Covariance-based Principal Component Analysis PCA was used to identify relations between concentrations of environmental variables and sediment depth. In analyses of molluscan community composition and diversity, 2-cm-thick increments in the top 20 cm were pooled into 4-cm-thick increments, and increments of equivalent sediment depths in two cores M28 and M29 were pooled into a single composite core.
This diversity measure transforms standard diversity indices onto a linear, easy-to-visualize scale representing the number of species in perfectly even communities [ 60 ]. Differences in effective species richness between six temporal bins were tested using the Wilcoxon rank test and the median number of species. Differences in molluscan community composition between consecutive bins were quantified by the pseudo-F statistics of permutational multivariate analysis of variance PERMANOVA , and its significance assessed by randomization of increment attribution to particular temporal bins.
This statistics represents the ratio between the variation explained by temporal bins and the unexplained variation. Non-metric multi-dimensional scaling NMDS based on Bray-Curtis distances and fit to two dimensions was used to visualize changes in the mollusc community structure down the sediment core no rare species or singletons were removed. This ordination method is a powerful tool to examine gradients in species composition [ 15 , 61 ]. Bray-Curtis distances are based on square-root transformed relative abundances. Correlations between NMDS-axes scores and environmental factors enabled us to explore the sources of compositional variation in ordination space.
This procedure was performed only for those core intervals for which measurement data of environmental factors are available. Using square-root transformed relative abundances i. Hellinger transformation , compositional distances among mollusc assemblages in RDA ordinations tend to accurately preserve Euclidean distances among them [ 63 ].www.iricom.com.ua/images/2019-08-08/surveiller-un-mobile.html
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All statistical analyses were performed in R-studio version 3. The sediment composition in Panzano Bay is very homogeneous throughout the cores. For PAHs, no thresholds are plotted because measured concentrations are considerably lower. Down-core concentrations are highly variable for the investigated heavy metals, nutrients, and organic pollutants Fig 3 and S2 Dataset.
Two groups of geochemical variables showing distinct stratigraphic distribution are evident in the PCA Fig 4. On one hand, concentrations of Pb, Zn, PAH, PCB, TOC, Ntot, and sand percentage are low in the deepest core layers, increase slightly in the intermediate section and rise to peak values in the uppermost sediment layers, starting from about 35 cm core depth Fig 3.
On the other hand, Cu, Cr, Ni and Hg decrease towards the surface and reach minimum values in the upper 35 cm. Concentrations of Cd follow a similar trend, with values decreasing in the upper 75 cm, but remaining high throughout the lower part.
The concentrations of all heavy metals except Cr and organic pollutants drop conspicuously in the uppermost 5 cm of sediment. Hg concentration peaks at Some heavy metal concentrations markedly exceed pollution thresholds Fig 3. The peak concentration of Hg In total, the two sediment cores contained bivalve individuals 50 species , gastropods 50 species , scaphopods 3 species , and one polyplacophoran see S1 Dataset for a full list of species with down-core abundances. The most abundant bivalves are the commensal galeommatid Kurtiella bidentata , the shallow-burrowing suspension-feeders C.
The gastropod community is dominated by Turritella communis , Nassarius pygmaeus , and Aporrhais pespelecani. Antalis inaequicostata is the most abundant scaphopod. Total molluscan abundance in cm thick increments is low in the deeper part of the cores 90— cm , oscillating around a mean of individuals per increment, with the smallest abundances in the middle part early 19 th century and a minimum value of shells in the cm increment. Higher up in the cores, abundance increases steeply and peaks at shells between 12 and 16 cm. A sudden drop to almost half this value characterizes the transition to the shallowest sediment layers Fig 2.
This overall abundance fluctuation negatively correlates with sedimentation rates Fig 2. Several species increase in relative abundance up-core N.
Species are sorted according to their weighted average occurrence along the cores, from top left side to bottom right side. Exceptions: maximum occurrence of Weinkauffia turgidula : 5; relative abundance of Pitar rudis : 2. For some species, the 0-value of the x-axis has been omitted for better readability. The first marked temporal changes in community structure based on species relative abundances occur in the cm layer late 18 th century with a strong increase of K. This trend continues up the core max. The opportunistic bivalve C.
Formerly abundant species such as T.
The molluscan community significantly differs between three pairs of consecutive temporal bins: 0—16 vs 16—30 cm, 16—30 vs 30—65 cm, 65—95 vs 95— cm Table 2. Other pairs of temporal bins do not differ significantly in composition 95— vs — cm; 30—65 vs 65—95 cm and are pooled in analyses of diversity. Thus, four distinct phases of molluscan community states can be distinguished: the deep section th centuries, —95 cm , the intermediate section 19th century, 95—30 cm , the subsurface section early 20 th century, 30—16 cm , and the surface section late 20 th century.
Species are sorted according to their abundance rank in the deepest core section.
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Only main categories are shown see Table 1 , and categories with low abundances are omitted for better readability. Percentages of epifaunal molluscs left column , scavengers middle column , and echinoderm commensals right column increase while infauna and ascidian commensals decline towards the 20 th century. Absolute abundances of infauna also increase towards the 20 th century, in contrast to relative abundances. Raw molluscan species richness ranges between 23 — cm core depth, 18 th century and 51 4—8 cm, late 20 th century species per increment and slightly increases from the bottom of the core to the surface, with a marked low in the middle part 50—90 cm, 19 th century, Fig 2B.
The observed number of species changes with total abundances. Bivalves generally display higher species numbers than gastropods.
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The effective species richness exp H remains relatively constant throughout the cores except for the uppermost layers, where a sharp increase is visible in the s Fig 2C and 2D. The percentages of the individual guilds remain largely constant throughout the core. There are, however, two peaks in detritivores at 95 and 50 cm, respectively, with corresponding lows of filter feeders.
From 30 cm sediment depth upwards, a major shift can be observed, with scavengers becoming the second most important feeding guild instead of detritivores, while filter feeders decrease. At 12 cm, this trend reverses again Fig 7. Relative abundances of infaunal species decline upwards, whereas their absolute abundances strongly increase in the upper third of the core 19—20 th century. The largest group with twelve species is associated with echinoderms and predominantly includes K.
A similar trend characterizes ascidian-associated molluscs mainly M.
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Crustacean- and mollusc-associated species are rare Fig 7. Differences between the temporal bins emerge not only with regard to species composition, but also on a functional level Table 2. For feeding guild and host association, significant differences appear between deep and intermediate 30—95 vs 95— cm and intermediate and subsurface 16—30 vs 30—95 cm core sections following the pattern found for compositional differences.
Organism-substrate relations behave slightly differently; here, changes are most marked in the transition from deep to intermediate and from subsurface to surface 16—30 vs 0—16 cm section. Feeding guilds and host associations do not change significantly within the uppermost 30 cm. NMDS ordinations reveal a marked community shift on the functional level as well as in species composition along the first axis at a core depth of about 40 cm, corresponding to the second half of the 19 th century Fig 8A—8D.
For feeding guilds, this shift is mainly influenced by the growing importance of scavengers and a simultaneous decline of most other feeding guilds. A conspicuous drop in epibionts and ectoparasites is the main driver for shifting organism-substrate relations between the subsurface and surface interval. Host associations display an overall decline in the uppermost 30 cm. This trend is reflected by the first axis in the ordination. The two groups separated along the second axis represent a shift towards a dominance of echinoderm associations in the intermediate section with a gradual decline of ascidian-associated species.
Ordinations are based on down-core species abundances A , feeding guilds B , organism-substrate relations C , and host associations D.
Greyscale shadings of dots mark the four main molluscan community phases as listed in panel C. The three dominant species K. The — cm sample is dissociated from the deep cluster due to a sharp peak in the relative abundance of T. Changes in feeding guilds are furthermore significantly correlated with the concentrations of Cu und Ni.
The concentrations of Ntot and PCB are those with the highest correlation coefficients and are also strongly correlated with changes in organism-substrate relations. Table 3. No significant correlations were found between environmental variables and changes in host association. Likewise, for all aspects of community change, the correlation between NMDS-axis-2 scores and the measured environmental variables is insignificant. In the RDA, total nitrogen content explains the largest proportion of down-core variation in molluscan community composition.
This variable effectively separates the assemblages of the uppermost core section at least down to 12 cm depth from the rest of the core.
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The same result is obtained for RDAs based on organism-substrate relations and feeding guilds. Total nitrogen separates the topmost core intervals from the rest of the core and explains a significant amount of variation in relative abundance of species A , feeding guilds B , organism-substrate relations C , and host associations D. The second axis represent the first principal component in A-C because the stepwise selection selected one environmental variable only. Despite the relatively long time-span covered by the two sediment cores from Panzano Bay, their sedimentological structure is very homogeneous, with silt and clay as the main sediment constituents almost uniformly distributed throughout the cores.