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.     

Tracing energy flow in stream food webs using stable isotopes of hydrogen

1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation. 2. Recent studies suggest that stable hydrogen isotopes (δD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in δD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in δD in stream food webs in a forested catchment where variation in δ¹³C has been described previously. 3. Although algal δD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67‰) from terrestrial carbon sources. Environmental conditions known to affect algal δ¹³C, such as water velocity and stream productivity did not greatly influence algal δD, and there was no evidence of seasonal variation. In contrast, algal δ¹³C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial δ¹³C in all streams with catchment areas larger than 10 km². 4. While knowledge of isotopic exchange with water and trophic fractionation of δD for aquatic consumers is limited, consistent source separation in streams suggests that δD may provide a complementary food web tracer to δ¹³C in aquatic food webs. Lack of significant seasonal or spatial variation in δD is a distinct advantage over δ¹³C for applications in many aquatic ecosystems.     

Diel,size-differential drift patterns of three macroinvertebrate species in the lower Mississippi River,Louisiana (USA)

We explored macroinvertebrate size-differential drift in the lower Mississippi River (a 9th order system). Because this river system is highly turbid, we hypothesized that visually-dependent vertebrate predators feeding on drifting organisms would be at a disadvantage. Thus, size-differential drift should not occur. For one 24-hour period in both January and April, six drift nets were used to sample surface drift. Nets were emptied once every four hours. Individual intra-ocular distances of three macroinvertebrate species (Hydropsyche orris: Trichoptera, Hexagenia limbata: Ephemeroptera, Macrobrachium ohione: Crustacea) were measured. Percentages of size classes in the drift were determined. In both months, large individuals of H. orris and H. limbata were prevalent in the nocturnal but scarce in the diurnal drift. In January, large M. ohione drifted regardless of time. In April, large M. ohione predominated the nocturnal drift. Our results could not be attributed solely to vertebrate predator avoidance. Other mechanisms such as diel microhabitat migration and current velocity may have accounted for the results.     

Seasonal succession of the phytoplankton in the upper Mississippi River

Species composition and seasonal succession of the phytoplankton were investigated on the upper Mississippi River at Prairie Island, Minnesota, U.S.A. Both the numbers and volume of individual species were enumerated based on cell counts with an inverted microscope. A succession similar to algal succession in the local lakes occurred. The diatoms were dominant during the spring and fall and blue-green algae were dominant during the summer. The algal concentrations have increased up to 40 fold the concentrations of the 1920's, since the installation of locks and dams. The maximum freshweight standing crop was 4 mg · l^–1 in 1928 (Reinhard 1931), 13 mg · l^–1 in 1975 a wet year, and 47 mg · l^–1 in 1976, a relatively dry year with minimal current discharge. The diatoms varied from 36–99%, the blue-green algae from 0–44% and the cryptómonads from 0–50% of the total standing crop. The green algae were always present but never above 21% of the biomass. The dominant diatoms in recent years were centric -Stephanodiscus andCyclotella spp. (maximum 50,000 ml^–1). The dominant blue-green algae wereAphanizomenon flos-aquae (L.) Ralfsex Born.et Flahault andOscillatoria agardhii Gomont (maximum 800 ml^–1). These algal species are also present in local lakes. Shannon diversity values indicated greatest diversity of algae during the summer months.     

高寒草旬消费者种群稳定碳、氮同位素组成的海拔分异

通过测定青藏高原东部高寒区不同海拔主要植物和消费者种群（雀形目鸟类和小型哺乳类）的稳定碳、氮同位素比值,研究了高寒草甸消费者种群同位素组成特征及其影响因素。结果表明：植物平均稳定碳同位素随海拔升高表现出明显的增加趋势;消费者种群稳定碳、氮同位素随海拔的升高有明显的富集效应。雀形目鸟类种群稳定碳、氮同位素随海拔的升高均呈明显增加趋势;小型哺乳类稳定碳同位素随海拔上升不明显,而稳定氮同位素具有明显的增大趋势。消费者种群稳定碳、氮同位素沿海拔梯度的富集效应,一方面与植物稳定同位素在海拔梯度上的富集密切相关;另一方面与海拔和纬度变化所引起一系列环境因子的变化在一定程度上影响到动物的稳定同位素分布模式。另外,在高寒草甸地区,与雀形目鸟类相比,小型哺乳类在动物稳定同位素组成的代谢过程中更容易受到环境改变的影响。     

Biochemical tracers reveal intra-specific differences in the food webs utilized by individual seabirds

Food web structure regulates the pathways and flow rates of energy, nutrients, and contaminants to top predators. Ecologically and physiologically meaningful biochemical tracers provide a means to characterize and quantify these transfers within food webs. In this study, changes in the ratios of stable N isotopes (e.g., δ¹⁵N), fatty acids (FA), and persistent contaminants were used to trace food web pathways utilized by herring gulls (Larus argentatus) breeding along the shores of the St Lawrence River, Canada. Egg δ¹⁵N values varied significantly among years and were used as an indicator of gull trophic position. Temporal trends in egg δ¹⁵N values were related to egg FA profiles. In years when egg δ¹⁵N values were greater, egg FA patterns reflected the consumption of more aquatic prey. Egg δ¹⁵N values were also correlated with annual estimates of prey fish abundance. These results indicated that temporal changes in aquatic prey availability were reflected in the gull diet (as inferred from ecological tracer profiles in gull eggs). Analysis of individual eggs within years confirmed that birds consuming more aquatic prey occupied higher trophic positions. Furthermore, increases in trophic position were associated with increased concentrations of most persistent organic contaminants in eggs. However, levels of highly brominated polybrominated diphenyl ether congeners, e.g, 2,2′,3,3′,4,4′,5,5′,6,6′-decabromoDE (BDE-209), showed a negative relationship with trophic position. These contrasting findings reflected differences among contaminant groups/homologs in terms of their predominant routes of transfer, i.e., aquatic versus terrestrial food webs. High trophic level omnivores, e.g., herring gulls, are common in food webs. By characterizing ecological tracer profiles in such species we can better understand spatial, temporal, and individual differences in pathways of contaminant, energy, and nutrient flow.     

长江上游弥陀漫滩水体鱼类食物网动态的季节性变化

长江上游是我国鱼类生物多样性最为丰富的地区之一,漫滩作为河流生态系统的重要组成部分对维持区域鱼类生物多样性具有重要作用。于2019年丰水期(8月份)和枯水期间(11月份)应用碳、氮稳定同位素技术并结合Bayesian混合模型和SIBER分析方法,对长江上游弥陀漫滩水体鱼类食物网结构动态的季节性变化特征进行了研究。结果显示：丰水期基础碳源的δ¹³C、δ¹⁵N平均值分别为-23.02‰和2.58‰,范围分别为-31.01‰—-11.2‰(δ¹³C)和-0.51‰—6.84‰(δ¹⁵N);枯水期其δ¹³C、δ¹⁵N平均值分别为-21.93‰、7.22‰,范围分别为-26.31‰—-15.36‰(δ¹³C)和4.89‰—8.81‰(δ¹⁵N)。丰水期鱼类食物网能量主要依赖于外源性营养物质,食物链长度达到3.6级;枯水期内源性营养物质是食物网能量的主要贡献者,食物链长度为2.6级。相对于枯水期,丰水期间鱼类群落拥有更大的生...     

Flow intermittency influences the trophic base,but not the overall diversity of alpine stream food webs

Alpine streams can exhibit naturally high levels of flow intermittency. However, how flow intermittency in alpine streams affects ecosystem functions such as food web trophic structure is virtually unknown. Here, we characterized the trophic diversity of aquatic food webs in 28 headwater streams of the Val Roseg, a glacierized alpine catchment. We compared stable isotope (δ¹³C and δ¹⁵N) trophic indices to high temporal resolution data on flow intermittency. Overall trophic diversity, food chain length and diversity of basal resource use did not differ to a large extent across streams. In contrast, gradient and mixing model analysis indicated that primary consumers assimilated proportionally more periphyton and less allochthonous organic matter in more intermittent streams. Higher coarse particulate organic matter (CPOM) C:N ratios were an additional driver of changes in macroinvertebrate diets. These results indicate that the trophic base of stream food webs shifts away from terrestrial organic matter to autochthonous organic matter as flow intermittency increases, most likely due to reduced CPOM conditioning in dry streams. This study highlights the significant, yet gradual shifts in ecosystem function that occur as streamflow becomes more intermittent in alpine streams. As alpine streams become more intermittent, identifying which functional changes occur via gradual as opposed to threshold responses is likely to be vitally important to their management and conservation.     

The 1994 experimental opening of the Bonnet Carre Spillway to divert Mississippi River water into Lake Pontchartrain, Louisiana

A diversion of Mississippi River water into Lake Pontchartrain, Louisiana, USA by way of the Bonnet Carre Spillway has been proposed as a restoration technique to help offset regional wetland loss. An experimental diversion of Mississippi River water into Lake Pontchartrain was carried out in April 1994 to monitor the fate of nutrients and sediments in the spillway and Lake Pontchartrain. Approximately 6.4×10⁸ m³ of Mississippi River water was diverted into Lake Pontchartrain over 42 days. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82–83%, nitrite+nitrate (NO_x) of 28–42%, in total nitrogen (TN) of 26–30%, and in total phosphorus (TP) of 50–59%. 3.9±1.1 cm of accretion was measured in the spillway. Nutrient concentrations at the freshwater plume edge in Lake Pontchartrain compared to the Mississippi River were lower for NO_x (44–81%), TN (37–57%), and TP (40–70%), and generally higher for organic nitrogen (−7–57%). The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge. Nutrient stoichiometric ratios indicate water at the plume edge was not silicate limited, suggesting conditions favoring diatomic phytoplankton.     

An inventory of 13C abundances in coastal wetlands of Louisiana,USA: vegetation and sediments

Summary Organic carbon-rich sediments from the surface of fresh, intermediate, brackish and salt marshes of coastal Louisiana were sampled and analyzed for their ¹³C content. The average ¹³C from all sites within each wetland type was-27.8,-22.1,-16.9, and-16.2, for fresh, intermediate, brackish and salt marshes, respectively. Means from the fresh, intermediate and brackish marshes were significantly different at the 0.01 level. A mixing model using measurements of standing crop and ¹³C of plant carbon was applied to estimate the contribution of each species to the sedimentary carbon at four of the marsh sites. Sedimentary ¹³C values generally reflected that of the dominant species present at each site. Brackish and salt marsh samples, however, showed a negative shift of ¹³C with respect to whole plant carbon. We interpret these depeleted ¹³C values to be the result of more extensive organic matter decomposition and selective preservation of ¹³C-depleted refractory components in sediments from saline sites. The results of this study suggest that ¹³C composition of sedimentary carbon may offer a valuable tool for distinguishing subtle changes in paleohydrology of wetlands resulting from relative sea level changes.     

Autotrophic energy sources for Paracheirodon axelrodi (Osteichthyes, Characidae) in the middle Negro River, Central Amazon, Brazil

The cardinal tetra (Paracheirodon axelrodi) is the most abundant species of the Brazilian ornamental fish trade, constituting more than 80% of the ornamental fish collected in the middle Negro River basin. Stable isotope analyses were used to identify the autotrophic carbon sources and trophic position for the cardinal in relation to the plant groups at the base of its foodchain. Filamentous algae, tree and plant leaves and cardinals were collected in stream habitats, flooded forest and interfluvial swamps (campos) during peak flood, falling water and low water periods. δ¹⁵N values of the cardinal in relation to the plants at the base of the food chain indicated a trophic position of omnivore. Values of δ¹³C for the plants ranged from −43.1 to −26.4‰, with averages of −37.6, −30.4, and −29.4‰ for filamentous algae, flooded forest leaves, and campo leaves, respectively. The δ¹³C values for the cardinal ranged from −35.0 to −27.9‰, with an average of −31.4‰. Relative contributions of plants to fish carbon were estimated in a two end-member mixing model which determined that the leaves (flooded forest and campo leaves combined) and filamentous algae had average relative contributions to cardinal carbon of 71% and 29%, respectively. However, seasonal variation in the relative contributions was encountered throughout the hydrological cycle. The cardinals least enriched in ¹³C were encountered in November during the falling water period, indicating that they had perhaps recently migrated down from interfluvial campos where filamentous algae production is significant. Considering that algal production has been reported to be less than 1% of total primary production in the Negro River, these results could suggest some evidence of selective herbivory in the cardinal’s food chain. Handling editor: J. M. Melack     

Allochthonous and autochthonous carbon flows in food webs of tropical forest streams

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Trophic relationships of juvenile blue crabs (Callinectes sapidus) in estuarine habitats

Salt marshes and shallow-water macroalgal beds are known to provide nursery habitat for many species of fish and invertebrates. The role of these habitats as refuge from predation is well established, but the degree to which indigenous primary production within the nursery provides food for growth and development of estuarine species remains unresolved. In this study, we tested the hypothesis that juvenile blue crabs depend on indigenous primary production, directly or indirectly, during their entire stay within the nursery. To test this hypothesis, we conducted isotopic studies and stomach content analyses of juveniles from habitats near the mouth of Delaware Bay and from an adjacent lagoonal estuary (ca. 39.5° N, 75.1° W). Primary producers, marsh detritus, various life-history stages of blue crabs and potential prey species were sampled in the main estuary and in an adjacent marsh during the summer and early fall of two consecutive years. Newly settled juveniles (<15 mm carapace width) from the marsh were about 1.8‰ lighter in carbon (−17.2‰) relative to larger juveniles from the marsh (15–30 mm carapace width) and appeared to have retained a carbon isotopic signature indicative of the phytoplankton-based food web associated with larval stages. However, the signature of juveniles changed as a function of size. Large juveniles and crabs >60 mm were enriched in δ¹³C (−14.7 ± 0.1‰) compared to small crabs, suggesting a gradual shift in diet from a planktonic to a detritus-based food web with increasing size. As with crabs from Delaware Bay, the δ¹³C signature of juvenile crabs sampled from macroalgal beds in the lagoonal estuary (Rehoboth Bay) changed as a function of size. Also, δ¹³C ratios of crabs varied among the various species of macroalgae. The δ¹⁵N composition of primary producers in the marsh and main estuary also was reflected in the δ¹⁵N values of crabs and other benthic consumers in the respective habitats. Results of stomach-content analysis in this study were consistent with isotope data. Observed changes in prey preferences were related to changes in size of juvenile crabs and also differed among habitats. Gut content analyses of the three size classes of juveniles in macroalgal beds from Rehoboth Bay indicated that the crabs depend heavily on various amphipod species that occur on the seaweeds. These amphipods graze directly on the macroalgae and are among the most abundant invertebrates in the macroalgal beds. This implies a direct trophic relationship between the juvenile crabs and the macroalgae. In summary, our study provides strong evidence that the value of nursery areas such as salt marshes and macroalgal beds goes beyond that of providing refuge from predation, and that species using these nurseries (e.g. juvenile blue crabs) are ultimately dependent on primary production originating in benthic plants indigenous to the nursery.     

Relative abundance of macroinvertebrates found in habitats associated with backwater area confluences in Pool 13 of the Upper Mississippi River

Ephemeroptera showed the greatest diversity and Diptera (due mainly to Chaoboridae) the greatest absolute abundance among the macroinvertebrate taxa collected in and near three backwater areas of the Upper Mississippi River from 24 April to 24 August 1983. Of the 32 taxa identified, all were insect larvae, pupae, or adults except for one amphipod and one copepod. Three peaks in total abundance were observed — a major one in late July and August and smaller ones in May and June. These peaks corresponded to periods of high temperatures and low flows. When the macroinvertebrates were divided into groups, depending on whether they were found predominantly in the backwater areas, in the main channel, or in a combination of the two, total densities were greater in the backwater areas than in either of the main-channel habitats. In the two main-channel habitats, densities were slightly greater downstream than upstream from the confluences of the backwaters with the main channel, possibly indicating that macroinvertebrates were drifting out of the backwater areas or that nutrients and zooplankton were drifting out of the backwater areas, increasing productivity in downstream areas.     

Backwater areas as nursery habitats for fishes in pool 13 of the Upper Mississippi River

Samples of larval and juvenile fishes were collected at two depths weekly during spring and summer 1983 near the mouths of backwater areas in Pool 13 of the Upper Mississippi River. The study was conducted to determine the relative value of these habitats as nursery areas for fishes present and to note any interactions that might occur between the backwaters which are being rapidly lost to siltation, and the main channel. The larvae and juveniles collected represented 13 families divided into 27 lower taxa. Cyprinidae, Clupeidae, and Sciaenidae made up 90% of the total catch. Both larvae and juveniles were more abundant near the surface than near the bottom. Densities differed greatly among the three backwater areas studied. Larval fishes were grouped on the basis of their relative abundance in the backwaters or main channel. Overall, more larvae were captured in the backwaters than in main-channel habitats, indicating that backwaters were more productive. In the main channel, densities were greater downstream from the mouths of the backwaters than upstream-possibly indicating that (1) larval fish drifted out of the backwater areas, (2) water rich in nutrients or zooplankton that flowed into the main channel created productive downstream sites that were used as nursery areas, or (3) adult fishes selected downstream sites as spawning areas. Juvenile forms were more abundant in the backwater areas then in the main-channel habitats, some bottom-dwelling fishes excepted. The backwater areas were judged to be important nursery areas for larval and juvenile fishes, and seemed to benefit downstream main-channel sites. Any loss of these habitats would be detrimental to the Mississippi River as a whole.     

Phytoplankton communities in navigation pool no. 7 of the Upper Mississippi River

A study of the phytoplankton community dynamics in Navigation Pool No. 7 of the Upper Mississippi River was conducted from May through October, 1982. The objectives of this study were to estimate total standing crops, determine the taxonomic composition and examine the seasonal succession of the phytoplankton community. Four sampling sites were established: two in Lake Onalaska, a large backwater lake on the Wisconsin side of the main channel; one in the main channel near Dakota, Minnesota; and one in the main channel just upstream from Lock and Dam No. 7.The phytoplankton communities at all sampling sites were dominated by diatoms except during July and August when a bloom of blue-green algae was observed. The dominant diatoms from May through mid-July were Melosira italica, Stephanodiscus niagarae, Stephanodiscus hantzschii, Stephanodiscus astrea, and Synedra ulna. Aphanizomenon flos-aquae and Microcystis aeruginosa were the most prevalent blue-green algae during the mid-summer bloom. The diatoms Melosira italica and Melosira granulata were dominant in September and October. Lesser amounts of green algae, cryptomonads and euglenoids were also observed at various times of the sampling period.Total standing crops based on cell volume were usually greatest at the Lock and Dam No. 7 site. The maximum standing crop (10.4 mm³ 1^–1) was observed at the Lock and Dam No. 7 site on 4 September; the minimum standing crop (0.4 mm³ 1^–1) was observed at the eastern Lake Onalaska site on the same date. Concentrations of nitrogen, phosphorus, and silica remained at high levels throughout the study period and did not appear to limit phytoplankton standing crops.     

Phycoperiphyton in selected reaches of the Upper Mississippi River: community composition,architecture, and productivity

Attached algal communities were studied during ice-free periods along the borders of the main channel and in backwaters of the Upper Mississippi River. Diatoms strongly dominated the phycoperiphyton except during late June through August when the green alga Stigeoclonium was abundant. Two distinct assemblages were apparent: a diverse, complex assemblage during spring and late fall annd a less complex, adnate, two-dimensional summer assemblage dominated by Cocconeis placentula var. euglypta and Navicula tripunctata var. schizonemoides. Simultaneous studies revealed two-dimensional periphytic colonization in the main channel and more complex communities in backwaters. Greater physical turbulence (e.g., surface waves) in the main channel may have favored adnate taxa and two-dimensional architecture, whereas lesser turbulence (e.g. protected backwaters) favored the more diverse, complex community. Community architecture and species composition were similar among communities on artificial substrates within each navigation pool (median SIMI0.87), but communities on artificial substrates were not taxonomically very similar (median SIMI=0.44) to epiphytic communities on Cladophora.Accrual of algal cells, chlorophyll a, and aufwuchs ash-free dry weight was usually greater in Pool 5 than in Pool 9. This may have been due to differences in discharges and/or sampler placement. The greatest accrual of cells and chlorophyll a occurred during summer and early fall. Chironomid and tricopteran larvae were common at that time and are known to affect algal accumulation by grazing.     

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.     

Macrobenthic distribution and community structure in the upper navigation pools of the Upper Mississippi River

The northern section of the Upper Mississippi River supports a diverse macrobenthic assemblage. Distribution of this benthic fauna, benthic community structure, and factors which influences which influence both of these phenomena in these upper pools are reviewed. Dumping of heavy loads of municipal and industrial wastes from the Minneapolis-St. Paul metropolitan area has severely stressed the benthic community. Once abundant, pollution-sensitive mayflies, Hexagenia bilineata and H. limbata, are noticeably absent, replaced by pollution-tolerant oligochaetes and midges (notably Chironomus). Harmful effects of this pollution are not restricted to the area immediately downstream from the Twin Cities. In Lake Pepin, the Hexagenia population has suffered a drastic decline. The benthic community is characterized by low species diversity and a dominant, pollution-tolerant Chironomus plumosus — Oligochaeta — Sphaeriidae — Hirudinea community complex. Farther south, effects of the high organic load which originates approximately 226 km upstream are ameliorated. Inundation of large, diverse land areas contributes to the great ecological diversity in Pools No. 7 and No. 8. In Navigation Pool No. 7, benthic standing crops in the backwater pool areas (biomass range: 2.08–26.96 g m^–2) exceed those in the main channel (biomass range: 0.05–1.02 g m^–2). Greater numbers of burrowing mayflies and mollusks were found in the pool areas. Of 131 taxa collected from 1976–1977 in Lake Onalaska, which occupies most of Pool No. 7, eight dominant groups — Oligochaeta, Hirudinea, Isopoda, Amphipoda, Lepidoptera, Diptera, Gastropoda, and Pelecypoda — accounted for 90–93% of the macroinvertebrates. In Pool No. 8, over half of the 144 benthic taxa collected during the summer of 1975 were insect nymphs and larvae. Oligochaetes were by far the most ubiquitous and dominant macroinvertebrates. Habitat preferences of particular benthic forms reflected distributional relationships between macroinvertebrates and physical-chemical conditions. Benthic production, in terms of total wet weight m^–2 and macroinvertebrate density in each study area, was generally greater in the more eutrophic areas. However, fewer taxa were supported in these areas. These taxa were generally pollution-tolerant organisms, such as oligochaetes and certain chironomids, which were capable of burrowing into depositional-type substrates. More taxa and greater numbers of gill breathers and filter feeders, such as caddisflies, mayflies, stoneflies, and dipterans, were collected from less eutrophic areas.     

Distribution of early life history stages of fishes in selected pools of the Upper Mississippi River

Effective management of the fishery resources of the Upper Mississippi River and successful mitigation of the loss of critical habitat depend in part on an understanding of the reproductive and early life history requirements of the affected fishes. However, little is known about the use of nursery areas by fishes in the river. Of the nearly 130 species identified in the adult ichthyofauna, only a few are represented proportionally in the available data on early life stages because study designs have not included consideration of the early stages, collection gears have not adequately sampled the young, and eggs and larvae of some species are difficult to sample by conventional approaches. For the species collected, information is available on seasonal variations in total densities, composition, and catch among different habitat types. However, the data are most accurate for species with buoyant early life stages, such as freshwater drum (Aplodinotus grunniens) and gizzard shad (Dorosoma cepedianum). Eggs and larvae of freshwater drum dominate collections made in the main channel, whereas other larval fishes are usually most abundant in backwater habitats. The species found there usually deposit eggs on the substrate or on vegetation. Habitat preferences (as indicated by relative abundance) often shift as development proceeds and physical and behavioral changes occur in the larvae. Only limited information is available on the distribution of larvae within habitats, but it is clear that variations within habitats are significant.