Reproductive Output, Synchrony across Depth and Influence of Source Depth in the Development of Early Life stages of Kelp

Abstract

Ecklonia radiata is the main foundation species in Australian temperate reefs, yet little has been published on its reproduction and how this may change across its depth range (1–50+ m). In this study, we examined differences in sporophyte morphology and zoospore production during a reproductive season and across four depths (7, 15, 25, and 40 m). Additionally, we examined differences in germination rate, survival, and morphological traits of gametophytes obtained from these four depths, cultured under the same light and temperature conditions. Multivariate morphology of sporophytes differed significantly between deep (~40 m) and shallow sites (7 and 15 m), but individual morphological traits were not significantly different across depths. Total spore production was similar across depths but the peak of zoospore release was observed in February at 15 m of depth (6,154 zoospores · mm−2 of tissue) and the minimum observed in January at 7, 25, and 40 m (1,141, 987, and 214 zoospores · mm−2 of tissue, respectively). The source depth of zoospores did not have an influence in the germination rate or the survival of gametophytes, and only gametophytes sourced from 40 m sites presented significantly less surface area and number of branches. Overall, these results indicate that E. radiata’s reproductive performance does not change across its depth range and that kelp beds reproducing in deeper areas may contribute to the replenishment of their shallow counterparts. We propose that deep kelps may constitute a mechanism of resilience against climate change and anthropogenic disturbances.

Density‐dependence and seasonal variation in reproductive output and sporophyte production in the kelp, Ecklonia radiata

“…2013a, 2013b, Giraldo‐Ospina et al. 2021). However, in Tasmania, a peak in zoospore release occurs during autumn and winter, which coincides with falling seawater temperature and is lowest in summer when ocean temperatures are highest.…”

“…2013b), although the peak in zoospore production appears to be delayed in deeper water (Giraldo‐Ospina et al. 2021).…”

Intergrading reef communities across discrete seaweed habitats in a temperate–tropical transition zone: Lessons for species reshuffling in a warming ocean

“…Despite the suppression of upwelling by the Leeuwin Current (Twomey et al, 2007), sporadic localized upwelling can occur under the right circumstances, and there are more upwelling days per year in the Abrolhos compared with the rest of the WA coast (Rossi et al, 2013). This local upwelling could also act as a vector for propagule dispersal from more deeper, cooler seaweed communities and bringing more diversity (Giraldo‐Ospina et al, 2021). Alternatively, the large amplitude internal waves (LAIW) may reduce the heat stress on shallow subtidal ecosystems (Reid et al, 2019).…”

Cross‐depth connectivity shows that deep kelps may act as refugia by reseeding climate‐vulnerable shallow beds

“…In the Perth region, the season starts in early summer and extends through spring (Mohring et al, 2013). Deep populations of E. radiata (40 m) have been shown to produce similar reproductive output and viable zoospores as shallower populations (Giraldo‐Ospina, Kendrick, & Hovey, 2020). After an extreme marine heat wave occurred in Western Australia during the summer of 2010–2011, the distribution of shallow beds of E. radiata contracted ~100 km (Smale & Wernberg, 2013; Wernberg et al, 2016).…”

“…To achieve this, we first estimated the total number of zoospores released by one individual of E. radiata at each of our target depths. For each depth, we calculated the area of reproductive tissue per kelp individual by using the average laminar area of kelp individuals (collected at each release depth from Giraldo‐Ospina, Kendrick, & Hovey, 2020; see Appendix S2: Table S1 for parameters used for this estimation) and the known maximum percentage area of reproductive tissue for E. radiata measured in Mohring et al (2013), which was maintained constant across depth, since there were no estimates across depth (Appendix S2: Table S1). Next, we calculated the total amount of zoospores released at all release sites (each site of 50 × 50 m) by multiplying the depth‐specific zoospore production by the average density of kelp sporophytes in the region (25 individuals m − 2 ) obtained from Kirkman (1984).…”