The influence of abiotic and biotic conditions on lifecycle stages is critical for estuarine seagrass resilience

Abstract

Abiotic and biotic factors influence seagrass resilience, but the strength and relative importance of the effects are rarely assessed over the complete lifecycle. This study examined the effects of abiotic (salinity, temperature, water depth) and biotic (grazing by black swans) factors on Ruppia spp. over the complete lifecycle. Structures were set up in two estuaries ( – 33.637020, 115.412608) that prevented and allowed natural swan grazing of the seagrasses in May 2019, before the start of the growing season. The density of life stage(s) was measured from June 2019 when germination commenced through to January 2020 when most of the seagrass senesced. Our results showed that swans impacted some but not all life stages. Seedling densities were significantly higher in the plots that allowed natural grazing compared to the exclusion plots (e.g. 697 versus 311 seedlings per m-2), revealing an apparent benefit of swans. Swans removed ≤ 10% of seagrass vegetation but a dormant seedbank was present and new propagules were also observed. We conclude that grazing by swans provides some benefit to seagrass resilience by enhancing seedling recruitment. We further investigated the drivers of the different lifecycle stages using general additive mixed models. Higher and more variable salinity led to increased seed germination whilst temperature explained variation in seedling density and adult plant abundance. Bet-hedging strategies of R. polycarpa were revealed by our lifecycle assessment including the presence of a dormant seedbank, germinated seeds and seedlings over the 8-month study period over variable conditions (salinity 2–42 ppt; temperatures 11–28 °C). These strategies may be key determinants of resilience to emerging salinity and temperature regimes from a changing climate.

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