1. Increasing blooms of Microcystis aeruginosa have unknown impacts on the copepods Eurytemora affinis and Pseudodiaptomus forbesi, which are the dominant zooplankters and key prey species for ...endangered larval fish in the upper San Francisco Estuary. 2. Laboratory feeding experiments were designed to measure the effect of Microcystis on copepod survival and to distinguish the effects of toxicity and nutrition. In a series of survival tests, copepods were fed a mixed diet of algae plus one of two strains of Microcystis, either producing (MC+) or lacking microcystin (MC-). 3. Microcystis significantly reduced survival even when it was a small proportion of the diet, indicating that toxicity was the major cause of mortality. Contrary to expectation, however, the MC+ strain did not result in higher mortality, suggesting that non-MC metabolites of Microcystis can be toxic to copepods. 4. Across treatments, survival of P. forbesi was greater than that of E. affinis, although the two copepods responded differently to both the ratio and the strain of Microcystis in their food. Survival of P. forbesi was greater on the MC+ strain and was inversely proportional to the ratio of dietary Microcystis (MC+ or MC-). In contrast, survival of E. affinis declined similarly across treatments and was not related to the proportion or strain of dietary Microcystis. Results indicate that the copepod P. forbesi can coexist with Microcystis while the other copepod E. affinis cannot. 5. Regardless of species, dietary Microcystis caused significant mortality to copepods, and it may cause adverse impacts to the potentially food-limited zooplankton community of the San Francisco Estuary. These impacts may not be related to the cellular MC concentration because Microcystis contains other metabolites that negatively affect copepods.
Plans for the restoration of aquatic ecosystems are increasingly focusing on the restoration and rehabilitation of self-sustaining native fish communities. Such efforts have not traditionally adopted ...an ecosystem-based perspective, which considers species as embedded within a broader food web context. In this study, we quantify food web changes in Lake Tahoe (California-Nevada) over the last century based on stable isotope analysis of museum-archived, preserved fish specimens collected during 4 historical periods and under present conditions. We also examine the contemporary food web of nearby Cascade Lake, which is free from most exotic species and contains a species assemblage resembling that of Lake Tahoe prior to historical species introductions. During the last century, the freshwater shrimp Mysis relicta and lake trout (Salvelinus namaycush) have been introduced and established in Lake Tahoe, and the native top predator, Lahontan cutthroat trout (Oncorhynchus clarki henshawi; hereafter LCT), has been extirpated. Isotope analysis indicates that lake trout now occupy a trophic niche similar to that of historical LCT. Fish production has shifted from benthic to pelagic, corresponding with the eutrophication of Lake Tahoe during recent decades. The current Cascade Lake food web resembles that of the historical Lake Tahoe food web. Our isotope-based food web reconstructions reveal long-term food web changes in Lake Tahoe and can serve as the basis for setting historically relevant restoration targets. Unfortunately, the presence of nonnative species, particularly Mysis and lake trout, have dramatically altered the pelagic food web structure; as such, they are barriers to native fish community restoration. Fish community restoration efforts should focus on adjacent ecosystems, such as Cascade Lake, which have a high likelihood of success because they have not been heavily affected by nonnative introductions.
This study investigates the toxicity and post-exposure effects of dissolved microcystin (MC-LR) on the dominant copepods of the upper San Francisco Estuary (SFE), where blooms of the toxic ...cyanobacteria
Microcystis aeruginosa coincide with record low levels in the abundance of pelagic organisms including phytoplankton, zooplankton, and fish. The potential negative impact of
Microcystis on the copepods
Eurytemora affinis and
Pseudodiaptomus forbesi has raised concern for further depletion of high quality fish food. Response of copepods to MC-LR (MC) was determined using a 48-h standard static renewal method for acute toxicity testing. Following exposure, a life table test was performed to quantify any post-exposure impacts on survival and reproduction. The 48-h LC-50 and LC-10 values for MC were 1.55 and 0.14 mg/L for
E. affinis; and 0.52 and 0.21 mg/L for
P. forbesi. Copepod populations recovered once dissolved MC was removed and cultures returned to optimal conditions, suggesting no post-exposure effects of MC on copepod populations. Dissolved microcystin above 0.14 mg/L proved likely to have chronic effects on the survival of copepods in the SFE. Since such high concentrations are unlikely, toxicity from dissolved microcystin is not a direct threat to zooplankton of the SFE, and other mechanisms such as dietary exposure to
Microcystis constitute a more severe risk.
An additive semi-analytic model of water clarity for the forward problem of calculating apparent optical properties (AOPs) of diffuse attenuation and Secchi depth from the inherent optical properties ...(IOPs) due to suspended matter in oligotrophic waters is presented. The model is general in form, taking into account algal concentration, suspended inorganic sediment concentration, particle size distribution, and dissolved organic matter to predict Secchi depth and diffuse attenuation. The model's application to ultra-oligotrophic Lake Tahoe, California-Nevada, USA is described. The function of the clarity model is to quantify the relative effect of phytoplankton or phytoplankton-derived organic materials, other particles such as suspended mineral sediment, and dissolved organic matter on the lake's clarity. It is concluded that suspended inorganic sediments and phytoplanktonic algae both contribute significantly to the reduction in clarity, and that suspended particulate matter, rather than dissolved organic matter, are the dominant causes of clarity loss.
Models predict that food-web structure is regulated by both consumers and resources, and the strength of this control is dependent on trophic position and food-web length. To test these hypotheses, a ...meta-analysis was conducted of 11 fish (consumer)-by-nutrient (resource) factorial plankton community experiments. As predicted, zooplankton biomass was under strong consumer control but was weakly stimulated by nutrient additions; phytoplankton biomass was under strong resource control with moderate control by fish. However, the phytoplankton and zooplankton responses to nutrient additions did not follow theoretical predictions based on the number of trophic levels in the food web.
We investigated the effects of cultural eutrophication on the coupling between pelagic primary producers and benthic consumers in Lake Tahoe. Spatial and temporal changes in zoobenthos energetics ...were documented by measuring >40 yr of change in pelagic primary production through 14C incubations, reduction in clarity by Secchi and light measurements, and sedimentation rates. Effects on zoobenthic primary consumers (oligochaete and chironomid) and an obligate benthic secondary consumer (Catostomus tahoensis) were determined by comparing δ 13C values of historical and contemporary samples. A model that considers primary production (benthic or pelagic) contributions and their respective δ 13C signals was used to examine the factors contributing to zoobenthic energy shifts. Spatially, zoobenthos exhibited a strong positive relationship between lake depth and pelagic isotopic signals. For depths at which ambient 1% light levels have shifted with time (50-85 m), pelagic primary producer and zoobenthic consumer coupling was positive. Historically, zoobenthos from this depth zone obtained 27% of their energy from phytoplankton sources. After 43 yr of eutrophication, they obtained 62% from pelagic sources. A simple model indicated that increased pelagic production and resultant export of matter combined with the loss of benthic primary production contributed to the change in zoobenthos energetics. This change was passed on to higher consumers, with the benthic fish Tahoe sucker (Catostomus tahoensis) now deriving ∼21% of its energy from pelagic primary production sources. This study demonstrates how lake eutrophication increases the coupling between pelagic and benthic habitats.
Many common PET segmentation methods for malignant lesions use surrounding background activity as a reference. To date, background has to be measured by drawing a second volume of interest (VOI) in ...nearby, undiseased tissue. This is time consuming as two VOIs have to be determined for each lesion. The aim of our study was to analyse whether background activity in different organs and body regions could be calculated from the tumour VOI by histogram analyses. The institutional review board waived informed consent for this retrospective study. For each of the following tumour types and areas - head and neck (neck), lung, hepatic metastasis (liver), melanoma (skin), and cervix (pelvis) - 10 consecutive patients with biopsy-proven tumours who underwent (18)F-fluorodeoxyglucose-PET in January 2012 were retrospectively selected. One lesion was selected and two readers drew a cubical VOI around the lesion (VOItumour) and over the background (VOIBG). The mean value of VOIBG was compared with the mode of the histogram, using equivalence testing with an equivalence margin of ±0.5 SUV. Inter-reader agreement was analysed for the mean background, and the mode of the VOItumour histogram was assessed using the concordance correlation coefficient. For both readers, the mode of VOItumour was equivalent to the mean of VOIBG (P<0.0001 for R1 and R2). The inter-reader agreement was almost perfect, with a concordance correlation coefficient of greater than 0.92 for both the mode of VOItumour and the mean of VOIBG. Background activity determined within a tumour VOI using histogram analysis is equivalent to separately measured mean background values, with an almost perfect inter-reader agreement. This could facilitate PET quantification methods based on background values without increasing workload.
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