Abstract
Biodiversity differentials between macrobenthic assemblages associated with adjacent intertidal and subtidal areas of a single seagrass system were investigated for the first time. Assemblage metrics of conservation relevance—faunal abundance and its patchiness, faunal richness, and beta diversity—were examined at four contrasting dwarf-eelgrass localities in the Knysna estuarine bay, part of South Africa's Garden Route National Park but a system whose intertidal areas are heavily impacted anthropogenically. Faunal assemblages were significantly different across all localities and between subtidal and intertidal levels at each locality although their taxonomic distinctness was effectively constant. Although, as would be expected, there were clear trends for increases in overall numbers of species towards the mouth at all levels, few generalities relating to the relative importance of the subtidal seagrass habitat were evident across the whole system—magnitude and direction of differentials were contingent on locality. Shore-height related differences in assemblage metrics were minor in the estuarine and lagoonal zones but major in the marine compartment, although the much greater subtidal faunal abundance there was largely consequent on the superabundance of a single species (the microgastropod Alaba pinnae), intertidal zones then displaying the greater species diversity due to greater equitability of species densities. Along its axial channel, the Knysna subtidal seagrass does not support richer versions of the intertidal polychaete-dominated assemblages fringing it; instead, it supports different and more patchily dispersed gastropod-dominated ones. At Knysna at least, the subtidal hardly constitutes a reservoir of the seagrass biodiversity present intertidally.
Highlights
Seagrass is markedly under-appreciated, for example receiving \ 1.5% of the total media attention devoted to coastal systems, compared to coral reef’s 72.5% (Duarte et al 2008; Dennison 2009; van Keulen et al 2018)
Macrofaunal sampling was conducted over a 10 week period during the 2020 austral summer in the Z. capensis beds of the Knysna estuarine bay, the research being approved by SANParks, and conducted in accordance with their scientific research regulations and requirements
The differential abundance of just five of the 116 species encountered in the seagrass were responsible for generating 54% of this overall pattern; that of A. pinnae alone accounting for 29% (SIMPER)
Summary
Seagrass is markedly under-appreciated, for example receiving \ 1.5% of the total media attention devoted to coastal systems, compared to coral reef’s 72.5% (Duarte et al 2008; Dennison 2009; van Keulen et al 2018) It plays one of the planet’s most important ecosystem-service roles (Costanza et al 2014). Per annum, 1 ha of seagrass can sequester carbon equivalent to that emitted by a car travelling 3350 km, making it a globally significant carbon stock with an average of some 14 kg buried C m-2, and it can absorb the nutrients released in the treated effluent of 200 people (McKenzie and Yoshida 2013; Adams 2016; Lefcheck et al 2019; Githaiga et al 2019). In spite of these known benefits, anthropogenic destruction of seagrass beds continues at a very high rate, i.e. a global loss of 7% year-1 since 1990 (Waycott et al 2009), and this could be releasing 300 Tg of blue carbon annually (Fourqurean et al 2012)
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