Abstract
Tropical seagrass meadows are formed by an array of seagrass species that share the same space. Species sharing the same plot are competing for resources, namely light and inorganic nutrients, which results in the capacity of some species to preempt space from others. However, the drivers behind seagrass species competition are not completely understood. In this work, we studied the competitive interactions among tropical seagrass species of Unguja Island (Zanzibar, Tanzania) using a trait-based approach. We quantified the abundance of eight seagrass species under different trophic states, and selected nine traits related to light and inorganic nutrient preemption to characterize the functional strategy of the species (leaf maximum length and width, leaves per shoot, leaf mass area, vertical rhizome length, shoots per meter of ramet, rhizome diameter, roots per meter of ramet, and root maximum length). From the seagrass abundance we calculated the probability of space preemption between pairs of seagrass species and for each individual seagrass species under the different trophic states. Species had different probabilities of space preemption, with the climax species Thalassodendron ciliatum, Enhalus acoroides, Thalassia hemprichii, and the opportunistic Cymodocea serrulata having the highest probability of preemption, while the pioneer and opportunistic species Halophila ovalis, Syringodium isoetifolium, Halodule uninervis, and Cymodocea rotundata had the lowest. Traits determining the functional strategy showed that there was a size gradient across species. For two co-occurring seagrass species, probability of preemption was the highest for the larger species, it increased as the size difference between species increased and was unaffected by the trophic state. Competitive interactions among seagrass species were asymmetrical, i.e., negative effects were not reciprocal, and the driver behind space preemption was determined by plant size. Seagrass space preemption is a consequence of resource competition, and the probability of a species to exert preemption can be calculated using a trait-based approach.
Highlights
Seagrasses are a polyphyletic assemblage of angiosperm plants that inhabit coastal areas and undergo their entire life cycle in the water
This study addresses our incipient understanding of competition among tropical seagrass species (Ooi et al, 2011) and the role traits play in competitive outcomes
Sites clustered in two groups marking areas subject to oligotrophic and mesotrophic conditions, whereas a single site was markedly different from all others given its high eutrophication (Figures 1, 4A). δ15N in the sediment was double in the eutrophic site than in the oligo- and mesotrophic areas (Figure 4B), indicating discharge of human waste water in the eutrophic site
Summary
Seagrasses are a polyphyletic assemblage of angiosperm plants that inhabit coastal areas and undergo their entire life cycle in the water Their ecosystems are highly productive habitats that support considerable biomasses of associated species diversity (Orth et al, 1984) and offer highly valued ecosystem services (Costanza and Folke, 1997), including climate regulation (Duarte and Chiscano, 1999) and nutrient filtering (Hemminga et al, 1991). Global warming directly affects seagrasses through thermal stress (Ontoria et al, 2019), reducing their biomass and eventually causing mortality (Rasheed and Unsworth, 2011) The influence of these drivers results in a decline in the coverage of seagrass worldwide (Orth et al, 2006; Waycott et al, 2009; Unsworth et al, 2018). This trend may be reversed in temperate and sub-tropical areas (de los Santos et al, 2019; Schäfer et al, 2021), the situation in tropical areas is largely underreported
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