Long-term Temporal Variation Research Articles

Long-term effective population sizes, temporal stability of genetic composition and potential for local adaptation in anadromous brown trout (Salmo trutta) populations.

We examined the long-term temporal (1910s to 1990s) genetic variation at eight microsatellite DNA loci in brown trout (Salmo trutta L) collected from five anadromous populations in Denmark to assess the long-term stability of genetic composition and to estimate effective population sizes (Ne). Contemporary and historical samples consisted of tissue and archived scales, respectively. Pairwise thetaST estimates, a hierarchical analysis of molecular variance (amova) and multidimensional scaling analysis of pairwise genetic distances between samples revealed much closer genetic relationships among temporal samples from the same populations than among samples from different populations. Estimates of Ne, using a likelihood-based implementation of the temporal method, revealed Ne >or= 500 in two of three populations for which we have historical data. A third population in a small (3 km) river showed Ne >or= 300. Assuming a stepping-stone model of gene flow we considered the relative roles of gene flow, random genetic drift and selection to assess the possibilities for local adaptation. The requirements for local adaptation were fulfilled, but only adaptations resulting from strong selection were expected to occur at the level of individual populations. Adaptations resulting from weak selection were more likely to occur on a regional basis, i.e. encompassing several populations. Ne appears to have declined recently in at least one of the studied populations, and the documented recent declines of many other anadromous brown trout populations may affect the persistence of local adaptation.

Species richness and environmental conditions of fish along the Norwegian Skagerrak coast

Long-term temporal variation in the community structure of fish species richness over the period 1953–1994 along the Norwegian Skagerrak coast is analysed in relation to abiotic environmental factors (oxygen saturation, salinity and temperature respectively at the sea surface, 10 and 50 m depth). Data on the observed number of fish species derive from a long-term and ongoing beach-seine monitoring programme being conducted each autumn. Data on observed fish species richness are analysed using multivariate techniques especially designed for the coupling of environmental and biological tables (Co-inertia analysis). The correlation structure of the abiotic variables is investigated by applying the STATIS (“ S tructuration des T ableaux AT rois I ndices de la S tatistique”) multivariate technique. We demonstrate that environmental variables in the deeper waters are linked to the community structure of the coastal zone of the Southern Skagerrak Coast. It is suggested that this effect probably operates through the Norwegian Coastal Current (NCC) and is a proxy measure for advective processes.

Dynamics of the lower thermosphere consistent with satellite observations of 5577 Å airglow: I. Method of analysis

Numerical solutions of the continuity equation for atomic oxygen in the altitude range 80 to 120 km have been obtained for the purpose of investigating the various possible causes of latitudinal and seasonal variation in the atomic oxygen layer. The calculations take into account both photochemical and dynamical processes. Included in the dynamical processes are transport due to molecular diffusion, eddy turbulence, and time-independent macroscopic winds. The model allows the influence of these transport processes on the atomic oxygen altitude profile to be quantitatively evaluated. It is designed to provide a method to characterize the atomic oxygen profile by physical parameters for subsequent use in the interpretation of long-term temporal and latitudinal variation of 5577 Å airglow observations.

Spatial and long-term temporal variation of meiobenthic-hyperbenthic copepods in Lake Pontchartrain, Louisiana

The distribution and abundance of the meiobenthic-hyperbenthic copepods of Lake Pontchartrain (a large, 1630 km 2, shallow, mean depth of 3·7m, brackish, 0·3–5‰, lake in Southeastern Louisiana) were characterized monthly from August 1978 to August 1979. Ten stations, all with water depth > 2m, were established and sampled quantitatively by removing four benthic subsamples (containing as much as 25 cm overlying water) from replicated box cores. Averaged across stations over time, total copepod densities ranged from 31–89 × 10 cm −2. A total of only 15 copepod species were identified from benthic samples; 8 harpacticoids, 4 cyclopoids and 3 calanoids. The copepod assemblage in Pontchartrain differs from all other known benthic assemblages in that the species composition is dominated by species which are often associated with the water column; true infaunal species are rare or found in reduced abundance. Four species comprised 90% of all individuals collected. All four are epibenthic or commonly collected as zooplankton and include Scottolana canadensis which comprised 47% of all individuals collected, Halicyclops fosteri which dominated fall collections, Acartia tonsa and Pseudobradya sp. Burrowing species were largely restricted to the more saline eastern stations. The rarity of burrowing species might be related to low salinity, however the highly unstable bottom in Pontchartrain with silty-clayey sediments commonly resuspended into the water column, may also influence burrowers. Cluster analysis reveals diffuse seasonal grouping of stations for all seasons except fall, with spatial groupings from the central and western sides. Physical processes appear to dictate the community structure of benthic-hyperbenthic copepods in Lake Pontchartrain.

Long‐term Temporal Variation and Community Dynamics Meiobenthic Copepods

To determine the organization, dynamics, and controlling mechanisms of an estuarine benthic copepod assemblage, 2 stations were established 1 km apart in a South Carolina estuary in January 1973, and sampled at monthly intervals for 3 yr. These sand and mud faunas displayed unique dynamic properties, yet species diversity, used as a measure of community structure, did not discriminate community differences seasonally, within or between the 2 locations. The mud fauna, distributed to a depth of 1 cm, has a randomly varying community abundance and diversity over time, and diversity was controlled by a marked seasonal cycling of species suites which recurred regularly. The sand fauna diversity decreased with time and was correlated to an increasing abundance trend. Sand faunal seasonality was not marked, however, diversity was maintained by a high spatial niche overlap of fauna through a 10—cm depth distribution. The communities were different in species composition, seasonal responses, environmental regimes, dominance relationships, and controlling factors, yet maintained equal diversity. Diversity, however, was maintained by different and independent mechanisms.