78 research outputs found
Theoretically-Efficient and Practical Parallel DBSCAN
The DBSCAN method for spatial clustering has received significant attention
due to its applicability in a variety of data analysis tasks. There are fast
sequential algorithms for DBSCAN in Euclidean space that take work
for two dimensions, sub-quadratic work for three or more dimensions, and can be
computed approximately in linear work for any constant number of dimensions.
However, existing parallel DBSCAN algorithms require quadratic work in the
worst case, making them inefficient for large datasets. This paper bridges the
gap between theory and practice of parallel DBSCAN by presenting new parallel
algorithms for Euclidean exact DBSCAN and approximate DBSCAN that match the
work bounds of their sequential counterparts, and are highly parallel
(polylogarithmic depth). We present implementations of our algorithms along
with optimizations that improve their practical performance. We perform a
comprehensive experimental evaluation of our algorithms on a variety of
datasets and parameter settings. Our experiments on a 36-core machine with
hyper-threading show that we outperform existing parallel DBSCAN
implementations by up to several orders of magnitude, and achieve speedups by
up to 33x over the best sequential algorithms
Seasonal Variations of Dissolved Organic Matter by Fluorescent Analysis in a Typical River Catchment in Northern China
Fluorescence (excitation-emission matrices, EEMs) spectroscopy coupled with PARAFAC (parallel factor) modelling and UV-Vis (ultraviolet visible) spectra were used to ascertain the sources, distribution and biogeochemical transformation of dissolved organic matter (DOM) in the Duliujian River catchment. Dissolved organic carbon (DOC), chromophoric dissolved organic matter (a335) (CDOM), and hydrophobic components (a260) were higher in summer than in other seasons with 53.3 m−1, while aromaticity (SUVA254) was higher in spring. Four fluorescent components, namely terrestrial humic acid (HA)-like (A/C), terrestrial fulvic acid (FA)-like (A/M), autochthonous fulvic acid (FA)-like (A/M), and protein-like substances (Tuv/T), were identified using EEM-PARAFAC modelling in this river catchment. The results demonstrated that terrestrial HA-like substances enhance its contents in summer ARE compared with BRE, whilst terrestrial FA-like substances were newly input in summer ARE, which was entirely absent upstream and downstream, suggesting that rain events could significantly input the terrestrial soil-derived DOM in the ambient downward catchments. Autochthonous FA-like substances in summer BRE could derive from phytoplankton in the downstream waters. The results also showed that DOM from wetland exhibited lower fluorescent intensity of humic-like peak A/C and fulvic-like peak A/M, molecular weight (SR) and humification index (HIX) during the low-flow season. Built-up land, cropland, and unused land displayed higher a335 (CDOM). A higher proportion of forest and industrial land in the SCs showed higher SUVA254 values. Humic-like moiety, molecular weight and aromaticity were more responsive to land use during stormflow in summer. Rainfall could increase the export of soil DOM from cropland and unused land, which influences the spatial variation of HIX. The results in this study highlighted that terrestrial DOM has a significant influence on the biogeochemical alterations of DOM compositions and thus water quality in the downward watershed catchments, which might significantly vary according to the land-use types and their alterations by human activities
Reviews and Syntheses: Ocean acidification and its potential impacts on marine ecosystems
Ocean acidification, a complex phenomenon that lowers seawater pH,
is the net outcome of several contributions. They include the dissolution of
increasing atmospheric CO<sub>2</sub> that adds up with dissolved inorganic
carbon (dissolved CO<sub>2</sub>, H<sub>2</sub>CO<sub>3</sub>, HCO<sub>3</sub><sup>−</sup>, and
CO<sub>3</sub><sup>2−</sup>) generated upon mineralization of primary producers (PP) and
dissolved organic matter (DOM). The aquatic processes leading to inorganic
carbon are substantially affected by increased DOM and nutrients via
terrestrial runoff, acidic rainfall, increased PP and algal blooms,
nitrification, denitrification, sulfate reduction, global warming (GW), and
by atmospheric CO<sub>2</sub> itself through enhanced photosynthesis. They are
consecutively associated with enhanced ocean acidification, hypoxia in
acidified deeper seawater, pathogens, algal toxins, oxidative stress by
reactive oxygen species, and thermal stress caused by longer stratification
periods as an effect of GW. We discuss the mechanistic insights into the
aforementioned processes and pH changes, with particular focus on processes
taking place with different timescales (including the diurnal one) in
surface and subsurface seawater. This review also discusses these collective
influences to assess their potential detrimental effects to marine organisms,
and of ecosystem processes and services. Our review of the effects operating
in synergy with ocean acidification will provide a broad insight into the
potential impact of acidification itself on biological processes. The
foreseen danger to marine organisms by acidification is in fact expected to
be amplified by several concurrent and interacting phenomena
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