L. Cruz-Pizarro

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.

Sediment phosphate fractionation and interstitial water phosphate concentration in two coastal lagoons (Albuferas de Adra, SE Spain)

The concentrations of o-phosphate and dissolved inorganic carbon were monitored in the interstitial water of the sediment in two highly eutrophic coastal lagoons, Laguna Honda and Laguna Nueva (SE, Spain) from July 2000 to August 2001. Additionally, the organic matter concentration and the P-fractions in the sediment were analysed. Despite their proximity, these two lagoons showed many significant differences (P<0.05). The P-concentration in the interstitial water of the top sediment was higher in the Laguna Honda (up to 1 mg l−1), especially during summer, when both sites registered their highest values. By contrast, the concentration of organic matter and organic P-fractions were higher in the sediment of the Laguna Nueva. The concentration of the organic matter in the vertical profile of the sediment decreased with depth in the Laguna Nueva (from 17% at 0 cm to 7% at 15 cm, annual mean value) while it increased in the sediment of Laguna Honda (from 9% at 0 cm to 12% at 15 cm, annual mean value). The P-concentration in the interstitial water and the organic matter in the top sediment of Laguna Honda followed a seasonal pattern and, were both correlated with the temperature of the bottom water (r=0.706, P<0.05 and r= -0.929, P<0.01, respectively). The inorg-P fractions comprised 63% of the Tot-phosphate in the sediment of Laguna Honda, whereas org-P fractions (68%) dominated in the sediment of Laguna Nueva. The concentrations of Tot-P in the sestonic material collected at three different depths of the water column averaged 2.2 mg g−1d.w. in Laguna Honda and 2.5 mg g−1d.w. in Laguna Nueva. The average atomic C:P ratio of the sestonic material was significantly higher in Laguna Honda than in Laguna Nueva, but the top sediment in Laguna Nueva had a C:P ratio significantly higher than that of the sestonic material (average: 416 and 162, respectively).The different biodegradability of the organic matter in the top sediment probably explained the differences found in sediment composition between these two lagoons. We suggest that the P-reclying from the sediment to the interstitial water was faster in Laguna Honda due to the higher biodegradability of the organic matter. By contrast, the lower biodegradability of the organic matter in the top sediment of Laguna Nueva was probably due to a higher contribution of vascular plants which could explain the organic matter accumulation detected in this lagoon.

Setting up High Gradient Magnetic Separation for combating eutrophication of inland waters.

To find new approaches to devise technologies for handling with eutrophication of inland waters is a global challenge. Separation of the P from water under conditions of continuous flow is proposed as an alternative and effective method. This work is based on using highly magnetic particles as the seeding adsorbent material and their later removal from solution by High Gradient Magnetic Separation (HGMS). Contrast to other methods based on batch conditions, large volumes of water can be easily handled by HGMS because of decreasing retention times. This study identifies the best working conditions for removing P from solution by investigating the effects of a set of four different experimental variables: sonication time, flow rate (as it determines the retention time of particles in the magnetic field), magnetic field strength and the iron (Fe) particles/P concentration ratio. Additionally, the change of P removal efficiency with time (build up effect) and the possibility of reusing magnetic particles were also studied. Our results evidenced that while flow rate does not significantly affect P removal efficiency in the range 0.08-0.36 mL s(-1), sonication time, magnetic field strength and the Fe particles/P concentration ratio are the main factors controlling magnetic separation process.

The influence of pH on manganese removal by magnetic microparticles in solution

An extensive experimental work is reported that aims to assess the efficiency in manganese (Mn) removal from aqueous solution by carbonyl iron microparticles using magnetic separation techniques. A set of batch experiments are performed to explore the effect of pH, adsorbent concentration, surface coating and contact time for achieving the highest Mn removal efficiency. Mn removal efficiency is extremely high (>98%) for pH values larger than 9 as a result of the chemisorption of Mn oxides onto magnetic microparticles. In contrast, Mn removal efficiency for pH < 9 was significantly reduced as Mn remains as a soluble cation. In this manuscript we demonstrate that the efficiency clearly increases when increasing the adsorbent concentration and when using MnOx(s) coated magnetic particles instead of bare particles. Desorption rates from Mn-loaded magnetic particles at different pHs were always lower than 15%. Furthermore, Mn removal efficiency remained at a very high value (>95%) when reused particles were employed in the adsorption process.

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.

Patterns in the composition of the rotifer communities from high mountain lakes and ponds in Sierra Nevada (Spain)

On the basis of periodic collections of rotifers from 29 lakes and ponds over 2500 m above sea level in the Sierra Nevada (Southern Spain), patterns of species richness, distribution and community composition were evaluated. Results allow us to distinguish communities which fall into two major lake types. One is defined by the presence of typically planktonic species as well as lower specific richness whereas the other includes communities of mainly benthic and periphytic species. Both lake types seem to be related to small differences in their physical and chemical characteristics. These relationships and the influence of littoral vegetation are discussed.

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.

Effects of fluctuating temperatures on the population dynamics of Hexarthra bulgarica (Wiszniewski) from high mountain lakes in Sierra Nevada (Spain)

We examine the development of Hexarthra bulgarica (Wisniewski) populations in relation to thermal stability in natural environments. A high frequency sampling program was developed simultaneously in two high mountain lakes: a shallow one, with daily large temperature changes but little surface-bottom temperature difference and a deeper one with more stable temperature but vertical heterogeneity in the water profile. Since the capacity of H. bulgarica to perform vertical migrations in these lakes of Sierra Nevada is already known, we have studied the relationship between egg ratios and chlorophyll-a concentration, mean temperatures and temperature instability (measured as the daily rate of temperature change — TCR — as well as the surface-bottom temperature difference — SBT -) in both lakes. Results show that the intensity of temperature fluctuations has a positive effect on the egg-ratios, as TCR is only correlated with that variable in the shallow lake and SBT is only correlated with egg-ratios in the deeper one.