Mollusc aquaculture homogenizes intertidal soft-sediment communities along the 18,400 km long coastline of China

Abstract Aim Molluscs are important grazers, filter and deposit feeders, scavengers and predators, which in turn are food for shorebirds, fish and people. Some species, targeted as human food, have been cultured along the Chinese coast for hundreds of years. To examine whether aquacultural practices have meanwhile affected biodiversity gradients, we measured mollusc community structure along the coast of China in habitats which are intensively used by humans. Location Chinese coast. Methods We sampled 21 intertidal sites spanning 20 latitudinal degrees and 18,400 km of coastline. We assessed alpha diversity to verify whether mollusc communities exhibit the expected biodiversity gradient with latitude and beta diversity gradients with distance. To examine whether human activities such as transportation and culturing could have affected these patterns, we distinguished commercial from non-commercial mollusc species and compared the differences in distribution, density, alpha diversity and beta diversity. Results We found non-commercial species showed the expected biodiversity gradients. Commercial species (a) dominated the intertidal mollusc communities at 19 of the 21 sites and compared with non-commercial species, (b) exhibited wider geographical distributions, (c) showed no significant change in Bray-Curtis index (abundance-based beta diversity) with either geographical or climatic distance, (d) exhibited lower average dissimilarities and (e) did not show a decrease in species richness and Shannon diversity with latitude. Combining all species, trends were the same as for the commercial species. Main conclusions A few cultured species dominated the intertidal mollusc communities in high densities along the Chinese coastline, taking over the ecological roles of the native species but not driving them extinct. In this way, aquacultural practices have exerted a homogenizing influence strong enough to erase basic biodiversity gradients. Since molluscs are food for the growing human population and the shrinking populations of migratory animals, coastal planning and management of both intertidal habitats and the exploitative activities employed need to incorporate these dimensions

Exploring macroinvertebrate species distributions at regional and local scales across a sandy beach geographic continuum.

Exposed sandy beaches are highly dynamic ecosystems where macroinvertebrate species cope with extremely variable environmental conditions. The majority of the beach ecology studies present exposed beaches as physically dominated ecosystems where abiotic factors largely determine the structure and distribution of macrobenthic communities. However, beach species patterns at different scales can be modified by the interaction between different environmental variables, including biotic interactions. In this study, we examined the role of different environmental variables for describing the regional and local scale distributions of common macrobenthic species across 39 beaches along the North coast of Spain. The analyses were carried out using boosted regression trees, a relatively new technique from the field of machine learning. Our study showed that the macroinvertebrate community on exposed beaches is not structured by a single physical factor, but instead by a complex set of drivers including the biotic compound. Thus, at a regional scale the macrobenthic community, in terms of number of species and abundance, was mainly explained by surrogates of food availability, such as chlorophyll a. The results also revealed that the local scale is a feasible way to construct general predictive species-environmental models, since relationships derived from different beaches showed similar responses for most of the species. However, additional information on aspects of beach species distribution can be obtained with large scale models. This study showed that species-environmental models should be validated against changes in spatial extent, and also illustrates the utility of BRTs as a powerful analysis tool for ecology data insight

Two typical snapshots of raw one-day period plankton movements (w-data) around −1000 m at mooring LOC164 (22.5°N, 27°W) for winter and summer.

<p>In winter, the more diffuse downward migration peaks later than expected as compared with the summer-data. The discrepancy between the absolute time difference between plankton movement (w-peaks) in summer and winter (dashed lines in lower panel) is larger in the morning than in the evening. This explains the larger amplitude of DVM-variations with day length observed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064435#pone-0064435-g005" target="_blank">Figure 5</a> for peak-w (*) by about 40 minutes on average.</p

Near-equatorial observations.

<p>(a) As <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064435#pone-0064435-g003" target="_blank">Figure 3a</a>, but for z = −587 m at the equator (0°N, 37°W, mooring LOC144 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064435#pone-0064435-t001" target="_blank">Table 1</a>). (b) Composite mean relative echo intensity from upward looking ADCP at (0°N, 37°W) between −973 and −483 m and downward looking ADCP at (1°N, 38°W, mooring LOC164 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064435#pone-0064435-t001" target="_blank">Table 1</a>) between −1075 and −1565 m. (c) The associated sunrise and sunset profiles with depth with respect to the minimum morning and maximum afternoon values in dI, or plankton, variation (first derivative of, black lines). The filled blue fields indicate errors. For reference, similar profiles have been computed from minimum morning and maximum afternoon w (red dashed lines). Times of local sunrise and sunset are followed (to within errors) down to about −650 m; for z<−650 m the times between minima and maxima for both d(dI) and w decrease roughly linearly with increasing depth; at large depths (z<−1400 m) errors become large, especially for w for which values become unrealistic.</p

Seasonal DVM-variations in day length experienced by the plankton, as calculated in half-overlapping steps from monthly data as in Figure 1b (o) and Figure 1e (*) for z = −1021 m.

<p>Thick solid lines indicate the times of true local day length determined from the sun. (a) Total day length; dashed line is the solid line multiplied by a factor of 77%. This factor is the average day length determined from plankton DVM, being slightly larger for dI-determined day length (o) and slightly smaller for w-determined day length (*). (b) Near sunrise and sunset.</p

Details moored Teledyne RDI 75-kHz, 20°-beam angle Longranger ADCPs.

<p>U-direction is zonal (positive East), V-direction is meridional (positive North).</p

Overview of relative echo intensity data (dI), amount of plankton, and vertical current data (w), movement of plankton, from an upward looking ADCP in the sub-tropical Canary Basin (at 22.5°N, 27°W, mooring LOC163 (Table 1), ranging vertically between z = −1476 and −886 m.

<p>Below −1150 m noise is increased, due to lower amounts of plankton once they have migrated upwards after dusk. Time in 2007 is yearday +365. (a) Entire 18 months of depth versus local solar time series of plankton, or echo intensity, relative to its time mean. (b) Monthly averaged summer time data from white rectangle in a. Symbols ‘s’ indicate local sunrise and sunset, ‘t’ nautical twilight and ‘a’ astronomical twilight. (c) Month of July raw data (rectangle in a.) of which b. is the average. (d) as c., but for w. (e) as b., but for w.</p

DVM-variations in day length inferred from raw data of Figure 1 at z = −1400 m, grouped in 24 hour (daily) periods.

<p>(a) Relative echo intensity. (b) Vertical current; note the much clearer upward migration of the plankton, confined in a shorter period, compared with the less clear downward migrations. In both panels, the black lines denote times of local sunrise and sunset.</p

Latitudinal dependence of DVM-amplitude of seasonal variation in day length determined from (derivative of) daily relative dI-values (o, derivative of) and from daily w-values (*) around 1000 m (except 1400 m for mooring LOC113 at 33°N) using harmonic analysis on a one-year periodicity (single frequency of 1/365 cpd) for all moorings in Table 1.

<p>(a) Day length amplitudes. The red graph indicates the suns variation in day length according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064435#pone.0064435.e003" target="_blank">equation (2)</a>. The dashed red line indicates the same graph shifted by 0.6 hours (see text). Note that at 1000 m the average acoustics data residual is 77% (9.3 hours) of the sun's day length. (b) Normalized residual variance, following harmonic analysis. High values are expected near the equator, where a yearlong harmonic is expected to be small. For those data, vertical averages over 200 m were computed and an expected reduced “error” is indicated by purple symbols. Similarly, averages over 4 bins are computed for data from 15°N, which showed unexpected high normalized residual variance.</p

Thermal tolerance ranges and climate variability: A comparison between bivalves from differing climates

The climate variability hypothesis proposes that in variable temperate climates poikilothermic animals have wide thermal tolerance windows, whereas in constant tropical climates they have small thermal tolerance windows. In this study we quantified and compared the upper and lower lethal thermal tolerance limits of numerous bivalve species from a tropical (Roebuck Bay, north western Australia) and a temperate (Wadden Sea, north western Europe) tidal flat. Species from tropical Roebuck Bay had higher upper and lower lethal thermal limits than species from the temperate Wadden Sea, and Wadden Sea species showed an ability to survive freezing temperatures. The increased freezing resistance of the Wadden Sea species resulted in thermal tolerance windows that were on average 7 °C greater than the Roebuck Bay species. Furthermore, at a local-scale, the upper lethal thermal limits of the Wadden Sea species were positively related to submersion time and thus to encountered temperature variation, but this was not the case for the Roebuck Bay species. A review of previous studies, at a global scale, showed that upper lethal thermal limits of tropical species are closer to maximum habitat temperatures than the upper lethal thermal limits of temperate species, suggesting that temperate species are better adapted to temperature variation. In this study, we show for the first time, at both local and global scales, that the lethal thermal limits of bivalves support the climate variability effect in the marine environment