The ecology of rafting in the marine environment - Bedim

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Average % in Stomachs 60 50 40 30 20 MARTIN THIEL & LARS GUTOW 397 stomachs 372 algal clumps C P 10 CI Ha 0 0 10 20 30 40 50 60 Average % in Floating Algae Figure 16 Relationship between frequency of potential prey organisms on floating algal patches and in fish stomachs of Cyclopterus lumpus sampled with these patches. Figure modified after Ingólfsson & Kristjánsson 2002. that on biotic items (macroalgae). In contrast, Brooks & Bell (2001b) assumed that foraging success of predators around detached macroalgae might be suppressed (compared with the water column) due to the structural complexity of the algae. While the exact influence of substratum characteristics on predation rates may require research attention in the future, in general it can be summarised that predation pressure around floating items is high. The rafting food web As shown above, the rafting community includes primary producers and various levels of consumers. Zaitsev (1970) provided a schematic view of the ecologic relationships in the surface layer of the open ocean. He emphasised the fact that dead organic matter is imported into the system, and that a variety of predators contribute to export of living organic matter (Figure 17A). Cheng (1975) also noted the importance of predator-prey pathways in her scheme of pleustonic food-chain relationships (Figure 17B). Stomach analysis of common rafters and of fishes associated with floating items supported these insights by confirming that many rafters feed on detritus and microorganisms supplied to the raft, and that fishes associated with rafts feed both on rafters as well as on smaller fish. Sano et al. (2003) found that some rafters consume phytoplankton and small detritus while others have a more carnivorous diet, feeding mainly on crustaceans (Figure 17D). Besides these, there are many rafters that have a less catholic diet, being characterised mainly as omnivores. The analysis of Sargassum macrofauna by Butler et al. (1983) and Stoner & Greening (1984) also revealed the high proportion of omnivorous rafters, feeding on detritus, sessile and motile fauna (Figure 17C). This dominance of opportunistic secondary consumers among the rafting community is not surprising since floating items gather in convergence zones where also particulate organic matter accumulates. Morris et al. (1976) remarked that “the limits of food availability within the Sargassum community have imposed a necessity for the largest organisms (crabs, shrimps, and fish) to have the greatest number of alternative food sources”. Several authors also emphasised the fact that excretions of rafting fauna may benefit either floating macroalgae themselves or epiphytic algae (Culliney 1970, Deudero & Morales-Nin 2001a). Similar to that reported for benthic habitats (Taylor & Rees 1998), these dissolved nutrients probably contribute to enhanced growth and, thus, to the longevity of floating or rafting macroalgae and 370 H
B A Organic Matter Small Predators Lift Predators Large Predators Phytoplankton RAFTING OF BENTHIC MARINE ORGANISMS Aquatic Predators Zooplankton Dead Organic Matter Large Predators D C 371 Sargassum & Epiphytes Floating Seaweeds Plankton Herbivores Detritus Hydroids Bryozoans Epiphytes Herbivores Phytoplankton Anemones Small Predators Herbivores Suspension Feeders Zooplankton Copepods Omnivorous Omnivorous Small Predators Omnivorous Specialist Predators Carnivorous Carnivorous Carnivorous Large Predators Figure 17 Schematic representations of qualitative food web models for the neuston, pleuston and floating Sargassum communities. Figures modified after Zaitsev 1970 (A), Cheng 1975 (B), Butler et al. 1983 (C), and Sano et al. 2003 (D).
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1 Oceanography and Marine Biology:
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RAFTING OF BENTHIC MARINE ORGANISMS
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Table 2 RAFTING OF BENTHIC MARINE O
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Table 2 (continued) Species Region
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Porifera RAFTING OF BENTHIC MARINE
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Cnidaria (Anthozoa) RAFTING OF BENT
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Page 41 and 42: RAFTING OF BENTHIC MARINE ORGANISMS
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The ecology of rafting in the marine environment - Bedim

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