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1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external). 2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling. 3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elem...

1. An in situ experiment was carried out in a high‐mountain oligotrophic lake (La Caldera) to quantify the specific release rates of ammonium and phosphorus in different phases of the seasonal succession of plankton. The zooplankton community was dominated by the calanoid copepod Mixodiaptomus laciniatus. 2. The rates of release of N and P (from non‐detectable to 3.58 μg N mg–1 DW h–1 and from 0.26 to 2.29 μg P mg–1 DW h–1, respectively) and the N : P released ratio (not detectable to 4.77) fell within the range typical of oligotrophic lakes dominated by copepodite assemblages. 3. The values of the zooplankton N : P ratio (5 : 1 to 10 : 1 by weight) were lower than those established for other species of calanoids, and followed a well‐established pattern of seasonal variation from year to year with higher values after ice thaw and lower values as individual size increased (ontogenetic development). 4. The elemental composition of the zooplankton depended on individual size, while the released N : P ratio was inversely related to the N : P ratio of the food. This pattern conforms to the predictions of the homeostasis theory of Sterner (1990). 5. A feedback regulation was established between the stoichiometric composition of the zooplankton, their food and the released N : P ratio that can help explain changes in pelagic community structure during the ice‐free period.

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Moreno-Ostos, E., Cruz-Pizarro, L., Basanta, A., George, D. G. (2009). The influence of wind-induced mixing on the vertical distribution of buoyant and sinking phytoplankton species. Aquatic Ecology, 43 (2), 271-284.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.Peer reviewe

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 nonuniform 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 effect of sediment desiccation and re-wetting on phosphate adsorption and desorption properties was examined in two oligotrophic high mountain lakes (La Caldera and Río Seco, altitude *3000 m) in the Sierra Nevada National Park, Spain. Decrease in phosphate sorption properties was observed in transects from the littoral zone to dry land (up to the high water mark) in both lakes concomitantly with loss of amorphous oxides of iron and aluminum as revealed by oxalate and dithionite extractions. X-ray diffraction did not indicate increased amounts of crystalline metal oxides, but there was a loss of fine particles (\20 lm) with distance from the shore, probably due to wind erosion. Likely this explains the loss of amorphous metal oxides as they are often enriched in the fine sediment fraction. Changes in P-speciation toward less available pools were also observed as a result of desiccation. When re-wetted under oxic conditions, the sediments, especially those closest to the shore, released phosphate to the overlying water. The loss of adsorption capacity for phosphate upon re-wetting was associated with loss of amorphous iron oxides, and the changes appear to be non-reversible. Thus, both desiccation and re-wetting will lead to a decrease in sediment phosphate sorption capacity and increased water level fluctuations, a possible scenario in climate change, can therefore increase P availability in the water column of these oligotrophic systems.