Abstract
Intensive surveys have been conducted to unravel spatial patterns of benthic infauna communities. Although it has been recognized that benthic organisms are spatially structured along the horizontal and vertical dimensions of the sediment, little is known on how these two dimensions interact with each other. In this study we investigated the interdependence between the vertical and horizontal dimensions in structuring marine nematodes assemblages. We tested whether the similarity in nematode species composition along the horizontal dimension was dependent on the vertical layer of the sediment. To test this hypothesis, three-cm interval sediment samples (15 cm depth) were taken independently from two bedforms in three estuaries. Results indicated that assemblages living in the top layers are more abundant, species rich and less variable, in terms of species presence/absence and relative abundances, than assemblages living in the deeper layers. Results showed that redox potential explained the greatest amount (12%) of variability in species composition, more than depth or particle size. The fauna inhabiting the more oxygenated layers were more homogeneous across the horizontal scales than those from the reduced layers. In contrast to previous studies, which suggested that reduced layers are characterized by a specific set of tolerant species, the present study showed that species assemblages in the deeper layers are more causal (characterized mainly by vagrant species). The proposed mechanism is that at the superficial oxygenated layers, species have higher chances of being resuspended and displaced over longer distances by passive transport, while at the deeper anoxic layers they are restricted to active dispersal from the above and nearby sediments. Such restriction in the dispersal potential together with the unfavorable environmental conditions leads to randomness in the presence of species resulting in the high variability between assemblages along the horizontal dimension.
Highlights
The sediment is a three-dimensional habitat for a vast number of infauna species
We observed that assemblages living in the top more oxygenated layers are more abundant, species rich and at the same time are less variable, in terms of species presence/absence and relative abundances, than assemblages living in the deeper reduced layers
We observed species typical of reduced sediments, such as Sabatieria sp.3 and Spirinia sp.1 in samples classified as ‘‘strongly reduced’’, this sediment class showed very low similarity along the horizontal scale suggesting that there is not a specific set of species living under these conditions
Summary
The sediment is a three-dimensional habitat for a vast number of infauna species In the sediment, these benthic organisms are spatially structured through of a variety of environmental factors, such as granulometry, salinity and oxygen and food availability, along both the horizontal and vertical dimensions [1,2,3]. Based on the current understanding of the vertical distribution of the fauna and of the redox profile, it can be hypothesized that the species poor assemblages inhabiting the more reduced layers of the sediment will be characterized by few tolerant species If this pattern proves to be consistent at multiple sites (horizontal scale), we can expect that the deep samples will have the same set of tolerant species and high similarity in the multivariate analysis. Given the high organic conditions of estuaries, reduced conditions are usually present at the deeper layers of the sediment [26]
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