Food and habitat choice in floating seaweed clumps: the obligate opportunistic nature of the associated macrofauna

Abstract

The species composition of macrofauna associated with floating seaweed rafts is highly variable and influenced by many factors like spatial and temporal variation, period since detachment and probably also the seaweed species. The presence of seaweed preferences was assessed by a combination of in situ seaweed samplings and multiplechoice aquarium experiments in a controlled environment, using the seaweedassociated grazing organisms Idotea baltica and Gammarus crinicornis. Results from sampling data confirm that the seaweed composition has an effect on macrofaunal species composition and abundance: samples dominated by Sargassum muticum displayed higher densities but lower diversities compared to samples dominated by Ascophyllum nodosum and Fucus vesiculosus. Seaweed preference was also apparent from the multiple choice experiments, but did not exactly match the results of the community analysis: (1) I. baltica had high densities in seaweed samples dominated by F. vesiculosus and A. nodosum, while in the experiments this isopod was most frequently associated with Enteromorpha sp. and F. vesiculosus, and fed mostly on S. muticum, A. nodosum and Enteromorpha sp.; (2) G. crinicornis had high densities in seaweed samples dominated by F. vesiculosus, while in the experiments this amphipod was most frequently associated with S. muticum, but fed most on A. nodosum and F. vesiculosus. It is clear from the laboratory experiments that preference for habitat (shelter) and food can differ among seaweed species. However, food and habitat preferences are hard to assess because grazer preference may change if choices are increased or decreased, if different sizes of grazers are used, or if predators or other grazers are added to the experiments. Effects of seaweed composition may also be blurred due to the obligate opportunistic nature of a lot of the associated macrofaunal species.

Spatio-temporal distribution of floating objects in the German Bight (North Sea)

“…We observed densities ranging from 1 to more than 1000 items km − 2 , which is comparable to values reported from the fjords of Southern Chile (Hinojosa et al, 2010) and coastal waters of California (Kingsford, 1995), New Zealand (Kingsford, 1992) and Japan (Segawa et al, 1960). Despite previous reports of floating seaweed and their associated fauna from the North Sea (Franke et al, 1999;Vandendriessche et al, 2006) the abundances of these items had so far not been estimated for this area. We found highest abundances of floating seaweed to the south-east of the island of Helgoland and in the western area off the East Frisian coast.…”

“…The higher proportion of A. nodosum and S. muticum observed in Belgian coastal waters (Vandendriessche et al, 2006), as well as the higher abundances of these species in the western area of the East Frisian sector most likely indicate an origin from the British Channel. Similarly, Kornmann and Sahling (1977) also inferred that H. elongata, stranded on Helgoland in late summer, probably comes from sources in the British Channel because this seaweed often harbors algal epiphytes that are common in Brittany and southern England.…”

“…The high densities in waters near Helgoland could be related to accumulations in the quasistable estuarine front in this area (Skov and Prins, 2001, Fig. 1), while the accumulation in the western area of the East Frisian sector might also receive input of floating seaweeds from the British Channel (see also Vandendriessche et al, 2006).…”

“…For example, Vandendriessche et al (2006) found approximately equal proportions of Fucus spp. (33%), A. nodosum (31%), and S. muticum (22%) in Belgian coastal waters (SW-North Sea), while we observed N90% Fucus spp.…”

“…In the North Atlantic, rafting on floating objects has been repeatedly inferred as a common dispersal process for marine organisms (e.g. Ingólfsson, 1995;Vandendriessche et al, 2006). It was suggested to explain the occurrence of apparently disjoint populations and/or the (re)colonization of isolated shores by marine invertebrates without a planktonic larval stage (Johannesson, 1988;Johannesson and Warmoes, 1990;Ingólfsson, 1992;Thiel and Haye, 2006).…”