Rythmes nutritionnels et organisation trophique d'une population de crustacés pélagiques (Euphausiacea)

The nature of the food (animal, plant or mixed) and the fullness of the stomachs at different times of the day have been studied through dissection of 18620 specimens representative of almost all the euphausiid species of the Central and Western Tropical Pacific Ocean. Animal food predominates in 22 of the 28 species studied, while 12 can be considered as omnivorous; only 4 are mainly phytophagous. The trophic level of a given species is more or less the same in different zones, but scarcity of phytoplankton in oligotrophic tropical regions results in an increase of the trophic level of herbivorous and omnivorous species. There is no correlation between trophic level and vertical distribution of a species. Each species follows a clearly defined feeding rhythm, usually characteristic for each genus: nutrition most active by night in Euphausia, continuous in Thysanopoda, restricted to light hours in Stylocheiron, mainly from noon to midnight in Nematoscelis and Nematobrachion. As a rule, it is obvious that the smaller the daily vertical migration, the more pronounced the feeding rhythm: the range of fluctuations in fullness of stomachs over 24 h is weak or non-existent in migrating species, maximum in non-migrating ones. Nevertheless, daily vertical migration does not appear to be “advantageous” from the point of view of trophic efficiency: assuming that the stomachal transit is the same for all species (a speculative proposition), it is shown that the ratio “total biomass of species: food consumed during 24 h”, i.e., “biomass permanently available for the upper link: biomass eaten daily at the expense of the lower link” is 4 times higher in non-migrating than in migrating species. It is thus considered that daily vertical migration is an expensive manner to transfer energy from link to link, and therefore fulfills other functions, amongst which diffusion throughout the whole water column of the biomass produced in the upper levels is probably one of the most important.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). III. Potentiel alimentaire du groupe

The position of a group of pelagic organisms among the food webs depends upon (1) its abundance in the biomass; (2) the factors allowing its utilization by predators; among these, the size distribution of animals and their bathymetric repartition appear to prevail. In the equatorial and southtropical Pacific Ocean, euphausiids represent 8% of the total macroplanktonic and micronektonic biomass, i.e., approximately 13 g/1000 m³ wet weight (somewhat less in oligotrophic tropical zones, a little more in the richer equatorial belt); Euphausia diomedae accounts for more than 50% of this biomass. Individuals measuring 9 to 18 mm in total length (4 to 37 mg wet weight) constitute 84% of the whole biomass of the group; the importance of each species in terms of size groups is discussed. Depth distribution is examined at the specific level. By night, 75% of the biomass concentrates in the 0 to 160 m water layer (ca. 10 g/1000 m³ wet weight; main species Euphausia diomedae), 22% between 160 and 300 m (3 g/1000m³ main species Nematoscelis spp.), and 3% (0.3g/1000 m³) in deeper layers. During the day, the only species abundant at depths less than 400 m belong to the genera Stylocheiron and Nematoscelis; specimens smaller than 15 mm and 20 mg remain at depths shallower than 200 m, individuals between 15 and 22 mm (20 to 65 mg) appear around 200 to 400 m, and larger animals are restricted to depths below 400 m.     

Leachate recirculation: moisture content assessment by means of a geophysical technique

Bioreactor technology is a waste treatment concept consisting in speeding up the biodegradation of landfilled waste by optimizing its moisture content through leachate recirculation. The measurement of variations in waste moisture content is critical in the design and control of bioreactors. Conventional methods such as direct physical sampling of waste reach their limits due to the interference with the waste matrix. This paper reviews geophysical measurements such as electrical direct current and electromagnetic slingram methods for measuring the electrical conductivity. Electrical conductivity is a property, which is linked to both moisture and temperature and can provide useful indications on the biodegradation environment in the waste mass. The study reviews three site experiments: a first experimentation shows the advantages (correlation between conductive anomaly and water seepage) but also the limits of geophysical interpretation; the two other sites allow the leachate recirculation to be tracked by studying the relative resistivity variation versus time from electrical 2D imaging. Even if some improvements are necessary to consider geophysical measurements as a real bioreactor monitoring tool, results are promising and could lead to the use of electrical 2D imaging in bioreactor designing.     

WATERSHED URBANIZATION AND CHANGES IN FISH COMMUNITIES IN SOUTHEASTERN WISCONSIN STREAMS1

ABSTRACT: We compared watershed land‐use and fish community data between the 1970s and 1990s in 47 small streams in southeastern Wisconsin. Our goal was to quantify effects of increasing urbanization on stream fishes in what had been a predominantly agricultural region. In the 43 test watersheds, mean surface coverage by agricultural lands decreased from 54 percent to 43 percent and urban lands increased from 24 percent to 31 percent between 1970 and 1990. Agriculture dominated the four reference watersheds, but neither agriculture (65–59 percent) nor urban (4.4–4.8 percent) land‐uses changed significantly in those watersheds during the study period. From the 1970s to the 1990s the mean number of fish species for the test stream sites decreased 15 percent, fish density decreased 41 percent, and the index of biotic integrity (IBI) score dropped 32 percent. Fish community attributes at the four reference sites did not change significantly during the same period, although density was substantially lower in the 1990s. For both the 1970s and 1990s test sites, numbers of fish species and IBI scores were positively correlated with watershed percent agricultural land coverage and negatively correlated with watershed urban land uses, as indexed by percent effective connected imperviousness. Numbers of fish species per site and IBI scores were highly variable below 10 percent imperviousness, but consistently low above 10 percent. Sites that had less than 10 percent imperviousness and fewer than 10 fish species in the 1970s suffered the greatest relative increase in imperviousness and decline in species number over the study period. Our findings are consistent with previous studies that have found strong negative effects of urban land uses on stream ecosystems and a threshold of environmental damage at about 10 percent imperviousness. We conclude that although agricultural land uses often degrade stream fish communities, agricultural land impacts are generally less severe than those from urbanization on a per‐unit‐area basis.     

Offshore–inshore and vertical distributional patterns of heteropod mollusks off leeward Oahu, Hawaii

Replicated zooplankton tows were taken during day and night periods at neritic (1 nmi offshore) and oceanic (5 and 15 nmi offshore) stations off the leeward side of Oahu, Hawaii during fall (September) and spring (April). Plankton samples were obtained with a 1-m² MOCNESS net system towed obliquely through seven depth strata to 80 m (neritic station) and eight depth strata to 200 m (oceanic stations). Five more species were collected in April (17) than in September (12). During both months the number of species decreased from the 15 to the 1 nmi stations; from 11 to 9 in September and 15 to 11 in April. Species diversity, as measured by species accumulation (rarefaction) curves, was higher at the 15 than the 1 nmi stations during both months. Total nighttime water column density in April was about double that in September. In September, the density decreased sevenfold from the 15 to 1 nmi stations, but in April densities were comparable among the three stations; mainly as a result of the shoreward increase in density of Atlanta plana. The most abundant species in both months were A. plana, A. lesueuri, A. inflata, A. peroni, and Protatlanta souleyeti, which in combination accounted for 93% (September) and 91% (April) of the total nighttime water column densities. The vertical ranges of six species were limited to the upper 100 m, and nocturnal migration was suggested for two (A. helicinoides during both months and A. turriculata in April). The ranges of the remaining 11 species extended to 160 or 200 m, and among them nocturnal migration was suggested for five (A. plana, A. meteori and Pterotrachea hippocampus during both months, and A. peroni and Protatlanta soueleyti in April).     

A process-based model designed for filling of large data gaps in tower-based measurements of net ecosystem productivity

In this paper we present a simple hybrid gap-filling model (GFM) designed with a minimum number of parameters necessary to capture the ecological processes important for filling medium-to-large gaps in Flux data. As the model is process-based, the model has potential to be used in filling large gaps exhibiting a broad range of micro-meteorological and site conditions. The GFM performance was evaluated using “Punch hole” and extrapolation experiments based on data collected in west-central New Brunswick. These experiments indicated that the GFM is able to provide acceptable results (r² > 0.80) when >500 data points are used in model parameterization. The GFM was shown to address daytime evolution of NEP reasonably well for a wide range of weather and site conditions. An analysis of residuals indicated that for the most part no obvious trends were evident; although a slight bias was detected in NEP with soil temperature. To explore the portability of the GFM across ecosystem types, a transcontinental validation was conducted using NEP and ancillary data from seven ecosystems along a north-south transect (i.e., temperature–moisture gradient) from northern Europe (Finland) to the Middle East (Israel). The GFM was shown to explain over 75% of the variability in NEP measured at most ecosystems, which strongly suggests that the GFM maybe successfully applied to forest ecosystems outside Canada.     

The effects of landscape structure on space competition and alternative stable states

Many species that compete for space live on heterogeneous landscapes and interact at local scales. The quality, amount, and structure of landscapes may have considerable impact on the ability of species to compete or coexist, yet basic models of space competition do not include that level of detail. We model space competition between two species with positive feedback through recruitment facilitation, which creates the potential for alternative stable states to occur. We compare the predictions of a spatially implicit model with a simulation model that includes explicit space and landscape structure. We create structured landscapes in which we specify the amount of habitat and degree of fragmentation and ask how landscape structure, dispersal strategy, and scale affect the presence of alternative stable states, or bistability. We find that structured landscapes can reduce the range of parameter values that lead to bistability in our model, but they do not eliminate bistability. The type of landscape and the dispersal distance for each species also influence the amount of environmental change needed for abrupt community shifts to occur. Coexistence of the two competitors is possible under certain conditions when connectivity is low. Consequently, landscape structure may lead to considerable disparity between the predictions of simple models and actual dynamics on complex landscapes during environmental change.     

Use of structured expert judgment to forecast invasions by bighead and silver carp in Lake Erie

Identifying which nonindigenous species will become invasive and forecasting the damage they will cause is difficult and presents a significant problem for natural resource management. Often, the data or resources necessary for ecological risk assessment are incomplete or absent, leaving environmental decision makers ill equipped to effectively manage valuable natural resources. Structured expert judgment (SEJ) is a mathematical and performance‐based method of eliciting, weighting, and aggregating expert judgments. In contrast to other methods of eliciting and aggregating expert judgments (where, for example, equal weights may be assigned to experts), SEJ weights each expert on the basis of his or her statistical accuracy and informativeness through performance measurement on a set of calibration variables. We used SEJ to forecast impacts of nonindigenous Asian carp (Hypophthalmichthys spp.) in Lake Erie, where it is believed not to be established. Experts quantified Asian carp biomass, production, and consumption and their impact on 4 fish species if Asian carp were to become established. According to experts, in Lake Erie Asian carp have the potential to achieve biomass levels that are similar to the sum of biomasses for several fishes that are harvested commercially or recreationally. However, the impact of Asian carp on the biomass of these fishes was estimated by experts to be small, relative to long term average biomasses, with little uncertainty. Impacts of Asian carp in tributaries and on recreational activities, water quality, or other species were not addressed. SEJ can be used to quantify key uncertainties of invasion biology and also provide a decision‐support tool when the necessary information for natural resource management and policy is not available. El Uso de Juicio Experto Estructurado para Predecir Invasiones de Carpas Asiáticas en el Lago Erie