Context dependency in the effect of Ulva‐induced loss of seagrass cover on estuarine macrobenthic abundance and biodiversity

Abstract

1 An extensive area of seagrass covering the shore of part of the South African estuary ranked highest in overall national conservation and biodiversity importance (the Knysna estuarine bay) was killed by a series of seasonal green-tidal blankets of Ulva starting in the late austral summer of 2015, leaving bare muddy sediment. An earlier (2011) survey of then seagrass macrobenthic assemblages at 27 stations along over 1 km of that shore was exactly repeated in 2018 after this event.2 After loss of seagrass, intertidal macrofauna became significantly less speciose (1:0.61), spatially and systematically more uniform (1:1.30 and 1:1.63), but more (not the usual less) abundant (0.62:1). Assemblage composition also changed, with polychaete numbers (small nereidids, spionids, fabriciids and an endemic paraonid) increasing from <50% of total macrofaunal individuals to >90% (although cirratulids decreased), whereas small malacostracan crustaceans (except the amphipod Grandidierella sp.) and microgastropods diminished to insignificant levelsmicrogastropods for reasons possibly unconnected with the green tide.3 Comparison with areas not vegetated in 2014, however, indicated that although macrofaunal abundance was greater in the 2018 Ulva-induced bare areas, levels of per unit area biodiversity (number of species, species and taxonomic diversity per station) were comparable. This suggests that the effect of local loss of seagrass cover is context dependent, particularly with regard to pre-existing local seagrass versus adjacent bare-sediment differentials, and does not necessarily lead to any "degraded" state.4 Decreased local importance of small malacostracans may severely impact the region's usage by the adult and juvenile fish for which it is an important nursery and feeding ground.

Do beds of subtidal estuarine seagrass constitute a refuge for macrobenthic biodiversity threatened intertidally?

“…This may well account for the restriction in numbers of infaunal polychaetes subtidally (Hughes et al 2000). Even the smaller subsurface biomass of intertidal seagrass may affect the numbers of burrowing worms in that there was a marked increase in their abundance after local loss of the eelgrass on the shores of Knysna's Steenbok Channel (Barnes 2019a;and see Pillay et al 2010b). If that is so, then areas of subtidal seagrass are unlikely ever to provide a suitable refuge for such polychaetes.…”

“…It has not, however, been designated a Marine Protected Area, notwithstanding that it is ranked South Africa's premier estuary in terms of its biodiversity and overall conservation importance (Turpie and Clark 2007;Van Niekerk et al 2019). Seagrasses are subject to many pressures (Gubbay 2016;Mvungi and Pillay 2019;Unsworth et al 2019) and at Knysna two of the main ones are green-algal blooms (Human et al 2016;Barnes 2019a;Pollard et al 2019) and heavy exploitation for bait largely for subsistence and recreational fishing (Hodgson et al 2000a;Napier et al 2009;Simon et al 2019;Claassens et al 2020).…”

Nutrient enrichment as a threat to the ecological resilience and health of South African microtidal estuaries

“…Polychaete numbers increased, but small malocostracan crustaceans and microgastropods diminished to insignificant levels. These changes altered the availability of food for adult and juvenile fish and reduced the estuary's value as a nursery and feeding ground (Barnes 2019). The deterioration of water quality in the Knysna Estuary is cause for concern, because it has been ranked most important in the country for ecosystem services and biodiversity conservation (Turpie et al 2002;Turpie and Joubert 2005).…”

A multistressor model of carbon acquisition regulation for macroalgae in a changing climate

“…Bloom‐forming species in the intertidal zone are poised to exploit changes in environmental conditions through rapid regulation of production. Macroalgal blooms in the intertidal zone have disproportionately large ecosystem effects because of the intensity of primary production per unit area in the biotically diverse land–sea interface (Aires et al 2019; Barnes 2019; Zhang et al 2019). The shifting ratios of resources, warming, and more acidified oceans may change the species composition of coastal ecosystems to favor noncalcifying phototrophs and animals over calcifying species (Wootton et al 2008; Koch et al 2013) and weedy species over long lived ones.…”