J. Lucena

Hydrological cycle and interannual variability of the aquatic community in a temporary saline lake (Fuente de Piedra, Southern Spain)

Fuente de Piedra is a shallow, temporary saline lake whoseseasonal behavior is strongly dependent on the annual hydrologicalbudget. In this study, we outline the characteristics of Fuente dePiedra Lake for two years that had different hydrological budgets.The high precipitations in 1989–90 caused the lake not to dry asusual, and decreased both salinity and the amplitude of changes.There were also differences in nutrient dynamics, with generallylower concentrations of soluble reactive phosphorus and ammonium,whereas in the more humid year nitrate showed a distinct maximum inwinter. Winter bloom chlorophyll a concentrations were alsomuch higher in 1989–90 (>600 μg l-1) but there wasalso a winter productive phase that was presumably poorly coupledwith consumption processes that predominate in spring. Planktonicassemblages were different between years. Highly halotolerantphytoplankton species (Dunaliella salina and D. viridis) became scarcer, and especially two previouslyunrecorded diatoms (Cyclotella sp. and Chaetoceros sp.)became dominant in the bloom time in the wet year. The speciesrichness of the zooplankton increased in the wet year, with newspecies appearing that were not collected during 1987–88(Branchinectella media, Daphnia mediterranea, Macrothrix sp.,Arctodiaptomus salinus, Cyclops sp., Sigara sp...).There was also a much higher development of macrophytes (Ruppiadrepanensis, Althenia orientalis, Lamprothamnium papulosum)and bird populations, especially flamingoes (Phoenicopterusruber).Important interannual variations in this sort of system pointto the need for long term studies to observe the whole range ofstates that define the lake as an entity.

Space-time heterogeneities of the zooplankton distribution in La Concepción reservoir (Istán, Málaga; Spain)

Space-time distribution of the zooplankton (Rotifera, Cladocera and Copepoda) density along the central axis (horizontally and vertically) and the shores of La Concepción reservoir have been studied for one year. Results show that the distribution along the central axis and the shores follows similar patterns both in space and time. Rotifera and Copepoda mainly appear during the first part of the mixing period and Cladocera during stratification. Horizontal distribution is characterized by the occurrence of density gradients from the intake to the dam. Vertical distribution present density maximum at the Secchi depth during mixing and at two times Secchi depth, during stratification. Zooplankton distribution is related to physical-chemical and biotic factors.

Sedimentation of phosphorus fractions and temporal variation in the C:P ratio in La Concepción reservoir, southern Spain

Abstract The settling flux and content of organic carbon (C), organic matter, and different phosphorus (P) fractions were measured fortnightly during a 6‐month stratified period (from early April to early November) with sediment traps located at five different depths in La Concepcion, a mesotrophic and P‐limited reservoir in southern Spain. Characterisation of P fractions in the collected material was performed by a sequential extraction procedure. Sedimentation rates exhibited a distinct seasonal pattern, that allowed us to differentiate three periods: (1) a biogenic sedimentation period (BS), in which the sedimentation rates and total suspended matter concentration were lowest and the result of material of autochthonous origin; (2) a calcium carbonate precipitation period (CCP), in which there were high calcium concentrations in the settling seston; and (3) a rainfall period (R), in which the highest sedimentation rates occurred. Organic C, organic matter, and organic P content were highest in the settling material during the BS period and lowest during the R period. However, during the R period, inorganic‐P sedimentation rates were highest and the major P fraction was apatite‐P. Total organic, and inorganic (Fe‐ and Al‐bound and apatite) P, increased substantially, whereas total and organic C decreased with depth in the material collected by sediment traps during the BS and CCP periods. Consequently, the total C:P (Ct:Pt) and organic C:P (Corg:Porg) ratios decreased significantly with increasing depth of sediment traps.

Nitrate Reductase Activity in an Eutrophic Reservoir duringthe Stratification Cycle

Nitrate reductase activity was measured during the stratification cycle of the eutrophic reservoir La Concepcion. Nitrate was consumed in the epilimnion mainly through assimilatory reduction, and in the hypolimnion through dissimilatory reduction triggered by anoxic conditions. Nitrate reductase activity (NR) scaled to particulate nitrogen (PN) was positively correlated with the external nitrate concentration and the NO3 ‐ /NH4 + ratio. Using NR as a tool for the assessment of the N cycle dynamics, it can be suggested that at the beginning of the stratification cycle, the phytoplanktonic community was mainly using NO3 ‐ as N source; and since N was not likely the limiting nutrient at that time of the year, NR was close to its maximum possible.

Significance of phosphomonoesterase activity in the regeneration of phosphorus in a meso-eutrophic, P-limited reservoir.

The significance of phosphomonoesterase (PMEase) activity in the regeneration of phosphorus was investigated in a phosphorus-limited reservoir during the growing season (April‐October) using the artificial substratep-nitrophenylphosphate (pNPP). Despite the relatively high concentration (up to 0.45 mmol l 21 ) of soluble nonreactive phosphorus (SNP) in the reservoir and the high total PMEase activity (up to 1144 nmol pNP l 21 h 21 ), the rate of phosphate release from SNP accounted for less than 0.6% of the direct orthophosphate uptake by the planktonic community. The high apparent affinity and maximum velocity for pNPP may underestimate the velocities of Pi release by PMEase. More critical studies using natural substrates are needed to assess the role of PMEase as Pi supplier for the planktonic community. PMEase activity was apparently not related to soluble or particulate phosphorus fractions, suggesting that PMEase activity may not be useful as an indicator of P-limitation, unless other phosphorus pools (e.g. intracellular phosphorus) are considered. q 2000 Elsevier Science Ltd. All rights reserved.

Pigment estimations and photosynthesis of Ruppia drepanensis Tin. ex Guss. in a hypersaline environment

We collected the ephemeral macrophyte Ruppia drepanensis Tin. ex Guss. from the athalassic shallow lake Fuente de Piedra (Málaga. Southern Spain). This lake, situated in an endorheic basin, shows great seasonal changes in depth and Total Dissolved Solids (TDS).Dissolved oxygen evolution studied in the laboratory at 17 different photon flux densities (PFD) showed a maximum rate of photosynthesis of 0.55 mg C g dry wt−1 h−1, a light compensation point at 86 µE m−2 s−1 and a saturation point at 333 µE m−2 s−1. A moderate photoinhibition (\ = 1.68 10−4) was found above 695 µE m−2 s−1.Estimates of pigment concentrations revealed 10 times more carotenoids than chlorophyll.The adaptation of the plants to high irradiances and to the particular features of their hypersaline environment are discussed.

EL PLANCTON DE LAS LAGUNAS DE SIERRA NEVADA: NECESIDAD DE ESTUDIOS TRANSDISCIPLINARES

La correcta valoracion de los procesos ecologicos, como base para el mantenimiento de la integridad ecologica del ecosistema, se ha convertido en una de las tareas de mas dificil cumplimiento en las labores de conservacion de los ecosistemas mediterraneos. La busqueda y desarrollo de herramientas en la deteccion y prevencion de alteraciones ambientales nos ha llevado a centrar la atencion en las lagunas de alta montana del Parque Nacional de Sierra Nevada, ecosistemas que presentan un singular valor ecologico dentro del ambiente mediterraneo, por su caracter oligotrofico, por su situacion a gran altitud y por la adaptacion de sus comunidades a condiciones ambientales extremas. El presente texto describe una propuesta de estudio integral en estos ecosistemas con una aproximacion basada en el analisis de las estructuras de tamano de las comunidades del plancton como herramienta descriptora y predictiva de las respuestas de esas comunidades a los cambios ambientales.

Assessment of a previous study on nitrous oxide production in the Swale—Ouse river system, England

Atmospheric N,ü increased annually by 0.8 ppbv during the 1980s (IPCC 1996), so its role in the destruction o f the oron e layer ( CICERONE 1987) has created a need for estimates from natural environments and predictions of future changes. The microbiological processes involved are denitrification, nitrification and dissimilatory nitrate reduction; denitrification may form or remove N,O, depending on conditions (KNOWLES 1981). N,Q production in freshwater sediments resulting from these processes has been quantified in various studies (see MEYER et al. 1996), but these are insufficient to estimate its overall significance. Part of the recent UK Land Ocean lnteraction Study was to assess the impact of biota on nutrient dynamics on passing down the River Swale-Ouse in Yorkshire. Accounts of denitrification and N,O production (PATTINSON et al. 1998; GARCfA-RUJZ et al. 1998a,b,c,d, and in press) apparendy represent the first for both processes throughout a whole river system. However, there will surely be an increasing demand for such studies based on whole rivers. The present aim is to evaluate the programme and methods used for the Swale-Ouse project as an aid to future studies. The emphasis is placed on N,O production, as this is the process for which there is a particular need for more information. Past results are reviewed only in so far as they provide insight to practical methods.