Aaron P. Potito

Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area.

Subfossil Chironomids As Indicators Of Recent Climate Change In Sierra Nevada, California, Lakes

ABSTRACT High-resolution chironomid (Insecta: Diptera) stratigraphies were developed for three subalpine lakes in the Sierra Nevada of California to assess whether these lakes have been impacted by recent climate change evident in regional instrumental records for the 19th and 20th centuries. Detrended correspondence analysis (DCA) of the chironomid fauna indicates that the lakes have experienced similar unidirectional change in community composition over the 20th century, with two of the lakes showing particularly sharp gradients of change since the 1970s. Application of a chironomid-based surface water temperature inference model (r2jack = 0.73, RMSEPjack = 1.1°C, and a maximum bias of 1.24°C) to the subfossil chironomid assemblages preserved in the lake sediment provided quantitative estimates of surface water temperature changes and revealed the existence of similar water temperature trends between the late 19th century and the present. Above average water temperatures characterized the late 20th century and below average surface water temperatures occurred between a.d. 1910 and a.d. 1980. Fluctuations in the surface water temperature of these lakes closely track changes in mean July air temperature as measured in Fresno, California, over the period a.d. 1895–2001. It appears that 20th century climate change has had an overriding influence on the composition of the chironomid communities within these three lakes. This study demonstrates that subfossil chironomid analysis can provide detailed records of community response to local and regional climatic changes at subdecadal time scales. It also suggests that chironomid communities in subalpine lakes in the Sierra Nevada are already recording the impact of recent climate warming.

Prolonged California aridity linked to climate warming and Pacific sea surface temperature

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

The Effects of Aridity on Conifer Radial Growth, Recruitment, and Mortality Patterns in The Eastern Sierra Nevada, California

ABSTRACT Understanding natural variability in precipitation and drought, and the resulting effects on Sierra Nevada forests, is crucial for successful resource management in this environmentally sensitive area of California. This study assessed the species-specific influence of precipitation variations on radial growth, recruitment, and mortality patterns for three conifer species (Pinus jeffreyi, Juniperus occidentalis, and Pinus contorta) in two mid-elevation lake catchments over the past 550 years. The P. jeffreyi chronology was the most highly correlated with winter precipitation patterns, although the other two species also exhibited significant correlations. Ring-width patterns suggest the influence of the Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) on winter precipitation over the length of the records. Recruitment patterns displayed significant, though directionally distinct, correlations with winter drought: P. contorta exhibited increased recruitment during extended drought periods, while P. jeffreyi and J. occidentalis showed increased recruitment during wetter intervals. Finally, a ring of dead trees around both lakes is evidence of a late 20th century water level rise, likely caused by earlier snowmelt and/or wetter conditions. Moisture availability has exerted a strong influence on Sierra Nevada forests through time, but the strength of tree-growth response, and even the sign of tree population response, has been species-specific.

Response of chironomids to Neolithic land-use change in north-west Ireland

This study provides the first decadally resolved chironomid and organic geochemistry record of the Irish Neolithic from a small lake adjacent to the Carrowkeel-Keshcorran complex in County Sligo, Ireland. Chironomid (non-biting midge fly) sub-fossils and lake sediment geochemistry (δ13C, δ15N and C:N) from the Templevanny Lough core were used to assess the timing and magnitude of within-lake responses to Neolithic farming activity. When compared with decadally resolved pollen and macroscopic charcoal records from the same core, the limnological data show a direct influence of prehistoric farming on a freshwater lake system through nutrient loading and lake eutrophication. Elevated nutrient levels, suggesting a more productive lake system, and a subsequent turnover in the chironomid community indicate a period of intensive farming activity from c. 3790–3620 BC in the early Neolithic. This was followed by a decline in farming with short periods of small-scale human activity, exemplified through nutrient loading and short-lived increases in eutrophic chironomid taxa during the middle to late Neolithic. A return of farming activity can be seen in all proxy data in the late Neolithic (c. 2720–2480 BC). The chironomid community composition typically lagged land-use change by c. 10–20 years and exhibited predictable and proportional responses to agricultural activity. The timing and magnitude of limnological changes show that land-use, rather than climate, is the main control on chironomids at Templevanny Lough, thus showing the potential prominence of the anthropogenic signal during the Neolithic.

Impact of early prehistoric farming on chironomid communities in northwest Ireland

This study explored the utility and performance of chironomid (Diptera: Chironomidae) autecology in the investigation of prehistoric farming impacts on freshwater lake systems. Chironomid subfossils, lake sediment geochemistry (δ13C, δ15N and C:N), pollen and macroscopic charcoal analyses were used in a comparative limnological assessment of three archaeologically rich study sites in northwest Ireland. At all three study sites, pastoral farming and its associated nutrient inputs, as represented by non-arboreal pollen indicative of grassland/pasture (NAPp) and lake sediment geochemistry, are concomitant with increases in eutrophic chironomid taxa. Redundancy analysis (RDA) and partial RDAs established that δ15N and NAPp were controlling factors of chironomid community compositional change during the Neolithic (4000–2500 BC) and Bronze Age (2500–600 BC). Bronze Age farming had a considerably greater impact on the lake systems than Neolithic farming, as indicated by a higher proportion of eutrophic taxa and increases in δ15N, C:N and δ13C values, consistent with increased erosion and agricultural inputs. Findings emphasise the importance of identifying the natural, pre-impacted state of a lake system to determine the extent of agricultural impact accurately. The timing and magnitude of change show that Neolithic and Bronze Age farming exhibited a strong control over chironomid communities at all three sites.

Impact of grass cover on the magnetic susceptibility measurements for assessing metal contamination in urban topsoil

&NA; In recent decades, magnetic susceptibility monitoring has developed as a useful technique in environmental pollution studies, particularly metal contamination of soil. This study provides the first ever examination of the effects of grass cover on magnetic susceptibility (MS) measurements of underlying urban soils. Magnetic measurements were taken in situ to determine the effects on &kgr; (volume magnetic susceptibility) when the grass layer was present (&kgr;grass) and after the grass layer was trimmed down to the root (&kgr;no grass). Height of grass was recorded in situ at each grid point. Soil samples (n=185) were collected and measurements of mass specific magnetic susceptibility (χ) were performed in the laboratory and frequency dependence (χfd%) calculated. Metal concentrations (Pb, Cu, Zn and Fe) in the soil samples were determined and a gradiometry survey carried out in situ on a section of the study area. Significant correlations were found between each of the MS measurements and the metal content of the soil at the p<0.01 level. Spatial distribution maps were created using Inverse Distance Weighting (IDW) and Local Indicators of Spatial Association (LISA) to identify common patterns. &kgr;grass (ranged from 1.67 to 301.00×10−5 SI) and &kgr;no grass (ranged from 2.08 to 530.67×10−5 SI) measured in situ are highly correlated [r=0.966, n=194, p<0.01]. The volume susceptibility datasets in the presence and absence of grass coverage share a similar spatial distribution pattern. This study re‐evaluates in situ &kgr; monitoring techniques and the results suggest that the removal of grass coverage prior to obtaining in situ &kgr; measurements of urban soil is unnecessary. This layer does not impede the MS sensor from accurately measuring elevated &kgr; in soils, and therefore &kgr; measurements recorded with grass coverage present can be reliably used to identify areas of urban soil metal contamination. Graphical abstract: Figure. No caption available. HighlightsStrong correlations were found between metal concentrations and both MS readings.MS was taken in situ with/without a grass layer to study its effect on measurements.MS measured with/without grass is effective in identifying high MS in urban soils.MS hotspots can aid in locating areas at risk in relation to soil metal pollution.MS measured with a grass layer reduces labour and costs involved in sampling.

Mid-Late Holocene Vegetational History and Land-Use Dynamics in County Monaghan, Northeastern Ireland—The Palynological Record of Lough Muckno

Abstract We conducted high-resolution palynological analysis on a sediment core obtained from Lough Muckno, County Monaghan, Ireland. The results presented represent the first paleoecological account of Mid-Late Holocene vegetational change and land-use dynamics in the study region. Human activity and agriculture is first recorded during the Early Neolithic (ca. 3870–3500 B.C.). After a period of undiscernible human activity of ∼900 years, farming resumes during the Early Bronze Age (ca. 2600 B.C.). Henceforth, human presence on the landscape is constant with fluctuating levels of intensity. During the Bronze Age, anthropogenic activity is most pronounced during ca. 2000–1750 B.C. and ca. 1500–1300 B.C. followed by a phase of reduced intensity in the Late Bronze Age (ca. 1000–650 B.C.). Farming activity increases during the Iron Age and is disrupted with the onset of a period of rapid woodland regeneration from ca. 200 B.C. to A.D. 200. During the prehistorical period agriculture has a strong focus on pastoral grazing with a limited arable component. An upsurge in agricultural activity is recorded in the historical period from ca. A.D. 400 in which a mixed agricultural economy placing more emphasis on cereal-crop cultivation is adopted. Arable farming attains its maximum levels ca. A.D. 990–1140. Evidence of farming disruptions in the pollen record may reflect of a period of local “conflict” during the Viking Age/Medieval period (ca. A.D. 800–1190). We explore the characterizing features of the pollen assemblage of this large lake system and its use in reconstructing past cultural landscape change.

A GIS-based approach for simulating of air temperature in hydroelectric reservoir region

Understanding spatial distribution of surface air temperature (SAT) is important for regional management and the efficient utilization of climate resources in the Ertan reservoir region, southwest China. The creation of a hydroelectric reservoir results in the formation of vast surface areas of water, which will lead to local climatic changes. In view of this, a series of models were developed to analyze the effect of Ertan reservoir on regional temperatures. A modified multiple regression model was developed which takes into account the influence of the reservoir on SATs in the region. This modified model was able to predict SAT in the study region with considerably higher accuracy than a conventional model that used only elevation and location as temperature predictors.