Enrique Moreno-Ostos

The influence of wind-induced mixing on the vertical distribution of buoyant and sinking phytoplankton species

In this study we exploit recent advances in high-resolution autonomous monitoring to investigate the impact of short-term variations in wind-induced mixing on the surface biomass and vertical distribution of buoyant and sinking phytoplankton species. An autonomous platform (the Automatic Water Quality Monitoring Station) moored in a Mediterranean reservoir provided minute-by-minute records of wind speed and the phytoplankton fluorescence during winter and summer. This information was then used here to quantify the impact of short-term changes in the weather on the vertical distribution of diatoms and cyanobacteria. Additionally, we apply an empirical model to determine the extent of entrainment of diatoms and cyanobacteria within the turbulent upper layers of the water column. During winter, the surface time series of fluorescence was positively correlated with the short-term variations in wind speed. In contrast, during the summer, fluorescence was negatively correlated with wind speed. In the latter case, turbulence overcame the flotation velocity of buoyant cyanobacteria, thus homogenizing their vertical distribution and decreasing surface biomass. In both cases, the dynamic response of surface phytoplankton biomass to short-term changes in wind stress was rapid, within the minute scale. As far as we know from the literature, this is the first study in which the interaction between wind stress and surface phytoplankton fluorescence has been quantified on such a fine temporal scale. Finally, relevance for forecasting and reservoir management is pointed out.

The spatial distribution of different phytoplankton functional groups in a Mediterranean reservoir

A new high-resolution spectrofluorimetric probe and an automatic water-quality monitoring station (AWQMS) have been used to record seasonal variations in the spatial distribution of three functional groups of phytoplankton in a Mediterranean water-supply reservoir. In comparison with classical methods, the combined use of these innovative techniques enables development of faster and less laborious spatial distribution surveys, thus favouring higher-frequency and spatially more detailed measurements, and, consequently, a better understanding of phytoplankton dynamics. The results show that the observed variations can be explained by the interaction between the buoyancy properties of the phytoplankton and the mixing characteristics of the reservoir. During the winter, when the lake was isothermal and the phytoplankton was dominated by diatoms, there was no significant spatial variation. In the spring, when the phytoplankton was dominated by chlorophytes there was also very little variation but some motile species formed patches when the wind speed was low. The most pronounced non-uniform distributions of phytoplankton were observed during the summer when the phytoplankton community was dominated by positively buoyant cyanobacteria. Then there was a very strong link between the vertical and horizontal gradients which were also related to the prevailing meteorological conditions.

The influence of external perturbations on the functional composition of phytoplankton in a Mediterranean reservoir.

The changes in abundance and composition experienced by phytoplankton communities in lakes and reservoirs occur in response to variations in the physical (light climate or energy) and the chemical (nutrient availability or resources) constraints for algal growth. Mediterranean reservoirs are very dynamic systems, subject to frequent changes in the physical environment as a result of water management operations, which suggests that phytoplankton communities might also undergo frequent changes. The phytoplankton community composition, abundance and seasonal dynamics of El Gergal, a medium-size Mediterranean reservoir, is analyzed and interpreted in terms of changes in the nutrient-energy balance. It is demonstrated that the seasonal scale changes in the physical environment trigger the seasonal predictable autogenic dynamics of the phytoplankton community. In addition, frequent short-term external perturbations of the physical environment may also induce allogenic shifts and reversions in the succession. The physical changes occur mainly as a result of variations in the outflows. Results are discussed in terms of phytoplankton functional groups life cycle strategies and water quality management.

Tailoring dam structures to water quality predictions in new reservoir projects: assisting decision-making using numerical modeling.

Selection of reservoir location, the floodable basin forest handling, and the design of dam structures devoted to water supply (e.g. water outlets) constitute relevant features which strongly determine water quality and frequently demand management strategies to be adopted. Although these crucial aspects should be carefully examined during dam design before construction, currently the development of ad hoc limnological studies tailoring dam location and dam structures to the water quality characteristics expected in the future reservoir is not typical practice. In this study, we use numerical simulation to assist on the design of a new dam project in Spain with the aim of maximizing the quality of the water supplied by the future reservoir. First, we ran a well-known coupled hydrodynamic and biogeochemical dynamic numerical model (DYRESM-CAEDYM) to simulate the potential development of anoxic layers in the future reservoir. Then, we generated several scenarios corresponding to different potential hydraulic conditions and outlet configurations. Second, we built a simplified numerical model to simulate the development of the hypolimnetic oxygen content during the maturation stage after the first reservoir filling, taking into consideration the degradation of the terrestrial organic matter flooded and the adoption of different forest handling scenarios. Results are discussed in terms of reservoir design and water quality management. The combination of hypolimnetic withdrawal from two deep outlets and the removal of all the valuable terrestrial vegetal biomass before flooding resulted in the best water quality scenario.

The Role of Allochthonous Inputs of Dissolved Organic Carbon on the Hypolimnetic Oxygen Content of Reservoirs

Hypolimnetic oxygen content in lentic ecosystems has traditionally been modeled as a function of variables measured at the epilimnion, or that are supposed to drive epilimnetic processes, like total phosphorus load. However, in man-made reservoirs the river inflow can plunge into deep layers, directly linking the hypolimnion with the surrounding watershed. In these circumstances, organic matter carried by the river can influence the hypolimnetic oxygen content without important intervention of epilimnetic processes. Taking long-term data from two reservoirs in Spain, we applied an empirical regression approach to show that the dissolved organic matter carried by the river is the main driver shaping the hypolimnetic oxygen content. By contrast, typical variables commonly included in the modeling of the oxygen content in the hypolimnion (nutrient concentrations, chlorophyll a, and dissolved organic carbon measured in the water column) did not show any significant correlation. Interpretations from this regression approach were supported by a comparison between the monthly oxygen consumption in the hypolimnion and the monthly dissolved organic carbon load from the river inflow. We also revisited the prediction of the year-to-year variability of the Nürnberg’s anoxic factor in four reservoirs from Spain and the USA, explicitly including the allochthonous sources in the equations. These sources were significant predictors of the anoxic factor, especially in those systems subject to relatively high human impact. Thus, effects of allochthonous dissolved organic carbon should always be considered in empirical modeling and management of reservoir hypolimnetic processes related to oxygen content (for example, anoxia, nutrient internal loading, or phosphorus cycle resilience).

LOW PREDICTABILITY IN THE DYNAMICS OF SHALLOW LAKES: IMPLICATIONS FOR THEIR MANAGEMENT AND RESTORATION

This study was conducted in two eutrophic shallow lakes (Lake Honda LH and Lake Nueva LN) that share geographic proximity but have contrasting hydrology, meteorology, biogeochemistry, and geomorphology. Our objective was to explore the inter-annual, seasonal, and daily variability in selected biological, physical, and chemical variables of these two systems. Although the study lakes demonstrated a notable inter-annual and seasonal variation in nutrient concentrations, water transparency was the only variable that was consistently more variable in LH than LN. The reason for the greater temporal variability in water transparency of LH is its major susceptibility to wind and rain events. The impact of wind events in this lake is favored by its shallowness and by its silty surface sediment; the high ratio of catchment area to lake area is responsible for the relatively higher susceptibility of LH to rain events than LN. By contrast, in the younger and deeper LN, ground-water discharge buffers certain water chemistry parameters such as conductivity, turbidity, and alkalinity. Interestingly, differences in turbidity and ground-water discharge do not seem to affect the variability in nutrient concentrations, which was similar between the lakes, although these factors may explain differences between the lakes in nutrient concentrations. This paper reveals that the unpredictability and frequency of events in Mediterranean aquatic ecosystems makes it necessary to increase data collection frequency to obtain more accurate simulations in water quality models.

Thermal structure and energy budget in a small high mountain lake: La Caldera, Sierra Nevada, Spain

Abstract This work examines the diel change of energy storage and its associated patterns of thermal stratification during the ice‐free period in a high mountain lake (La Caldera Lake, Sierra Nevada, Spain), in response to meteorological conditions. Bihourly data have been implemented to a standard methodology of surface heat exchange calculations in lakes. Strong variations have been observed on the diverse components of the energy budget at different time scales, ranging from diel to seasonal. Additionally, time‐series analyses have been applied to reveal the underlying periodicities involved in relation to the different variables studied. The results obtained from this study provided realistic conditions for the environmental modelling of such processes, which are very sensitive in time scale.

On non-Eltonian methods of hunting Cladocera, or impacts of the introduction of planktivorous fish on zooplankton composition and clear-water phase occurrence in a Mediterranean reservoir

Among the topics covered by Hutchinson’s Santa Rosalia article, the question of the shortening and lengthening of food webs occupies a central role. As Hutchinson realized, at the time scales of ecological studies, the impact of invader species on established food webs is the fastest shortcut to the shortening or lengthening of the food webs. The construction of thousands of dams in Spain during the last century has offered ecologists a good opportunity to test the effects of invader fish species on the plankton dynamics of these systems. In this article, a series of data related to the food web structure of Sau Reservoir is analyzed for the period 1997–2005. Parameters such as Secchi depth and chlorophyll concentration, as well as abundance and size structure of zooplankton, have been matched to the zooplankton dynamics in the reservoir. Most of the changes detected within this period are attributed to the introduction of zooplanktivorous fish in the reservoir. The Secchi depth measurements have showed a progressive diminution in the clear-water phase during recent years. These changes have been related to the decrease in the abundance of Daphnia and to the reduction of the size of zooplankton, which help to explain concomitant increases in the chlorophyll concentration in the same period. Other observed changes in the composition of the zooplankton community have been the substitution of Daphnia by Bosmina and the increase in the abundance of rotifers. Thus, the annual average abundance of Bosmina in 1997 was 70% of cladocerans, while in 2005 it reached 98%. In parallel, the percentage occurrence of individual rotifers was 40% of total zooplankton numbers but had risen to 85% at the end of the period. All these changes are attributed to the artificial expansion of the food web through stocking of the reservoir with zooplanktivorous fish (Rutilus rutilus and Alburnus alburnus). This study improves our understanding of the trophic relationships in the food web prior to the introduction of the fish.

Response of waterbirds to alternating clear and turbid water phases in two shallow Mediterranean lakes

Albufera de Adra (Southern Spain) constitutes an internationally-recognised marsh for waterbirds; important populations of some endangered species such as White-headed Duck and Red-crested Pochard overwinter and breed in its two shallow permanent lakes (Lake Honda and Lake Nueva). In a recently published article, we revealed the factors responsible for the irregular alternation between phytoplankton-dominated turbid phases and macrophyte-dominated clear water phases in Lake Honda and Lake Nueva. In this note, we try to clarify the impact of such an alternation of equilibrium states on the waterbird dynamics. Marked increments in abundance, and brood recruitment of dabbling and diving waterbirds were recorded during the clear water phases in contrast with the turbid water phases, as the increase of macrophytes associated with increased water transparency attract waterbirds for available food. Implications for ecosystem management, restoration and conservation are identified.

The role of river inputs on the hypolimnetic chemistry of a productive reservoir: implications for management of anoxia and total phosphorus internal loading

Abstract The effect of DOC and nitrate river inputs on summer hypolimnetic oxygen and nutrient dynamics in the advective-dominated, canyon-shaped Sau Reservoir (Spain) was investigated using 11 years of monitoring data. River water entering the reservoir during summer was the main driver defining hypolimnetic oxygen and nutrient concentration. Thus, volume-normalized hypolimnetic oxygen concentration was highly correlated with the river DOC, but not significantly correlated with surrogates of the epilimnetic primary production or with in-lake features. Also, the areal extent of anoxia and nitrate concentration controlled total phosphorus content in the hypolimnion, suggesting that the river DOC and nitrate inputs control internal load of phosphorus. Because improvement of the river water quality was the consequence of implementation of advanced wastewater treatment plants in the reservoir watershed, we advocate these solutions to manage reservoir eutrophication problems. Our results should prompt reservoir limnologists to always take into account the probable, direct effect of allochthonous sources in the hypolimnetic oxygen content and nutrient dynamics, especially in human-impacted systems.