Deep Sea Research Part I: Oceanographic Research Papers
Effects of megafauna exclusion on nematode assemblages at a deep-sea site
Introduction
Mechanisms of diversity maintenance in the deep sea have been the subject of speculation for decades (Snelgrove and Smith, 2002). The matter of how so many species coexist within nutrient-poor sediments is especially intriguing since the deep sea lacks the obvious spatial and temporal complexities that contribute to diversity in other high-diversity environments (Gage, 1996). On the other hand, the deep-sea environment is generally homogeneous in terms of temperature, sediment composition, physical disturbance and broad topography. At the small scale, however, there is increasing evidence of temporal and spatial variability in biogenic disturbance and biogenic microhabitats. Their importance for diversity maintenance is the focus of ongoing research (Snelgrove and Smith, 2002; Thistle, 2003).
Habitat heterogeneity and small-scale biological disturbances, although through different mechanisms, are both expected to enhance diversity. The habitat heterogeneity hypothesis assumes that structurally complex habitats may provide more diverse ways of exploiting the environmental resources, thereby increasing species diversity (Bazzaz, 1975), whereas disturbance should enhance diversity by reducing populations of dominant competitors (Connell, 1978; Huston, 1979). In the deep sea, very modest sources of disturbance are considered important because they are operating in a comparatively stable, low-energy environment. Disturbance of the surface sediment due to the feeding activities of relatively larger organisms, for example, may enhance diversity because their effects could persist long enough to be used over different successional stages by a broad spectrum of organisms (Snelgrove and Smith, 2002). In the same way, biologically generated habitat heterogeneity might persist in the sediment for years and therefore contribute to the high species coexistence in the deep sea (Thistle, 1981; Gage, 1996).
Small-scale disturbances and biogenic heterogeneity in the deep sea are created partly by the activities of epibenthic megafaunal organisms (Gray, 1974; Wheatcroft et al., 1989; Kukert and Smith, 1992), which are conspicuous elements of deep-sea communities and occur in virtually all bathyal and abyssal habitats (Gage and Tyler, 1991). These comparably large organisms are dominated by deposit feeders which rework particles at or immediately below the sediment–water interface (Gray, 1974; Thistle, 2003). The low quantities of organic matter within deep-sea sediments means that deposit feeders need to ingest and forage large amounts of sediment in order to gain a net input of energy. This requirement implies that the level of impact they have on the sediment surface can be substantial, both in terms of bioturbation and biochemical reworking (Sibuet and Lawrence, 1981; Roberts et al., 2000) and in terms of utilization and redistribution of particulate organic carbon (Lauerman et al., 1996; Kaufmann and Smith, 1997; Miller et al., 2000; Bett et al., 2001). At the same time, some larger benthic organisms may also prey on smaller organisms, potentially altering prey abundances (Iken et al., 2001; Thistle, 2003).
By documenting correlations between macro- and meiofaunal organisms and biogenic features, several studies have provided support for the importance of biological disturbance for smaller-sized organisms in the deep sea (Thistle, 1979; Levin et al., 1986; Thistle and Eckman, 1990). However, because of the inherent difficulties of conducting experiments in the deep sea, the direct effects of disturbance and habitat heterogeneity caused by megafaunal organisms on the diversity and community structure of these organisms have been difficult to assess. So far, small-scale manipulative experiments carried out in the deep sea have involved mainly colonization of defaunated sediment trays and nutrient-enriched sediment trays (Grassle, 1977; Levin and Smith, 1984; Desbruyères et al., 1985; Smith, 1985; Grassle and Morse-Porteous, 1987; Levin and Di Bacco, 1995; Snelgrove et al., 1992, Snelgrove et al., 1996). Exceptions include Smith et al. (1986) and Kukert and Smith (1992), who investigated the effects of artificial sediment mounds on macrofauna communities. All of these experiments have focused on macrofaunal groups. No equivalent experimental data are available on the impact of biological disturbance on deep-sea nematodes, the most abundant and perhaps, at the local scale, the most species-rich metazoan group in deep-sea sediments (Lambshead, 1993). Given the differences in lifestyles and dispersal potential between macrofauna and meiofauna, it is quite conceivable that processes regulating diversity differ between the groups (Warwick, 1984; Snelgrove and Smith, 2002). This paper reports on the first investigation of the effects of megafaunal exclusion on nematode assemblages at a deep-sea site (2500 m). Our hypothesis is that in the absence of megafauna disturbance and predation inside cages, nematode assemblages will be more abundant, but less species rich and diverse, than those in control sediments nearby.
Section snippets
Study site
Experiments were conducted at 2500 m water depth at the deep-sea long-term observatory HAUSGARTEN (Soltwedel et al., 2005), situated in the eastern Fram Strait west of Spitsbergen at approximately 79°N/4°E (Fig. 1). Circulation patterns in Fram Strait result in highly variable sea-ice cover, with our study site being generally ice-free during summer months. Photo/video footage from towed camera systems and remotely controlled vehicles has revealed the conspicuous presence of megafauna and
Environmental variables
Environmental variables from the first 2 cm of sediment did not differ between inside and outside the cages. The only exceptions were chl a and phaeopigments, which were both significantly higher inside the cages (Table 1). Mean chl a and phaeopigment concentrations were, respectively, 2.8 and 1.5 times higher inside the cages than in the control samples (Table 1).
Ordination by a correlation-based PCA of the environmental data showed a relatively clear distinction between cage and control
Discussion
At the genus level, nematode assemblages at our study site (regardless of treatment) were rather similar to those in other Arctic deep-sea regions. For instance, the seven most abundant genera, Thalassomonhystera, Microlaimus, Tricoma, Daptonema, Acantholaimus, Molgolaimus and Halalaimus, are in accordance with previous studies in the Arctic deep sea (Vanaverbeke et al., 1997; Vanreusel et al., 2000; Fonseca and Soltwedel, 2007; Hoste, 2006; Hasemann, 2006). The greater number of deposit
Acknowledgments
We thank the operational team of the remotely operated vehicle ‘Victor 6000’ (IFREMER/GENAVIR) and the crew of R.V. ‘Polarstern’ for their helpful support during the summer expeditions of 1999 and 2003. We are grateful to Ann Vanreusel for kindly making sediment analysis facilities at the Marine Biology Section of Ghent University available and Danielle Schram for performing the sediment analysis. Special thanks are due to Sérgio Netto, Tom Moens, Dr. Andrew Gooday and 3 anonymous reviewers for
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