Types and occurrence of morphological anomalies in Scaphirhynchus spp. of the Middle and Lower Mississippi River

Sturgeon specimens encountered in the wild that exhibit visible signs of gross physical trauma often look to the naked eye to be in otherwise good condition. Visible morphological anomalies were observed in 9.1% of 176 pallid (Scaphirhynchus albus) and 4.6% of 4904 shovelnose (Scaphirhynchus platorynchus) sturgeon specimens captured in the Middle (mouth of Missouri River to mouth of Ohio River) and Lower (below mouth of Ohio River) Mississippi River from 1997 to 2004. Frequencies among the types of anomalies differed between the lower and middle river reaches. In the lower river, deformities from foreign objects (typically rubber bands) comprised almost one‐third of anomalies observed and may have contributed to other types of anterior injury which, if combined, would comprise the majority of lower river anomalies. In the middle river, nearly half of the observed anomalies involved damage to the caudal peduncle, usually a missing tail. Power regressions from length–weight relationships were compared for anomalous and non‐anomalous specimens and demonstrated no significant disparity, verifying the resiliency of river sturgeons.     

Physiological and behavioral variation in estivation among mud turtles (Kinosternon spp.)

Kinosternid mud turtles, a primarily aquatic group, exhibit variable degrees of terrestrial activity in the Sonoran and Chihuahuan Deserts. We compared behavioral and physiological responses to dry conditions in four populations representing three species, Kinosternon sonoriense, Kinosternon flavescens, and Kinosternon hirtipes. All four groups were subjected to simulated dry season conditions in the laboratory, during which activity was monitored and physiological responses (blood chemistry and rates of resting metabolism and evaporative water loss) were measured. Kinosternon flavescens and K. hirtipes represented extremes in apparent ability to estivate, based on activity and rate of increase of plasma osmolality. Two populations of K. sonoriense exhibited intraspecific differences in behavioral and physiological measures that were related to extant environmental conditions. Large numbers of K. sonoriense from Arizona and K. hirtipes, the poorest estivators, had to be rehydrated after only 30 d out of water. Kinosteron flavescens had the lowest metabolic rates, but no evidence of metabolic depression during dehydration was found for any of the four populations. We conclude that the differences in capacity for estivation among populations are primarily linked to variable behavioral responses to dry conditions, though high rates of evaporative water loss in K. hirtipes represent a probable physiological constraint.     

Microsatellite variation among River Sturgeons of the genus Scaphirhynchus (Actinopterygii: Acipenseridae): a preliminary assessment of hybridization*

Microsatellite variation from 13 disomic loci is reported for a total of 208 individuals of the genus Scaphirhynchus. This includes 105 individuals of the pallid sturgeon (Scaphirhynchus albus) from the lower Mississippi River, 11 pallid sturgeon from the Upper Mississippi and Missouri rivers, 65 shovelnose sturgeon (S. platorynchus) from the lower Mississippi River, six Alabama sturgeon (S. suttkusi), and 21 individuals of sturgeon identified as intermediate between S. albus and S. platorynchus. Results indicate that all five of the above population/species units are significantly differentiated from one another based on pairwise F_ST estimates. Locus Spl‐7 was diagnostic for the Alabama sturgeon and serves to further differentiate this allopatric species from other Scaphirhynchus. Classification of genotypes with and without a priori designations failed to clearly delineate the species and intermediates in the latter case but was successful for the species but not in the intermediates in the former case. The presence of six unique alleles in five of the 21 morphologically ‘intermediate’ sturgeon examined requires additional evaluation but suggests that these individuals are possibly not the result of hybridization. We hope that raising these important issues will bring all stakeholders to the table to establish a concerted effort needed for both morphological and molecular analyses to adequately address the question of hybridization and the origin of the morphological variation in these fishes.     

Characterization of Pallid Sturgeon (Scaphirhynchus albus) Spawning Habitat in the Lower Missouri River

Acipenseriformes (sturgeons and paddlefish) globally have declined throughout their range due to river fragmentation, habitat loss, overfishing, and degradation of water quality. In North America, pallid sturgeon (Scaphirhynchus albus) populations have experienced poor to no recruitment, or substantial levels of hybridization with the closely related shovelnose sturgeon (S. platorynchus). The Lower Missouri River is the only portion of the species’ range where successful reproduction and recruitment of genetically pure pallid sturgeon have been documented. This paper documents spawning habitat and behavior on the Lower Missouri River, which comprises over 1,300 km of unfragmented river habitat. The objective of this study was to determine spawning locations and describe habitat characteristics and environmental conditions (depth, water velocity, substrate, discharge, temperature, and turbidity) on the Lower Missouri River. We measured habitat characteristics for spawning events of ten telemetry-tagged female pallid sturgeon from 2008–2013 that occurred in discrete reaches distributed over hundreds of kilometers. These results show pallid sturgeon select deep and fast areas in or near the navigation channel along outside revetted banks for spawning. These habitats are deeper and faster than nearby river habitats within the surrounding river reach. Spawning patches have a mean depth of 6.6 m and a mean depth-averaged water-column velocity of 1.4 m per second. Substrates in spawning patches consist of coarse bank revetment, gravel, sand, and bedrock. Results indicate habitat used by pallid sturgeon for spawning is more common and widespread in the present-day channelized Lower Missouri River relative to the sparse and disperse coarse substrates available prior to channelization. Understanding the spawning habitats currently utilized on the Lower Missouri River and if they are functioning properly is important for improving habitat remediation measures aimed at increasing reproductive success. Recovery efforts for pallid sturgeon on the Missouri River, if successful, can provide guidance to sturgeon recovery on other river systems; particularly large, regulated, and channelized rivers.     

Evaluation of Reforestation in the Lower Mississippi River Alluvial Valley

Only about 2.8 million ha of an estimated original 10 million ha of bottomland hardwood forests still exist in the Lower Mississippi River Alluvial Valley (LMAV) of the United States. The U.S. Fish and Wildlife Service, the U.S. Forest Service, and state agencies initiated reforestation efforts in the late 1980s to improve wildlife habitat. We surveyed restorationists responsible for reforestation in the LMAV to determine the magnitude of past and future efforts and to identify major limiting factors. Over the past 10 years, 77,698 ha have been reforested by the agencies represented in our survey and an additional 89,009 ha are targeted in the next 5 years. Oaks are the most commonly planted species and bare-root seedlings are the most commonly used planting stock. Problems with seedling availability may increase the diversity of plantings in the future. Reforestation in the LMAV is based upon principles of landscape ecology; however, local problems such as herbivory, drought, and flooding often limit success. Broad-scale hydrologic restoration is needed to fully restore the structural and functional attributes of these systems, but because of drastic and widespread hydrologic alterations and socioeconomic constraints, this goal is generally not realistic. Local hydrologic restoration and creation of specific habitat features needed by some wildlife and fish species warrant attention. More extensive analyses of plantings are needed to evaluate functional success. The Wetland Reserve Program is a positive development, but policies that provide additional financial incentives to landowners for reforestation efforts should be seriously considered.     

Effects of temperature and hydrology on growth of shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820) in the lower Mississippi River

We evaluated the effects of thermal and hydrologic conditions on the growth of shovelnose sturgeon (Scaphirhynchus platorynchus) in the lower Mississippi River, USA. Duration of water temperatures 12–24°C had a positive influence and temperatures below 11°C had a negative influence on annual growth increment, but these two variables accounted for less than 15% of the variation in growth. Duration of water temperatures above 28°C, duration of floodplain inundation, duration of low water, and minimum and maximum river stage did not influence annual growth increment. Growth of shovelnose sturgeon in the lower Mississippi River appears to be positively influenced by duration of moderate water temperatures but minimally influenced by hydrologic conditions. The low variation accounted for by thermal and hydrologic variables suggests annual growth increment may be largely influenced by additional abiotic or biotic factors.     

Gonadosomatic index and fecundity of Lower Missouri and Middle Mississippi River endangered pallid sturgeon estimated using minimally invasive techniques

Minimally invasive, non‐lethal methods of ultrasonography were used to assess sex, egg diameter, fecundity, gonad volume, and gonadosomatic index, as well as endoscopy to visually assess the reproductive stage of Scaphirhynchus albus. Estimated mean egg diameters of 2.202 ± 0.187 mm and mean fecundity of 44 531 ± 23 940 eggs were similar to previous studies using invasive techniques. Mean S. albus gonadosomatic indices (GSI) for reproductive and non‐reproductive females were 16.16 and 1.26%, respectively, while reproductive and non‐reproductive male GSI were 2.00 and 0.43%, respectively. There was no relationship between hybrid status or capture location and GSI. Mean fecundity was 48.5% higher than hatchery spawn estimates. Fecundity increased as fork length increased but did so more dramatically in the upper river kilometers of the Missouri River. By examining multiple fish over multiple years, the reproductive cycle periodicity for hatchery female S. albus was found to be 2–4 years and river dwelling males 1–4 years. The use of ultrasonic and endoscopic methods in combination was shown to be helpful in tracking individual gonad characteristics over multi‐year reproductive cycles.     

Population characteristics of pallid sturgeon (Scaphirhynchus albus (Forbes & Richardson, )) in the Lower Missouri River

Population characteristics of pallid sturgeon Scaphirhynchus albus in the lower Missouri River are relatively unknown. Therefore, data collected from the Nebraska Game and Parks Commission Pallid Sturgeon Population Assessment Program was synthesized to (i) document the population structure of pallid sturgeon by origin (hatchery‐reared or wild), gender, and reproductive readiness, (ii) document the minimum size and age‐at‐maturity by gender, and (iii) document the fecundity rates of the fish that were successfully spawned in the hatchery. During this 4‐year study (2008–2011), relative abundance for wild and hatchery‐reared pallid sturgeon collected with gill nets did not significantly change whereas relative abundance for wild fish using trot lines declined significantly. The proportion of hatchery‐reared pallid sturgeon increased annually, with the population being composed primarily of hatchery‐reared fish. The proportion of reproductively ready females to non‐reproductively ready females was 1 : 2.0, compared to male ratios at 1 : 0.9. Minimum fork length‐at‐maturity was estimated for females at 788 mm and for males at 798 mm. Minimum age‐at‐maturity for hatchery‐reared released fish was age‐9 for females and age‐7 for males. Highest relative fecundity, based on the ovosomatic index, was 10% with an overall mean of 7%. The number of eggs per ml (egg size) was not correlated with fork length (P = 0.0615) or weight (P = 0.0957). Relative condition factor (Kn) for females was significantly different by reproductive condition (P = 0.0014) and Kn for males did not differ between reproductive conditions (P = 0.2634). Detecting shifts in population characteristics are essential not only to understand population dynamics since hatchery inputs and natural perturbations continue to change the population structure but also to assess species recovery efforts to ensure long‐term species sustainability.     

Field endoscopy for identifying gender, reproductive stage and gonadal anomalies in free-ranging sturgeon (Scaphirhynchus) from the lower Mississippi River

Thirty-six sturgeon ( Scaphirhynchus spp.) were captured from the lower Mississippi River using baited trot-lines, anesthetized using tricaine methanesulfonate, and subjected to endoscopic evaluation of their coelom (coelioscopy) on the river bank. Using a 2.7 mm rod-lens telescope and gravity–fed sterile saline infusion, gender and reproductive stage were determined in the field, and later re-evaluted and confirmed by histology of endoscopic biopsies in the laboratory. Gender and reproductive stage were determined endoscopically in all fish without complication, and proved to correlate 100% with biopsy histology. Modern rod-lens telescopes and sterile saline insufflation can be used under field conditions, provide superior visualization of sturgeon gonads, and offer a high degree of accuracy even with inactive gonads. Rigid endoscopy is recommended as a minimally-invasive means of assessing the reproductive organs of sturgeon and other fish.     

Spring and summer macroinvertebrate drift in the Lower Mississippi River,Louisiana

Macroinvertebrate drift was sampled dielly each month at three stations across the lower Mississippi River, Louisiana. At least 134 taxa were identified from seven phyla. Total drift catches varied with respect to month, time of day, and station. Relative abundance was greatest in June and August, moderate in July, and lowest in April and May. Dipterans, ephemeropterans and trichopterans numerically dominated the catches and each tended to have its zenith in July, August and June respectively. Important non-insect groups were hydroids, oligochaetes, pelecypods and amphipods and with the exception of pelecypods, were most abundant in April. The presence of certain apparent exotic oligochaetes suggested that at least some elements of the drift may be derived from rather remote areas — perhaps even the drainage headwaters. Diel periodicity was most pronounced during the summer months of high total drift densities. Ephemeropterans exhibited strong, nocturnally — oriented diel periodicity, both at the ordinal and lower taxonomic levels, trichopterans and dipterans showed varying tendencies with respect to individual lower taxa. Catches for the dielly-periodic taxa indicated a preponderance of the bigeminus forms. But each major insect order may have had one lower taxon exhibiting the alternans pattern. The stations were decreasingly productive from right-shore to left-shore, while taxonomic richness was greater at midstream and approximately equal along the shores. But spatial patterns of occurrence varied according to months.     

Microbial Manganese Oxidation in the Lower Mississippi River: Methods and Evidence

Abstract

Recent work has led to the suggestion that biologically-mediated redox processes might be important in the regulation of dissolved trace element concentrations in rivers, especially with regard to manganese. Here, we focus on the removal of dissolved Mn from lower Mississippi River water. Experiments indicate that dissolved Mn can be rapidly removed from lower Mississippi River water on a timescale of days or less and that Mn oxides are formed. However, demonstrating a biological origin for this removal is problematic. Experiments reveal that commonly used microbial controls, including NaN₃ mixtures, HgCl₂, heat sterilization, and sonification all affect fluvial particulate Mn through dissolution, disaggregation, interference with adsorption, or particle ageing. Thus, these microbial controls may affect abiotic as well as biological processes. Evidence supporting microbial removal of dissolved Mn from lower Mississippi River water includes a temperature optimum for the process (～30°C), a lower activation energy than reported for heterogeneous inorganic Mn oxidation, and a faster rate than reported for autocatalytic inorganic Mn oxidation. This rapid Mn oxidation process occurs at essentially the same rate in the dark as well as the light. Observation of Mn removal at similar rates in a blackwater river in addition to the lower Mississippi, suggests that this is a common phenomenon in fluvial systems. If, as has been shown by other lab studies, the freshly biologically precipitated Mn oxides have a high specific surface area, then our observations provide a potential link between microbial activity, Mn cycling, and the cycling of other particle-reactive trace elements in rivers. Our results also indicate that unfiltered river water samples for dissolved Mn analysis should be filtered as soon as possible or at least stored cold if immediate filtration is not possible.     

Effects of river stage height and water temperature on diet composition of year‐0 sturgeon (Scaphirhynchus spp.): a multi‐year study

The diet composition was quantified of year‐0 Scaphirhynchus sturgeons during sampling years (2004–2008) from the Middle Mississippi River to evaluate trends in diet. River stage height varied among years and seasons. Water temperature differed among seasons but did not differ among years. The presence of empty stomachs was evident across years, seasons, and size classes of sturgeon. The overall Multivariate Analysis of Variance for frequency of occurrence was significant, suggesting that macroinvertebrate taxa (e.g. Ephemeroptera, Diptera pupae, Chironomidae, ‘Other’) differed among size classes of sturgeon, but not seasons or years. Furthermore, Tukey's pairwise comparisons indicated Ephemeroptera frequency of occurrence differed between medium and large size classes; Diptera pupae differed between small and medium size classes and small and large size classes; Chironomidae differed between small and large size classes and medium and large size classes; the macroinvertebrate taxa category ‘Other’ differed between small and large size classes. Ultimately, because of the link between diet and growth of year‐0 Scaphirhynchus sturgeon, the early life history may affect eventual recruitment to adult stages.     

Larvae provide first evidence of successful reproduction by pallid sturgeon, Scaphirhynchus albus, in the Mississippi River

The pallid sturgeon (Scaphirhynchus albus) was not described until 1905, when it was commonly caught by commercial fishers. This species began to decline in the early 1900s presumably because of overharvest and habitat degradation. The U.S. Fish and Wildlife Service listed S. albus as an endangered species in 1990. Because S. albus live in deep, turbid rivers that are difficult to sample, very little is known about its reproductive timing and spawning habitat. The act of spawning has never been observed in nature. Captures of wild young S. albus verifying natural reproduction are rare, the last being a 4‐year‐old fish taken in 1978. In this paper, we describe the first collection of a larval S. albus from the wild and subsequent larval collections in the Mississippi River from 1998 to 2000 using a modified slingshot balloon trawl (the Missouri Trawl) designed to capture small fishes in deep, turbulent rivers. We captured larval Scaphirhynchus spp., including verified S. albus, in association with island habitats often in heavy detritus, especially at downstream tips. We postulate that Scaphirhynchus spp. spawned at the heads of islands upstream from where we collected larvae, but we cannot be certain. The capture of larval S. albus verifies reproduction possibly from the lower Missouri River to the upper and lower Mississippi River. However, we found no evidence of recruitment of S. albus from 1998 to 2000 as we were unable to capture juveniles after 374 trawl hauls that captured over 21 735 fish.     

Potential responses of the Lower Missouri River Shovelnose Sturgeon (Scaphirhynchus platorynchus) population to a commercial fishing ban

We developed an age‐structured population matrix model to perform population viability analysis for Lower Missouri River (LMR) shovelnose sturgeon (Scaphirhynchus platorynchus). We investigated potential effects of the commercial fishing moratorium put in place to help protect the similar‐appearing pallid sturgeon (S. albus). The model applies different components of total variance in life history parameters at different levels: sampling variance (parameter uncertainty) between model iterations; temporal variance (temporal environmental fluctuations) between time steps within iterations; and individual variance (individual differences) within each time‐step. The model predicted annual rates of population increase of 1.96% under historic fishing and 2.67% with removal of historic fishing. We identified combinations of fishing and harvest size restrictions that would permit a sustainable harvest of shovelnose sturgeon. Overall, the ban on commercial fishing of shovelnose sturgeon in the LMR due to similarity of appearance to pallid sturgeon should help the LMR shovelnose sturgeon population begin to rebound while decreasing any negative effects it may have had on pallid sturgeon populations.     

Modelling bioenergetic and population-level impacts of invasive bigheaded carps (Hypophthalmichthys spp.) on native paddlefish (Polyodon spathula) in backwaters of the lower Mississippi River

1. While invasions of large rivers by exotic fish species are well documented, assessing actual or potential impacts on native species is a challenge. Rapid assessments may be possible through the application of a combination of bioenergetic and population dynamic models.
2. Paddlefish (Polyodon spathula) is a native species in the central USA with a history of population decline due to waterway development and overharvesting for roe. It is not known whether paddlefish are impacted by resource competition from invasive bigheaded carp populations, including silver (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis), which have expanded dramatically in the Mississippi River.
3. We used bioenergetic models to project the potential impact of invasive silver and bighead carp on zooplankton density and paddlefish somatic growth in backwater habitat. Bioenergetic outputs were translated to impacts on fecundity, becoming inputs for 50-year metapopulation simulations of backwater habitat connected to the main-stem Mississippi River by episodic flood events.
4. Competition with carp reduced growth and increased the risk of population decline for paddlefish. Impacts increased disproportionately with increased carp abundance and were further exacerbated in scenarios with increased diet overlap or decreased zooplankton abundance.
5. We also analysed paddlefish condition data collected at sites near the lower Mississippi River with varying histories of carp invasion. These data give credence to the bioenergetic model output; paddlefish had reduced body condition at sites with long-established, high-density carp populations.
6. We conclude that invasive bigheaded carps have great potential to reduce paddlefish growth, fecundity, and abundance. The pairing of bioenergetics and population models is likely to be broadly useful in assessing the risks posed by other invasive species.

     

Gonadal development and sex-specific demographics of the shovelnose sturgeon in the Middle Mississippi River

Harvest of the shovelnose sturgeon, Scaphirhynchus platorynchus for caviar has increased. To determine whether populations can withstand increased harvest, detailed information regarding sexual demographics is needed. We describe gender and reproductive development of 306 shovelnose sturgeon from the Middle Mississippi River (River km 0–322) during September 2001 through December 2003. Using dissection and histology, we identified three of the four gonadal stages described previously for male lake sturgeon and all seven stages for females. Males reached maturity at a smaller size than did females. Gonads can be rapidly inspected for sex and stage of development for the shovelnose sturgeon. The sex ratio was not different from 1 : 1. Seven intersexual fish occurred. Female fecundity was positively related to body weight (number of eggs = 30.24 × body weight − 8392; P = 0.013; r² = 0.45) and weakly related to fork length (number of eggs = 146.37 × fork length − 66 176, P = 0.053, r² = 0.23).     

Roost-site selection among flat-haded bats (Tylonycteris spp.)

During six years, 1962–68, the biology of the bats Tylonycteris pachypus and T. robustula has been studied at the University of malaya Field Studies Centre, Ulu Gombak, Selangor, Malaysia. Both species were often found roosting within internodes of the bamboo Gigantochloa scortechinii , entering and leaving through narrow vertical slits caused by the beetle Lasiochila goryi . In this paper the roost sites are described and evidence is presented that each species of bat actively selected roosts of a different range of dimensions. The ecological implications of the separation of the species by this factor are discussed.     

Allozyme variation and evolutionary relationships of grain amaranths (Amaranthus spp.)

Summary Allozyme studies in amaranth provided useful assays of genetic variation in order to verify the patterns inferred from morphological traits, for elucidating the genetic structure of landraces, and for the studies of evolutionary relationships among wild, weedy and crop species. Thirty-four populations of cultivated New World amaranths were surveyed along with 21 weedy New World populations for allozyme variation at nine electrophoretic enzyme loci. Eleven populations of cultivated amaranths from the Indian State of Uttar Pradesh and six from Nepal were also surveyed for a comparison. In the New World populations, heterozygosity was low, and different populations ranged from 0 to 44% polymorphic loci. Adjacent populations were often fixed for different alleles or had very different allele frequencies at certain loci, with no apparent geographical patterns. Diversity index H was partitioned into the intra- and interpopulation as well as the interspecific components of variability. The crop versus weed genetic distances were the largest, whereas the intra- and interpopulation components of H were about equal. Genetic structure of all three species of the New World amaranths together can be described as a collection of distinct populations, each more or less a heterogeneous collection of highly homozygous individuals. The North Indian populations showed relatively less allozyme variability with the most common alleles same as those of Mexican landraces. Alleles at several loci proved to be diagnostic of the crop and weed groups, and of the three individual crop species. Genetic distances based on pooled gene frequencies showed the three crop species to be generally more closely related inter se than they were to their putative weedy progenitor species, respectively (with the exception of the weed-crop pair A. quitensis and A. caudatus). This implies a single domestication event involving A. hybridus as the common ancestor rather than three separate domestication events. Close similarity between A. caudatus and A. quitensis might have resulted from transdomestication based on a weedy or semi-domesticated species having migrated from Meso-America to South America. This preliminary report must now be expanded by further ecogeographical, cytogenetic and population studies on new extensive collections from the areas of early domestication. Some evidence of recent introgression and/or segregation of crop-weed hybrids between A. caudatus and A. retroflexus is available in the form of rare individuals in crop populations with crop allozyme genotypes except for a single homozygous weedy allele.     

The impact of cropland cover on river nutrient levels in the Mississippi River Basin

Aim This study developed high‐resolution datasets of the area of maize, soybean and wheat cultivation in the United States for 1950, 1970 and 1992. The datasets were used to examine the relationship between individual crop cover and nutrient levels in rivers across the Mississippi River Basin. Location This study was conducted in the United States, with focus on the Mississippi River Basin. Methods County and state‐level estimates of the planted area of maize, soybean and wheat were integrated with a satellite‐derived pattern in total cropland to develop 5′ × 5′ resolution datasets of the fractional area of maize, soybean and wheat in the United States for 1950, 1970 and 1992. Regression analysis was used to determine the relationships between mean levels of nitrogen, phosphorous and silica in 25 rivers across the Mississippi Basin, and the extent of maize, soybean and wheat cultivation in the watersheds. Results The crop datasets provide a spatially explicit representation of the area of the three primary crops in 1950, 1970 and 1992. The datasets clearly depict the rapid expansion of soybean cultivation, the increased specialization of individual agricultural regions, and the accompanying reduction in agricultural biodiversity since 1950. The statistical analysis identified strong correlations between the extent of maize and soybean cover in a watershed and the level of nitrogen (N) and phosphorous (P) at the watershed outlet. In particular, the area of maize appears to be strongly related to the concentration of total‐N and the ratio of dissolved inorganic‐N and organic‐N at the watershed outlet. Conclusions The intensification of cultivation of the three primary crops — maize, soybean and wheat — has altered land cover and water quality across the U.S. since 1950. The high present‐day rate of fertilizer use on maize and the expansion of soybean cultivation could be largely responsible for the change in nutrient ratios in the Mississippi River since 1950.     

Bacterial production in the Lower Mississippi River: importance of suspended sediment and phytoplankton biomass

The roles and metabolic activity of heterotrophic bacteria, along with factors controlling their activity, are poorly known for large, turbid rivers. The aim of this study was to evaluate temporal patterns in heterotrophic bacterial production (BP) in the main channel of the Lower Mississippi River (LMR) in relation to several seasonally dynamic environmental factors. We hypothesized that whole-water BP would vary with levels of temperature, as well as phytoplankton biomass and suspended sediment concentration. Further, we hypothesized that bacteria attached to suspended sediment would comprise an important component of whole-water BP, their importance varying with sediment concentration. Measurements were made at three locations on the LMR for up to 29 months. Bacterial production in whole-water ranged over an order of magnitude between summer and winter, with little variation among sites. Peaks in whole-water BP were associated with periods of high suspended sediment concentrations in spring, and elevated phytoplankton biomass in summer. Attached BP was correlated with all the measures of sediment concentration, especially particulate phosphorus, and accounted for a large majority of water-column BP. After temperature, the only positive correlate of free-living cells was with phytoplankton biomass. Rates of BP in the LMR during summer were much higher than measurements made previously in the Mississippi River plume, similar to the Hudson River, but lower than in three large tributaries of the LMR. Determination of bacterial population dynamics is an essential step in analysis of the food web structure and biogeochemical processes of large rivers. This is the first study of heterotrophic bacterial production in the main channel of the LMR.