Prevailing sea surface temperatures inhibit summer reproduction of the kelp Laminaria digitata at Helgoland (North Sea)

Abstract

The impact of abiotic factors on kelp sporophyte reproduction has rarely been investigated. Laminaria digitata (Hudson) J.V. Lamouroux is one of the few summer fertile Laminaria species worldwide and reproduction is subjected to relatively high water temperatures. We investigated the impact of prevailing summer temperatures (~18°C in August) on the induction of sporangia, meiospore release, and germination at the island of Helgoland (North Sea). At Helgoland, fertile sporophytes are found between April and December with a maximum in late summer. While released meiospore numbers were constant between June and October, germination rates decreased significantly in summer. Short-term exposure of mature sori to 17°C-22°C induced a significantly higher meiospore release indicating enhancement of sporulation by elevated temperatures. Induction of sporangia on vegetative blade disks was not possible at 20°C, and fertility was only 20% at 18°C-19°C, but it was 100% in cool temperatures of 1°C-10°C. It was shown for the first time in a kelp species that "sporogenesis" is the life-cycle process with the narrowest temperature window compared to growth or survival of the sporophyte or reproduction, growth, and survival of the gametophyte. We incorporated several parameters (induction time, fertile area, and relative fertility) into a "Reproductive efficiency index." This indicates that sporogenesis of L. digitata is a cold-adapted process with an optimum at (5)-10°C. The results show that the population at Helgoland is at its reproduction limit despite the existence of other geographically more southerly located populations.

Status, trends and drivers of kelp forests in Europe: an expert assessment

“…The reasons for this reduction are unknown but may be a consequence of competition between species, namely caused by the expansion of L. hyperborea. Recently it was shown that the reproductive efficiency of the infralittoral population of L. digitata was negatively affected in summer conditions, and that this upper population may become eradicated intermittently after warm summers (Bartsch et al 2013). In the Baltic, where L. digitata reaches its salinity limits, the recorded changes might be due to hydrological reasons.…”

“…comm. ) and are under pressure in the southern North Sea (Bartsch et al 2013). In Kongsfjorden (western Spitsbergen, Svalbard) this species considerably increased in biomass since 1996/98 at shallow sublittoral depths (Bartsch et al 2016).…”

Interactions of daylength, temperature and nutrients affect thresholds for life stage transitions in the kelp Laminaria digitata (Phaeophyceae)

“…(2) As high summer temperatures (>17°C) delay gametogenesis and recruitment, the potential recruitment period is shifted to cooler autumn to winter conditions. (3) As all vegetative gametophyte cells are omnipotent and may become fertile later when conditions become cooler again, zoospores released under unfavourable summer conditions are not necessarily lost from the population, as described by Bartsch et al (2013). This sequence of events would prevent the formation of juvenile sporophytes in stressful summer conditions (low nutrients, high temperatures and high irradiation) but nevertheless ensures recruitment later in the year.…”

Wasted effort: recruitment and persistence of kelp on algal turf

“…Increasing ocean temperature may drive losses of kelp canopy by limiting reproduction (Bartsch et al 2013) and growth (Bolton & Lüning 1982, Simonson et al 2015, Gao et al 2016 or through tissue degradation that increases fragmentation or mortality (Lüning 1984, Wernberg et al 2013, Simonson et al 2015. Proliferation of turf algae following canopy removal is intensified by factors that directly stimulate turf growth, including increases in ocean temperatures (Connell & Russell 2010).…”