Browsing by Subject "Interannual variability"

Coastal plain ponds are an understudied and threatened wetland ecosystem with many unique environmental attributes. Research in these ponds can investigate species-environment relationships, while simultaneously providing ecosystem-specific information crucial to their continued conservation and management. This dissertation explores patterns in coastal plain pond vegetation composition and species-environment relationships across space, through time, and in the seed bank and standing vegetation.

In a two-year field study at 18 coastal plain ponds across the island of Martha's Vineyard, Massachusetts, I investigated species-environment relationships within and among ponds. I identified vegetation species presences and abundances within 1 m2 quadrats, which ran continuously along transects established perpendicular to the water's edge. Species data were analyzed against local and landscape-scale environmental data. I also conducted a one-year seed bank study in which sediments from four coastal plain ponds were incubated in growth chambers and composition was compared to the standing vegetation. One hundred and thirty-four plant species were identified during vegetation sampling and 38 species were identified from incubated sediments.

I found significant compositional change across space in response to environmental gradients, with patterns in species composition occurring at both local and landscape scales. Elevation was the only local factor strongly correlated with species composition. Significant landscape-scale environmental factors included surficial geology and pond water salinity. Species composition was significantly correlated with hydrologic regime in 2005 but not in 2006. Overall patterns in vegetation species composition and abundance were more closely related to landscape-scale environmental variables than to local environmental variables.

I also found that coastal plain ponds undergo significant compositional change from one year to the next. Interannual variability disproportionately affected certain ponds and quadrats more than others, highlighting patterns in the relationships between compositional change and environmental attributes. Specifically, ephemeral ponds, ponds located on the moraine, ponds with high specific conductance values, and quadrats located closer to the waterline exhibited greater compositional change from 2005 to 2006 than permanent ponds, ponds located on the outwash plain, ponds with low specific conductance values, and quadrats located further from the waterline.

Finally, I found that coastal plain ponds exhibit a low degree of similarity between composition in sediments and standing vegetation. More species were identified in the standing vegetation than in the seed bank, and in most cases average species richness per quadrat was higher in the standing vegetation than in the seed bank. Seed bank and standing vegetation samples from ponds with different surficial geology were compositionally distinct. Seed bank samples from permanent and ephemeral ponds were compositionally distinct whereas standing vegetation samples were not.

This study examines the seasonal and interannual variabilities of carbonaceous aerosols, including black carbon (BC) and organic carbon (OC), over the years of 2005-2016 by using outputs from the NASA GISS ModelE simulations and observations from the OMI instrument aboard Aura, AERONET stations in Amazon region, and the GoAmazon aircraft campaigns.

Simulated seasonal variations and spatial distributions of surface concentrations of BC and OC in Amazon agree well with those of biomass burning emissions. The concentrations are the highest in the dry season (July-September) and lowest in the wet season (February-May), and the locations of high concentrations follow those of high emissions. ModelE is found to underestimate concentrations of OC and BC. Comparisons of the vertical profiles of OC from ModelE with GoAmazon observations in 2014 show that ModelE underestimates OC at all altitudes. In the dry season, when biomass burning dominates, ModelE captures 42%-86% of OMI AAOD in Amazon over 2005-2016, suggesting a low bias in simulated BC concentrations. Simulated seasonal variations in AOD and AAOD in ModelE differ from OMI observations; simulated AOD (AAOD) values are the highest in the dry season, while OMI observed AOD (AAOD) values are the highest in October-January.

Interannual variations in BC and OC are quantified by relative deviation from the mean (RDEVM). Interannual variations of BC and OC in dry season are much higher than those in wet season. RDEVM values are in the range of -63.2% to 127.2% (-70.8% to 143.8%) for BC (OC) in dry season and in the range of -17.8% to 32.7% (-26.3% to 53.4%) for BC (OC) in wet season. Simulated OC concentrations have larger interannual variability than simulated BC for both the dry and wet seasons. We also found that, compared with OMI observations, ModelE overestimates the interannual variability of AOD and AAOD in the Amazon region for both the dry and wet seasons.

Results from this study contribute to the understanding of aerosol distributions in the Amazon and have implications for the impact of carbonaceous aerosols on climate on an interannual timescale.