Control of macroalgal blooms at early developmental stages: Pilayella littoralis versus Enteromorpha spp.

  • Lotze H
  • Schramm W
  • Schories D
 et al. 
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Abstract

Although blooms of opportunistic fast-growing macroalgae now occur
frequently in coastal ecosystems affected by eutrophication, their
initiation and control is little understood. Most previous studies
have focused on the ecophysiology of adult algae only. We show that
spores and/or germlings may represent critical stages in the life
cycles and mass-developments of co-occurring opportunistic macroalgae
in the Baltic (Pilayella littoralis and Enteromorpha spp.). We investigated
the overwintering of spores, timing of germination, subsequent growth,
and grazing on spores and germlings, in order to explain the initiation
of mass blooms and species dominance patterns. In the field, Enteromorpha
spp. showed 10- to 50-fold higher abundances of overwintering microscopic
forms (up to 330 individuals cm−2) than P. littoralis. Moreover,
we found continuous production of spores (up to 1.2 million settling
spores m−2 h−1) from April to October in Enteromorpha spp., while
there was evidence of only a short reproductive period in Pilayella.
However, in spring, germlings and adults of P. littoralis appeared
earlier in the field and reached a 10-fold higher biomass than Enteromorpha
spp. In factorial laboratory experiments including temperature and
light, there were clear differences in timing of germination. P.
littoralis germinated at 5°C whereas Enteromorpha spp. required temperatures
of 10–15°C for germination. In contrast, we detected only minor differences
in growth response among adults of P. littoralis and Enteromorpha
spp. Germination, not growth of adults, appeared to be the ecophysiological
bottleneck for initiating mass spring development. Following the
spring Pilayella bloom, Enteromorpha germlings occurred massively
in the field (April–September), but rarely developed into adults.
In laboratory feeding experiments we tested whether crustacean mesograzers
common in summer may control development of Enteromorpha germlings.
Both germination of settled spores and growth of germlings were reduced
by 93–99% in the presence of grazers (Idotea chelipes and Gammarus
locusta). Thus in addition to ecophysiological constraints, grazers,
if present, may play a decisive role in the early life stages of
macroalgal mass developments. These results mirror patterns of overwintering
of seeds, germination control, seed and seedling predation in terrestrial
plant communities.

Author-supplied keywords

  • Baltic Sea
  • Coastal eutrophication Crustacean grazers germination
  • Overwintering propagules

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Authors

  • Heike K. Lotze

  • Winfrid Schramm

  • Dirk Schories

  • Boris Worm

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