William M. Lewis

Toxicity of Nitrite to Fish: A Review

Abstract Nitrite, an intermediate in the oxidation of ammonium to nitrate, changes hemoglobin to methemoglobin, which does not carry oxygen; nitrite may thus cause anoxia in fish and other aquatic organisms. The published literature on nitrite toxicity to fish, which consists of about 40 papers, shows that the ratio of the 24-h LC50 (concentration lethal to half of the test organisms in 24 h) to the 96-h LC50 has a median value of 2.0 and is fairly uniform across species; toxicity tests of differing duration can therefore be standardized to a common duration. In general, chronic effects are difficult to detect at concentrations below one-fifth of the 96-h LC50. Most fish concentrate nitrite in fresh water; chloride in the external environment offsets the toxicity of nitrite by competing with nitrite for uptake through the chloride cells of the gills. The strength of the chloride effect is greatest for the least-sensitive species and smallest for the most-sensitive species. The addition of 1 mg/L chloride ...

Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data

Stable isotope data and indirect methods for estimating consumer production were used in a foodweb analysis for the Orinoco River floodplain, Venezuela. The ratio of annual production to mean annual biomass (Pa/B̄) was estimated from mass at maturity for all major categories of consumers. Field data on B̄ then were used to estimate Pa for each major category. Carbon sources for all categories of consumers were identified through δ13C analysis, and trophic shifts in δ15N were used in assigning trophic levels to consumers. The ultimate C source for both invertebrates and fish was algae (phytoplankton and periphyton), even though macrophytes and litterfall from the floodplain forest composed 98% of potentially available C. Production of invertebrate consumers (benthos, rhizofauna, and zooplankton), which occurred almost entirely through 1st-level consumption, was estimated as 14 g C m−2 y−1. Fish, which produced 11 g C m−2 y−1, showed a mean δ15N trophic shift of 1.8 units above the algal C source, suggesting that ∼20% of fish production was directly accountable to consumption of algal C (1st-level consumption), and that most of the remainder was attributable to 1st-level carnivory. Data on production and trophic shifts implied trophic efficiency of 5% for invertebrates and 20% for fish. Although the food web is focused on a very small fraction of potentially available primary C (algae), this C source can account quantitatively for the observed production of both invertebrates and fish. The food web showed marked trophic compression (large consumers supported by trophic levels 1 and 2), which is the only means by which high production of large consumers can be sustained on a small fraction of the potentially available C.

Ecological Determinism on the Orinoco Floodplain

ll oo dplains of l a r ge rivers are among the most produ ctive of eco s ys tem s ; they are also among the f i rst to be altered by econ omic devel opm ent and population growth.Floodplains are vulnerable not only to direct agents of change, such as agriculture and settlement, but also to changes in the hydrology or quality of water in the rivers to wh i ch they are con n ected . Thu s , even floodplains not yet clear ed or diked may show drastic ecological change caused by shifts in flo oding regime, sediment transport, or n utrient budgets as a result of human activities upstream. Because the floodplains of large rivers are vulnerable to a diverse array of anthropogenic influences, their natural functions cannot be obse rved or documented in an y region of dense population or where natural resources and a gri c u l tu ral po ten tial have been ex p l oi ted ex ten s ively. Nearly all of the major floodplain rivers in the Northern Hemisphere,and many in the Southern Hemisphere,have been le veed or impounded f or na vigation, agriculture, power g eneration, or flo od c ontrol (P ower et al. 1995, Vitousek et al.1997).Only large rivers of the far north and in some parts of the tropics now offer a view of large floodplains as they existed b efore human intervention (Cushing and Cummings 1995). Information on the ecological characteristics o f these few floodplains is the only practical means of understanding the natural functioning of floodplains, and is p robably the most useful basis for planning the restoration or protection of floodplains. All large floodplains give the impression of great ecological complexity. The combination of constant physical change, which is driven by the rise and fall of the river, spatial heterogeneity of vegetation reflecting varied depth and duration of inundation, and longitudinal gradients of f l oodplains proj ects intract a ble mu l ti d i m en s i on a l i ty. Closer inspection shows, however, a sur prising degree o f order and repetition in the physical and chemical environment, which in turn suggests the possibility of order and predictability in the biotic components of the system. The most feasible candidate for a test of this idea is a large floodplain whose physical characteristics remain essentially natural.

The role of vegetation and bed-level fluctuations in the process of channel narrowing

Abstract A catastrophic flood ire 1965 on Plum Creek, a perennial sandbed stream in the western Great Plains, removed most of the bottomland vegetation and transformed the single-thalweg stream into a wider, braided channel. Following eight years of further widening associated with minor high flows, a process of channel narrowing began in 1973; narrowing continues today. The history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5-km reach of Plum Creek near Louviers, Colorado. Sixty-three of these plants were excavated in order to determine the age and elevation of the germination point. The reconstructed record of channel change was verified from historical aerial photographs, and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach. Channel narrowing at Plum Creel: occurs in two ways. First, during periods of high flow, sand and fine gravel are delivered to the channel, temporarily raising the general bed-level. Subsequently, several years of uninterrupted low flows incise a narrower channel. Second, during years of low flow, vegetation becomes established on the subaerial part of the present channel bed. In both cases, surfaces stabilize as a result of vegetation growth and vertical accretion of sediment.

Metabolism of a tropical rainforest stream

Abstract Gradients in photosynthesis (P) and respiration (R) were measured on an unperturbed portion of the Rio Mameyes, a tropical stream in the Luquillo Experimental Forest, northeastern Puerto Rico. Rates of P, which were similar to those of streams in temperate-deciduous forests, were low in the heavily canopied headwaters (<70 g O2 m−2 y−1) and were higher (453–634 g O2 m−2 y−1) in middle and lower reaches. Periphyton biomass did not show the expected increase as the canopy opened downstream, probably because of increasing herbivory in downstream reaches. Rates of R, which were much higher than in most temperate streams, also were lower in the headwaters (767 g O2 m−2 y−1) than in the middle and lower reaches (1550–1660 g O2 m−2 y−1). High rates of R and suppressed periphyton abundance caused annual P/R to be ≪1 from headwaters to lower reaches. Results for the Rio Mameyes suggest that intense herbivory, which is favored by the presence of large herbivores and consistently high temperatures, may be more typical of tropical than temperate streams. Results also show that the tropical rainforest provides the stream with sufficient amounts of labile organic C to support high rates of respiration over long distances across the basin.

Direct and indirect effects of mine drainage on bacterial processes in mountain streams

The effects of mine drainage on 2 bacterial processes were examined in Rocky Mountain streams affected by mine drainage. Thymidine incorporation into bacterial DNA was measured as an index of bacterial production, and nitrification was examined because it is sensitive to a variety of stressors. These processes, as well as pH, concentration of dissolved Zn, and deposition rate of metal oxides, all of which are influenced by mine drainage, were measured at 37 sites. The concentrations of inorganic nutrients and dissolved organic carbon (DOC), as well as algal biomass (as chlorophyll a), also were measured at each site. The rate of incorporation of thymidine into DNA was negatively related to deposition of metal oxides and positively related to pH, algal biomass, and concentration of DOC. pH, DOC, and algal biomass together explained 59% of the variation in thymidine incorporation among sites in a multiple regression. Metal oxide deposition probably influenced thymidine incorporation through suppression of algal biomass and through sorption of a fraction of the DOC. The nitrification potential for microbes on gravel substratum removed from streams was estimated as the rate of NO3− production following addition of NH4+. Nitrification was undetectable at pH < 5.3 or Zn > 2 mg/L. Low pH (<6, as a categorical variable), concentration of Zn (which had a negative effect), and algal biomass (which had a positive effect) explained 77% of the variation in nitrification potential. The results suggest that bacterial processes vary in their sensitivity to the stressors from mine drainage, and that they can be disrupted both directly and indirectly. Nitrification was sensitive to direct influences of acid mine drainage, namely acidity and dissolved Zn. Low pH also affected thymidine incorporation. In contrast, acid mine drainage indirectly affected both thymidine incorporation and nitrification potential through its influence on algal biomass, which declined in response to deposition of metal oxides.

Ecological energetics of Chaoborus in a tropical lake

SummaryEcological energetics of Chaoborus brasiliensis from Lake Valencia, Venezuela, were studied between February 1979 and February 1980. Direct measurements were made of the respiration rate, assimilation efficiency, and growth rate of all 4 larval instars and of the pupae. For the larval stages, respiration increased as the 0.67 power of body mass. Respiration rates of the larvae, when corrected for body size and temperature, were extraordinarily low by comparison with the rates for most aquatic insects. The respiration rates of pupae were 3 times as high as those of larvae the same size. Assimilation rates increased significantly with body size for the larvae and differed slightly but significantly among food types. Assimilation efficiencies fell within the expected range for carnivores. The growth efficiencies were exceptionally high for instars II-IV by comparison with other small aquatic organisms. High growth efficiency for Chaoborus brasiliensis, and possibly for Chaoborus generally, is explained by a very low maintenance cost and may be a significant explanation for the wide distribution and high degree of ecological success in this primary carnivore of plankton communities.

Metabolic responses to temperature change in a tropical freshwater copepod (Mesocyclops brasilianus) and their adaptive significance

SummaryMetabolic rates of Mesocyclops brasilianus from Lake Valencia, Venezuela, were determined at several temperatures spanning the environmental range (22–28° C). The QO2s (oxygen consumption per unit weight) of all Mesocyclops stages from Lake Valencia are higher than most but not all QO2s from temperate copepod species that have been studied. The QO2 is essentially static through naupliar development and shows a sudden jump between N6 and CI, which probably results from the major change in morphology and behavior at this point in the life history. QO2 declines steadily between CI and adult stages. Acclimated copepodite and adult Mesocyclops show a decreasing metabolic rate with increasing temperature (i.e. Q10< 1.0) over the temperature range 26–28° C. This is the range of temperatures normally encountered during the daily vertical migration when the lake is thermally stratified (April–November). Since vertical migration would result in a compromise between a fully acclimated and an acute response, a nearly constant metabolic rate or a slight decline in metabolic rate in the warmer water would be expected in field populations. The results thus show that the metabolic rate of Mesocyclops is not reduced when it moves into deeper (cooler) water, as would be predicted by certain energy-based hypotheses that have been used to explain vertical migration. In contrast to the low Q10s between 26 and 28° C, copepodites and adults have very high Q10 values in the range 22–26° C. This indicates an adaptive decrease in metabolic rate which is thermally programmed to coincide with the cooler temperatures that are encountered during the mixing season (December–March), when a drastic change in ecological conditions occurs in the lake.Nauplii show evidence of the same seasonal response but without the superimposed plateau at high temperatures, which they would not need because they are weak migrators. Nauplii show a plateau at the lowest temperatures, however, which suggests that a fixed metabolic reduction occurs at the onset of mixing and metabolism is not altered thereafter with declining temperature.The change in QO2 with temperature generally supports the hypothesis that all Mesocyclops stages are adapted to hold a high, constant metabolic rate through the diel cycle but experience a seasonal reduction in metabolic rate in response to major ecological changes in the lake at the time of seasonal mixing.

Effect of the May–June Mount St. Helens eruptions on precipitation chemistry in central Colorado

Abstract Bulk precipitation chemistry for the interval 17 May to 12 June 1980, when the Mount St. Helens volcano was intermittently erupting, was compared with similar data over the same interval in previous years at a site in central Colorado. Although phosphorus, chloride and paniculate loading values were higher than in previous years, no significant differences exist for these or other variables, indicating that the volcanic effect on chemical loading from the atmosphere at this site was within the ordinary range of variability for bulk precipitation chemistry.

Nutrient Enrichment Studies

The purpose of a nutrient enrichment study is to determine the actual or incipient limitation of phytoplankton by specific nutrients. The literature on nutrient enrichment contains a wide variety of experimental designs and methods for such studies. This variation in methodology is in part explained by differences in the exact purpose of the studies. In general, nutrient enrichment studies can involve enrichment of standard algal cultures, or enrichment of natural assemblages over a few hours, a few days, or many days.