Aquatic and terrestrial organic matter in the diet of stream consumers: implications for mercury bioaccumulation

The relative contribution of aquatic vs. terrestrial organic matter to the diet of consumers in fluvial environments and its effects on bioaccumulation of contaminants such as mercury (Hg) remain poorly understood. We used stable isotopes of carbon and nitrogen in a gradient approach (consumer isotope ratio vs. periphyton isotope ratio) across temperate streams that range in their pH to assess consumer reliance on aquatic (periphyton) vs. terrestrial (riparian vegetation) organic matter, and whether Hg concentrations in fish and their prey were related to these energy sources. Taxa varied in their use of the two sources, with grazing mayflies (Heptageniidae), predatory stoneflies (Perlidae), one species of water strider (Metrobates hesperius), and the fish blacknose dace (Rhinichthys atratulus) showing strong connections to aquatic sources, while Aquarius remigis water striders and brook trout (Salvelinus fontinalis) showed a weak link to in-stream production. The aquatic food source for consumers, periphyton, had higher Hg concentrations in low-pH waters, and pH was a much better predictor of Hg in predatory invertebrates that relied mainly on this food source vs. those that used terrestrial C. These findings suggest that stream biota relying mainly on dietary inputs from the riparian zone will be partially insulated from the effects of water chemistry on Hg availability. This has implications for the development of a whole-system understanding of nutrient and material cycling in streams, the choice of taxa in contaminant monitoring studies, and in understanding the fate of Hg in stream food webs.     

Collapse and reorganization of a food web of Mwanza Gulf, Lake Victoria

Lake Victoria in East Africa is the world's second largest freshwater system. Over the past century the ecosystem has undergone drastic changes. Some 30 years after the introduction of Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) in the 1950s, the highly diverse community of native haplochromines collapsed, leaving a system dominated by only four species: the native cyprinid dagaa (Rastrineobola argentea) and shrimp (Caridina nilotica), as well as the introduced Nile perch and Nile tilapia. More recently, an unexpected resurgence of haplochromines has been reported. To understand these changes in terms of ecosystem functioning and of changes in growth of trophic groups, we created mass balances of the food web near Mwanza, Tanzania, before, during, and after the Nile perch boom (1977, 1987, and 2005), using the application ECOPATH. We connected these mass balances with a dynamic model assuming linear trends in net growth rates of the trophic groups. Our analysis suggests that the Nile perch boom initially altered the biomass distribution over trophic levels. Also, results indicate that not only fishing but also changes at the detritivores' trophic level might have played an important role in driving changes in the system. Both the mass balances and the dynamic model connecting them reveal that, after a major distortion during the Nile perch boom, the biomass distribution over the main trophic levels had largely recovered its original (1977) state by 2005. However, no such return appeared in terms of community structure. Biodiversity in the new state is dramatically lower, consisting of introduced species and a few native surviving species. We conclude that at an aggregate level Lake Victoria's ecosystem has proved to be resilient in the sense that its overall trophic structure has apparently recovered after a major perturbation. By contrast, its intricate functional structure and associated biodiversity have proved to be fragile and seem unlikely to recover.     

Midwest U.S. landscape change to 2020 driven by biofuel mandates

Meeting future biofuel targets set by the 2007 Energy Independence and Security Act (EISA) will require a substantial increase in production of corn. The Midwest, which has the highest overall crop production capacity, is likely to bear the brunt of the biofuel-driven changes. In this paper, we set forth a method for developing a possible future landscape and evaluate changes in practices and production between base year (BY) 2001 and biofuel target (BT) 2020. In our BT 2020 Midwest landscape, a total of 25 million acres (1 acre = 0.40 ha) of farmland was converted from rotational cropping to continuous corn. Several states across the Midwest had watersheds where continuous corn planting increased by more than 50%. The output from the Center for Agriculture and Rural Development (CARD) econometric model predicted that corn grain production would double. In our study we were able to get within 2% of this expected corn production. The greatest increases in corn production were in the Corn Belt as a result of conversion to continuous corn planting. In addition to changes to cropping practices as a result of biofuel initiatives we also found that urban growth would result in a loss of over 7 million acres of productive farmland by 2020. We demonstrate a method which successfully combines economic model output with gridded land cover data to create a spatially explicit detailed classification of the landscape across the Midwest. Understanding where changes are likely to take place on the landscape will enable the evaluation of trade-offs between economic benefits and ecosystem services allowing proactive conservation and sustainable production for human well-being into the future.     

Prey size increases with nestling age: are provisioning parents programmed or responding to cues from offspring?

As offspring grow, parents often feed them with different sizes or taxa of prey to suit the changing nutritional or energetic demands. We investigated whether such changes in prey types were innate and inflexible or whether they were based on the proximate cue of offspring size. We created experimental broods in which parent blue tits Cyanistes caeruleus and pied flycatchers Ficedula hypoleuca received either older nestlings simulating rapid development or younger nestlings simulating delayed development. The size of prey increased over time in all brood types, suggesting a strong programmed pattern of foraging by parents. However, delayed broods were provisioned with smaller prey than control or advanced broods indicating some plasticity in the response by the parents to the size of nestlings. Although female birds brought smaller prey than male birds, both sexes showed the same rate of increase of prey size with time and brought similar types of prey items. The proportion of soft, digestible larva prey in the nestling diet decreased over time in pied flycatchers but increased in blue tits. Counter to the hypothesis that spiders provide unique and preferred nutrients for young nestlings, the proportion of arachnids in the diet did not change with nestling age for either species. The lack of treatment effect on the taxa of prey delivered suggests that temporal shifts in diet composition are not driven by the proximate cues of nestling age or size but that the feeding patterns are fairly innate and fixed in these altricial birds.     

Stalked tunicates Boltenia ovifera form biogenic habitat in the rocky subtidal zone of Nova Scotia

The stalked tunicate Boltenia ovifera is widely distributed in the Arctic and North Atlantic oceans on rocky substrata at 10–300 m depth, although its ecological role in benthic communities is poorly understood. The distribution and abundance of B. ovifera were recorded at 10–100 m depth in towed-video transects in November 2011 and in June, July, and November 2012 off a wave-exposed headland near Halifax, Nova Scotia. Specimens also were collected at 30 m depth using SCUBA (44°26.88′N, 63°31.59′W) in September 2012. Areas dominated by B. ovifera had densities of 10–100 individuals m^?2 on rocky substrata at 30–60 m depth. These tunicate beds often were associated with sparse kelp (Agarum clathratum) and turf-forming red algae. A generalized additive model indicated that depth, substrate type, and benthic algal type were strong predictors of tunicate abundance. Twenty-two epibiotic species were found on specimens of B. ovifera, including juvenile conspecifics. Filter-feeding macroinvertebrates, including anemones and soft corals, were more abundant in areas with B. ovifera than in areas without these tunicates. Our findings suggest that beds of B. ovifera can act as biogenic habitat to enhance local species richness in the rocky subtidal zone of Nova Scotia.     

Near-term ecological forecasting for dynamic aeroconservation of migratory birds

Near-term ecological forecasting has the potential to mitigate negative impacts of human modifications on wildlife by directing efficient action through relevant and timely predictions. We used the U.S. avian migration system to highlight ecological forecasting applications for aeroconservation. We used millions of observations from 143 weather surveillance radars to construct and evaluate a migration forecasting system for nocturnal bird migration over the contiguous United States. We identified the number of nights of mitigation required to reduce the risk of aerial hazards to 50% of avian migrants passing a given area in spring and autumn based on dynamic forecasts of migration activity. We also investigated an alternative approach, that is, employing a fixed conservation strategy based on time windows that historically capture 50% of migratory passage. In practice, during both spring and autumn, dynamic forecasts required fewer action nights compared with fixed window selection at all locations (spring: mean of 7.3 more alert days; fall: mean of 12.8 more alert days). This pattern resulted in part from the pulsed nature of bird migration captured in the radar data, where the majority (54.3%) of birds move on 10% of a migration season's nights. Our results highlight the benefits of near-term ecological forecasting and the potential advantages of dynamic mitigation strategies over static ones, especially in the face of increasing risks to migrating birds from light pollution, wind energy infrastructure, and collisions with structures.     

Use of Pb blast furnace slag as a partial substitute for fine aggregate in cement mortar

The technical properties of cement mortars containing natural fine aggregate that is replaced by lead blast furnace slag at 25 and 35% level were assessed at fixed water-to-cement (W/C) ratio and at fixed flow table value. The leachabilities of some toxic elements from the cement mortars were also assessed to test the environmental suitability of the slag for use in preparation of cement mortar. At fixed W/C ratio, the strength of the mortar decreased with increase of the slag content. On the other hand, at fixed consistency, strength increased with increasing slag content in the mortar composition. The concentrations of some toxic elements in the leachates collected from the mortars containing slag were slightly higher than for the control mortar, but the concentrations in the leachates remained within the regulatory limits for recycling in construction applications. For most elements, leaching from a mortar containing 35% of slag was similar to that from a mortar containing 25% of slag. Therefore, 35% of natural sand can be beneficially replaced with Pb slag to produce cement mortar without affecting the mechanical and leaching properties studied in this work.