The distribution of Heterotrissocladius oliveri Saether (Diptera: Chironomidae) in Lake Michigan

Fifty one chironomid species were identified from 504 samples collected at depths ranging 8 to 267 m in Lake Michigan, U.S.A. Heterotrissocladius oliveri Saether occurred in 32% of these samples and had an average abundance of 22 m^–2 which was similar to other estimates from the Great Lakes. Maximum average lake-wide density was at 30 to 60 m (41 m^–2). At depths 60 m, H. oliveri was the dominant chironomid species comprising 75% of total Chironomidae. The substrate preference of H. oliveri differed within each depth regime considered: at 30–60 m, 2–3 ; at 60–120 m, 3–5 , 7–9 ; and at 120–180 m, 6–8 . Abundance was notably reduced at all depths in substrates characterized as medium silt (5–6 ). On a lake-wide basis, the distribution pattern suggested H. oliveri was most numerous from 30 to 60 m along the southwestern, eastern, and northern shorelines and at 60–120 m depths along the southern and eastern shorelines. Increased abundance in the South Basin was concurrent with evidence of increased sedimentation at 60 to 100 m. However, in several other areas of the lake, high densities were associated with medium to very fine sands relatively free of silts and clays. This observation suggested occurrence of H. oliveri was minimally affected by sediment type.Widely variable, but generally elevated water temperatures likely prevent H. oliveri from establishing a substantial population density at depths < 30 m. With increased depth, temperature fluctuation is negligible and food is more stable, though the source is variable. Factors limiting abundance of H. oliveri at depths 30 m were related to decreased food supply due to distance from shore, food sources of lower value (clays), and, most importantly, to reproductive replenishment.Although still oligotrophic in nature, high density occurrences in both high and low sedimentation areas of the lake suggest the trophic indicator status of H. oliveri might be broader than previously thought.     

Evaluation of ion gradient-dependent H+ transport systems in isolated enterocytes from the chick

Summary The experiments reported here evaluate the capability of isolated intestinal epithelial cells to accomplish net H⁺ transport in response to imposed ion gradients. In most cases, the membrane potential was kept constant by means of a K⁺ plus valinomycin voltage clamp in order to prevent electrical coupling of ion fluxes. Net H⁺ flux across the cellular membrane was examined at pH 6.0 (the physiological lumenal pH) and at pH 7.4 using methylamine distribution or recordings of changes in media pH. Results from both techniques suggest that the cells have an Na⁺/H⁺ exchange system in the plasma membrane that is capable of rapid and sustained changes in intracellular pH in response to an imposed Na⁺ gradient. The kinetics of the Na⁺/H⁺ exchange reaction at pH 6.0 [K _t for Na⁺=57mm,V _max=42 mmol H⁺/liter 3OMG (3-O-methylglucose) space/min] are dramatically different from those at pH 7.4 (K _t for Na⁺=15mm,V _max=1.7 mmol H⁺/liter 3OMG space/min). Experiments involving imposed K⁺ gradients suggest that these cells have negligible K⁺/H⁺ exchange capability. They exhibit limited but measurable H⁺ conductance. Anion exchange for base equivalents was not detected in experiments performed in media nominally free of bicarbonate.     

Antarctic nemerteans: Heteronemertea– descriptions of new taxa, reappraisals of the systematic status of existing species and a key to the heteronemerteans recorded south of latitude 50oS

Twenty-three species representative of 15 heteronemertean genera are recorded from Antarctic and Subantarctic waters south oflatitude 50^oS. Five new genera are established and seven new species are fully described and illustrated. These are Cephalurichus antarcticus gen. et sp. nov., Dokonemertes magellanensis gen. et sp. nov., Dokonemertes macquariensis sp. nov., Micrellides multiperula gen. et sp. nov., Paramicrurinella falklandica gen. et sp. nov., Parborlasia landrumae sp. nov. and Zodionemertes chilensis gen. et sp. nov. Baseodiscus antarcticus Baylis, Antarctolineus scotti (Baylis), Lineus longifissus (Hubrecht) and Parborlasia fueguina Serna de Esteban & Moretto are redescribed and illustrated and the nature of rhynchodaeal pouches protruding into the cephalic blood lacunae of Parborlasia corrugatus (Mcintosh) is reexamined and discussed. The main anatomical characters of the remaining 11 species are recorded and it is concluded that of these only Parapolia grytvikenensis Wheeler, Cerebratulus malvini Wheeler, Chilineus glandulosa (Burger), Huilkia ushuaiensis Serna de Esteban & Moretto, Micrura pacifica Friedrich and Wiolkenia friedrichi Serna de Esteban & Moretto can at present be accepted as valid, although incompletely described, taxa. Cerebratulus larseni Wheeler, Cerebratulus validus Bürger, Lineus autrani Joubin and Lineus turqueli Joubin are so poorly and inadequately described that their taxonomic status cannot be verified, and the recorded occurrence of Lineus nigricans Burger in southern latitudes cannot be substantiated on the available evidence. Maps showing the recorded distribution of all the species are included and a key to the 18 validated species is provided.     

A root observation and sampling chamber (rhizotron) for pot studies

Summary A plant root observation chamber (rhizotron) was designed to examine soil-grown roots under a stereomicroscope and to sample roots and soil during the growth period of a pot study. The mini-rhizotrons are inexpensive to construct and are suitable for replicated, multitreatment experiments. Illustrative data on root hair and lateral development are presented for seedlings of four crop species.Vermont Agric. Exp. Stn. Journal Article No 572.     

Ecology of nematophagous fungi: vertical distribution in a deciduous woodland

Summary The distribution of nematophagous fungi in soil collected from a deciduous woodland is compared to various biotic and abiotic soil factors. The microfungi are isolated at all depths down to a maximum of 35 cm. Predators forming constricting rings, adhesive branches and adhesive knobs are restricted to the upper litter and humus layers. The net forming predators and endoparasites are isolated at all depths, although they are significantly more abundant in the lower mineral rich soils. A much greater species diversity of nematophagous fungi is recorded in the upper organic zones.Preliminary soil analysis indicates thatCephalosporium balanoides is independent of all soil variables, while predators able to form traps spontaneously are restricted to the organic soils which are rich in nematodes. Non-spontaneous trap forming predators, which are excellent saprophytes, are isolated from the deeper soils which are low in nutrients. The ecological significance of these results is discussed.     

Vertical distributions of zooplankton and population dynamics of Daphnia in a meromictic lake

Vertical distributions of zooplankton were studied in relation to profiles of temperature, oxygen and chlorophyll a in Roi Lake, a small meromictic lake in central Alberta. Zooplankton were distributed fairly evenly through the oxygenated part of the water column in early summer, but a gradual descent of several species became evident in June. The vertical distribution of chlorophyll was dominated by a huge peak at the 8- to 9-m-deep chemocline. the location of a plate of photosynthetic sulfur bacteria. Ambient concentration of chlorophyll was a poor predictor of the numbers of zooplankton and the fecundity of Daphnia pulicaria at different depths, and per capita birth rates of Daphnia were usually highest in the surface waters. The reproductively disadvantageous restriction of daphnids to deep water by late summer and their catastrophic decline in the face of high ambient concentrations of chlorophyll suggest that factors other than temperature and food supply are important in influencing the dynamics and distribution of zooplankton in this lake.     

Distribution and abundance of fishes in the San Francisco Bay estuary between 1980 and 1982

In 1980 a long-term study of the fishery resources of the San Francisco Bay estuary was initiated in an effort to delineate the importance of freshwater inflow to fish and invertebrate abundance and distribution in the bay. An analysis of the trawl data collected between January 1980 and December 1982 illustrates the influence of the timing and magnitude of freshwater inflows on fish fistribution and abundance in this estuary from the perspective of monthly, seasonal and annual time scales. Normally found in the delta, Suisun Bay and San Pablo Bay during periods of increased salinity, pelagic species moved downstream after the two peak flows studied, while demersal species usually found in Central San Francisco Bay moved upstream. Such upstream movements may be due in part to transport by strong density-driven currents.Timing and magnitude of monthly catches of some species varied on a seasonal cycle coincident with variations of freshwater inflow. Most species, especially the marine species, showed no consistent cycle of monthly catches. In the wet years of 1980 and 1982 the distributions of freshwater, estuarine and anadromous species were extended downstream into San Pablo, Central and South San Francisco Bays and some marine species, including the flatfish, were more abundant in the upstream areas. In the dry year of 1981 when bay salinities were higher, few marine species extended their distributions upstream into San Pablo and Suisun Bays. Jacksmelt was the only fish of the 15 most abundant species with its peak abundance in 1981. Most marine species were more abundant in the San Francisco Bay estuary in the wet years.     

Studies on ciliated protozoa in eutrophic lakes: 2. Field and laboratory studies on the effects of oxygen and other chemical gradients on ciliate distribution

An investigation into the spatial distribution of hypolimnetic ciliates in three small eutrophic lakes during the period of summer stratification was carried out. Peak ciliate densities were found to occur at the oxic/anoxic boundary, ciliate numbers declining with increasing depth within the hypolimnion. The ciliates only occurred in aerobic water where oxygen levels were less than about 0.5 mgl^–1 Laboratory experiments demonstrated that the ciliates swim upwards under anaerobic conditions but swim rapidly downwards under aerobic conditions. Further laboratory experiments showed that although the bulk of the population occured within anaerobic water, the hypolimnetic ciliates are aerobes and cannot survive indefinite anoxia. Despite the demonstrable toxicity of high levels of ammonia and sulphide, it was probably excesive distance from an available source of oxygen that excluded the ciliates from the lowest levels of the hypolimnion. Possible mechanisms which allowed these aerobic ciliates to colonise anaerobic water are considered.