Structure of subtidal algal assemblages on soft-bottom sediments: fauna/flora interactions and role of disturbances in the Bay of Brest, France

Abstract

Algal and fauna1 assemblages living on soft bottom sediments were studied in the Bay of Brest, France, using quantitative descriptions of the distribution, diversity and biomass of epibenthic species. Similar classification and ordination analyses were used on floral and animal data. Dense assemblages of macrophytes, mainly Rhodophyceae, were found to grow on sandy mud sediments. Sediments were essentially covered by maerl beds of the calcareous red alga Lithothamnion corallioides and various shell substrates. Five maln algal assemblages extended along a west-east decreasing gradient of hydrodynamic activity (i.e. tidal currents, waves and turbidity). The high frequency of storms during the year is the main feature which disturbs both flora and epifauna. Interactions between flora and fauna were either direct, e.g. grazing and spatial competition, or indirect, most of the substrata occupied by macrophytes being shells of dead bivalves and gastropods. Moreover attachment of algae was largely dependent on population dynamics of shell species living in the area. As a consequence, in the most disturbed area, the macrophytes found were opportunistic species (Polysiphonia fibrillosa and Polysiphonia urceolata). In areas where the assemblage was unstructured, a fact which prevented the development of the herbivorous species, the animal assemblage was then dominated by suspension feeders (FicuLina ficus and Phallusia marnmillata) which competed for space with the macrophytes. In the least disturbed area, the herbivorous species dominated the fauna assemblage while flora assemblage had a high level of organization. However, in those places eutrophication and high turbidity prevented the establishment of many algal species, causing the ubiquitous ones to domInate (Ulva sp., Ceramium rubrum). Between these extreme areas, the living Lithotharnnion beds were more developed and were defined as a characteristic assemblage of diversified Rhodophyceae species such as Chyloclad~a verticillata, Halarachnion ligulaturn, Crytopleura ramosa. Finally, the role of wave disturbance as the main factor controlling algal assemblages of shallow soft-bottom sedlments is discussed.