Fish recruitment, dispersal, and trophic interactions in a heterogeneous lotic environment

I examine how dispersal of juvenile creek chubs (Semotilusatromaculatus) from beaver ponds into adjacent stream environments interacts with temporal abiotic variability to influence fish foraging, growth, and long-term persistence in the lotic ecosystem. Minnow trapping in upstream and downstream beaver ponds, along with weir traps used to monitor directional movement, indicated that most chubs colonized the stream from the downstream beaver pond. Large annual fluctuations in density of age 0 creek chubs occurred in the stream over a 10-year sampling period. Multiple regression analysis indicated that stream temperature, precipitation, and the density of reproductive creek chubs were not correlated with summer density of age 0 chubs in the stream. The factor most strongly associated with increased density of age 0 creek chubs was creation of the downstream beaver pond during the 6th–7th years of the study, suggesting dispersal from the pond was the primary factor determining age 0 fish density in the stream. Most individuals in the strong year classes neither persisted in the stream through their first winter nor resulted in an increased abundance of older age classes in later years. Comparison of age 0 fish density in summer to the proportion of fish surviving to age 1 in spring suggested that overwinter mortality was density dependent. Furthermore, a comparison of the size structure for age 0 individuals in summer to age 1 individuals the following spring indicated that winter mortality was size dependent. Experiments in an artificial stream adjacent to the natural channel revealed that fish growth was strongly density dependent, decreasing as fish density increased across both spring and summer, and elevated and low discharge. The decline in invertebrate prey captured by the fish and the subsequent decline in fish growth appeared to be particularly pronounced under low discharge in summer. Changes in juvenile creek chub density had no significant effect on benthic insect or crustacean abundance, suggesting that exploitative competition for limited invertebrate drift resources was a more important cause of density- dependent growth than depressed local benthic invertebrate abundance. These results suggest that lotic regions adjacent to beaver ponds act as potential reproductive “sinks” for dispersing juveniles confronting seasonal and flow-mediated restrictions on resource acquisition and growth, and the occurrence of seasonal bottlenecks to their survival, especially harsh winter conditions. Received: 9 September 1996 / Accepted: 8 August 1997     

Population genetics and reproductive strategies of two Notostraca (Crustacea) species from winter ponds in Israel

Fluorescent-amplified fragment length polymorphism (FAFLP) fingerprinting assay was used to compare the genetic diversity within and between tadpole shrimps (Notostraca) populations of Lepidurus apus (n=7) and Triops cancriformis (n=2) from rain pools in Israel. Each ephemeral water body has revealed a unique fingerprint pattern with an entailed genetic drift between nearby ponds. High similarity of genotypic diversity within each geographic area led to three clusters of water bodies, north, south and center of Israel. FAFLP assays on several newly hatched individuals of T. cancriformis revealed high identity amongst kin, as compared to L. apus where newly hatched from the same maternal source showed high diversity. Results indicate that T. cancriformis populations from Israel are probably parthenogenetic as indicated by clonal structures. The higher genetic variability in the L. apus populations and in laboratory-hatched specimens indicates the existence of sexual reproduction.     

Copepods and branchiopods of temporary ponds in the Doñana Natural Area (SW Spain): a four-decade record (1964–2007)

The Doñana Natural Area includes a large array of wetlands with the highest degree of environmental protection in Spain, and so it has long attracted many studies. We present a cumulative list of zooplankton taxa (Copepods and Branchiopods) based on a collection of 18 publications (1964–2007) and 4 unpublished studies. Seventy-eight taxa have been recorded in a set of 55 ponds, and 72 taxa at 38 sites spread over the Doñana marshland. In total, 96 taxa have been recorded, including 50% of all branchiopod species reported for the whole Iberian Peninsula. Taxa composition was significantly segregated between ponds and marshland during floods (ANOSIM test, R = 0.929, P < 0.01), but this segregation disappeared at a larger spatio-temporal scale when a non-metric MDS ordination produced a gradient from ponds to marshland (ANOSIM test, R = 0.272, P < 0.01). The lack of segregation between ponds and marshland sites, and among ponds with different hydroperiods, was not due to a large number of cosmopolitan species, but to a random distribution of a large number of low-occurrence species (67% of total taxa occurred with a frequency <15%). Long-hydroperiod ponds occupy a key position among the Doñana wetlands in terms of biodiversity as these ponds accumulated a high crustacean richness over time. They also supported a significantly higher cumulative number of cladoceran and harpacticoid taxa, while short-hydroperiod ponds accumulated the lowest number of diaptomid taxa. Our data indicate the need for recording biodiversity in the long term as richness on a short-temporal scale is not a good indicator of the number of crustacean species that would be encountered with a longer sampling period in Mediterranean temporary wetlands.     

Rare, threatened and alien species in the gastropod communities in the clay pit ponds in relation to the environmental factors (The Ciechanowska Upland, Central Poland)

The objectives of the present survey were to formulate a zoocenological analysis of the gastropod communities in clay pit ponds, as well as to determine the relationships between the gastropods and their environments, and to evaluate the water habitats in terms of their ecological-conservation value. The physical and chemical parameters of water, gastropod species and macrophytes occurring in the ponds indicate mesotrophic conditions. Principal Component Analysis (PCA) reveals a correlation between gastropod density and nitrates, a correlation between gastropod density and alkalinity, and additionally shows a correlation between gastropod density and chlorides. The pond surface area, nitrates, chlorides concentrate, alkalinity, and substratum have all affected these gastropod communities. The occurrence of the first permanent population of Ferrissia clessiniana (Jickeli, 1882) was recorded in Poland. Valvata naticina Menke 1845 has become a critically rare species (CR) due to the pollution of water environments. Five other species, e.g. Anisus vorticulus (Troschel, 1834) and Planorbis carinatus O.F. Müller, 1774 are recorded on the Polish Red List of Species. The clay pit ponds of the Ciechanowska Upland, because of their distinctive environmental features, provide a refuge where a number of rare, threatened and alien gastropod species live.     

Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

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Access denied: the anatomy of silence,immobilization and the gendered migrant

Abstract

This article argues that the status of migrant subjects is characterized by a loss of communication rights and locates the instances where this loss is most visible. It investigates the process of silencing and immobilization of migrants and the particular forms it takes for female migrants through the disenablement of communicative acts. In this process the detained migrant loses her status as an interlocutor, irrespectively of the instances and processes that allow her–or demand of her–to speak. The state of exceptionality assigned to detained migrants is supported in the criminalization of migration laws and securitization, which together with widespread policies of incarceration in the West have become the antipode of the fundamental principles of free movement and expression. Silence and immobilization constitute the ‘standard’ rather than exceptional conditions of people on the move that shadow them across every step of their way, geographically, politically, culturally, legislatively, socially.     

Carbon and nitrogen dynamics in a small pond in the maritime Antarctic

Small ponds and puddles are extremely common throughout the ice-free areas of the maritime Antarctic. The carbon and nitrogen dynamics in a typical pond on Livingston Island, South Shetland Islands were investigated during summer 1991. The pond vegetation consisted of a benthic mat of cyanobacteria, diatoms and chlorophytes. The mat was not limited by nutrient availability, both phosphorus and nitrogen being available in the overlying water and N:P ratios in both the water and the mat indicating a roughly balanced supply. Maximal rates of carbon fixation of 0.1–0.2 mgC g^–1 dry weight h^–1 were similar to those of other perennial Antarctic mat communities. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated. Although carbon fixation rates were low, carbon loss processes were minimal leading to an accumulation of material in the mat approximating to one doubling per year. Atmospheric nitrogen fixation was not a significant component of the nitrogen budget of the pond, accounting for only 0.1 % of the nitrogen accumulation by the mat. Nitrogen uptake was largely from dissolved nitrogen sources, in particular as dissolved organic nitrogen. It is concluded that ephemeral water bodies may play a significant role in the nutrient dynamics of maritime Antarctic ecosystems.     

Functional traits reveal environmental constraints on amphibian community assembly in a subtropical dry forest

The relationships between functional traits and environmental gradients are useful to identify different community assembly processes. In this work, we used an approach based on functional traits to analyse if changes in hydroperiod and tree covers of ponds are relevant for local amphibian community assembly processes. Ephemeral ponds with low vegetation cover are expected to impose constraints on different species with particular trait combinations and, therefore, to exhibit communities with lower functional diversity than more stable ponds with greater tree cover. Sampling was conducted in 39 temporary ponds located along vegetation and hydroperiod gradients in the most arid portion of the Chaco ecoregion. Seven functional traits were measured in each species present in the regional pool. Associations between these traits and environmental gradients were detected using multivariate ordination techniques and permutation test (RLQ and fourth‐corner analyses respectively). Functional diversity indices were then calculated and related to variations in the environmental gradients. The results obtained allowed us to identify different sets of traits associated with hydroperiod and tree cover, suggesting that these environmental variables are relevant for structuring amphibian communities according to interspecific variations in functional traits from both, larval and adult stages. Contrary to our expectations, communities associated with more stable ponds and with greater tree cover exhibited lower functional diversity than expected by chance (and were the ponds with highest species richness). This result indicates that the reduction in relative importance of environmental restrictions imposed by a very short hydroperiod and the lack of tree cover, favours different species of the regional pool that are similar in several functional traits. Accordingly, communities associated with stable ponds with high tree cover exhibited high functional redundancy.     

Benthic resting eggs of calanoid copepods in Norwegian enclosures used in mariculture: abundance, species composition and hatching

Abundance, species composition and viability of benthic resting eggs of calanoid copepods were investigated in the sediments of seven Norwegian enclosed pond systems used in mariculture as well as in the sediments of the nearby open sea. The ponds represented different types of morphometry and zooplankton communities and were located from approximately 60 to 70 °N. Differences in environmental conditions in deep water also typical with anoxia and the presence of hydrogen sulphide at most of the localities. The lowest and highest egg densities in the enclosures were 7.5 ± 6.4 × 10⁴ and 5.5 ± 2.5 × 10⁶ m^-2, respectively, higher than in the sediments of the nearby open sea. Highest densities were found at larger or median depths. Altogether, egg densities higher than 10³ m^-2 were found of only four calanoid species; Eurytemora affinis, Acartia teclae, Acartia clausi, Temora longicornis. Hatching success was variable (0–92%), and no significant correlation to the presence of anoxia and hydrogen sulphide was found. This high hatching success was achieved with eggs exposed to hydrogen sulphide for several months. The recruitment potential to the plankton of nauplii was estimated from 4.8 to 1682 nauplii l^-1 for the different localities. The results showed that resting egg production is a common life history characteristic of some neritic calanoid species in Norwegian waters. In addition to copepod eggs, low densities (< 10³ m^-2) of eggs from rotifers and cladocerans were found in most of the localities.