Steven L. Kohler

Disturbance and recovery of large floodplain rivers

Disturbance in a river-floodplain system is defined as an unpredictable event that disrupts structure or function at the ecosystem, community, or population level. Disturbance can result in species replacements or losses, or shifts of ecosystems from one persistent condition to another. A disturbance can be a discrete event or a graded change in a controlling factor that eventually exceeds a critical threshold.The annual flood is the major driving variable that facilitates lateral exchanges of nutrients, organic matter, and organisms. The annual flood is not normally considered a disturbance unless its timing or magnitude is “atypical.” The record flood of 1973 had little effect on the biota at a long-term study site on the Mississippi River, but the absence of a flood during the 1976–1977 Midwestern drought caused short- and long-term changes. Body burdens of contaminants increased temporarily in key species, because of increased concentration resulting from reduced dilution. Reduced runoff and sediment input improved light penetration and increased the depth at which aquatic macrophytes could grow. Developing plant beds exerted a high degree of biotic control and were able to persist, despite the resumption of normal floods and turbidity in subsequent years.In contrast to the discrete event that disturbed the Mississippi River, a major confluent, the Illinois River, has been degraded by a gradual increase in sediment input and sediment resuspension. From 1958 to 1961 formerly productive backwaters and lakes along a 320-km reach of the Illinois River changed from clear, vegetated areas to turbid, barren basins. The change to a system largely controlled by abiotic factors was rapid and the degraded condition persists.Traditional approaches to experimental design are poorly suited for detecting control mechanisms and for determining the critical thresholds in large river-floodplains. Large river-floodplain systems cannot be manipulated or sampled as easily as small streams, and greater use should be made of man-made or natural disturbances and environmental restoration as opportunistic experiments to measure thresholds and monitor the recovery process.

PATHOGEN OUTBREAKS REVEAL LARGE-SCALE EFFECTS OF COMPETITION IN STREAM COMMUNITIES

The ability of small-scale experiments to predict dynamics and patterns observed at larger scales is an important issue in ecology. In this paper, we describe responses of benthic, trout stream communities to replicated, whole-ecosystem perturbations resulting from pathogen-induced reductions in populations of a dominant grazer, the cad- disfly Glossosoma nigrior. Previous small-scale experiments suggested that Glossosoma had strong competitive effects on other grazers (through exploitation) and small-bodied filter-feeders (through interference). Glossosoma populations collapsed in a number of Michigan trout streams since the late 1980s and have been maintained at low levels by recurrent pathogen outbreaks. Here we address the effects on periphyton, grazers, and filter- feeders of parasite-induced Glossosoma reductions in six streams for which considerable pre- and post-collapse data are available, and we compare the results of this large-scale perturbation to responses predicted from a small-scale experiment that had been previously conducted in one of these streams. Periphyton and most grazers and filter-feeders showed marked increases in abundance following reduction in Glossosoma populations. Several grazers that were rare or absent prior to- Glossosoma collapse have established sizable populations, suggesting that they had been excluded by competition with Glossosoma. The small-scale experiment successfully predicted the direction of response of most taxa to whole-stream reductions in Glossosoma abundance, but it tended to underestimate the extent and magnitude of Glossosomas effects in the community. In these systems, observations at larger spatial and temporal scales have been essential to developing a clearer under- standing of the mechanisms structuring communities.

BIRDS ARE OVERLOOKED TOP PREDATORS IN AQUATIC FOOD WEBS

Most freshwater food web models assume that fish occupy the top trophic level. Yet many diet studies and a few caging and artificial stream experiments suggest that birds may be top predators in many freshwater systems. We conducted a large-scale field experiment to test whether avian predators affect the size distribution and abundance of fish in two midwestern streams. We used a combination of netting and perches to manipulate predation by Great Blue Herons (Ardea herodias) and Belted Kingfishers (Ceryle alcyon), and measured the response in the fish assemblage. Bird exclusions caused significant increases in medium size classes of two common prey, striped shiners (Luxilus chrysocephalus) and central stonerollers (Campostoma anomalum). We show that these species of piscivorous birds can alter the abundance of common prey and thus need to be considered more fully when attempting to explain the structure of aquatic food webs. Corresponding Editor: O. J. Schmitz.

Effects of hydroperiod and predation on a Mississippi River floodplain invertebrate community

Abstract The objective of this study was to determine if pond permanence and vertebrate predation (by fish and waterfowl) affect invertebrate community structure in the mudflat habitat of floodplain ponds. Invertebrate communities were studied for 1 year in four Mississippi River floodplain ponds with different hydroperiods. Pond 1 experienced five dry periods, pond 2 experienced four, pond 3 dried once, and standing water remained in pond 4 for the entire year. Vertebrate predator exclusion treatments (all access, no access, small-fish access and cage controls) were placed in all ponds. As pond duration increased, predatory invertebrate richness and abundance increased while overall invertebrate richness and abundance decreased. With the exception of the cladoceran Diaphanosoma, all commonly encountered taxa were strongly affected by pond permanence in terms of abundance, biomass and, generally, individual biomass. Taxa were nearly early divided between those that were more abundant in less permanent ponds and those that were more abundant in longer-duration ponds. Invertebrate taxa richness, abundance, and total biomass were lower in the all-access treatment than in the treatments that restricted predator access, and these effects were stronger in the more permanent ponds. In general, there were no significant differences in responses to the treatments with small-fish access and no access. These results support models that predict relatively weak effects of predation in frequently disturbed habitats.

Evaluating Passive Integrated Transponder Tags for Marking Mottled Sculpins: Effects on Growth and Mortality

Abstract We examined the efficacy of marking mottled sculpins Cottus bairdii with passive integrated transponder (PIT) tags in a 28-d laboratory study. A 2 × 3 factorial experiment was used to compare growth and mortality of tagged fish with those of a control group (i.e., not injected with tags) among three size-classes (55–59, 60–69, and ≥70 mm total length [TL]). Fish were measured on the day of tagging and each week thereafter. Among 26 tagged fish (56–83 mm TL), both survival and tag retention were 96% or greater. Survival was 100% among 25 control fish (56–85 mm TL). Instantaneous growth rates (calculated on a cumulative basis for each sampling period based on mass) for tagged fish were significantly lower than those for control fish during the first 14 d, suggesting that mottled sculpins recovered from PIT tagging after 14–21 d. Additionally, fish size did not significantly affect the instantaneous growth rate of PIT-tagged fish relative to that of the control group. More importantly, we found (on ...

Influence of stream location in a drainage network on the index of biotic integrity

Abstract The index of biotic integrity (IBI) has become a widely used tool for assessing the condition of stream fish communities and the overall biological status of streams. Because the location of a stream in a drainage network can influence the species richness offish communities and because species richness is an important component of the IBI, we examined the influence of stream spatial location on the IBI. We found that IBI scores for headwater streams in three Illinois drainage basins were significantly lower than those calculated for tributary streams of similar size connecting directly to larger streams. This difference in IBI was related to the increased species richness and to a greater number of sucker and darter species in tributaries that drain into larger, main-channel streams. Because of the influence of tributary location on the IBI, expected values for headwater tributary streams should be developed independently from those developed for main-channel tributary streams. Failure to do so ...

Atrazine exposure affects growth, body condition and liver health in Xenopus laevis tadpoles.

Six studies were performed regarding the effects of atrazine, the most frequently detected pesticide in fresh water in the US, on developing Xenopus laevis tadpoles exposed 5 days post-hatch through Nieuwkoop Faber Stage 62. The levels of atrazine tested included those potentially found in puddles, vernal ponds and runoff soon after application (200 and 400 μg/L) and a low level studied by a number of other investigators (25 μg/L). One study tested 0, 25 and 200 μg/L, another tested 0, 200 and 400 μg/L, while the remaining four studies tested 0 and 400 μg/L. During all exposures, mortality, growth, metamorphosis, sex ratio, fat body (a lipid storage organ) size and liver weights, both relative to body weight, were evaluated. In selected studies, feeding behavior was recorded, livers and fat bodies were histologically evaluated, liver glycogen and lipid content were determined by image analysis, and immunohistochemical detection of activated caspase-3 in hepatocytes was performed. The NOEC was 25 μg/L. None of these exposure levels changed sex ratios nor were intersex gonads noted, however, no definitive histological evaluation of the gonads was performed. Although a marginal increase in mortality at the 200 μg/L level was noted, this was not statistically significant. Nor was there an increase in mortality at 400 μg/L versus controls. At the 400 μg/L level, tadpoles were smaller than controls by 72 h of exposure and remained smaller throughout the entire exposure. Appetite was not decreased at any exposure level. Slowed metamorphosis was noted only at 400 μg/L in two of five studies. Livers were significantly smaller in the study that tested both 200 and 400 μg/L, yet no pathological changes or differences in glycogen or lipid stores were noted. However, livers from 400 μg/L exposed tadpoles had higher numbers of activated caspase-3 immunopositive cells suggesting increased rates of apoptosis. Fat body size decreased significantly after exposure to 200 and 400 μg/L although these organs still contained some lipid and lacked any pathology. Since this was noted across all studies, it was considered the most sensitive indicator of atrazine exposure measured. The changes noted in body and organ size at 200 and 400 μg/L atrazine indicated exposure throughout development compromised the tadpoles. Significant reductions in fat body size could potentially decrease their ability to survive the stresses of metamorphosis or reduce reproductive fitness as frogs rely on stored lipids for these processes.

Virulence is context-dependent in a vertically transmitted aquatic host-microparasite system.

Recent work has suggested that the outcomes of host-symbiont interactions can shift between positive, neutral and negative depending on both biotic and abiotic conditions. Even organisms traditionally defined as parasites can have positive effects on hosts under some conditions. For a given host-parasite system, the effects of infection on host fitness can depend on host vigour, route of transmission and environmental conditions. We monitored sublethal microsporidian infections in populations of Gammarus pseudolimnaeus (Amphipoda: Gammaridae) from four cool water streams in southwestern Michigan, USA. Our objectives were to: (i) infer the mechanism of transmission (horizontal, vertical or mixed) from observed effects of infection on host fitness, (ii) determine if the magnitude of the effects on host fitness is a function of parasite load (infection intensity) compared with simple presence or absence of infection, and (iii) determine if there is variation in parasite effects on host fitness in isolated populations. PCR and DNA sequence analyses revealed that there were two microsporidia present among the four host populations: Dictyocoela sp. and Microsporidium sp. PCR screening of a subset of infected hosts showed that Dictyocoela sp. accounted for 90% of infections and was present in all four G. pseudolimnaeus populations, while Microsporidium sp. was found in two populations but was only relatively common in one. We found very low prevalence in males (∼5%), but high prevalence in females (range: 37-85%). Female fitness was positively associated with infection in two streams, resulting from either higher fecundity or more reproductive bouts. Infection had a negative effect on the number of reproductive bouts in a third population, and no effect on fecundity in a fourth population. Infection intensity explained additional variation in fecundity in one population; females with intermediate infection intensity had higher fecundity than females with either light or heavy infection intensity. Given the high prevalence of infection in females compared with males and the generally weak negative fitness effects coupled with some positive fitness effects, it is likely that both Dictyocoela sp. and Microsporidium sp. are primarily vertically transmitted, feminizing microsporidia. Our results suggest that microsporidian effects on G. pseudolimnaeus fitness were context-dependent and varied with host sex and local environment.

Complex postglacial recolonization inferred from population genetic structure of mottled sculpin Cottus bairdii in tributaries of eastern Lake Michigan, U.S.A.

This study used analyses of the genetic structure of a non-game fish species, the mottled sculpin Cottus bairdii to hypothesize probable recolonization routes used by cottids and possibly other Laurentian Great Lakes fishes following glacial recession. Based on samples from 16 small streams in five major Lake Michigan, U.S.A., tributary basins, significant interpopulation differentiation was documented (overall FST = 0·235). Differentiation was complex, however, with unexpectedly high genetic similarity among basins as well as occasionally strong differentiation within basins, despite relatively close geographic proximity of populations. Genetic dissimilarities were identified between eastern and western populations within river basins, with similarities existing between eastern and western populations across basins. Given such patterns, recolonization is hypothesized to have occurred on three occasions from more than one glacial refugium, with a secondary vicariant event resulting from reduction in the water level of ancestral Lake Michigan. By studying the phylogeography of a small, non-game fish species, this study provides insight into recolonization dynamics of the region that could be difficult to infer from game species that are often broadly dispersed by humans.

Oncolytic Tanapoxvirus Expressing Interleukin-2 is Capable of Inducing the Regression of Human Melanoma Tumors in the Absence of T Cells

BACKGROUND Oncolytic viruses (OVs), which preferentially infect cancer cells and induce host anti-tumor immune responses, have emerged as an effective melanoma therapy. Tanapoxvirus (TANV), which possesses a large genome and causes mild self-limiting disease in humans, is potentially an ideal OV candidate. Interleukin-2 (IL-2), a T-cell growth factor, plays a critical role in activating T cells, natural killer (NK) cells and macrophages in both the innate and adaptive immune system. OBJECTIVE We aimed to develop a recombinant TANV expressing mouse IL-2 (TANVΔ66R/mIL- 2), replacing the viral thymidine kinase (TK) gene (66R) with the mouse (m) mIL-2 transgene resulting in TANVΔ66R/mIL-2. METHODS Human melanoma tumors were induced in female athymic nude mice by injecting SKMEL- 3 cells subcutaneously. Mice were treated with an intratumoral injection of viruses when the tumor volumes reached 45 ± 4.5 mm3. RESULTS In cell culture, expression of IL-2 attenuated virus replication of not only TANVΔ66R/ mIL-2, but also TANVGFP. It was demonstrated that IL-2 inhibited virus replication through intracellular components and without activating the interferon-signaling pathway. Introduction of mIL-2 into TANV remarkably increased its anti-tumor activity, resulting in a more significant regression than with wild-type (wt) TANV and TANVΔ66R. Histopathological studies showed that extensive cell degeneration with a significantly increased peri-tumor accumulation of mononuclear cells in the tumors treated with TANVΔ66R/mIL-2, compared to wtTANV or TANVΔ66R. CONCLUSION We conclude that TANVΔ66R/mIL-2 is potentially therapeutic for human melanomas in the absence of T cells, and IL-2 expression resulted in an overall increase of therapeutic efficacy.