Denitrification in nitrate-rich streams: application of N2:Ar and 15N-tracer methods in intact cores.

Rates of benthic denitrification were measured using two techniques, membrane inlet mass spectrometry (MIMS) and isotope ratio mass spectrometry (IRMS), applied to sediment cores from two NO3(-)-rich streams draining agricultural land in the upper Mississippi River Basin. Denitrification was estimated simultaneously from measurements of N2:Ar (MIMS) and 15N[N2] (IRMS) after the addition of low-level 15NO3- tracer (15N:N = 0.03-0.08) in stream water overlying intact sediment cores. Denitrification rates ranged from about 0 to 4400 micromol N x m(-2) x h(-1) in Sugar Creek and from 0 to 1300 micromol N x m(-2) x h(-1) in Iroquois River, the latter of which possesses greater streamflow discharge and a more homogeneous streambed and water column. Within the uncertainties of the two techniques, there is good agreement between the MIMS and IRMS results, which indicates that the production of N2 by the coupled process of nitrification/denitrification was relatively unimportant and surface-water NO3- was the dominant source of NO3- for benthic denitrification in these streams. Variation in stream NO3- concentration (from about 20 micromol/L during low discharge to 1000 micromol/L during high discharge) was a significant control of benthic denitrification rates, judging from the more abundant MIMS data. The interpretation that NO3- concentration directly affects denitrification rate was corroborated by increased rates of denitrification in cores amended with NO3-. Denitrification in Sugar Creek removed < or = 11% per day of the instream NO3- in late spring and removed roughly 15-20% in late summer. The fraction of NO3- removed in Iroquois River was less than that of Sugar Creek. Although benthic denitrification rates were relatively high during periods of high stream flow, when NO3 concentrations were also high, the increase in benthic denitrification could not compensate for the much larger increase in stream NO3- fluxes during high flow. Consequently, fractional NO3- losses were relatively low during high flow.     

Modeling the beta diversity of coral reefs

Quantifying the beta diversity (species replacement along spatiotemporal gradients) of ecosystems is important for understanding and conserving patterns of biodiversity. However, virtually all studies of beta diversity focus on one-dimensional transects orientated along a specific environmental gradient that is defined a priori. By ignoring a second spatial dimension and the associated changes in species composition and environmental gradients, this approach may provide limited insight into the full pattern of beta diversity. Here, we use remotely sensed imagery to quantify beta diversity continuously, in two dimensions, and at multiple scales across an entire tropical marine seascape. We then show that beta diversity can be modeled (0.852 > or = r2 > or = 0.590) at spatial scales between 0.5 and 5.0 km2, using the environmental variables of mean and variance of depth and wave exposure. Beta diversity, quantified within a "window" of a given size, is positively correlated to the range of environmental conditions within that window. For example, beta diversity increases with increasing variance of depth. By analyzing such relationships across seascapes, this study provides a framework for a range of disparate coral reef literature including studies of zonation, diversity, and disturbance. Using supporting evidence from soft-bottom communities, we hypothesize that depth will be an important variable for modeling beta diversity in a range of marine systems. We discuss the implications of our results for the design of marine reserves.     

Potential Effects of a Forest Management Plan on Bachman's Sparrows (Aimophila aestivalis): Linking a Spatially Explicit Model with GIS

By combining a spatially explicit, individual-based population simulation model with a geographic information system, we have simulated the potential effects of a U.S. Forest Service management plan on the population dynamics of Bachman's Sparrow ( Aimophila aestivalis ) at the Savannah River Site, a U.S. Department of Energy facility in South Carolina. Although the Forest Service's management plan explicitly sets management goals for many species, most of the prescribed management strategy deals with the endangered Red-cockaded Woodpecker ( Picoides borealis ) because of legal requirements. We explored how a species (the sparrow) that is not the target of specific management strategies but that shares some habitat requirements with the woodpecker, would fare under the management plan. We found that the major components of the proposed management plan may allow the sparrow population to reach and exceed the minimum management goal set for this species, but only after a substantial initial decline in sparrow numbers and a prolonged transition period. In the model, the sparrow population dynamics were most sensitive to demographic variables such as adult and juvenile survivorship and to landscape variables such as the suitability of young clearcuts and mature pine stands. Using various assumptions about habitat suitability, we estimated that the 50-year probability of population extinction is at least 5% or may be much higher if juvenile survivorship is low. We believe, however, that modest changes in the management plan might greatly increase the sparrow population and presumably decrease the probability of extinction. Our results suggest that management plans focusing on one or a few endangered species may potentially threaten other species of management concern. Spatially explicit population models are a useful tool in designing modifications of management plans that can reduce the impact on nontarget species of management concern.     

Ecological Biodiversity of Marine Nematodes in Samples from Temperate, Tropical, and Deep-Sea Regions

Little is known about the biodiversity of free-living nematodes. We have attempted to provide baseline information about the natural diversities (those not influenced by pollution) that might be expected in six biotopes. Seventeen marine nematode data sets consisting of 197 samples were standardized to allow a comparison of alpha diversity, or sample diversity, from temperate estuarine, tropical sublittoral, temperate sublittoral, bathyal, abyssal, and hadal biotopes, which were selected on criteria of depth and latitude. The diversity analysis methods we employed were rarefaction curves; three weighted diversity indices of species richness, SR, H', and ES(X); and two equitability indices, J' and V. Diversity was significantly different in the six biotopes. The weighted indices of species richness were more capable of resolving differences between the biotopes than were the equitability indices, whose large standard errors suggested that they were more influenced by local, small-scale ecological factors. This suggests that species richness is a better measure than equitability for large-scale comparisons of biotopes or regions. The ES(X), which is robust to sample size variations, was more efficient than the weighted indices of species richness, which were easily influenced by sample size. There was a nonlinear relationship between depth and diversity with the bathyal and abyssal biotopes displaying the highest diversity. The tropical sublittoral biotope was not more diverse than the temperate sublittoral biotope. The lowest diversities were found in the physically challenging temperate estuarine and hadal biotopes.     

Deconstructing an assemblage of “turtle” barnacles: species assignments and fickle fidelity in Chelonibia

Barnacles in the genus Chelonibia are commensal with a variety of motile marine animals including sea turtles, crustaceans, and sirenians. We conducted a worldwide molecular phylogenetic survey of Chelonibia collected from nearly all known hosts to assess species relationships, host-fidelity, and phylogeographic structure. Using DNA sequences from a protein-coding mitochondrial gene (COI), a mitochondrial rRNA gene (12S), and one nuclear rRNA gene (28S), we found that of four species, three (C. testudinaria, C. patula, and C. manati) are genetically indistinguishable. In addition, we show each utilizes a rare androdioecious mode of reproduction involving complemental males. In contrast, the fourth species (C. caretta), which is hermaphroditic and specializes on turtles, is genetically distinct—leading to the conclusion that the three former taxa are morphotypes of the same species and should be synonymized under C. testudinaria. Phylogenetic analysis resulted in three geographic clades (Atlantic, Indian Ocean/western Pacific, and eastern Pacific) with haplotype parsimony networks revealing no shared haplotypes among geographic regions. Analysis of molecular variance detected significant differences among sequences by region (p < 0.005); conversely, there were no significant differences among sequences when grouped by host or taxonomic designation. Average pairwise genetic distances were lower between the eastern Pacific and Atlantic clades (0.053 ± 0.006) than between the eastern Pacific and Indian Ocean/western Pacific clades (0.073 ± 0.008), suggesting Atlantic and eastern Pacific populations were connected more recently, perhaps until the rise of the Isthmus of Panama. Host use by Chelonibia morphotypes is discussed along with speculation on possible ancestral hosts and support for a “turtle-first” hypothesis.     

Human health risk from Pb in urban street dust in northern UK cities

The presence of Pb in the environment can cause significant health problems. These issues are exasperated when the lead is in a more amenable form for potential ingestion. This study investigates the potential human health risk from Pb in urban street dusts. The lead levels in urban street dust in major city centres in northern UK have been compared to levels determined in 35 cities around the world. With a few exceptions, it was noted that the mean Pb levels in this study exceeded those found in other cities worldwide. Samples (n = 15) of urban street dust were collected across five city centres, and specifically in areas in which pedestrians are likely to concentrate, as well as near historical buildings. Typical total lead concentrations across all sampling sites ranged from 306 to 558 mg/kg. The human health risk was assessed using oral bioaccessibility testing of the urban street dust. The mean oral bioaccessibility data, irrespective of site and sample location, were in the range 43 ± 9 %. The total and bioaccessible concentrations of lead were compared to the estimated tolerable daily intake (TDI_oral) values based on unintentional soil/dust consumption. It is noted, in all cases, that the maximum estimated lead daily intake exceeded the TDI_oral. An alternative approach for assessing the daily intake is proposed based on the actual measured air quality in selected cities on the same day as the sampled urban dust.     

Spatial and social connectivity of fish-eating “Resident” killer whales (Orcinus orca) in the northern North Pacific

The productive North Pacific waters of the Gulf of Alaska, Aleutian Islands and Bering Sea support a high density of fish-eating “Resident” type killer whales (Orcinus orca), which overlap in distribution with commercial fisheries, producing both direct and indirect interactions. To provide a spatial context for these interactions, we analyzed a 10-year dataset of 3,058 whale photo-identifications from 331 encounters within a large (linear ~4,000 km) coastal study area to investigate the ranging and social patterns of 532 individually identifiable whales photographed in more than one encounter. Although capable of large-scale movements (maximum 1,443 km), we documented ranges generally <200 km, with high site fidelity across summer sampling intervals and also re-sightings during a winter survey. Bayesian analysis of pair-wise associations identified four defined clusters, likely representing groupings of stable matrilines, with distinct ranging patterns, that combined to form a large network of associated whales that ranged across most of the study area. This provides evidence of structure within the Alaska stock of Resident killer whales, important for evaluating ecosystem and fisheries impacts. This network included whales known to depredate groundfish from longline fisheries, and we suggest that such large-scale connectivity has facilitated the spread of depredation.     

Mass mortality and slow recovery of Diadema antillarum: Could compromised immunity be a factor?

Long-spined sea urchins (Diadema antillarum-a) began dying in January 1983 and over the next year virtually disappeared from the Caribbean. Decades later, Diadema densities remain low, with only very limited recovery in some areas. Ecological extinction of Diadema is a factor in the transformation of coral to barren algae-covered rock that has occurred in the Caribbean during the past 25 years. Working with sea urchins collected from St. Croix, United States Virgin Islands, we asked whether the immune system of Diadema differs from those of other common Caribbean urchins that did not experience the die-off. Using isolated coelomocytes, we tested humoral responses of the urchins with classic stimulators in numerous humoral immune assays. All coelomocytes responded to stimulators—with one notable and statistically significant exception—D. antillarum did not respond to lipopolysaccharide. Our results indicate that Diadema may have a weakness in its humoral defense response that is independent of local stressors and may help explain why Caribbean Diadema was vulnerable to the 1983 epidemic and has shown slow and limited recovery since then.     

Current versus future reproduction: an experimental test of parental investment decisions using nest desertion by mallards (<Emphasis Type="Italic">Anas platyrhynchos</Emphasis>)

Past investment in offspring may be important in determining a parent's ability to reproduce in the future and, hence, should affect the relative value of current offspring. However, there have been surprisingly few clear tests of whether animals actually adjust parental care in response to diminished opportunities for future reproduction. We modified the experimental protocol of Sargent and Gross [Behav Ecol Sociobiol (1985) 17:43–45] to examine offspring desertion by mallards ( Anas platyrhynchos), and decoupled the influence of past investment from expected current benefits by controlling for the effect of offspring age on clutch value. Using 9 years of nest mortality data, we accounted for the increasing prospects of egg survival with clutch age by calculating clutch sizes throughout incubation with equivalent expected benefits. Applying this approach, we experimentally reduced 203 clutches at two different incubation stages such that they had equivalent expected benefits but differed in the amount of past investment. Nest desertion rates did not differ between early- and late-incubated clutches that had equivalent expected benefits. Rather, the probability of desertion increased with the severity of the clutch reduction treatment. These results suggest that female mallards adjust parental care according to the expected benefits of current offspring, rather than to diminished prospects for future reproduction due to past investment. We further examined whether females assessed expected benefits on the basis of clutch size alone or clutch size adjusted for the age of the clutch. Using Akaike's Information Criterion, the most parsimonious model to explain the probability of deserting an experimentally reduced clutch included both the proportion of the clutch remaining and clutch age. Thus, female mallards appear to fine-tune their level of parental care not only according to the relative number of offspring in the clutch, but also to the increased prospects for offspring survival as they age.     

Density affects female and male mate searching in the fiddler crab, <Emphasis Type="Italic">Uca beebei</Emphasis>

In most species, only one sex searches for mates while the other waits. Models of sex-specific mate-searching behavior predict single-sex searching, but the factors that determine which sex searches are not understood. In this study, we examine the effects of density and predation risk on mate-searching behavior in the fiddler crab Uca beebei. U. beebei is one of the few fiddler-crab species in which both sexes search for mates. In a field experiment conducted in Panama, we manipulated crab density and perceived predation risk in replicate plots. Females searched more and males searched less at high densities. At high levels of perceived predation risk, both sexes similarly reduced their search rates. Observations of plots that naturally varied in crab density show that females were more likely to search for mates in areas of higher density, where there were more males. Females may preferentially search for mates in high-density areas because the abundance of nearby burrows, into which they can run to escape predators, decreases their costs of searching and because the abundance of males and male burrows facilitates comparisons and thus may increase their benefits from searching. Males at high densities decrease their mate-searching rate perhaps in response to the increase in female searching and to the corresponding increase in the intensity of their competitors' mate-attraction signals.