamilton Harbour, the stratification in the shallower littoral regions essentially switched between stratified and isothermal conditions over short timescales, which is predicted to lead to high temporal variability in detection range that must be accounted for during the analysis and interpretation of telemetry-derived data.

Contribution of the deep chlorophyll maximum to primary production, phytoplankton assemblages and diversity in a small stratified lake

AbstractThis 6-month study characterized the contribution of deep chlorophyll maximum (DCM) to lake phytoplankton diversity and primary production, in relation to stratification during the ice-free season. Phytoplankton and zooplankton dynamics were examined with environmental drivers in a small stratified lake that presents vertical gradients of light and nutrients. The phytoplankton, first composed of diatoms and chrysophyceae, shifted to cyanobacteria in mid-July. With stratification increase, surface nutrient limitation appeared to favor motile species characteristic of oligotrophic environments above a deep layer of filamentous cyanobacteria, fueled by the vertical nutrient fluxes from sediment. The DCM contributed on average to 33% (but up to 60%) of total production during the strongest summer stratification period. In late summer, as stratification was eroding, the vertical gradient of nutrients was reduced, but light attenuation with depth increased. Distinct assemblages were identified between surface and deep layer with shade-adapted species. The contribution of DCM was reduced to 10%. Zooplankton community varied in conjunction with phytoplankton and stratification. Our study demonstrates no benefit of DCM for taxonomic and functional diversity and a limited contribution to total production. The depths over which phytoplankton use separate spatial niches may be lesser in a 6-m-deep lake compared with deeper stratified lakes.

Inflow Dynamics in Weakly Stratified Lakes Subject to Large Isopycnal Displacements

AbstractThe effect of large‐amplitude isopycnal displacements, frequently observed in deep medium‐size arctic lakes during the ice‐free period, on the near‐ and far‐field fate of negatively buoyant river inflows is explored in this work. A three‐dimensional transport and hydrodynamic model of sub‐arctic Lake Lagarfljót was used to simulate the fate of river inflows during the summer stratification period. The intrusion dynamics are strongly affected by the amplitude and direction (downwelling/upwelling) of the isopycnal displacements induced by the wind near the river inlet. These displacements control the distance of travel of the river plume until reaching the layers with maximum density gradients (pycnocline) and, thus, the mixing ratio between the river plume and the lake water. Specifically, strong upwelling near the inlet causes the river to flow to the bottom as an underflow. Under downwelling, river plumes tend to form metalimnetic intrusions. The influence of the isopycnal displacements on the initial river fate can be parameterized using a time‐varying density Richardson number, which needs to be smoothed to account for the effects of unsteadiness. Large amplitude internal motions, of up to 70 m in Lake Lagarfljót, move deep underflows upwards to shallower basins where they could be readily incorporated into the surface mixed layer and rapidly flushed out of the lake. Metalimnetic currents associated with the V2H1 internal circulation can also accelerate riverine transport out of the lake.

New production across the shelf-edge in the northeastern North Sea during the stratified summer period

Abstract New production of organic matter from photosynthesis based on “new” nitrate transported into the illuminated surface layer fuels temperate ecosystems during periods of stratification when surface waters are nutrient limited. Published observations from the northeastern North Sea show a large spatial heterogeneity in vertical nitrate fluxes and suggest shelf edge mixing may be the major source for new production here during the stratified summer season. In the current study, we further investigate these empirical findings with a numerical model, where physical transports and mixing are evaluated against observations of temperature, salinity, nutrients and dissipation of turbulent kinetic energy. The relatively shallow central North Sea is separated from the deep Norwegian trench by a strong shelf edge current. This shelf edge frontal zone is characterized by a vertical separation of the surface and benthic boundary layers by an intermediate layer exhibiting low turbulence. A new nitrate assimilation model, driven by light and nitrate availability, is developed and applied for quantifying the potential for, and distribution of, new production in the area. New production in the frontal zone above the shelf edge is located in a narrow high productive (~100 mg C m−2 day−1) band. This is in qualitative accordance with observations. The model results also suggest, however, that new production of similar magnitude occurs above the deep Norwegian trench, where a shallow nutricline in combination with mesoscale eddy activity leads to increased transport of nitrate to the surface layer. Increased new production along the shelf edge could potentially impact ecosystem structure and may explain the relatively high species richness and fishing activity recorded in this part of the North Sea.

Insights into the dynamics of the deep hypolimnion of Lake Geneva as revealed by long‐term temperature, oxygen, and current measurements

AbstractIn order to identify or shed light on dominant long‐term processes of the deep hypolimnion of Lake Geneva (309 m depth), time series of temperature and horizontal currents and profiles of temperature and oxygen, taken for over a decade in the deepest part of the lake, were analyzed. The focus was on the summer stratification period (May to October). During that period, temperatures near the bottom always increased quasi‐linearly with the same gradient and small amplitude variability. Vertical mean temperature gradients in the lowest 60 m of the water column remained nearly constant over the years and were comparable to those of the deep ocean. Mean current velocity at the deepest point was 3.0 ± 0.5 cm s−1 and ever‐present inertial motions clearly dominated currents. Oxygen decreased quasi‐linearly and eutrophication appeared not to affect this rate of decrease. It is suggested that as in the deep ocean, breaking internal waves and friction from decaying inertial motion occurring in the deep hypolimnion strongly contribute to turbulent mixing in that layer. The climate change‐induced long‐term warming trend seen in the upper layers of the lake was not detected in the deep hypolimnion. Instead, year‐to‐year climate change‐induced variability strongly affected the deep hypolimnion. During cooling in cold winters, lateral advection contributed more to temperature decrease and oxygen renewal in the deep hypolimnion than vertical convection. The present analysis has shown that the dynamics of this layer are highly three‐dimensional and that the processes occurring therein cannot be correctly described by traditional one‐dimensional concepts.

Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities

In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.

Aerobic methane production by planktonic microbes in lakes

Methanogenesis in freshwater lakes has classically been considered to arise from anaerobic methanogens in oxygen-depleted sediments. However, the accumulation of supersaturated methane in fully oxygenated water columns is commonly observed in many lakes, and factors responsible for the formation of the subsurface methane maximum (SMM) remain largely unknown. The present study conducted in 14 Japanese freshwater lakes showed that the SMM formation during the summer stratification period is a common feature in large and deep oligotrophic lakes. The seasonal survey of a deep oligotrophic lake revealed that SMM formation may be uncoupled with the dissolution of atmospheric methane, as well as with the transport of methane from tributary rivers, littoral sediments, and hypolimnetic anoxic sources, suggesting the contribution of in situ methane production. In fact, batch-culture experiments confirmed that bacterioplankton present in lake subsurface waters produce methane aerobically through the decomposition of methylphosphonic acid. Moreover, the development of SMM was closely associated with the seasonal dynamics of planktonic cyanobacteria such as Synechococcus, which may carry the enzyme to catabolize organophosphonate compounds. Therefore, we suggest that the predominance of Synechococcus during the thermal stratification period plays a significant role in SMM formation, and likely the methane flux from lakes to the atmosphere.

Fish spawning strategies in the northwestern Mediterranean Sea

The Mediterranean is globally considered an oligotrophic sea. However, there are some places or certain seasons in which mechanisms that enhance fertility may occur. These mechanisms, and related processes, are especially relevant in maintaining fish populations when they take place during the period of larval development. This contribution analyzes how environmental conditions occurring in the NW Mediterranean, at local and seasonal scales, determine the temporal and spatial patterns of fish reproductive activity in the region. The structure of the bathymetry, diversity of adult fish habitats and hydrodynamic mechanisms conditioning the primary production of the region (e.g., shelf-slope density front and associated current, continental water inflows, winter mixing, stratification of the water column) determine the location of spawning and the distribution patterns of fish eggs and larvae. A pronounced seasonal variability regarding both the number of species and the number of fish larvae in the plankton can be observed throughout the year. Most NW Mediterranean fish reproduce during the spring–summer stratification period, when the phytoplankton biomass values at the upper layers of the water column are lower than in winter. The development of the Deep Chlorophyll Maximum in this period and the high zooplankton biomass associated to it offers an important food source for the larvae. Additionally, during this period the inputs of continental waters are one of the fertilization mechanisms of surface waters and some species, as anchovy, takes advantage of this situation. Autumn–winter is the period with lower ichthyoplankton diversity, being dominated by sardine. Vertical mixing during winter is one of the mechanisms that enhance productivity. Overall, fish species show reproductive strategies and larval fish behavior that allow them to take advantage of the available resources throughout the seasonal cycle. These strategies, together with the high ecological efficiency of oligotrophic systems, contribute to the relatively high yield of Mediterranean fisheries. In a context of global change, understanding of the mechanisms relating environmental changes to the extent of spatial and temporal location of suitable spawning habitats of fish is a key first step to predicting and projecting such future changes, and thereby adapting to these changes.

Decreased oxygenation demand following hypolimnetic oxygenation

Gantzer PA, Preece EP, Nine B, Morris J. 2019. Decreased oxygenation demand following hypolimnetic oxygenation. Lake Reserv Manage. 35:292–307. We analyzed long-term oxygenation dynamics in 3 waterbodies: Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR), which are water supply reservoirs, and North Twin Lake (NTL), a natural lake. Bubble plume line diffuser hypolimnetic oxygenation system (HOS) operation was for 13, 10, and 7 yr for SHR, CCR, and NTL, respectively. Water column profiles were measured in all 3 waterbodies. Additionally, remotely deployed sensors were used to monitor dissolved oxygen (DO) in NTL year-round, specifically to capture winter conditions during ice cover. Hypolimnetic oxygen demand (HOD) was observed to decrease after several years of HOS operation in all 3 waterbodies. Annual HOD decreased more for HOS operated year-round compared to recovery-based operation where HOS was started after DO decreased below 5.0 mg/L. Additionally, HOD throughout the summer stratified period was lower following maintenance of higher DO during spring prior to onset of stratification. An analogous result was observed during winter ice cover relative to DO conditions following fall turnover. Prior work showed increased HOD during HOS operation compared to pre-HOS. The increased HOD during HOS operation was termed diffuser-induced oxygen demand and implied the HOS was inducing oxygen demand. However, the current work shows HOS operation is not actually inducing oxygen demand; rather, it is addressing the existing oxygen demand already in the sediments. Our data suggest that a HOS is an effective tool to maintain hypolimnion DO in stratified lakes and reservoirs. When operated year-round or before DO decreases, HOS can decrease HOD and reduce overall long-term operational costs.