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

Abstract

The kelp, Ecklonia radiata, is an abundant subtidal ecosystem engineer in southern Australia. Density‐dependent changes in the abiotic environment engineered by Ecklonia may feedback to affect reproduction and subsequent recruitment. Here, we examined: 1) how the reproductive capacity of Ecklonia individuals in the field (zoospores released · mm−2 reproductive tissue) varied with adult density and time, and 2) how the recruitment of microscopic gametophytes and sporophytes was influenced by zoospore density at two times. Zoospore production did not vary with adult density, with only one month out of ten sampled over a 2‐y period showing a significant effect of density. However, zoospore production varied hugely over time, being generally highest in mid‐autumn and lowest in mid‐late summer. There were strong effects of initial zoospore density on gametophyte and sporophyte recruitment with both a minimum and an optimum zoospore density for sporophyte recruitment, but these varied in time. Almost no sporophytes developed when initial zoospore density was <6.5 · mm−2 in spring or <0.5 · mm−2 in winter with optimum densities of 90‐355 · mm−2 in spring and 21‐261 · mm−2 in winter, which resulted in relatively high recruitment of 4‐7 sporophytes · mm−2. Sporophyte recruitment declined at initial zoospore densities >335 · mm−2 in spring and >261 · mm−2 in winter and was zero at very high zoospore densities. These findings suggest that although adult Ecklonia density does not affect per‐capita zoospore production, because there is a minimum zoospore density for sporophyte production, a decline in population‐level output could feedback to impact recruitment.

Optimising the zoospore release, germination, development of gametophytes and formation of sporophytes of Ecklonia radiata

“…For example, Tatsumi and Wright (Tatsumi and Wright 2016) obtained an average of 5 female gametophytes mm −2 and 0.5 sporophytes mm −2 in a laboratory experiment after 14 and 30 days respectively, equating to a transition rate of 10%. Another study quantified the transition rate of E. radiata during June (austral winter) and November (austral summer) and found that the transition from female gametophyte to sporophyte was higher in winter, with 30-55% transition compared to 19-36% transition in summer (Tatsumi et al 2022). Notably, cultures were maintained under 12 °C in June and under 17 °C in November to represent ambient winter and summer water temperatures respectively in that study (Tatsumi et al 2022), suggesting that seasonality as well as a temperature may be important.…”

“…Another study quantified the transition rate of E. radiata during June (austral winter) and November (austral summer) and found that the transition from female gametophyte to sporophyte was higher in winter, with 30-55% transition compared to 19-36% transition in summer (Tatsumi et al 2022). Notably, cultures were maintained under 12 °C in June and under 17 °C in November to represent ambient winter and summer water temperatures respectively in that study (Tatsumi et al 2022), suggesting that seasonality as well as a temperature may be important. An effect of both seasonality and temperature on gametogenesis and transition to sporophyte has been previously demonstrated for kelp species (Lee and Brinkhuis 1988;Tala et al 2004;Murúa et al 2013).…”

“…Crucially, the transition to the sporophyte stage occurs only under favourable conditions (Nelson 2005;Carney and Edwards 2010;Martins et al 2017) and the effect of abiotic factors on the development of each of the life stages of E. radiata is poorly understood. Previous studies have identified low rates of transition to sporophytes, with more than half of the seeded surfaces failing to produce sporophytes (Tatsumi and Wright 2016) and transition rates from female gametophyte to sporophyte as low as 10% (Tatsumi and Wright 2016) to 55% (Tatsumi et al 2022). Therefore, a critical first step towards achieving the controlled cultivation of E. radiata for any application is to develop standardised protocols for the hatchery stage that result in high rates of germination and transition from gametophyte to sporophyte.…”

“…This is of particular importance when collection in the field is strongly weather-dependent or in a remote location resulting in a limited time window to collect biomass and process it upon return to the laboratory. Studies on E. radiata have initiated zoospore release on the day of collection (Mohring et al 2013;Suebsanguan et al 2021) or one day after collection (Tatsumi and Wright 2016;Tatsumi et al 2022). However, the effect of delaying the initiation of zoospore release on the quantity and viability of released zoospores remains unknown and needs to be considered, including whether initiation of zoospore release can be delayed for more than one day following collection.…”

A strategic review and research roadmap for offshore seaweed aquaculture—A case study from southern Australia

“…The species could also readily contribute to the bulk ‘kelp biomass’ market targeted at fertiliser and alginate production, and perhaps as a source of nutraceuticals (e.g., fucoidan) 1,63 . The first studies targeting nursery cultivation methods for the species that underpin near and offshore cultivation are emerging 55,64,65 . Despite the extensive ecological knowledge of the species, its aquaculture potential is largely unknown.…”