Temporal dynamics of estuarine phytoplankton: A case study of San Francisco Bay

Detailed surveys throughout San Francisco Bay over an annual cycle (1980) show that seasonal variations of phytoplankton biomass, community composition, and productivity can differ markedly among estuarine habitat types. For example, in the river-dominated northern reach (Suisun Bay) phytoplankton seasonality is characterized by a prolonged summer bloom of netplanktonic diatoms that results from the accumulation of suspended particulates at the convergence of nontidal currents (i.e. where residence time is long). Here turbidity is persistently high such that phytoplankton growth and productivity are severely limited by light availability, the phytoplankton population turns over slowly, and biological processes appear to be less important mechanisms of temporal change than physical processes associated with freshwater inflow and turbulent mixing. The South Bay, in contrast, is a lagoon-type estuary less directly coupled to the influence of river discharge. Residence time is long (months) in this estuary, turbidity is lower and estimated rates of population growth are high (up to 1–2 doublings d^–1), but the rapid production of phytoplankton biomass is presumably balanced by grazing losses to benthic herbivores. Exceptions occur for brief intervals (days to weeks) during spring when the water column stratifies so that algae retained in the surface layer are uncoupled from benthic grazing, and phytoplankton blooms develop. The degree of stratification varies over the neap-spring tidal cycle, so the South Bay represents an estuary where (1) biological processes (growth, grazing) and a physical process (vertical mixing) interact to cause temporal variability of phytoplankton biomass, and (2) temporal variability is highly dynamic because of the short-term variability of tides. Other mechanisms of temporal variability in estuarine phytoplankton include: zooplankton grazing, exchanges of microalgae between the sediment and water column, and horizontal dispersion which transports phytoplankton from regions of high productivity (shallows) to regions of low productivity (deep channels).Multi-year records of phytoplankton biomass show that large deviations from the typical annual cycles observed in 1980 can occur, and that interannual variability is driven by variability of annual precipitation and river discharge. Here, too, the nature of this variability differs among estuary types. Blooms occur only in the northern reach when river discharge falls within a narrow range, and the summer biomass increase was absent during years of extreme drought (1977) or years of exceptionally high discharge (1982). In South Bay, however, there is a direct relationship between phytoplankton biomass and river discharge. As discharge increases so does the buoyancy input required for density stratification, and wet years are characterized by persistent and intense spring blooms.     

Restriction fragment polymorphisms as probes for plant diversity and their development as tools for applied plant breeding

Summary Maize and tomato cDNA clones have been hybridized in Southern blotting experiments to plant genomic DNA prepared from different lines to detect restriction fragment polymorphisms (RFPs). In maize we have found that a high degree of genetic variability is present, even among domestic inbred lines. Most randomly chosen maize cDNA clones can be used to detect elements of this variability. Similar levels of polymorphism are observed when genomic DNA is digested with any of a number of different restriction enzymes and probed with individual clones. When a clone is hybridized to genomic DNAs prepared from several different maize lines, a number of different alleles are often detected at a single locus. At the same time one clone can often detect more than one independently segregating locus by cross hybridization to related sequences at other loci. As expected these markers are inherited as simple codominant Mendelian alleles from one generation to the next and colinkage of these markers can be demonstrated in the progeny from a heterozygous parent. In similar studies with tomato, remarkably different results were found. Few RFPs were demonstrable among domestic Lycopersicon esculentum lines although a higher level of variability could be detected when comparing esculentum with its wild Lycopersicon relatives. These results are discussed in relation to the applied uses of RFPs in plant breeding as well as the inherent variability of different plant genomes.This work was supported in part by funds from Sandoz Ltd. (Basel, Switzerland) and its subsidiary company, Northrup King Co. (Minneapolis, Minn., U.S.A.) as well as by NSF SBIR grant #BSR-8360870.     

Temporal constancy in grasshopper assemblies (Orthoptera: Acrididae)

ABSTRACT.

• 1 Temporal constancy in the structure of grasshopper assemblies (about forty-five species each) from two types of North American grasslands was assessed; one site was followed 25 years and the other 7 years.
• 2 Densities and relative abundances varied but composition of assemblies based on ranks suggested significant structure when three or more species were included in the analysis.
• 3 Results compared favourably with other insect herbivore assemblies which have been examined; variability in population change was intermediate along the spectrum of organisms which have been studied.

     

MORPHOLOGICAL VARIABILITY IN POPULATIONS OF CHRYSOPHYCEAN RESTING CYSTS. I. GENETIC (INTERCLONAL) AND ENCYSTMENT TEMPERATURE EFFECTS ON MORPHOLOGY1

Many chrysophycean species produce resting cysts (statospores) with purportedly species-specific morphology. I investigated variation in the cyst morphology of a single species that may result from genetic differences among the vegetative clones involved and from variation in the temperature of the environment during cyst development. Populations of Dinobryon cylindricum Imhof cysts were produced under defined conditions in vitro and then sampled for morphological analysis based on SEM micrographs. Morphological data is presented and then used in a multivariate discriminant analysis to determine the utility of each morphological character in distinguishing the six populations studied. Results suggest that some features of cyst morphology (i.e. cyst diameter) are invariant among the populations, while other features show distinctive variation. The density of spines covering the cyst body as well as the morphology of those spines appear correlated to the specific clones involved, and thus may represent useful phenotypic genetic markers. The length and definition of both the spines and the cyst collar, on the other hand, are markedly influenced by encystment temperature. The implications of these findings for paleoecological studies is discussed.     

Species temporal variability: epigaeic ant assemblages and management for abundance and scarcity

Summary Ant assemblages in five closely-spaced point habitats were studied over a three-year period in a subtropical area of South Africa. A total of 20,746 individuals in 56 species were sampled with pitfall traps. Rapid species turnover occurred during short periods after longer intervals of relatively constant species composition. There was annual cycling of total numbers of individuals. The temporal variance for total number of individuals within each habitat increased significantly with mean population size by a logarithmic regression. This indicates that population fluctuations are dependent upon the mean, with a distinctive relationship. The total number of species was cyclical, with annual and approximately half-yearly cycles. Each habitat nevertheless carried a characteristic mean number of species. The temporal variance of species richness showed a highly significant positive logarithmic relationship with the mean number of species. Pheidole megacephala (F.) accounted for 45% of all individuals. Its overall population showed annual cycles. Ph. megacephala population trend was the main determinant for seasonal variation of the whole assemblage in most habitats. Percentage dominance was positively linearly correlated with the variance and with mean population level. Although Ph. megacephala population variance increased with the mean, proportionate variability did not, indicating that high populations are no more variable than low populations. Five variability measures were compared with each other on ant assemblages containing only the common species (1st quartile) and then including the increasingly rare species (1st and 2nd, 1st, 2nd and 3rd, and all quartiles). Williamson's Variability Measure, using logarithmic or inverse sine transformations, gave variable results depending whether or not the rarer species were included in the analysis. The Coefficient of Fluctuation also gave variable results depending on assemblage size. The Coefficient of Variation and Proportionate Variability Measure gave similar and consistent results, indicating that common species are not more variable than rare ones. The rarer species are stenotopic and tend always to be rare, whereas the common species are eurytopic, highly competitive and demographically volatile. Control measures for common ants, and conservation of rare ants, should take cognizance of these trends.     

Multi-scale spatial pattern of recruitment in the barnacles Semibalanus cariosus at fishing ports on the Kameda peninsula, southern Hokkaido, Japan

The potential causes of the variable nature of recruitment of marine organisms can be inferred from the scales over which they vary. Sampling for recruits of Semibalanus cariosus on the intertidal concrete tetrapods at 21 fishing ports along the Kameda Peninsula, southern Hokkaido, Japan, was conducted at the end of the recruitment season in 1994 at three spatial scales: at each fishing port, separated by several km; at two sites at each fishing port, separated by several hundred m; and on three blocks at each site, separated by 1–2 m. At all spatial scales, recruitment intensity was independent of adult densities. Recruitment densities significantly varied within all spatial-scales, however, 85.6% of the total variances was estimated to be due to variation among ports. Such km-scale variation of recruitment intensities coincided with the hydrographic pattern of the direction of coastal current.     

Recolonisation and recruitment of fishes to intertidal rockpools at Wellington, New Zealand

Synopsis A study of recolonisation of rockpools by intertidal fishes on the Wellington south coast, New Zealand, found the assemblage to be resilient and seasonally stable. A total of 26 species from nine families were recorded, dominated by the Tripterygiidae (triplefins) and Gobiesocidae (clingfishes). A pattern of alternating species dominance occurred, with the triplefinsBellapiscis medius andForsterygion lapillum being numerically dominant in summer, but becoming less common in winter and replaced as dominants by the clingfishesTrachelochismus pinnulatus andGastroscyphus hectoris. Juvenile recruits of eleven species occurred in the samples from spring to early summer, however only the aforementioned four species recruit to the intertidal zone in large numbers. The speed of rockpool recolonisation by fishes after extractive sampling is seasonally dependent, being quicker in the summer than winter. In general, recolonisation takes at least one month, but probably fewer than three. While stochastic factors influence assemblage composition in the short term, overall regulation of the fish assemblage of rockpools appears to be primarily deterministic, resulting in an essentially predictable taxonomic structure.     

Genetic variability in two hydrophilous species ofPotamogeton,P. pectinatus andP. filiformis (Potamogetonaceae)

Genetic variability inPotamogeton pectinatus andP. filiformis was studied by means of isozymes. The overall levels of variability were similar to some other well studied hydrophilous species, and were shown to be distributed more between than within populations. This partitioning of variability was attributed to three main factors. (1) Clonal growth (as measured by the frequency of multi-enzyme phenotypes) was shown to be a major factor in both species, although more important inP. pectinatus. (2) Low levels of sexual reproduction were shown to be a likely contributor to the partitioning of variability inP. pectinatus; such reproduction is probably limited by seedling recruitment rather than by infrequent flowering. (3) Geographical isolation was indicated as a factor regulating gene flow at distances of more than about 1000 km in both species, with the data suggesting that dispersal between populations is mainly by seed rather than by vegetative means.     

Perceptual constraints on optimal foraging: The effects of variation among foragers

Summary We present a simple model of habitat selection in which individuals differ in their ability to discriminate between resource sites' profitabilities. The model investigates the effects of violating the ideal assumption of the well-known ideal free distribution (IFD). We show that (1) variability in perceptual limits within a population can significantly change the distribution of foraging animals even though the mean perceptual limit is the same, (2) the direction of this change depends on the proportion of the population that choose randomly between resource sites and (3) better perceivers are more likely to be found at individually more profitable sites, which, because of undermatching with respect to the IFD, are also the absolutely more profitable sites. We note that variability in perceptual limits almost always led to an undermatching of organisms to resources, thereby extending previous workers' results implying that the incorporation of any form of perceptual limits leads to undermatching with respect to the IFD.     

SEXUAL SELECTION IN THE BARN SWALLOW (HIRUNDO RUSTICA). IV. PATTERNS OF FLUCTUATING ASYMMETRY AND SELECTION AGAINST ASYMMETRY

The patterns of variation in fluctuating asymmetry were studied in four morphological characters of the barn swallow Hirundo rustica. The level of absolute and relative asymmetry was larger in the secondary sexual character “outer tail length” than in three nonsexual morphological traits (wing, central tail, and tarsus length). The extent of individual asymmetry in outer tail length was negatively correlated with tail-ornament size, whereas the relationship between asymmetry of all other morphological characters and their size was flat or U-shaped. Asymmetry in outer tail length was unrelated to asymmetry in other morphological characters, whereas asymmetries in the length of wing, central tail, and tarsus were positively correlated. Male bam swallows exhibited larger asymmetry in outer tail length than females. Asymmetry of most morphological traits exhibited intermediate repeatabilities between years, with the exception of male and female outer tail length, which were highly repeatable. Tail asymmetry of offspring weakly, though significantly, resembled that of their parents. Asymmetry in wing and outer tail length was also significantly related to several fitness components. Male barn swallows that acquired a mate were less asymmetric in wing and outer tail length than unmated males. Females with more asymmetrical tails laid eggs significantly later. Annual reproductive success was unrelated to fluctuating asymmetry. Male barn swallows that survived were less asymmetric in wing and outer tail length than nonsurvivors, whereas female survivors were less asymmetric in outer tail length than nonsurvivors. These results suggest that levels of fluctuating asymmetry in barn swallows are associated with differences in fitness.