Search

In contrast, active sonar systemsare widely used for a variety of civil and military applicationsin underwater environment. However, passive sensor systemssimilar to fish provide a huge advantage in “silent” observa-tion of the environment which does not reveal the source anddoes not interfere with moving vehicles and various lifeforms.

Population dynamics of chyorids from Elk Lake, Minnesota, were followed from May through October 1971 by means of three collecting methods. Emphasis was placed on data obtained by use of artificial substrate. This method was found to collect most species of chydorids living in the underlying substrate. A second method, funnel trapping, collected animals that migrated 10—20 cm vertically from their substrate into the funnel traps. Funnel trapping was found to be not very efficient at collecting all chydorid species, but to be useful for following overall population trends and for exploring the vertical movements of some species. A third method, called pattern sampling, was tried experimentally. It appears to collect most chydorid species, and also shows the mocro—distributions (or spatial arrangements) associated with population densities. Most chydorid species increase in population numbers during spring and fall, and decrease during midsummer. Chydoris have clumped or contagious patterns that fit the neg...

Microfossil remains from surficial sediments of 20 Danish and 14 northern Indiana Jakes were counted, and their relative percentages calculated. Using the function H = — Σp°ilog2°° species diversity was calculated for each lake. Indices of diversity for each lake and data on primary production and transparency were subsequently ranked and Spearman's rank correlation coefficient was determined between species diversity and production, and species diversity and transparency. Interpretation of results within the framework of information theory was not satisfactory. It is sugested that species diversity of chydorids is controlled primarily by complexity of the thier habitat, the rooted aquatic macrophytes of lakes. See full-text article at JSTOR

A 286—cm core of sediments was taken from the center of Potato Lake, Coconino Co., Arizona. Pollen, chemical, and physical analyses were made on the sediment. Pluvial conditions, evidenced by high PAP percentages, exist until approximately the 120—cm level of the core; at this level a major change in pollen types occurs with pine assuming dominance. A core section at 142—172 cm was dated at 14,000 plus or minus 300 BP. Post—pluvial climate, as reflected by the pollen diagram, was fairly stable. Loss on ignition at 550 degrees C of the sediment material implies that eutrophication of Potato Lake began at the close of the pluvial. Variations of iron, manganese, and calcium may be explained by lake succession from oligotrophic to eutrophic conditions See full-text article at JSTOR

Surficial sediments from 77 Danish lakes were processed for chydorid remains for the purpose of establishing relationships of species occurrence to lake type. A total of 35 species of chydorids was recovered; 3 species were new for Denmark, while 3 others previously reported from Danish lakes were not found. The lakes sampled were subjectively grouped according to physical—chemical measurements into 1 of 3 groups–clear—water lakes (1), ponds and bogs (2) and polluted, clear—water lakes (3). Correlations between chydorids and chemical—physical properties of lakes, plus evidence from relative abundances and frequencies of each species found in each lake type, suggests that most species prefer those waters best termed oligotrophic and dystrophic. The exceptions of Chydorus sphaericus and Alona rectangula which, when found in high percentages, typify eutrophic or polluted sites. These data correlated well with species diversity values–high values were found in clear—water lakes (1) and ponds and bogs (2), while low diversities were typical of polluted sites (3). These data suggested that species diversity is a function of available habitat (rooted aquatics), and hence to a large extent the distribution of most chydorid species within a region is determined by habitat diversity (structure). Grane Langso (an oligotrophic, Jutlandic lake) and Esrom S° (an eutrophic, Zealandic lake) were selected for paleo—studies because of their differences, which resulted in contrasting lake development. Postglacial gyttja sediment in Grane Langso extended to 431 cm below the mud—water surface interface. Pollen zonation, and a radiocarbon date of ±9500 BP placed the lowest organic deposits into Pre—Boreal time. Two chydorid species were important in Grane Langso Late—glacial sands underlying the gyttja (Alona affinis and Chydorus sphaericus). Chydorid faunas developed through the Pre—Boreal and early Boreal; by late Boreal chydorid populations were most diverse, and presumably stable. A decrease in species diversity during Atlantic time was due to unstable conditions within the lake, evidenced by deposition of sand; Chydorus piger was abundant during this period. Sub—Boreal organic deposits overlay the Atlantic sands. High percentages of Pleuroxus during mid—Sub—Boreal were attributed to man's activity, and a lower water level. The most recent change in chydorid assemblages occurred within the past 100 yrs; this change was increased percentages of Alonella nana–a species normally associated with low alkalinities in Denmark. Postglacial calcareous gyttja deposits in Esrom So extended to 380 cm below the mud water interface. Calcareous clay extended from 381—840 cm, and glacial till occurred below 840 cm. Pollen preservation was poor in Esrom, but the lowermost gyttja was probably Pre—Boreal (Zone IV). No pollen counts were attributable to Zone VI. Few chydorid remains were found in Late—glacial clays, and extensive development of populations began in Pre—Boreal time (370 cm). Chydorid assemblages characteristic of discriminant Group 1 were present throughout most of the postglacial deposits; benthic species were prevalent during most of the lake's history. Littoral forms did increase in Sub—Boreal time, but by Sub—Atlantic chydorid assemblages were again dominated by benthic groups. In Esrom, postglacial chydorid fluctuations were dampened by its morphological and chemical characteristics, while Grane Langso fossil assemblages were sensitive to minor variations in the lake's environs. Pioneer chydorid species in northern Europe are Chydorus sphaericus, Alona affinis, A. quadrangularis, Acroperus harpae, and Alonella nana.

Cladoceran microfossil remains from the surficial sediments of 19 Danish and 14 Indiana lakes were analyzed quantitatively. An attempt was made to correlate the numbers of remains with C14 primary productivity measurements by previous authors. The Spearman rank correlation method gave no correlation for lakes in different drainage basins. It is suggested that under special circumstances there may be a correlation when only lakes of the same drainage basin are considered. See full-text article at JSTOR