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Gambusia holbrooki es un pequeño pez Poecílido, de origen norteamericano que fue introducido a principios del siglo XX en humedales de la Península Ibérica para el control biológico de los mosquitos. Actualmente presenta poblaciones en muchos humedales y está considerada como una de las principales especies invasoras en la península. Estos peces se alimentan principalmente de invertebrados, pero son también susceptibles de ingerir y acumular residuos de microplásticos que presentan tamaños similares al tamaño de algunas de sus presas habituales. En el presente estudio analizamos los contenidos del tracto gastrointestinal de ejemplares adultos de esta especie, procedentes de dos lagunas costeras restauradas, con el objetivo de caracterizar la ingesta de microplásticos. Los restos de microplásticos encontrados fueron clasificados por tamaños, por categorías (fibras y fragmentos) y por colores. Se analizaron 156 ejemplares de Gambusia holbrooki: 92 hembras (59%) y 64 machos (41%). Las hembras presentaron un mayor peso (0.43 ± 0.31g) y longitud (3.42 ± 0.63 cm) que los machos (0.11 ± 0.03 g y 2.60 ± 0.26 cm). Un 44% de los peces presentó algún resto de microplástico (fibras o fragmentos) en el tracto gastrointestinal. La mayor parte de los microplásticos (48%) tenían un tamaño comprendido entre 100 y 400 µm. Las fibras azules fueron las más frecuentes (más del 60%), mientras que los fragmentos marrones fueron más abundantes (36%). En relación con el número medio de los microplásticos por individuo en los contenidos gastrointestinales, este fue ligeramente superior en machos respecto a las hembras, en muestras de primavera respecto a las de verano, y en la laguna G02 respecto a la L04, aunque únicamente se encontraron diferencias significativas entre las lagunas. Al analizar el número de microplásticos ingeridos en función del peso de los ejemplares de G. holbrooki, una ligera tendencia pone de manifiesto que los individuos con pesos inferiores presentaron mayor cantidad de microplásticos. En este sentido, la presencia de microplásticos en los contenidos gastrointestinales de peces puede ser un indicador del incremento de residuos de plásticos en ecosistemas costeros mediterráneos, pudiendo llegar a alterar las dinámicas tróficas de los organismos acuáticos.

Animal vectors are essential for the movement of invertebrate resting eggs between water bodies. However, differences in habitat preferences and feeding behaviour between bird species may result in variations in the dispersal of invertebrates via these birds, even if the different bird species live in the same lake. To test such effects, faecal samples from Anas platyrhynchos (collected in autumn and spring) and Tadorna tadorna (collected in spring) were cultured in water at 20° C and 12 L: 12 D conditions in order to quantify the resting eggs which could be internally transported by these birds. One half of each faecal sample was initially cultured at a conductivity of 0.6 mS cm −1 and the other half at 6 mS cm −1 . 1054 invertebrates hatched from a total of 60 faecal samples, including cladocerans, copepods, ostracods, rotifers and ciliates, with a wide variability among faeces. Autumn yielded a low proportion of samples with hatchlings (12.5%) compared to spring (90%). Significant differences were observed between birds, but not between conductivity treatments. Thus, our results imply different hatching dynamics affected by disperser and season, but most species transported as resting eggs by birds seem to have a wide tolerance to hatch under variable salinity conditions. These differences may largery influence the metacommunity dynamics of lake networks, and could be a key factor to consider in wetland conservation planning.

Biomass and size-based estimations provide relevant information regarding ecosystem functioning and biotic interactions. Our aims were to study the effect of fish and macrophytes on the size structure of invertebrate assemblages (from rotifers to insects) in a set of coastal water bodies, estimating the biomass (total and main invertebrate groups), the biomass-size spectra (model of Pareto) and size diversity. In fishless ponds, cladoceran and ostracod biomass were higher, and they presented greater size diversity. In fish ponds, rotifer biomass presented greater proportion; while in fishless ponds, cladocerans were usually the most abundant taxa and the largest organisms. The biomass size spectra showed more irregularities in fishless ponds, due to the low densities of small taxa (rotifers and copepod juveniles) and big taxa (malacostraceans or insects). Differences is size structure and diversity were also observed between spring and summer, suggesting a higher recruitment of juveniles in spring, and thus, a higher predation pressure upon zooplankton at that moment. Macrophyte cover did not apparently influence those parameters, except for the biomass of ostracods, copepods, and insects. Therefore, predation by fish strongly affected invertebrate biomass, reflecting their selective feeding, and allowing high densities of small taxa. Predation pressure decreased size diversity, by limiting the abundance of vulnerable taxa of specific size. Seasonal changes were likely related to the spring recruitment of fish juveniles. The presence of small fish and invertebrate predator taxa among the macrophytes, restrict their role as refuges for prey invertebrates.

The toxicity of Bacillus thuringiensis var. israelensis on zooplanktonic microcrustaceans was evaluated using individuals collected in coastal wetlands where this larvicide has been used for mosquito control over the last decades. We tested five zooplankton species that coexist with mosquito larvae: two copepods (both nauplii and adults of Tropocyclops prasinus and Acantocyclops americanus), and three cladocerans (Ceriodaphnia reticulata, Chydorus sphaericus, and Daphnia cf. pulex). Our experiments included seven replicates of six concentrations (Bti Vectobac12AS 1200 Bti ITU/mg): 0, 5, 25, 50, 250, and 500 mg L−1. We analyzed acute and sub-chronic effects after a single inoculation. Despite the high variability of responses among our tested organisms, we found a general pattern of increasing mortality with concentration and time. We conclude that negative effects at the community level are not unlikely as some species were affected at doses close to those used in field applications.

The red swamp crayfish (Procambarus clarkii), originally from North America, is one of the world's worst aquatic invaders. It is a favoured prey item for waterbirds, but the influence of this novel predator–prey relationship on dispersal of other organisms has not previously been considered. We investigated the potential for dispersal of plants and invertebrates by migratory waterbirds feeding on alien P. clarkii in European ricefields at harvest time. In November–December of 2014–2015, we collected propagules from the outside of 13 crayfish captured as they moved out of ricefields during harvest in Doñana, south‐west Spain. We also collected excreta (N = 76 faeces, 14 pellets) of lesser‐black backed gull (Larus fuscus). We recorded diaspores from at least 11 plant species (161 seeds from 10 angiosperm taxa, and 14 charophyte oogonia) on the outside of crayfish, together with 54 eggs from eight aquatic invertebrate taxa. Adults and juveniles of at least nine microcrustaceans, including the alien ostracods Hemicypris reticulata and Ankylocythere sinuosa, were also recovered from crayfish. No intact propagules were present in the digestive system of the crayfish. Contents of regurgitated pellets confirmed P. clarkii as the main food item for gulls. Diaspores from at least 12 plant species (154 seeds from 11 angiosperm taxa, and 17 charophyte oogonia) were recovered from gull excreta, together with 129 eggs of 12 aquatic invertebrate taxa. A statoblast of the alien bryozoan Plumatella vaihiriae was found in gull faeces. Seven of the plant species are important agricultural weeds, and two are alien to Spain. Diaspores from six plant taxa were germinated, confirming viability. These propagules were from a similar set of plants and invertebrates to those found on the outside of crayfish, suggesting that propagules in gull excreta were ingested inadvertently with their crayfish prey. Ricefields constitute a major artificial aquatic habitat covering an increasing proportion of the world's land surface and typically support native or alien crayfish. Crayfish invasion can lead to novel secondary dispersal pathways for plants and invertebrates through interactions with their predators, promoting the expansion of alien and native species (including weeds) through long‐distance dispersal via migratory waterbirds and increasing connectivity of organisms between artificial and natural ecosystems. This represents a previously overlooked impact of crayfish invasion on ecosystem services. Supporting Information

Human-mediated dispersal has rarely been considered in wetland conservation strategies at regional scales, yet high concern exists about this aspect for (inter-)national management considering invasive species in other aquatic systems. In this context, we aim at understanding the role of human-mediated dispersal by footwear in protected wetlands with high conservation value. Zooplankton and zoobenthos were sampled in 13 shallow lakes in central Spain and, at the same time, mud attached to waders was collected and later cultured in deionized water under laboratory conditions for 4 weeks. Two-hundred and four individuals belonging to 19 invertebrate taxa were recovered after hatching; Ostracoda (84 %), Cladocera (53 %), Copepoda (30 %), Anostraca (30 %), and Notostraca (7 %) were the most frequent groups among the hatched crustaceans. NMDS and PERMANOVA analyses showed significant differences between the dispersed (via footwear) and the source active metacommunity, suggesting different dispersal abilities among the species found. Human vectors facilitate dispersal among protected lakes, which could eventuality lead to biotic homogenization and faster spread of non-indigenous species. Preservation strategies and education campaigns associated to target humans in close contact with water bodies should be imperative in conservation management of protected lakes.

Malladas de El Saler is a mosaic of dune slacks scattered along a coastal sand bar in the Mediterranean region. Active populations of the fairy shrimp T. stagnalis are exclusively present in one of these ponds. We combined field observations and laboratory experiments to find the ecological requirements of T. stagnalis that determine this habitat exclusiveness and the possibility of future establishment in nearby ponds. Conductivity revealed as a determinant ecological constraint. In the field, the species was restricted to conductivity levels below 1.28 mS cm−1 and in experimental microcosms, hatchability, survival and reproductive potential were detrimental from 2.9 mS cm−1. The observation of life history characteristics from the experimental individuals confirmed the importance of the unpredictable character of Mediterranean temporary ponds in shaping life history strategies, as they hatched early, matured rapidly and produced a high number of cysts. We discuss the essential steps during colonization that may explain the presence or absence in our ponds, and revealed a harsh future for the species if the current conditions of habitat deterioration persist. We suggest several conservation measures, including a cyst bank storage or/and adult translocations to nearby suitable ponds. These measures will ensure (1) the preservation of genetic diversity and (2) healthy metapopulation dynamics with recolonization rates compensating for local extinctions.

Invertebrate aquatic assemblages in ponds from Mediterranean wetlands are composed of organisms belonging to different taxonomic groups, which present a wide range of sizes (from small rotifers to large crustaceans or insects). Although they are often sampled and analyzed separately, the ecological links among these organisms should be considered, especially in very shallow waters. In our study, invertebrate assemblages (including micro-, macrozooplankton and macroinvertebrates) were characterized in eight shal- low lakes from Mediterranean wetlands in Southeastern Spain. A great spatial and temporal variability in the assemblages and in some environmental features was observed. The community was dominated in abundance and species richness by rotifers in most of the water bodies. Fish greatly contributed to the differences in community composition, as fishless ponds presented abundant and diverse cladoceran populations. Nutrient and chlorophyll-a concentration, conductivity and macrophyte cover had also a great influence on these as- semblages, especially in micro- and macrozooplankton. The temporal changes observed in some of these vari- ables affect the proportion of invertebrate biomass of the different groups along the study period. All this environmental heterogeneity produces similar responses in the invertebrate groups, in general well adapted to salinity changes or high trophic conditions. Although the heterogeneity and human disturbances can increase regional diversity, they can cause the disappearance of valuable habitats, and finally, the homogeneity in invertebrate assemblages.

New habitats are an interesting tool to monitor colonization processes in the light of changes in genetic structure and evolutionary potential of populations. Cladocerans are cyclical parthenogens that offer the possibility to track genetic changes during the colonization process due to the alternation of sexual and asexual phases. The locally called “malladas” are shallow and temporary peridunal ponds in Eastern Spain that were silted in the 1960s and have been recently dug to their original basin. We here describe the colonization process in two of these newly restored ponds by the cladoceran Daphnia magna during their first 3 years. This colonization process followed three critical steps: population founding by the initial colonists, arrival of secondary immigrants and consolidation of the population structure. We found a low number of genotype colonists followed by the establishment of secondary immigrants and a slight decrease in gene flow with time. We discuss the importance of dispersal, founder effects, natural selection, inbreeding depression and genetic drift during the colonization process in the light of the most recent literature.

We compared two freshwater mountain ponds during an annual cycle by focusing on Rotifera and Crustacea. These ponds, Laguna de Bezas and Laguna de Rubiales, are located only 4.5 kilometers apart and share a similar altitude (ca. 1200 m.a.s.1) on the Iberian Mountain range, in Teruel (Spain). Bezas is near-permanent and dries out only in severe droughts; Rubiales is temporary, drying out periodically. They also show important differences in other limnological traits. Bezas has transparent water, harbors a fish population, and has a dense macrophyte cover, whereas Rubiales has elevated turbidity and neither fish nor macrophytes during most of the hydroperiod. These differences are probably why they have significantly different aquatic fauna. We found that rotifers were density-dominant in the zooplankton of Bezas, while Rubiales was dominated by microcustaceans. Ostracods were practically absent at Bezas, whereas the ostracod community at Rubiales was rich and variable.

The hatching of diapausing eggs from sediment samples of ponds and lakes is a fast, easy, and economical technique proven useful for the assessment of diversity and abundance of zooplankton in permanent habitats. Here, we extend and optimize this approach for temporary brackish ponds which may experience great within-year variation in salinity conditions. We studied the species and numbers of rotifers emerging from the sediments of several brackish ponds and saline lakes from Eastern Spain incubated at the full range of salinity conditions observed at the study sites. On average, 82 % of the total observed species in weekly sampling for one year were detected in a single sample of both the sediments and water column. No statistical difference was found between both methods. However, some rotifer species were only observed in the water column samples, but not in sediment hatchlings, and vice versa. We concluded that both methods are not exhaustive by themselves but are complementary. Salinity affected the estimation of both rotifer richness and the density of individuals of each rotifer species that emerged from the incubated sediments. Differences in the density of emerging animals were also found among sites and among combinations of sites and salinities. The latter result indicates that in order to obtain an estimate of the emergence rate for each rotifer species present at a site, it is necessary to incubate sediments at salinity conditions close to their local values.

Cladoceran assemblages were studied in littoral samples from 44 water bodies in Central Spain, showing great differences in salinity and permanence of water. Principal component analyses (PCA) were performed with data on cladoceran relative abundances to identify the main groups of species. Five main groups of species were found, defining the following types of environments: (I) Small but deep hard water lakes (II) permanent but shallow hard water lakes fed by surface springs, rich in macrophytes (III) ephemeral and shallow soft waters lakes (IV) subsaline and hyposaline lakes in their lower range, specially the permanent ones and (V) saline lakes (salinities>6 mg l−1) subject to short or long periods of dryness. The main species constituting these groups were: Group (I): Acroperus neglectus, Alona guttata, Pleuroxus truncatus, Daphnia longispina; group II: Phrixura leei, Latonura rectirostris, Eurycercus lamellatus; Alonella excisa, Daphnia pulicaria; group III: Ephemeroporus phintonicus, Macrothrix rosea, Moina micrura; group IV: Alona rectangula, Ceriodaphnia reticulata, Tetrocephala ambigua, Diaphanosoma mongolianum; group V: Alona salina, Pleuroxus letourneuxi, Dunhevedia crassa, Moina brachiata, Daphnia magna and Daphnia mediterranea. Sample scores from PCA extracted from the correlation matrix of cladoceran proportions were correlated with limnological measurements. Significant correlations with salinity, alkalinity anions and cations indicate that salinity is an important factor in the distribution of the species. However, species assemblages also reflect other factors, such as temporality and littoral development.

Grazing rates of zooplankton were analysed in the summer of 1999 in Yellow Belly Lake, an oligotrophic system in the Sawtooth Mountains of Idaho (U.S.A.). The colonial rotifer Conochilus unicornis was a dominant species in the epilimnion, with densities reaching 20 colonies 1−1 (ca. 400 ind. 1−1). Clearance rates were measured with an in situ Haney Grazing chamber and synthetic microspheres 5, 9 and 23/μm in diameter. At epilimnetic temperatures of around 14 °C, mean clearance rates for 5µm particles ranged from 30 to 65 µ1 ind.−1 h −1. Clearance rates were 2–9 times higher on the 5µm spheres than on the 9 µm spheres, and C. unicornis almost never fed on the 23 µm spheres. Grazing rates did not change over the diel cycle. Clearance rates declined more than 10-fold as temperatures declined from 14 °C in the epilimnion to 7 °C in the metalimnion. In the epilimnion, grazing by C. unicornis was more important than grazing by crustaceans in the community, at least on particles ≤9µm. The results show the importance of grazing by rotifers in lakes, and the significance of spatial variations that influence grazing rates.

Zooplankton structure and dynamics were studied in a freshwater wetland subject to strong hydrologic fluctuations. This wetland underwent a six-year drought that terminated at the end of 1996. At that time, inundation area and water level increased, diminishing macrophyte cover in some cases and enhancing a continuing eutrophication problem. Sampling was performed monthly from January 1996 to December 1998 in five shallow water sites. Zooplankton were a mixture of limnetic and littoral species (66 rotifer, 15 cladoceran, and 10 copepod taxa, plus unidentified oligotrichid and peritrichid ciliates), the composition of which changed coincident with the shift of the hydrologic conditions. The most common taxa were bdelloid rotifers and Lecane closterocerca. The microcrustancean Ceriodaphnia dubia, Daphnia curvirostris, and Acanthocyclops robustus were frequent. Ciliates were the most important component of zooplankton in terms of biomass, except in spring when cladocerans were dominant. Rotifer biomass was the lowest fraction of zooplankton. When community structure was described by taxonomic classes, mean biomass, or time biomass trajectories, only weak spatial patterns were found following the flux of water in the wetland. Each zooplankton group was distinctly affected by flood. Ciliates and rotifers increased biomass after the flooding, possibly as a result of the enhancement of eutrophication; cladoceran populations decreased after the flooding likely due to macrophyte loss that facilitated planktivorous fish control. Copepods increased biomass consequent to the stabilized water level. There was a clear seasonal trend of zooplankton biomass seemingly unaffected by flood. Ciliates and rotifers had fluctuating low biomass in winter-spring, with maximum biomass in summer-autumn; cladocerans appeared and peaked only in spring; copepods peaked in late winter and spring.

The study of diel vertical movements of zooplankton in a small meromictic karstic lake (lake La Cruz), presenting highly stratified waters, was performed using two different methodologies: (i) samples were taken along the vertical profile and were compared at different hours in a diel cycle; (ii) some plankton traps were located at different depths, covering different periods of time, to catch organisms going upwards and downwards. The main subject of this study has been the vertical movements affecting rotifers since they were dominant in the zooplankton of this lake, but we have also included the results obtained for other zooplankton groups. The results indicate an almost general movement in epilimnetic waters corresponding to the 'normal' pattern of migration (ascent at dusk and descent at dawn); in metalimnetic and hypolimnetic waters, vertical movements were greatly reduced and were more independent of the diel cycle. The results also confirm that the dense populations at these levels of the oxic-anoxic boundary are made up of active animals. Among the causes accounting for our results, the cycles of activity (mainly feeding) connected with the diel cycle could be important in explaining vertical movements of filter-feeder rotifers. In larger zooplankters, visual predation might be the main reason for such behavior.

The zooplankton communities from several lakes and pools in three zones of a karstic area in central Spain were studied in spring and early autumn, in relation to morphometric, physical and chemical characteristics of the lakes. Zooplankton diversity was higher in autumn than in spring and it was positively correlated with lake size (area and depth) in spring, but not in early autumn. This effect is partially explained by the sharp zooplankton stratification in deep lakes, in early autumn. Some morphometric characteristics of lake basins were also correlated with the structure of zooplank- ton communities. Principal component analyses (PCA) and Two-Way Indicator Species Analysis (TWINSPAN) performed on zooplankton data and a posterior correlation analysis with the extracted factors and main limnological parameters showed that trophic state in the first place, and mineral- ization in the second place, were the main sources of variance to explain our data. However, the results of these analyses also show that important sources explaining the variance of zooplankton data include some factors not quantified in the standard limnological analysis, but evidently related to lake typology, such as shallowness and type of water feeding and flux.

Reservoirs are the most common deep lakes in Spain, as a consequence of water needs and dry climate. Although these aquatic systems can play an important ecological role in such an area with few large natural lakes, they can also provide new habitats for exotic species, which can colonize ecosystems that native species have not explored yet. Here we present our results for a biannual survey of the ostracod fauna from 24 reservoirs in Xuquer River basin. We check which variables affect ostracod presence, test for differences between winter and summer assemblages, and compare our data with previous available ostracod records from the same river drainage network. Our results reveal that ostracod presence is positively related to high diversity of the invertebrate community and reservoir volume, and negatively with phosphorus concentration. Among the 22 species found, it is noteworthy the first Iberian record of Ilyocypris getica and the second European record of Candonocypris novaezelandiae . Ostracod assemblages significantly vary between summer and winter, and strong differences are found between ostracod communities of reservoirs and those from their connected rivers. Remarkably higher frequency of exotic species in the reservoirs confirms previous findings about the facilitation that artificial ecosystems confer to aquatic invasions.

The main source of variation of rotifer species distributions in lake Arcas-2, a small karstic lake near Cuenca (Spain), was explored by means of principal components factor (PCA) and canonical correlation (CCA) analyses. PCA was performed using rotifer densities and CCA using rotifer densities plus physical and chemical parameters. Factor 1 of PCA separated summer species from winter-spring species and Factor 2 accounted for the variation in the vertical profile. Three summer species with different food habits (Polyarthra dolichoptera, Hexarthra mira and Asplanchna girodi) were grouped together at the positive end of Factor 1, while Factor 2 separated the two hypolimnetic species (Filinia hofmanni and Anuraeopsis fissa) from the rest. The relative position of rotifer species in the space determined by the CCA was roughly the same. The most significant environmental factors that became paired with rotifer distribution in the CCA were temperature and oxygen, and parameters related to water inflow. Segregation of filter-feeding species in the spatio-temporal subenvironments is clearly shown by the multivariate analysis.

‘Oxiclinal’ rotifer species show high concentrations just above the oxic-anoxic interface in the hypolimnion of some lakes. The stratification of their populations is best shown by sampling at close depth intervals and quantifying their densities by the Utermohl technique. With this technique we were able to count males which otherwise pass through filters and more accurately count egg production. We evaluated female, male and egg numbers of the two main oxiclinal species of lake Arcas-2: Filinia hofmanni and Anuraeopsis fissa, during two annual cycles (1990–91). F. hofmanni was an exclusive oxiclinal species. It had an exponential growth phase at the onset of stratification giving a distinct spring peak. The population then maintained a high density during summer, but was almost absent the rest of the year. This cycle is repeated annually but population density can vary among years, depending on winter-spring circulation. Sexuality was always observed when the animal was present in the samples, with a maximum of males and resting eggs at the peak of the population. Resting eggs were always inside females. The annual cycle of A. fissa is displaced with respect to that of F. hofmanni: A. fissa attained greatest densities during summer, until the autumn overturn. Mixis in A. fissa was restricted to the end of the stratification period. Moreover, A. fissa occured throughout the vertical profile and secondarily occupied the oxicline.