Multi-year Trends Research Articles

Nitrogen fixation may not balance the nitrogen pool in lakes over timescales relevant to eutrophication management

We explored multiyear linear trends in nutrient concentrations, nitrogen (N) : phosphorus (P) ratio, and phytoplankton biomass within the 37‐yr, whole‐ecosystem nutrient enrichment experiment in Lake 227 of the Experimental Lakes Area, Canada. Based on experimental conditions, data were divided into subsets, which included (1) the period from 1969 to 1989 when the lake was fertilized with both N and P; (2) the period from 1990 to 2005 when the lake was fertilized with P alone; and (3) the period from 1997 to 2005 when the lake was fertilized with P alone and which also postdated a food web manipulation experiment, which left the lake without fish. After N fertilization was halted in 1990, total N concentrations decreased, which resulted in a decrease in the ratio of total N to total P and suggested increasing N deficiency. Chlorophyll ά concentration decreased over this same period. Phytoplankton biomass (mg m−3) was highly variable during the food web manipulation experiment but exhibited a clear decrease from 1997 to 2005, which was the longest period of monotonic change in phytoplankton biomass over the entire 37‐yr study. Collectively, these results suggest that Lake 227 has become increasingly N‐limited since N fertilization was halted and indicate that N fixation by cyanobacteria was not sufficient to offset the decrease in external N inputs to Lake 227. Furthermore, phytoplankton biomass decreased in response to decreased N availability, suggesting that the degree of eutrophication can be controlled by managing N inputs concurrently with P.

Raster-Based Analysis of Coastal Terrain Dynamics from Multitemporal Lidar Data

Abstract Multitemporal sets of lidar data provide a unique opportunity to analyze and quantify changes in topography in rapidly evolving landscapes. Methodology for geospatial analyses of lidar data time series was developed to investigate patterns of coastal terrain evolution, including the beach and dune systems. The diverse lidar-point data density, noise, and systematic errors were first quantified, and the results were used to compute a consistent series of high-resolution digital elevation models using spline-based approximation with optimized parameters. Raster-based statistical analysis was applied to the elevation-model time series to derive maps representing multiyear trends in spatial patterns of elevation change, to quantify dynamics at each cell using standard deviation maps, and to extract the core surface below which the elevation has never decreased. The methodology was applied to a North Carolina barrier island that was mapped by a sequence of 13 lidar surveys during the past decade, usin...

MINE‐DERIVED MERCURY: EFFECTS ON LOWER TROPHIC SPECIES IN CLEAR LAKE, CALIFORNIA

Considerable ecological research on mercury (Hg) has focused on higher trophic level species (e.g., fishes and birds), but less on lower trophic species. Clear Lake, site of the abandoned Sulphur Bank Mercury Mine, provides a unique opportunity to study a system influenced by mine-derived Hg. An exponentially decreasing gradient of total Hg (TotHg) away from the mine allowed us to evaluate Hg bioaccumulation in planktonic and benthic invertebrates and evaluate population- and community-level parameters that might be influenced by Hg. Studies from 1992-1998 demonstrated that TotHg in lower trophic species typically decreased exponentially away from the mine, similar to trends observed in water and sediments. However, a significant amount of invertebrate TotHg (approximately 60% for sediment-dwelling chironomid insect larvae) likely derives from Hg-laden particles in their guts. Spatially, whole-body methylmercury (MeHg) did not typically exhibit a significant decrease with increasing distance from the mine. Temporally, TotHg concentrations in plankton and chironomids did not exhibit any short-term (seasonal or annual) or long-term (multiyear) trends. Methylmercury, however, was elevated during late summer/fall in both plankton and chironomids, but it exhibited no long-term increase or decrease during this study. Although data from a 50-yr monitoring program for benthic chaoborid and chironomid larvae documented significant population fluctuations, they did not demonstrate population-level trends with respect to Hg concentrations. Littoral invertebrates also exhibited no detectable population- or community-level trends associated with the steep Hg gradient. Although sediment TotHg concentrations (1-1200 mg/kg dry mass) exceed sediment quality guidelines by up to 7000 times, it is notable that no population- or community-level effects were detected for benthic and planktonic taxa. In comparison with other sites worldwide, Clear Lake's lower trophic species typically have significantly higher TotHg concentrations, but comparable or lower MeHg concentrations, which may be responsible for the discrepancy between highly elevated TotHg concentrations and the general lack of observed population- or community-level effects. These data suggest that MeHg, as well as TotHg, should be used when establishing sediment quality guidelines. In addition, site-specific criteria should be established using the observed relationship between MeHg and observed ecological responses.

Phytoplankton growth and regulation in the Labrador Sea: light and nutrient limitation

The importance of light and nutrients in regulating phytoplankton growth in the Labrador Sea was evaluating using climatological data, 12 years of measurements made as part of the Labrador Sea Monitoring Program (LSMP) and physiological information from the literature. Light limits primary production and phytoplankton growth much of the year, even during summer when surface irradiance is at its seasonal peak. Nutrients, nitrogen (nitrate) and silicate, are reduced to low levels in surface waters in summer/autumn and can limit phytoplankton production and growth at this time of year. Nitrate appears to be the nutrient in shortest supply on the Labrador Shelf while silicate is in shortest supply in the central Labrador Basin. Multiyear trends in regional hydrography (increases in water temperatures, decreases in mixed layer depth) and changes in nitrate and silicate supply over the past decade (i.e. increases in nitrate and decreases in silicate) may be linked to changes in phytoplankton community composition and structure (i.e. decreases in large forms such as diatoms and increases in small picoplankton and nanoplankton forms).

Spatial and Temporal Variability of Ambient Air Toxics Data

This paper summarizes information on the spatial and temporal variability of selected air toxics pollutants collected on a national basis primarily for a period encompassing 1990–2003. Spatial information on pollutant concentrations is characterized in terms of within-city and between-city variability. Temporal information is summarized as diurnal and seasonal variability and in multiyear trends. The information on variability is presented in the framework of a larger need for systematic documentation of information on air toxics pollutants to assess progress in air pollution control programs.

Transition to the new race/ethnicity data collection standards in the Department of Veterans Affairs

BackgroundPatient race in the Department of Veterans Affairs (VA) information system was previously recorded based on an administrative or clinical employee's observation. Since 2003, the VA started to collect self-reported race in compliance with a new federal guideline. We investigated the implications of this transition for using race/ethnicity data in multi-year trends in the VA and in other healthcare data systems that make the transition.MethodsAll unique users of VA healthcare services with self-reported race/ethnicity data in 2004 were compared with their prior observer-recorded race/ethnicity data from 1997 – 2002 (N = 988,277).ResultsIn 2004, only about 39% of all VA healthcare users reported race/ethnicity values other than "unknown" or "declined." Females reported race/ethnicity at a lower rate than males (27% vs. 40%; p < 0.001). Over 95% of observer-recorded data agreed with self-reported data. Compared with the patient self-reported data, the observer-recorded White and African American races were accurate for 98% (kappa = 0.89) and 94% (kappa = 0.93) individuals, respectively. Accuracy of observer-recorded races was much worse for other minority groups with kappa coefficients ranging between 0.38 for American Indian or Alaskan Natives and 0.79 for Hispanic Whites. When observer-recorded race/ethnicity values were reclassified into non-African American groups, they agreed with the self-reported data for 98% of all individuals (kappa = 0.93).ConclusionFor overall VA healthcare users, the agreement between observer-recorded and self-reported race/ethnicity was excellent and observer-recorded and self-reported data can be used together for multi-year trends without creating serious bias. However, this study also showed that observation was not a reliable method of race/ethnicity data collection for non-African American minorities and racial disparity might be underestimated if observer-recorded data are used due to systematic patterns of inaccurate race/ethnicity assignments.

Early Life History and a Modeling Framework for Lobster (Homarus Americanus) Populations in the Gulf of Maine

Beginning in the late 1980s, lobster (Homarus americanus) landings for the state of Maine and the Bay of Fundy increased to levels more than three times their previous 20-year means. Reduced predation may have permitted the expansion of lobsters into previously inhospitable territory, but we argue that in this region the spatial patterns of recruitment and the abundance of lobsters are substantially driven by events governing the earliest life history stages, including the abundance and distribution of planktonic stages and their initial settlement as Young-of-Year (YOY) lobsters. Settlement densities appear to be strongly driven by abundance of the pelagic postlarvae. Postlarvae and YOY show large-scale spatial patterns commensurate with coastal circulation, but also multi-year trends in abundance and abrupt shifts in abundance and spatial patterns that signal strong environmental forcing. The extent of the coastal shelf that defines the initial settlement grounds for lobsters is important to future population modeling. We address one part of this definition by examining patterns of settlement with depth, and discuss a modeling framework for the full life history of lobsters in the Gulf of Maine.

EVALUATING THE STEADY STATE ASSUMPTION: SIMULATIONS OF GORILLA NEST DECAY

Large mammal surveys are often based on indices of animal abundance such as dung or, for great apes, sleeping nests. They also tend to rely on the “steady state” assumption that the rate at which an index is deposited in the environment is exactly balanced by the rate at which it disappears (decays). Here we use a data set on western gorilla (Gorilla gorilla) sleeping nest decay rates to show that this assumption is likely to be strongly violated often and in ways that have serious implications for our ability to accurately survey and monitor a wide range of endangered species, particularly in tropical forests. We first fit the data using a model describing the daily probability of nest decay as a function of nest age and monthly rainfall. We then use this decay model to simulate time series of gorilla nest standing stock, given observed rainfall regimes from a series of sites in the equatorial African country of Gabon. These simulations suggest that, within a given site, the relationship between nest standing stock and true gorilla density fluctuates wildly and is, on average, tens of percent away from the mean nest decay time for the empirical data set. The behavior of nest standing stock is extremely sensitive to variability in rainfall, not just annual mean rainfall. Differences between sites in rainfall variability produce counterintuitive differences between sites in nest standing stock. Multiyear trends in the mean and variance of rainfall produce the spurious impression of trends in animal abundance. Furthermore, heterogeneity in rainfall exists at all spatial and temporal scales, so that attempts to use regression models based on rainfall measurements taken at one location or time are not likely to accurately predict the nest decay regime at other places or times. We close with some suggestions on alternative estimation methods that do not rely on extrapolations of environmental conditions from one time or place to another.

Using the Content of Online Privacy Notices to Inform Public Policy: A Longitudinal Analysis of the 1998-2001 U.S. Web Surveys

In the United States, Congress has had a long-standing interest in consumer privacy and the extent to which company practices are based on fair information practices. Previously, public policy was largely informed by anecdotal evidence about the effectiveness of industry self-regulatory programs. However, the Internet has made it possible to unobtrusively sample web sites and their privacy disclosures in a way that is not feasible in the offline world. Beginning in 1998, the Federal Trade Commission relied upon a series of three surveys of web sites to assess whether organizations post online privacy disclosures and whether these disclosures represent the U.S. definition of fair information practices. While each year's survey has provided an important snapshot of U.S. web-site practices, there has been no longitudinal analysis of the multiyear trends. This study compares a subset of equivalent individual-level web-site data for the 1998, 1999, 2000, and 2001 web surveys. Implications for using this type of research to inform public policy are discussed.

Interaction between marine populations and fishing activities: temporal patterns of landings of La Rochelle trawlers in the Bay of Biscay

Marine populations are distributed heterogeneously in space and time because of the diversity of habitats and the requirements of species life cycles. Human exploitation of these resources also varies as a function of space, time and the type of fishing activities performed. These three factors determine fishing strategy at different levels of integration. The purpose of this study was to describe and analyse, with respect to different time scales, the relations between the modalities of resource exploitation and the biological or demographic characteristics of the species involved. These investigations relate to the more general task of acquiring the basic knowledge needed for spatialised management of fishing effort. A fleet of trawlers from La Rochelle, operating in the Bay of Biscay, was studied over a 15-year period (1979–1993), which led to the development of a reference resource exploitation scheme for these vessels. The degree of stability of this scheme over time was determined from landing profiles of the 18 most important species fished (94% of landings). An annual cycle for the species composition of landings was determined by multiple factor analysis. Two factors account for more than 55% of the inertia of the data. The first, of biological origin, is closely related to the breeding activity of species and associated migrations between the coast and the open sea. The second is spatial in nature, corresponding to the distribution and availability of resources according to a bathymetric gradient. This organisational scheme persisted over the first 13 years, but showed signs of change toward the end of the study period. Analysis of multi-year trends indicated four periods marked by sustained levels of landings per unit of effort for some species (decreasing for sole and wedge sole, or increasing for Norway lobster, striped red mullet, rays and the smallspotted catshark). These changes are attributable to restrictions on resource access imposed on the fleet (regulations and/or competition among the fishing gears for occupation of space), variations in the abundance of traditionally fished populations (hake, anglerfish) and changes in the professional behaviour of fishermen.

Using Resampling to Assess Reliability of Audio-Visual Survey Strategies for Marbled Murrelets at Inland Forest Sites

-Marbled Murrelets (Brachyramphus marmoratus) are threatened seabirds that nest in coastal oldgrowth coniferous forests throughout much of their breeding range. Currently, observer-based audio-visual surveys are conducted at inland forest sites during the breeding season primarily to determine nesting distribution and breeding status and are being used to estimate temporal or spatial trends in murrelet detections. Our goal was to assess the feasibility of using audio-visual survey data for such monitoring. We used an intensive field-based survey effort to record daily murrelet detections at seven survey stations in the Oregon Coast Range. We then used computer-aided resampling techniques to assess the effectiveness of twelve survey strategies with varying scheduling and a sampling intensity of 4-14 surveys per breeding season to estimate known means and SDs of murrelet detections. Most survey strategies we tested failed to provide estimates of detection means and SDs that were within ?20% of actual means and SDs. Estimates of daily detections were, however, frequently estimated to within ?50% of field data with sampling efforts of 14 days/breeding season. Additional resampling analyses with statistically generated detection data indicated that the temporal variability in detection data had a great effect on the reliability of the mean and SD estimates calculated from the twelve survey strategies, while the value of the mean had little effect. Effectiveness at estimating multi-year trends in detection data was similarly poor, indicating that audio-visual surveys might be reliably used to estimate annual declines in murrelet detections of the order of 50% per year. Received 1 7January 2001, accepted 29June 2001.

Scotian Shelf euphausiid and silver hake population changes during 1984–1996 measured by multi-frequency acoustics

A 13-year time series of integrated acoustic volume backscattering strengths from Emerald Basin on the Scotian Shelf at 12, 50, 122, and 200 kHz are examined here, together with zooplankton data from net sampling. The three highest acoustic frequencies delineate relative abundances of the euphausiid Meganyctiphanes norvegica while the 12-kHz frequency primarily reflects abundances of pelagic silver hake. Euphausiid populations derived from acoustics fluctuate by a factor of 20 combining both seasonal and inter-annual variation components. Acoustically derived euphausiid populations correlate significantly with both net-sampled euphausiid populations and acoustically derived silver hake abundances, but show no clear long-term (multi-year) trends. The correlation between Emerald Basin euphausiid and migratory pelagic fish abundances suggests the ecological influence of the Shelf Basin euphausiid stocks extends to the surrounding shelf areas.

Measurements of middle atmospheric water vapor from low latitudes and midlatitudes in the northern hemisphere, 1995–1998

We present middle atmospheric water vapor measurements made at 22 GHz using two Naval Research Laboratory Water Vapor Millimeter‐wave Spectrometers (WVMS2 and WVMS3). We include measurements from an intercomparison campaign at Table Mountain, California (34.4°N, 242.3°E), and measurements obtained since March 1996 from Table Mountain and from Mauna Loa, Hawaii (19.5°N, 204.4°E). The mesospheric data from Mauna Loa show both a mixing ratio peak at a higher altitude and smaller seasonal variations than those from the WVMS instrument at Table Mountain. These differences are qualitatively consistent with the changes in mixing ratio and the increase in mesospheric variability expected with decreasing latitude. The latitudinal variation in the mixing ratio profile observed in the WVMS data is very similar to that observed by the Halogen Occultation Experiment (HALOE) in the upper stratosphere and lower mesosphere. The WVMS measurements generally show higher water vapor mixing ratios than HALOE does in the upper stratosphere and lower mesosphere but smaller mixing ratios in the upper mesosphere. The winter of 1997–1998 shows an unusually large decrease in mesospheric water vapor, a result that is consistent with measurements from HALOE throughout the Northern Hemisphere for this winter. There appears to be a slight overall decrease in water vapor for all seasons over the period of the WVMS observations from Mauna Loa. The relatively small seasonal variations, combined with the small amount of tropospheric water vapor above Mauna Loa, make this an ideal site for the monitoring of multiyear trends in water vapor.

Evidence for increasing ultraviolet irradiance at Point Barrow, Alaska

This paper presents analysis of noontime ultraviolet irradiance measurements made at Point Barrow, Alaska (71.2° N, 159° W) between the years 1991 and 1996. Eighteen wavelengths of the solar irradiance spectrum between 290 nm and 340 nm were examined for multiyear trends. This spectral region is particularly critical as these wavelengths are known to cause biological damage. After reducing the effects of clouds, 305 nanometer wavelength increases ranging between 3 and 10% per year were found in all daylit months except June which exhibited an annual average decline of approximately −3% per year at this wavelength. The correlation between the absorption cross‐section of ozone and the spectral character of the UV‐B changes suggest that declines in column ozone are responsible for the dramatic 305 nm increases. The unexpected decline in June 305 nm irradiance may be due to an increase in column ozone over this location in June.

Glaciological studies at Siple Station (Antarctica): potential ice-core paleoclimatic record

AbstractThe quality and utility of the records of oxygen-isotopic abundances, dust concentrations and anionic concentrations preserved in the ice at Siple Station (75°55′ S, 84° 15′ W) are assessed from four shallow (20 m) cores. The combination of high accumulation (0.56 m a−1w.e.) and low mean annual temperature (—24°C) preserves the prominent seasonal variations in δ18Ο which are very spatially coherent. Sulfate concentrations vary seasonally and, in conjunction with δ18Ο, will allow accurate dating of deeper cores from Siple Station. The concentrations of insoluble dust are the lowest measured in Antarctica, making Siple Station an excellent location to examine large increases in atmospheric tubidity.The seasonal variations and annual fluxes of the anions are examined for the last two decades (AD 1966–85) with regard to probable sources. An unusually high sulfate flux in 1976 may reflect the February 1975 eruption of Mount Ngauruhoe, New Zealand. No annual signal in nitrate concentration is confirmed and no unusually high nitrate fluxes support the suggestion of nitrate production by large solar flares. However, nitrate flux is higher for the latter half of the 1970s and early 1980s, possibly reflecting the recent loss of stratospheric ozone.Finally, comparison of the δ18O record with available surface-temperature data (AD 1957–85) reveals that multi-year trends along the western coast of the Antarctic Peninsula are recorded at Siple. More importantly, comparison with areally weighted temperature reconstructions suggests that the δ18Ο record may reflect larger-scale, persistent trends in the high southern latitudes. The strong spatial coherence of the preserved records, the potential for accurate dating, and possible relevance to larger-scale processes make Siple Station an excellent site for paleoenvironmental reconstruction from ice cores.

Glaciological studies at Siple Station (Antarctica): potential ice-core paleoclimatic record

AbstractThe quality and utility of the records of oxygen-isotopic abundances, dust concentrations and anionic concentrations preserved in the ice at Siple Station (75°55′ S, 84° 15′ W) are assessed from four shallow (20 m) cores. The combination of high accumulation (0.56 m a−1 w.e.) and low mean annual temperature (—24°C) preserves the prominent seasonal variations in δ18Ο which are very spatially coherent. Sulfate concentrations vary seasonally and, in conjunction with δ18Ο, will allow accurate dating of deeper cores from Siple Station. The concentrations of insoluble dust are the lowest measured in Antarctica, making Siple Station an excellent location to examine large increases in atmospheric tubidity.The seasonal variations and annual fluxes of the anions are examined for the last two decades (AD 1966–85) with regard to probable sources. An unusually high sulfate flux in 1976 may reflect the February 1975 eruption of Mount Ngauruhoe, New Zealand. No annual signal in nitrate concentration is confirmed and no unusually high nitrate fluxes support the suggestion of nitrate production by large solar flares. However, nitrate flux is higher for the latter half of the 1970s and early 1980s, possibly reflecting the recent loss of stratospheric ozone.Finally, comparison of the δ18O record with available surface-temperature data (AD 1957–85) reveals that multi-year trends along the western coast of the Antarctic Peninsula are recorded at Siple. More importantly, comparison with areally weighted temperature reconstructions suggests that the δ18Ο record may reflect larger-scale, persistent trends in the high southern latitudes. The strong spatial coherence of the preserved records, the potential for accurate dating, and possible relevance to larger-scale processes make Siple Station an excellent site for paleoenvironmental reconstruction from ice cores.

Multiyear trends in snowpack ion accumulation and loss, northern Michigan

From December 1982 to May 1987 the quality and quantity of snowfall and snowpack were measured at four stations in a small (176 ha) gauged watershed adjacent to Lake Superior's south shore. Annual precipitation was less than and winter precipitation greater than observed east and south of the Lake Superior basin. Snowfall averaged 37% of annual precipitation but was quite variable. Snowfall significantly increased (from 21.1 to 35.7 cm, P &lt; 0.001) with 150 m increase in elevation. Hydrogen and SO42− were the dominant ions in precipitation and snowpack while Ca2+ and HCO3− dominated stream water. No significant time trends in winter precipitation chemistry were found. Snowpack ionic retention rarely matched precipitation input. Generally, midwinter freeze‐thaw periods resulted in elevated concentrations of solutes in snow meltwater and greatly reduced snowpack ion loads prior to peak stream water discharge. All ions exhibited pulses in snowmelt each year. Preferential elution from the snowpack of SO42− and K+ was observed. Stream water H+, NH4+ and most NO3− pulses were associated with midwinter thaws. Stream water SO42− pulses were rare. For those years exhibiting the most pronounced spring increase in stream H+, NO3− and SO42− concentration, the magnitude and rate of concentration change was as great in a year of small peak snowpack ion load as it was following a more rapid ion loss from a larger snowpack load. This suggests that the pattern of snowpack melt and especially the pathway the meltwater follows to the stream may be important factors in determining whether stream water chemical pulses occur.

Multi-year trends in snowpack ion accumulation and loss, northern Michigan

Multi-year trends in snowpack ion accumulation and loss, northern Michigan

The prediction of drought: a meteorological perspective

Prospects for drought prediction up to a few seasons in advance seem a bit brighter than a decade ago. This is due in part to advances in the numerical modelling of the coupled atmosphere-ocean-land surface system, and is particularly true for the tropics, where a better understanding of the relationship between sea surface temperature anomalies and interannual climate variability has emerged. On longer time-scales, efforts to predict runs of predominantly dry years or multi-year trends in precipitation will continue to be hazardous and highly controversial until the controlling processes are better understood.

Estimation of Pesticide Use Through the Cooperative Crop Monitoring System in Michigan Apple Production1

A computerized approach to gathering, summarizing, archiving, and analyzing data on pesticide use for apples grown across the state was developed. The approach characterizes multiyear (1980–1982) trends in the use of insecticides, acaricides, and fungicides in the Michigan apple industry. We discuss the implications of instituting this type of data-collection program as a part of an overall integrated pest management program, and also for other state and federal uses.