The ecology of rafting in the marine environment - Bedim

MARTIN THIEL & LARS GUTOW
that future comparisons should focus on biological traits of rafting organisms and on ecological
interactions.
Geographic and seasonal variations in rafting communities
The abundance and species composition of rafting communities varies throughout the world’s
oceans. This is partly due to the global variations in availability of different rafting substrata (see
Thiel & Gutow 2004). However, there are also regional differences in other environmental variables
(e.g., nutrient availability, primary productivity and temperature) that affect the presence and
persistence of rafting organisms on floating items.
Regional differences in the Sargassum community had been recognised early on. Timmermann
(1932) emphasised that the northern part of the Sargasso Sea, i.e., the area of the Gulf Stream,
features a higher diversity of rafting organisms than the southern part. Conover & Sieburth (1964)
revealed that bacterial population densities on floating Sargassum in the North Atlantic varied
geographically according to the specific production of chemical antifouling mechanisms. Stoner &
Greening (1984) provided a multifactorial comparison of the fauna associated with floating Sargassum
from the Sargasso Sea and the Gulf Stream and found only low similarities between faunal
assemblages from these regions. Sargasso Sea samples showed a relatively even distribution of
species, with the gastropod Litiopa melanostoma dominating the community with a mean proportion
of about 25% of the total fauna. Gulf Stream samples, however, were slightly less diverse and
strongly dominated by the shrimp Latreutes fucorum comprising about 70% of the Sargassum fauna
in that region. Consistently, the Gulf Stream fauna yielded a higher percentage of large omnivores
(sensu Butler et al. 1983), while in the Sargasso Sea smaller omnivores consuming primarily sessile
prey organisms were more abundant. Similarly, Smith et al. (1973) found higher numbers of rafting
species in the Slope Water (33 species) than in the Sargasso Sea (about 25 species). However,
densities of the animals in the Sargasso Sea (~4200 ind. g –1 wet weight [WW] Sargassum) were
about twice as high as in the Slope Water (~2000 ind. g –1 WW Sargassum). In the same region,
Keller (1987) found that the biomass of the three most abundant invertebrate species was linked
to the nutrient content of the surrounding waters. High epifaunal biomass was observed in the
centre of the nutrient-rich, cool slope waters north of the Gulf Stream while productivity was low
in the oligotrophic waters of the Sargasso Sea. Unexpectedly high productivity of epifauna was
detected in the Sargasso Sea in the track of a cold water ring from the Gulf Stream. The nutrientloaded
subarctic slope water allowed for high primary production inside the ring influencing even
the productivity of higher trophic levels such as the motile carnivorous epifauna on floating
Sargassum. Niermann (1986) suggested that the low degree of epibiont coverage (primarily bryozoans
and hydrozoans) on floating Sargassum in the Sargasso Sea proper could also be due to food
limitation.
Data presented by Winston et al. (1997) first hinted towards a latitudinal decrease in colonisation
of marine debris by sessile rafters, which was later confirmed by studies from Barnes & Sanderson
(2000). In general, items from low latitudes are densely colonised while items from high latitudes
are typically uncolonised (Winston et al. 1997, Barnes & Sanderson 2000, Barnes 2002). While
there exist latitudinal variations in abundance and type of floating substrata (Barnes 2002, Thiel &
Gutow 2004), differences in colonisation appear to be primarily due to ecological factors. In polar
regions sessile rafters (primarily bryozoans) were almost completely absent from floating debris
(Figure 22), and the authors discussed that this could be due to the harsh physical conditions at
high latitudes (Barnes & Sanderson 2000, Barnes 2002).
Seasonal variations of the abundance of rafting organisms are also more pronounced at high
latitudes (Figure 23). During the winter, many floating items collected in Icelandic waters (64°N)
were almost free of rafters. In particular, the occurrence of some colonisers on floating seaweed
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