80 J
. Norkko et al. J. Exp. Mar. Biol. Ecol. 248 2000 79 –104 Keywords
: Baltic Sea; Benthic macrofauna; Eutrophication; Macroalgae; Oxygen deficiency
1. Introduction
Benthic drift algae may increase habitat complexity on normally bare soft bottoms Langtry and Jacoby, 1996; Norkko, 1997; Raffaelli et al., 1998, thereby changing the
settings for interspecific interactions Norkko, 1998. High abundances of invertebrates have been recorded in sublittoral accumulations of drifting macroalgae and seagrass, and
the highest abundances of macrofauna recorded in marine benthic communities were found in accumulations of algal detritus off the West Coast of the USA Vetter, 1994,
1995. As opposed to other physical structures which add a three dimensional aspect to benthic habitats, benthic drift algae constitute a highly mobile structure. Although
ephemeral both in time and space, benthic drift algae have been shown to function as an alternative habitat for mobile benthic macrofauna, providing food and refuge from
predators Gore et al., 1981; Kulczycki et al., 1981; Virnstein and Carbonara, 1985; Virnstein and Howard, 1987; Holmquist, 1994, 1997; Vetter, 1995, 1998; Tzetlin et al.,
1997; Norkko, 1998. High abundances of macrofauna have also been recorded in intertidal Nicholls et al., 1981; Soulsby et al., 1982; Kirkman and Kendrick, 1997 and
pelagic Lenanton et al., 1982; Kingsford and Choat, 1985; Kingsford, 1992; Shaffer et
´ al., 1995; Ingolfsson, 1998 drift algal mats.
The effects of drift algal mats on infauna of normally bare soft bottoms depend on the spatial and temporal extent of algal coverage. Patchy occurrences of drift algae may be
beneficial to the benthic community and increase diversity Pihl et al., 1996; Norkko, 1997; Raffaelli et al., 1998, but at the same time high amounts of drift algae are known
to stress benthic infauna, the result often being an impoverished community with an altered species composition Nicholls et al., 1981; Soulsby et al., 1982; Thrush, 1986;
Hull, 1987; Bonsdorff, 1992; Everett, 1994; Peterson et al., 1994; Bonsdorff et al., 1995; Norkko and Bonsdorff, 1996a,b; Holmquist, 1997. When algae cover extensive areas
the habitat is homogenised and oxygen deficiency induced, resulting in decreased diversity in the benthic community. The seasonal and often sudden occurrence of
drifting algae will have varying effects on the behaviour, habitat use and survival of benthic species, depending on their functional group, mobility and tolerance to hypoxia
Norkko, 1998. If conditions under the algae are stressful, mobile benthic species could potentially avoid these by moving up into the overlying algae. Alternatively, mobile
species could actively colonise patches of drift algae in search of food and refuge from predators and thus benefit from the additional physical structure and food drifting algae
provide for the benthos.
Excessive growth of macroalgae is a well-known symptom of eutrophication Morand and Briand, 1996; Schramm and Nienhuis, 1996; Raffaelli et al., 1998, and due to the
increasing eutrophication of the Baltic Sea Elmgren, 1989; Cederwall and Elmgren, 1990; HELCOM, 1990; Rosenberg et al., 1990; Bonsdorff et al., 1991, 1997; Nehring
¨ and Matthaus, 1991; Blomqvist et al., 1999, the proliferation of annual filamentous
macroalgae in the littoral zone has increased in the northern Baltic Sea over the last
J . Norkko et al. J. Exp. Mar. Biol. Ecol. 248 2000 79 –104
81
¨ 10–15 years Makinen and Aulio, 1986; Kautsky, 1991; Bonsdorff et al. 1997. When
these algae are detached from their substratum they are transported to the sublittoral and deposited on relatively shallow
, 15 m soft bottoms as increasing amounts of drifting algae, covering ever increasing areas in late summer Olafsson, 1988; Bonsdorff, 1992;
Norkko, 1997. Drift algae occur both as small patches and as large homogenous mats, at times covering up to several hectares Norkko and Bonsdorff, 1996b. Small amounts
of drifting algae have always been present in this region, but the amounts recorded in recent years in the archipelago area are a new, eutrophication induced phenomenon. The
effects of the algae are therefore changing from stochastic to more predictable, and the algae have the potential to significantly alter the structure and function of this benthic
system Norkko, 1997.
No spatial and or temporal studies of species composition and abundance of the macrofauna associated with benthic drift algal mats have been conducted in the Baltic
Sea. The aim of this study was therefore to describe and quantify the invertebrate
˚
macrofauna occurring in benthic drift algal mats in the archipelago of the Aland Islands, northern Baltic Sea. We also experimentally tested differences in response to algal mats
in species representing different functional and taxonomic groups. We wanted thus to investigate the potential value of benthic drift algal mats as habitat for benthic fauna.
This was accomplished through sampling of algal mats in the field and through a series of laboratory experiments.
2. Methods
