Abstract
Seagrasses, as key ecosystem engineers in coastal ecosystems, contribute to enhancing diversity in comparison with nearby bare areas. It has been proved mainly for epifauna, but data on infauna are still scarce. The present study addresses how seagrass structural complexity (i.e., canopy properties) affects the diversity of infaunal organisms inhabiting those meadows. Canopy attributes were achieved using seagrass mimics, which were used to construct in situ vegetation patches with two contrasting canopy properties (i.e., shoot density and morphology) resembling the two seagrass species thriving in the inner Cadiz Bay: Zostera noltei and Cymodocea nodosa. After three months, bare nearby areas, two mimicked seagrass patches (‘Zostera’ and ‘Cymodocea’), and the surrounding natural populations of Zostera noltei were sampled in a spatially explicit way. Shifts in organism diets were also determined using 15N and 13C analyses in available food sources and main infaunal organisms, mixing models, and niche metrics (standard ellipse area). Seagrass-mimicked habitats increased the species richness (two-fold), organism abundance (three to four times), and functional diversity compared with bare nearby areas. The clam Scrobicularia plana (deposit/filter feeder) and the worm Hediste diversicolor (omnivore) were dominant in all of the samples (> 85%) and showed an opposite spatial distribution in the reconstructed patches: whilst S. plana accumulated in the outer-edge parts of the meadow, H. diversicolor abounded in the center. Changes in the isotopic signature of both species depending on the treatment suggest that this faunal distribution was associated with a shift in the diet of the organisms. Based on our results, we concluded that facilitation processes (e.g., reduction in predation and in bioturbation pressures) and changes in food availability (quality and quantity) mediated by seagrass canopies were the main driving forces structuring this community in an intertidal muddy area of low diversity.
Highlights
Species richness doubled that in bare sediment (BS) and was similar to Z. noltei natural populations (ZNNP). Such an increase was associated with a rise in functional diversity, where mainly carnivore polychaetes increased in number (Table 2)
The total contribution of both species to the total infauna was quite constant in all of the sampled points (>85%), it changed drastically when artificial patches were included in the system (Table 2); whereas H. diversicolor represented less than 7% in untreated plots (BS and ZNNP), its abundance increased up to 32% in Zostera noltei artificial patches (ZNAP) and Cymodocea nodosa artificial patches (CNAP) (Table 2)
It suggests that the alteration of the hydrodynamically driven food supply may be an important underlying factor contributing to structure seagrass communities
Summary
Seagrass ecologists broadly agree that anthropogenic activities are the main drivers of the current regression of seagrass ecosystems worldwide [1]. This brings into awareness that most of the crucial functions and services provided by these valuable ecosystems [2]. One of the most important functions of seagrasses is to act as ecosystem engineers [3], supporting higher biodiversity levels than bare areas [4,5,6]. The ongoing regression of seagrasses and/or associated diversity might impair the functioning of coastal ecosystems, with profound consequences for human welfare [7,8,9]. Deepening our understanding on the potential positive interactions between seagrasses and their associated communities is essential for successful conservation and restoration strategies [10,11]
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