Multi-scale effects of hydrological and landscape variables on macrophyte richness and composition in British lakes

Macrophytes are an integral component of lake littoral zones and play an irreplaceable role in maintaining the ecological balance of wetlands. Recent research has highlighted the role of lake-scale environmental factors (or “filters”) and catchment- and/or landscape-scale processes in explaining variation in macrophyte communities across different scales. In this work, the effects of land-use and connectivity on macrophyte communities were explored at two contrasting spatial scales (i.e. local catchment scale and topographic catchment scale). At the local catchment scale, the results revealed strong scale-dependency. The effects of land use on macrophyte richness were most apparent at fine spatial scales (within 0.5 to 1 km) and significantly outweighed the importance of hydrology. In terms of growth form composition, the effects of hydrological connectivity were stronger than those of land use, with the greatest effect observed at an intermediate distance (~ 5 km) from the lake. The study on the hydrologically-connected lake pairs indicated that environmental filters were more influential in explaining species turnover than lake connectivity. Interestingly, geographical connectivity explained more of the variability in species turnover than hydrological connectivity. Moreover, the relative importance of environmental filters and lake connectivity to species turnover was very sensitive to the degree of human disturbance. The multi-scale interaction analyses indicated the effect of lake alkalinity on macrophyte composition is strongly influenced by catchment scale variables including hydrological features and land use intensity. The turnover in macrophyte composition in response to variability in alkalinity was stronger in catchments with low lake and stream density and weaker in catchments with a more highly developed hydrological network. Lake abiotic variables were found to have more influence on macrophyte composition in lowland catchments with a higher intensity of human disturbance. Moreover, the catchment-scale factors promoting the establishment of different communities were found to vary between catchments depending on lake type, the degree of environmental heterogeneity and hydrological connectivity

The influence of hydrological and land use indicators on macrophyte richness in lakes – A comparison of catchment and landscape buffers across multiple scales

In biogeography it is well established that environmental variables often have scale-dependent effects on abundance and distribution of organisms. Here we present results from a study on scale-dependency of macrophyte (aquatic plant) richness to hydrology and land use indicators. Hydrological connectivity and land use within the landscape surrounding 90 \{UK\} lakes, at nine buffer sizes varying from 0.25 km to 10 km from the shoreline, with (catchment buffer) and without (landscape buffer) adherence to the catchment boundary, were constructed using GIS. These variables were used to explain variation in macrophyte richness derived from field surveys. The results revealed strong scale-dependency. The effects of land use were most apparent at small buffer sizes and grossly outweighed the importance of hydrology at all spatial scales. The total richness of macrophytes was most strongly determined by land use and hydrology within 1 km of the lake for landscape buffers and 500 m for catchment buffers. The nature of the scale-dependent effect also varied with macrophyte growth habit. In terms of growth form composition, the effects of hydrological connectivity were stronger than those of land use, being greatest at an intermediate distance (∼5 km) from the lake. Our results indicate the value of maintaining some lake catchments with less intensive land use, at least within 1 km of the lake shore, while also minimising alterations to catchment hydrology (e.g. through drainage or impoundment) over distances extending at least 5 km from the lake shore

Response of terrestrial net primary productivity (NPPT) in the Wujiang catchment (China) to the construction of cascade hydropower stations

The damming of rivers results in hydrological modifications that not only affect the aquatic ecosystem but also adjoining terrestrial systems. Thirteen dams commissioned along the Wujiang River have induced ecological problems, including decreased water turbidity and loss of biodiversity, which potentially influence ecosystem net primary production (NPP) and hence the sequestration, transformation, and storage of carbon. We used terrestrial NPP (NPPT) as a bioindicator to assess the impact of dams on carbon storage in the Wujiang catchment. MODIS satellite and meteorological data were used as inputs to the CASA model to calculate annual NPPT from 2000 to 2014. NPPT was calculated at the catchment and landscape scale to quantify the impact of dams on surrounding terrestrial ecosystems. Mean NPPT was calculated for concentric buffer zones covering a range of spatial extents (0–10 km) from the reservoir shoreline. We found a negligible impact from construction of a single dam on NPPT at the catchment scale. By contrast, the impact of dam construction was scale-dependent, with a stronger landscape-scale effect observed at short distances (i.e., 0–1 km) from the reservoir. Decreases in NPPT were mainly ascribed to the loss of vegetated land resulting from dam impoundment and subsequent urbanization of the surrounding area

Lake and catchment-scale determinants of aquatic vegetation across almost 1,000 lakes and the contrasts between lake types

Aim The factors controlling macrophyte (aquatic plant) composition are complex, recent research having shown that the well-studied effects of lake environmental factors (the so-called “environmental filter”) can be constrained by hydrological and landscape factors. We investigated the factors determining macrophyte composition in lakes over water body and catchment- scales and the transferability of this pattern across lake types. Location Almost 1000 lakes distributed across Britain. Taxon Lake macrophytes Methods Lakes were partitioned into five types based on subdivision of alkalinity and elevation gradients. Data from botanical surveys were used to compare the spatial turnover and nestedness components of beta diversity between lake types. The relative importance of lake environment (based on local physicochemical data), hydrology (e.g. lake and stream density), landscape (e.g. fragmentation indices, land cover) and spatial autocorrelation in explaining variation in macrophyte composition were derived from variance partitioning. Results Species composition showed strong spatial structuring, suggestive of overland dispersal, enhanced by spatially-correlated abiotic factors such as alkalinity and elevation. Catchment-scale factors (e.g. land use, connectivity) promoted the establishment of different communities (more or less diverse, or differing in composition) but were of secondary importance. Turnover in composition between upland lakes was lower than in other lake types, reflecting a more specialist flora and increased potential for propagule exchange due to spatial aggregation and higher hydrological connectivity. Main conclusions Vegetation composition in lakes is more spatially-structured than previously appreciated, consistent with the importance of dispersal limitation, but this does not apply evenly to all lakes, being most acute in lowland high alkalinity lakes. Thus, spatially-structured abiotic factors, such as alkalinity, influence macrophyte composition most (suggestive of niche filtering) in high alkalinity lakes where human impacts tend to be greatest, although nestedness was also lowest in such lakes. By contrast, hydrological connectivity has a proportionally stronger structuring role in upland lakes

Piercing Through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields: I. X-ray Spectral and Long-term Variability Analyses

We present a detailed X-ray spectral analysis of 1152 AGNs selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs ($N_{\rm H} > 10^{23}\ \rm cm^{-2}$). By fitting spectra with physical models, 436 (38%) sources with $L_{\rm X} > 10^{42}\ \rm erg\ s^{-1}$ are confirmed to be highly obscured, including 102 Compton-thick (CT) candidates. We propose a new hardness-ratio measure of the obscuration level which can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed logN-logS relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. 19% (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high-column-density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sightline may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic NH distribution and its evolution. The CT-to-highly-obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (~17 years in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of NH, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified due to large spectral fitting errors.Comment: 32 pages, 21 figures, 9 tables, accepted for publication in Ap

Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. II. Are Highly Obscured AGNs the Missing Link in the Merger-Triggered AGN-Galaxy Coevolution Models?

By using a large highly obscured ($N_{\rm H} > 10^{23}\ \rm cm^{-2}$) AGN sample (294 sources at $z \sim 0-5$) selected from detailed X-ray spectral analyses in the deepest Chandra surveys, we explore distributions of these X-ray sources in various optical/IR/X-ray color-color diagrams and their host-galaxy properties, aiming at characterizing the nuclear obscuration environment and the triggering mechanism of highly obscured AGNs. We find that the refined IRAC color-color diagram fails to identify the majority of X-ray selected highly obscured AGNs, even for the most luminous sources with ${\rm log}\,L_{\rm X}\, \rm (erg\ s^{-1}) > 44$. Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of $f_{\rm 24 \mu m}\, /\,f_R > 1000$ and $R - K > 4.5$ criteria, implying complex origins and conditions for the obscuring materials that are responsible for the heavy X-ray obscuration. The average star formation rate of highly obscured AGNs is similar to that of stellar mass- ($M_*$-) and $z$-controlled normal galaxies, while the lack of quiescent hosts is observed for the former. Partial correlation analyses imply that highly obscured AGN activity (traced by $L_{\rm X}$) appears to be more fundamentally related to $M_*$, and no dependence of $N_{\rm H}$ on either $M_*$ or SFR is detected. Morphology analyses reveal that 61% of our sources have a significant disk component, while only 27% of them exhibit irregular morphological signatures. These findings together point toward a scenario where secular processes (e.g., galactic-disk instabilities), instead of mergers, are most probable to be the leading mechanism that triggers accretion activities of X-ray-selected highly obscured AGNs.Comment: 23 pages, 12 figures, 3 tables, resubmitted to ApJ after addressing referee's comment