Peter J. Jacobson

Transport, retention, and ecological significance of woody debris within a large ephemeral river

The spatiotemporal patterns and ecological significance of the retention of coarse particulate organic matter and large woody debris have been intensively studied in perennial rivers and streams but are virtually unknown in ephemeral systems. We examined the influence of 2 features characteristic of ephemeral systems, downstream hydrologic decay and in-channel tree growth, on the distribution, transport, and retention of woody debris following a flood having a ∼2.6-y recurrence interval in the ephemeral Kuiseb River in southwestern Africa. A total of 2105 pieces of wood were painted at 8 sites along the river channel to measure retention patterns. The flood had a peak discharge of 159 m3/s at the upper end of the study area, decaying to <1 m3/s by 200 km downstream. Downstream export of wood from marking sites totaled 59.5% (n = 1253). Transport distances ranged from 1 to 124 km, and 34.8% (n = 436) of the exported wood was recovered. Marked wood retained within marking sites was significantly longer than exported wood (p < 0.001, t-test). Once in transport, there was little correlation between wood length and distance traveled (r = 0.11, correlation analysis, n = 369). Length influenced the site of retention; material retained on debris piles was significantly longer than that stranded on channel sediments (p < 0.001, t-test). In-channel growth of Faidherbia trees significantly influenced wood retention; 83.7% of marked wood not moved by the flood was associated with debris piles on Faidherbia trees. Similarly, 65% of the exported wood retained within downstream debris piles was associated with Faidherbia trees. In contrast to many perennial systems, we observed a general increase in wood retention downstream, peaking in the rivers lower reaches in response to hydrologic decay. Debris piles induced sediment deposition and the formation of in-channel islands. Following flood recession, debris piles and their associated sediments provided moist, organic-rich microhabitats, which were focal points for decomposition and secondary production, mimicking patterns reported from the channels of perennial streams and rivers. The ecological significance of retentive obstacles and associated organic debris is a feature common to all fluvial ecosystems, irrespective of their hydrologic regime.

Non-Rainfall Moisture Activates Fungal Decomposition of Surface Litter in the Namib Sand Sea

The hyper-arid western Namib Sand Sea (mean annual rainfall 0–17 mm) is a detritus-based ecosystem in which primary production is driven by large, but infrequent rainfall events. A diverse Namib detritivore community is sustained by minimal moisture inputs from rain and fog. The decomposition of plant material in the Namib Sand Sea (NSS) has long been assumed to be the province of these detritivores, with beetles and termites alone accounting for the majority of litter losses. We have found that a mesophilic Ascomycete community, which responds within minutes to moisture availability, is present on litter of the perennial Namib dune grass Stipagrostis sabulicola. Important fungal traits that allow survival and decomposition in this hyper-arid environment with intense desiccation, temperature and UV radiation stress are darkly-pigmented hyphae, a thermal range that includes the relatively low temperature experienced during fog and dew, and an ability to survive daily thermal and desiccation stress at temperatures as high as 50°C for five hours. While rainfall is very limited in this area, fog and high humidity provide regular periods (≥ 1 hour) of sufficient moisture that can wet substrates and hence allow fungal growth on average every 3 days. Furthermore, these fungi reduce the C/N ratio of the litter by a factor of two and thus detritivores, like the termite Psammotermes allocerus, favor fungal-infected litter parts. Our studies show that despite the hyper-aridity of the NSS, fungi are a key component of energy flow and biogeochemical cycling that should be accounted for in models addressing how the NSS ecosystem will respond to projected climate changes which may alter precipitation, dew and fog regimes.

Agricultural conversion of floodplain ecosystems: Implications for groundwater quality

With current trends of converting grasslands to row crop agriculture in vulnerable areas, there is a critical need to evaluate the effects of land use on groundwater quality in large river floodplain systems. In this study, groundwater hydrology and nutrient dynamics associated with three land cover types (grassland, floodplain forest and cropland) were assessed at the Cedar River floodplain in southeastern Iowa. The cropland site consisted of newly-converted grassland, done specifically for our study. Our objectives were to evaluate spatial and temporal variations in groundwater hydrology and quality, and quantify changes in groundwater quality following land conversion from grassland to row crop in a floodplain. We installed five shallow and one deep monitoring wells in each of the three land cover types and recorded water levels and quality over a three year period. Crop rotations included soybeans in year 1, corn in year 2 and fallow with cover crops during year 3 due to river flooding. Water table levels behaved nearly identically among the sites but during the second and third years of our study, NO₃-N concentrations in shallow floodplain groundwater beneath the cropped site increased from 0.5 mg/l to more than 25 mg/l (maximum of 70 mg/l). The increase in concentration was primarily associated with application of liquid N during June of the second year (corn rotation), although site flooding may have exacerbated NO₃-N leaching. Geophysical investigation revealed differences in ground conductivity among the land cover sites that related significantly to variations in groundwater quality. Study results provide much-needed information on the effects of different land covers on floodplain groundwater and point to challenges ahead for meeting nutrient reduction goals if row crop land use expands into floodplains.

Use of Neutral Red to Assess Survival of Juvenile Freshwater Mussels (Bivalvia: Unionidae) in Bioassays

The effectiveness of vital staining for assessing lethal and sublethal responses of juvenile mussels was examined. Neutral red was used to quantify survival of juvenile Villosa iris and Anodonta grandis after exposures to aqueous copper in 24-hour static bioassays. Live juveniles readily incorporated the stain, but dead individuals did not. Variation in stain intensity was associated with behavioral responses, permitting diagnosis of alive, dead, and sublethal responses of juvenile V. iris. The amber coloration of juvenile A. grandis prevented detection of variations in stain intensity, thus allowing only livingversus-dead determinations to be made. Responding to precipitous declines in populations of freshwater mussels (Unionidae), several workers recently conducted laboratory tests to measure sensitivity of juvenile stages to various pollutants (Johnson et al., 1990; Keller & Zam, 1991; Lasee, 1991). Both Johnson et al. (1990) and Keller & Zam (1991) determined post-exposure mortality from observations of internal anatomy, but did not detail any sublethal effects of the exposures. By contrast, Lasee (1991) assessed both post-exposure mortality and sublethal responses by individual inspection of the juvenile mussels. Juveniles were recorded as alive (active and moving), stressed (no foot movement but cilia beating), or dead (no foot or cilia movement). Toxicity tests depend on an accurate assessment of post-exposure condition and are complicated by the small size (<1 mm) of juvenile mussels. Healthy juveniles are typically active, extruding the foot and gaping (opening) their valves. If immobile or ungaped, their condition is not as apparent. Because juveniles of many species possess transparent valves, with visible internal structure, the reduction or absence of movement by the foot or cilia may be used to assess responses. This requires close, individual inspections, and the effort is time-intensive. A more rapid and equally precise means of assessing postexposure condition of juvenile mussels thus was desirable. Vital staining has been used successfully to distinguish living from dead We thank Mr. Lou Rifici and Ms. Lisa Wolcott for assistance in the laboratory with rearing juvenile mussels and vital staining procedures. This research was supported by a grant from the American Electric Power Company through the American Electric Power Service Corporation, Columbus, Ohio 43216, U.S.A. 2 Address: Department of Biological Sciences, Arkansas State University, State University, Arkansas 72467, U.S.A. 3 Address: U.S. Fish and Wildlife Service, Virginia Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. TRANS. AM. MICROSC. SOC., 112(1): 78-80. 1993. ? Copyright, 1993, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.220 on Fri, 02 Sep 2016 04:36:20 UTC All use subject to http://about.jstor.org/terms VOL. 112, NO. 1, JANUARY 1993 invertebrates. Dressel et al. (1972) used vital staining to sort copepods, and Crippen & Perrier (1974) used neutral red to determine mortality among marine plankters. Platter-Rieger & Frank (1987) successfully used neutral red to assess post-exposure effects of tributyltin on mussel larvae (Mytilus edulis). They defined three levels of staining: darkly stained (healthy), lightly stained (stressed and inactive), or not stained (dead). They found increasing percentages of lightly stained larvae in treatments with higher toxicant concentrations and assumed that staining intensity was related to the level of stress. The purpose of our investigation was to evaluate the effectiveness of vital staining with neutral red in determining post-exposure survival of juvenile freshwater mussels. Copper was chosen as the toxicant because it is a known molluscicide, highly toxic to invertebrates, and a common pollutant in riverine systems (U.S.E.P.A., 1985; Van Hassel & Gaulke, 1986). MATERIALS AND METHODS Juveniles of Villosa iris (I. Lea, 1829) and Anodonta grandis Say, 1829 were obtained following metamorphosis of glochidia encysted upon largemouth bass, Micropterus salmoides (Lacepede, 1802). We conducted 24-h, static exposures at eight concentrations ranging from approximately 0 to 200 tg Cu/L. Absolute metal concentration was determined by inductively coupled, argon-plasmaemission spectroscopy. We used two replicates of each toxicant concentration for V. iris and three for A. grandis, with 10 juveniles per replicate. Test containers were held in an incubator at 20?C; after exposure, juvenile response was determined by visual inspection and vital staining. Prior to vital staining, we examined juveniles at 12-50 x magnification with a stereomicroscope. Three classes of response were established: (1) gaped and alive, (2) gaped and dead, or (3) ungaped. Dead juveniles were characterized by their rigid, immobile foot and the absence of beating cilia. We stained juveniles after the initial inspection by the technique of Crippen & Perrier (1974) using a 1-h exposure to a 1:100,000 concentration of neutral red in water. Stained juveniles were stored overnight in a refrigerator at approximately 4?C prior to examination. The degree of staining was assessed using a stereomicroscope at 12-20 x magnification. We defined three levels of staining: (1) brightly stained, (2) lightly or partially stained, and (3) unstained. Classes 2 and 3 were combined to yield a total number affected both by visual inspection and vital

The autecology of Battarrea stevenii in ephemeral rivers of southwestern Africa

In September 1990, 74 sporocarps of Battarrea stevenii were observed on the floodplain of the ephemeral Kuiseb River in western Namibia. Herein we report subsequent studies of the distribution, abundance, nutritional role, phenology, and sporocarp development of this fungus in the hyper-arid Namib Desert. Included are full descriptions of developing and mature sporocarps. B. stevenii is a common associate of riparian forests on silty floodplain terraces, but does not form mycorrhizal associations with the dominant woody species, Faidherbia albida or Tamarix usneoides. Rather, clamped mycelium extends throughout floodplain soils decomposing coarse and fine particulate organic material (4—7% of soil dry weight). Sporocarp production occurs 4.5—12 mo post-flooding in response to soil desiccation at depths of 20—35 cm. The extensive mycelium, duration of vegetative growth post-flooding, and large size and abundance of B. stevenii sporocarps suggest that it is an important component of the subsurface decomposer community in the Namibs ephemeral rivers. Given that the fungus has also been recorded from floodplain soils of Angola, Hungary, and New Mexico (U.S.A.), and is known to have a world-wide distribution, we predict that further biogeographical studies will reveal that B. stevenii is a characteristic element of the riparian biota in dryland rivers, which drain approximately one-third of the earths land surface.

Field observation of diurnal dissolved oxygen fluctuations in shallow groundwater.

Dissolved oxygen (DO) concentrations influence many biogeochemical processes in groundwater systems but studies of temporal variability in DO are lacking. In this study, we used an optical DO probe to measure rapid changes in concentration due to plant-groundwater interaction at an alluvial aquifer field site in Iowa. Diurnal DO concentrations were observed during mid- to late-summer when soil conditions were dry, fluctuating approximately 0.2 to 0.3 mg/L on a daily basis. DO fluctuations in groundwater were out-of-phase with diurnal water table fluctuations, increasing during the day and decreasing at night. DO consumption at night is likely due to increased soil autotrophic and heterotrophic respiration linked with patterns of carbon supply derived from daytime photosynthetic activity, and consistent with available literature on diurnal soil respiration patterns. Although more work is needed to quantify specific processes, our results indicate the potential usefulness of the new optical DO technology to reveal insights regarding many ecohydrological processes.

Field evidence from Namibia does not support the designation of Angolan and Namibian subspecies of Welwitschia mirabilis Hook

Welwitschia mirabilis is an ancient, endemic gymnosperm found in numerous disjunct populations in western Namibia north of the Namib Sand Sea to the Bentiaba River in southwestern Angola. Based on six plants grown in the Berlin-Dahlem Botanical Garden, Leuenberger [2001, Welwitschia mirabilis (Welwitschiaceae), male cone characters and a new subspecies. Willdenowia 31: 357–381] differentiated two groups of plants using eight male inflorescence characters. He concluded that the two groups represented geographical subspecies: W. mirabilis subsp. mirabilis in Angola and W. mirabilis subsp. namibiana in Namibia, based on putative location data of the seed sources for the plants. Leuenberger called for field data collection, especially in northern Namibia, to verify his results. However despite this lack of data, numerous authors have cited his work as definitive in scientific papers and online plant descriptions and keys. We analysed five distinguishing male cone characters from 13 Namibian plant populations and found significant variation that encompasses both descriptions of the Namibian and Angolan subspecies. Data collected from plants grown from Angolan and Namibian seed in the Kirstenbosch Botanical Garden also fell within the range of the Namibian field populations. We found significant evidence of latitudinal gradients in cone and peduncle length, but cone colour, secondary branch length and bract shape were highly variable within and between the Namibian populations sampled. Our data do not support the designation of an Angolan and Namibian subspecies. W. mirabilis has probably been evolving in this landscape for more than 100 million years with unknown metapopulation dynamics predetermining current geographical populations. Our study calls for molecular characterisation in conjunction with a thorough investigation of both male and female plant characteristics throughout the current range of the plant to further clarify the phylogenetic relevance of the variation observed in male cone characters.

Supporting a teaching and learning community across borders: Grinnell College at Gobabeb

For the past fourteen years, two Grinnell College graduates have been selected each year for a competitive one-year Grinnell Corps Service Fellowship at the Gobabeb Research and Training Centre in Namibia. A recent survey of the 28 Fellows that completed the program revealed that this is an invaluable experience for the participants, with more than half revealing that it was “life-changing” and a further 43% reporting that it re-affirmed their life goals. Most Fellows felt that their most valuable contribution to Gobabeb was teaching and mentoring Namibian secondary and tertiary students. All Fellows are now either in graduate school or employed in a diverse array of positions reflecting their professional interests that in some fashion “support the common good”, an integral part of Grinnells mission statement. Synergistic activities that have arisen from this Grinnell–Gobabeb relationship include visiting lecturer opportunities at Grinnell for Namibians; internships for Grinnell students and service fellowships for Grinnell faculty in Namibia; a faculty development seminar in Namibia resulting in more class modules on dryland and southern African topics; a course with embedded travel to the Namib Desert; and independent research projects for students. These exchanges are invaluable for cultivating the cross-cultural understanding needed to address the vexing global environmental challenges of this century.