Craig D. Sandgren

The effect of temporal environmental heterogeneity on community structure: a replicated experimental study

SummaryPeriodicity, predictability and stochasticity of environmental perturbations are shown to influence the community structure that develops in microcosms. Sets of replicate, microalgal communities were subjected to different temporal patterns of rarefaction and resource resupply and their species-abundance patterns after 120 days of such manipulations were determined. Perturbations having, 1, 7, and 28 day periodicities differentially effected community structure. The predictability of these perturbations had a less profound influence on the communities which developed than the average perturbation periodicity.

HABITAT MATTERS FOR INORGANIC CARBON ACQUISITION IN 38 SPECIES OF RED MACROALGAE (RHODOPHYTA) FROM PUGET SOUND, WASHINGTON, USA1

Measurements of pH drift were used to assess the ability of 38 red algal seaweeds to use bicarbonate and to deplete the dissolved inorganic carbon pool (DIC) from seawater medium. Subtidal algae were typically restricted to the use of DIC in the form of dissolved CO2, reducing the initial DIC by only 9%. Intertidal species used both dissolved CO2 and bicarbonate and reduced initial DIC by as much as 70%. DIC reductions and pH compensation points for the intertidal species tested were strongly correlated with their vertical zonation on the rocky shoreline (analysis of variance). DIC acquisition efficiency increased with tidal height, but species from the upper edge of the intertidal demonstrated a reversal of this trend. This general pattern associated with tidal height was observed not only among intertidal red algae in general, but also among four species of the genus Porphyra (P. torta V. Krishnamurthy, P. papenfussii Krishnamurthy, P. perforata J. Agardh, P. fucicola Krishnamurthy) and among four populations of the broadly distributed species Mastocarpus papillatus (C. Agardh). The Mastocarpus observations suggest either that individuals of this species may be able to express alternate strategies for carbon acquisition or that intertidal height may select for survivorship of genotypes with different carbon acquisition strategies. Taken together, these data suggest that the carbon acquisition strategy of intertidal red algae may be an important physiological set of adaptations that is under active evolutionary selection. These physiological differences were not related to phylogeny, tested as membership in red algal families and orders.

Chrysophycean cysts: indicators of eutrophication in the recent sediments of Frains Lake, Michigan, U.S.A.

The recent sediments of Frains Lake, Michigan contain a rich and well preserved association of chrysophycean cysts. Forty one forms are revealed by scanning electron microscopy (SEM) and light microscopy (LM). Taxonomic descriptions. and SEM micrographs are provided for the dominant forms. The three dominant taxa throughout the sediments, Cysta minima, C. modica and C. subbavaricum, do not show significant shifts in proportional abundance associated with European settlement and the onset of cultural eutrophication. However, certain subdominant taxa do show clear trends. Density counts indicate a dramatic decline in cyst concentration (by volume and by dry mass) and a sharp increase in absolute accumulation (net annual influx) following settlement.The Frains Lake profile of chrysophycean cysts is compared to sequences of other North American and European temperate lakes. The utility of chrysophycean cysts as paleoenvironmental indicators is considered on the basis of these results.

A stratified sampling approach to compensating for non-random sedimentation of phytoplankton cells in inverted microscope settling chambers

A pronounced “edge effect” resulting in non-random cell sedimentation is documented for standard-design inverted microscope counting chambers. Comparison of mean cell densities of two green algae from random fields in central and peripheral regions of chambers substantiate a marked peripheral settling bias. The non-random settling pattern is independent of phytoplankton cell size and population density over the ranges encountered in this analysis. If this problem is not compensated for, the magnitude of the bias is such that large errors in estimates of mean cell density/chamber can result. A stratified sampling approach is developed to minimize the error resulting from non-random settling without increasing the amount of time spent enumerating cells. Counting of random fields rather than patterned transects is strongly recommended as the standard procedure in phytoplankton studies.

CHARACTERISTICS OF SEXUAL AND ASEXUAL RESTING CYST (STATOSPORE) FORMATION IN DINOBRYON CYLINDRICUM IMHOF (CHRYSOPHYTA)1

The reproductive significance of siliceous cysts (statospores) produced by the common vernal chrysophyte Dinobryon cylindricum Imhof has been investigated under defined culture conditions. Three types of statospores have thus far been induced in culture: 1) uninucleate, asexual; 2) binucleate, asexual (potentially autogamic); 3) binucleate, sexual (zygotic). The production of each type of cyst responds differently to an array of nutrient deficiencies (P, N, vitamins, micrometals). An individual clone may be capable of participating in the production of all or only a subset of these types of resting cysts. All D. cylindricum statospores are morphologically identical regardless of their reproductive significance. Sexual reproduction leading to zygotic statospore formation is anisogamous, heterothallic, and involves a gametogenic hormone (erogen) that is apparently continuously released from female clones. Only a single bipolar mating group is documented here and clonal compatibility varies considerably within the mating group.

Chrysophyte reproduction and resting cysts: A paleolimnologist's primer

Chrysophyte algae produce siliceous resting cysts (stomatocysts) that are becoming an increasingly useful class of paleoecological indicator microfossils. This paper provides a review of the role that stomatocysts play in the life cycle and reproductive ecology of freshwater planktonic chrysophytes. Such information provides paleolimnologists with greater insight into the ecology of the vegetative, planktonic growth phase of species contributing stomatocysts to lacustrine microfossil assemblages. Specific chrysophyte reproductive characteristics discussed include: temporal dynamics of vegetative growth and encystment, cyst induction, cyst survivorship, germination requirements and recruitment strategies. This information serves as an introduction to a special issue of the Journal of Paleolimnology which is dedicated to the ‘Application of Chrysophyte Stomatocysts in Paleolimnology’.

The ultrastructural bases of chrysophyte systematics and phylogeny

By means of light microscopy certain main lines in chrysophyte taxonomy were demonstrated during the first half of this century. However, the increasing use of electron microscopy from the 1950s and 1960s, together with other modern methods such as pigment analysis and advanced culture techniques, have improved our knowledge considerably and have made necessary even quite fundamental changes, both as regards the delimitation of the Chrysophyceae, and as to the relationships between the various groups within this class. On the species level, electron microscopy has been especially useful as regards the silica‐scale bearing groups, and the main problems are here now to judge how small differences in scale structure really should be given taxonomic value.

Heterothallic sexuality and density dependent encystment in the chrysophycean alga Synura petersenii Korsh.

Sexual reproduction of the common planktonic chrysophyte Synura petersenii is described from observations made on clonal isolates grown in defined culture. Sexual fusion was isogamous and heterothallic, with cells of normal appearance from compatible clones serving as hologametes. No special culture conditions were required to induce sexual behavior; actively growing cell populations appeared to be continually receptive to mating when mixed with a sufficient number of cells from a compatible clone. A single, bipolar mating group was documented containing five of the seven clones tested. Zygotic statospores were found to be binucleate and to contain 4 chloroplasts at maturity. Production rates of zygospores were low for even the most highly compatible clones, with batch culture yields ranging from 1‐20% of final cell density under the culture conditions utilized. Six of the clones tested were also capable of very low frequency (0.001‐0.01%) homothallic statospore production but the reproductive significance of these cysts remains enigmatic. The dynamics of sexual encystment suggest that the process proceeds during periods of active population growth and is density dependent. Based on the characteristics of cyst induction and encystment dynamics, it is concluded that chrysophycean flagellates may have a perennation strategy quite different from that of the majority of planktonic diatoms, dinoflagellates, and green algae for which resting cyst production requires an exogenous trigger usually associated with physiological stress and periods of negative growth.

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.

An experimental evaluation of diversity indices as environmental discriminators

Seven diversity indices were calculated for each of fifty-eight microcosm communities. All fifty-eight communities were initiated from equal density inoculations of fourteen algal species. Each microcosm developed in one of six controlled experimental environments; the environments differing only in their temporal patterns of disturbance. Five linear discriminating techniques were used to evaluate the diversity index most useful for discriminating between these environments. The Shannon-Wiener index was best according to two of the discriminating methodologies and second best using the other techniques. Evenness was best when the Shannon-Wiener index was second best and vice versa.