Composition of Rafting Assemblages on Floating Litter

Taxonomic Overview

A review of 82 publications revealed a total of 387 marine litter rafting taxa, of which 244 were identified to the species, and 143 to the genus level (for complete species list see Appendix 1). In this review we included publications that report on organisms associated with floating litter in the field as well as experimental studies on the colonization of anthropogenic flotsam. We did not consider the many experimental studies on the succession of fouling communities on rigidly fixed artificial substrata because these items do not display the specific floating behavior, which probably affects the colonization by marine biota. To avoid potential overlaps, taxa identified at genus level were excluded if a species-level identification existed for the same genus. The identification of some micro-organisms was vague despite the use of advanced analytical methods such as electron microscopy and RNA analysis. Most taxa (335) were associated with plastic substrata (domestic waste, plastic fragments or buoys made of plastic), which constitute the large majority of anthropogenic floating litter in the oceans (Galgani et al. 2015). Accordingly, only few taxa (17) were recorded from other floating litter items consisting of metal, glass and paper. For 83 taxa, the floating substrata were of unknown composition or were composed of various materials. The given numbers exceed the total number of 387 taxa because some species have been found on more than just one substratum type. 132 taxa were recorded from items, which previously served maritime purposes (mainly buoys and fishing gear). A large proportion (60 %) of the rafting taxa was sampled in situ, associated with their floating substrata, whereas 35 % of the taxa are only known from beached litter. For 2 %, the ability to raft on floating litter was inferred from floating experiments (Bravo et al. 2011) and the remaining 3 % consist of taxa that could not be reliably identified but were assigned to a certain genus or species by the respective authors.

Fig. 6.4 Number of observed rafting taxa on floating marine litter (number of studies in brackets) in major oceanic regions (from top left Arctic, North Atlantic, Mediterranean, North Pacific, South Pacific, South Atlantic, Indian Ocean, Southern Ocean). The symbols represent reports of frequently observed rafting species on marine litter: Circles = Jellyella tuberculata, squares = Lepas anatifera, triangles = Idotea metallica, stars = Fiona pinnata. The two crosses represent the northernand southernmost observations of rafters on marine litter

The highest numbers of rafting taxa on floating litter were found in the Pacific and North Atlantic, which might be explained by the overall high research effort undertaken in these regions (Fig. 6.4). A considerable number of rafters were also found in the Mediterranean while only few taxa were reported from the South Atlantic and from the Indian Ocean. Some rafters have even been found in the Arctic at 79°N (Barnes and Milner 2005) and in Antarctica at approximately 67°S (Barnes and Fraser 2003). The percentage of anthropogenic litter items colonized varied significantly with latitude. Barnes and Milner (2005) found that at low latitudes (0–15°) about 50 % of all beached litter items were colonized by marine biota while at higher latitudes (15–40°) only 25 % of the litter items had attached organisms. This rate decreased further to 5–10 % at 40–60° latitude and beyond 60° colonization of marine litter was rarely observed (Fig. 6.5). This geographic pattern was evident for remote sites as well as for sites close to the continental shore (Barnes 2002). A similar latitudinal decrease of the colonization rate was evident on a smaller spatial scale for the Indian Ocean (Barnes 2004).

Numerous taxa of bacteria, protists and algae (most prominently diatoms and Rhodophyta) form part of the rafting community on marine fl litter (Table 6.1). Four studies examined the microbiota associated with marine microplastics (i.e. plastic particles in the size range of millimetres and a few centimetres—Fortuño et al. 2010; Carson et al. 2013; Zettler et al. 2013; Reisser et al. 2014) and found a total

Fig. 6.5 Proportion of marine litter colonized according to latitude (modified after Barnes and Milner 2005)

of 44 bacteria and 56 Chromista taxa. Micro-organisms seem to be ubiquitous on marine litter as Carson et al. (2013) found microbes on each plastic item sampled in the North Pacifi gyre. Plastic litter offers a habitat for various functional microbial groups including autotrophs, symbionts, heterotrophs (including phagotrophs) and predators (Zettler et al. 2013). Harmful micro-organisms were also found on fl litter, including potential human and animal pathogens of the genus Vibrio (Zettler et al. 2013), the ciliate Halofolliculina sp., which causes skeletal eroding band disease in corals (Goldstein et al. 2014) and the dinofl Ostreopsis sp., Coolia sp. and Alexandrium taylori, known to form harmful algal blooms under favorable conditions (Masó et al. 2003). The composition of the microbial community clearly differs from the surrounding seawater suggesting that plastic litter forms a novel habitat for microbiota (termed 'microbial reef' by Zettler et al. 2013). Some organisms found on plastic samples are otherwise strictly associated with open seawater and their presence was probably the result of entanglement (Zettler et al. 2013). Carson et al. (2013) characterized the encountered microbial community in the North Pacifi gyre as dominated by rod-shaped bacteria and pennate diatoms, each at densities of roughly 1,000 cells m−2. Less frequent microbiota on plastic samples comprised coccoid bacteria, centric diatoms, dinofl coccolithophores, and radiolarians. A surprisingly low morphological diversity among the abundant diatoms was mentioned.

Macroalgae have occasionally been found attached to floating marine litter, among them red (11 taxa), brown (6 taxa) and some green algae (4 taxa). However, rarely was a single taxon encountered more than once. Diatoms (29 taxa), dinoflagellates (5 taxa) and foraminiferans (7 taxa) seem to be more common, although likewise, very few taxa were reported more than once, probably owing to the low number of studies focusing on micro-organisms.

The most common invertebrate groups on marine litter are crustaceans, bryozoans, molluscs and cnidarians (Table 6.1). The composition of taxa retrieved from beached litter tends to be biased towards sessile organisms with hard (calcifi structures such as bryozoans, foraminiferans, tubeworms and barnacles (Stevens et al. 1996; Winston et al. 1997; Gregory 2009). Mobile organisms such as crustaceans and annelids are more frequently observed on rafts collected while afl (Astudillo et al. 2009; Goldstein et al. 2014). Some taxa have repeatedly been observed associated with fl litter (Fig. 6.4) and thus, may not just be accidental rafters.

Table 6.1 Taxonomic overview of marine litter rafters (for complete taxonomic list see Appendix 1)

Kingdom

Phylum

Class

Order

Number of taxa

Bacteria

44

Chromista

Ciliophora

2

Foraminifera

7

Myzozoa

Dinophyceae

5

Haptophyta

7

Ochrophyta

Bacillariophyceae

29

Phaeophyceae

6

Plantae

Charophyta

1

Chlorophyta

3

Rhodophyta

11

Animalia

Porifera

2

Cnidaria

Anthozoa

10

Hydrozoa

26

Nemertea

1

Annelida

Polychaeta

27

Arthropoda

Pycnogonida

1

Insecta

3

Ostracoda

1

Maxillopoda

Kentrogonida

1

Lepadiformes

11

Sessilia

15

Malacostraca

Decapoda

22

Amphipoda

21

Isopoda

8

Tanaidacea

1

Mollusca

Gastropoda

18

Bivalvia

21

Echinodermata

3

Bryozoa

Table 6.1 (continued)

Kingdom

Phylum

Class

Order

Number of taxa

Gymnolaemata

66

Stenolaemata

10

Chordata

Ascidiacea

4

Total

387

Stalked barnacles of the genus Lepas are by far the most frequently encountered hitchhikers in all major oceanic regions except for the Arctic and Southern Ocean. Seven Lepas species have been found rafting on litter, the most frequently observed and widespread being L. anatifera and L. pectinata. Lepas are prominent fouling species and readily colonize a variety of floating objects, a process likely facilitated by their extended planktonic larval stage (Southward 1987).

Isopods of the genus Idotea are frequently found on marine litter in the Atlantic, Pacific and Mediterranean. While I. metallica and I. baltica have repeatedly been reported on floating litter items other species such as I. emarginata are less common. Idotea metallica is an obligate rafter without benthic populations, and the constant replenishment of an otherwise not self-sustaining population in the North Sea illustrates its conformity with the rafting environment (Gutow and Franke 2001). Idotea metallica shows specific adaptations to the rafting life-style such as reduced “locomotive activity and a tight association to the substratum” and low food requirements compared to its congener I. baltica (Gutow et al. 2006, 2007). The latter species predominantly colonizes algal rafts, which are rapidly consumed by this voracious herbivore (Gutow 2003; Vandendriessche et al. 2007). Other frequently encountered crustaceans include the three pelagic species of crab, Planes major, P. marinus and P. minutus, found in the Atlantic, Pacific and Indian Ocean; and five species of the diverse amphipod genus Caprella, whose members show morphological adaptations in the form of reduced abdominal appendages enabling them to cling to flotsam (Takeuchi and Sawamoto 1998).

Bryozoans from the closely related genera Membranipora and Jellyella were found rafting on marine litter in the Atlantic, Pacific, Mediterranean and even in Arctic waters. Jellyella tuberculata was the most frequently encountered species in the Atlantic and Pacific and is known to colonize a wide range of substrata including plastic litter and macroalgae (Winston et al. 1997). The species typically occurs at tropical and subtropical latitudes (Gregory 1978), however, sightings on marine litter are reported from all major oceanic regions with the exception of polar seas (Fig. 6.4). The most common gastropod on floating litter, Fiona pinnata, was sighted in the Pacific and Mediterranean. According to Willan (1979), F. pinnata has a cosmopolitan distribution and commonly inhabits floating wood and macroalgae where it can exploit its Lepas prey, growing on the same substratum.

 
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