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Marine Ecology Progress Series

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MEPS 199:127-136 (2000)  -  doi:10.3354/meps199127

Flagging greens: hydrobiid snails as substrata for the development of green algal mats (Entero- morpha spp.) on tidal flats of North Atlantic coasts

Dirk Schories1,*, Jaime Anibal2, Annelise S. Chapman3, Elisabeth Herre4, Ingela Isaksson5, Ana I. Lillebø6, Leif Pihl5, Karsten Reise4, Martin Sprung2, Martin Thiel7

1Center for Tropical Marine Ecology, Fahrenheitstr. 1, 28359 Bremen, Germany
2Centro de Ciências do Mar, Unidade de Ciências e Tecnologias dos Recursos Aquáticos, Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal
3(formerly: A. S. Albrecht) Dept. of Biol. Sciences, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA, UK
4Wattenmeerstation Sylt, Alfred-Wegener-Institut für Polar- und Meeresforschung, 25992 List/Sylt, Germany
5Kristineberg Marine Research Station, 45034 Fiskebäckskil, Sweden
6IMAR Institute of Marine Research, Dept. of Zoology, University of Coimbra, 3004-517 Coimbra, Portugal
7Facultad de Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile

ABSTRACT: During the past 3 decades, dense mats of green algae (especially Enteromorpha spp.) have been recorded regularly from tidal flats worldwide. The development of green algal mats on tidal flats may be initiated by overwintering and regrowth of adult plants or by the formation and release of small propagules, i.e. vegetative fragments, zoospores and zygotes. On soft sediments, macroinvertebrates may constitute prime substrata for germination of algal spores. Hydrobiid (mud-) snails are widespread along North Atlantic soft sediment shores and were identified previously as important substrata for Enteromorpha spp. germlings in 1 of our study areas. To test the generality of this phenomenon, we investigated the presence of Enteromorpha spp. germlings attached to hydrobiid snails from November 1995 to December 1996 on 6 tidal flats of North Atlantic coasts (Trälebergskile, Sweden; Königshafen Bay, Germany; Mondego Estuary, Portugal; Ria Formosa, Portugal; Cole Harbour, Nova Scotia, Canada; Lowes Cove, Maine, USA). With 1 exception, hydrobiid snails were present in all areas studied, and intensive growth of Enteromorpha spp. occurred during summer. Throughout winter 1995/96, hardly any Enteromorpha spp. germlings were found on snail shells, but over the following months germlings developed on up to 60% of the hydrobiid snails present. In 2 areas (Königshafen, Germany; Lowes Cove, USA), germling abundance on hydrobiids began to rise before the peak of green algal mat development. In Trälebergskile, Sweden, high mat abundance occurred simultaneous to and after increased germling abundance on Hydrobia ulvae. Densities of snails were very low, however, and hydrobiids appeared to be unimportant as substratum. No clear temporal pattern between high germling abundance on snails followed by mat development was found in the other 3 study areas (Ria Formosa, Portugal; Mondego Estuary, Portugal; Cole Harbour, Canada). In Lowes Cove, USA, germlings and juveniles of Enteromorpha spp. first grew at the site with high Hydrobia abundance and were subsequently drifted to another site where they developed into full mats. We conclude that initiation of green algal mats by germination on Hydrobia spp. may be a general phenomenon, but that other modes of development also occur frequently. Pelagic drift of overwintering thalli to new sites, followed by prolific growth, might be of similar or greater importance.

KEY WORDS: Algal blooms · Eutrophication · Epibionts · Tidal flat · Hydrobia · Enteromorpha · Germination

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