Community function refers to the dynamics of the living components of assemblages of organisms. In the context of deep-ocean habitats this is considered to include such variables as growth, feeding, reproduction, recruitment, predation, mortality, respiration, and excretion (Figure 7.76). It can also include responses of the latter list to variables such as pollution, organic matter input, temperature, oxygen, and currents. In the deep ocean, these features of a
Figure 7.73. Violin plotsof sampling depths for the top ten most common species (with highest occurrence) from (a) Shelf Break, (b) Upper Slope, (c) Upper-to-Mid-Slope, (d) Mid-to-Lower and Lower Slope, and (e) Lower-Slope-to-Abyssal Groups.Colorsindicate different sampling times.
The violin plotis a combination of box plot and kernel density plot (See Fig.7.72). When the sampling depths were equal or fewer than three observations, the raw depth values are shown (from Wei et al.2012b).
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community are substantially more difficult to assess than in shallow environments or compared to community structure characteristics (e.g., biomass, species composition, and diversity).
Methods for measuring community function include sediment traps to assess input of POC to the seafloor; use of natural and introduced radionuclides to define rates of change in time (Yeager et al.2004; Santschi and Rowe2008; Prouty et al.2011); stable isotopes to infer food web structure; incubations in the laboratory or in situ to determine uptake rates of biologically active compounds such as oxygen, nitrate, and sulfide (Rowe et al.2002,2008a); and numerical simulations that solve for rates that are impossible to measure (Cordes et al. 2005b; Rowe et al.2008b; Rowe and Deming2011).
In the deep Gulf a number of studies have been undertaken to determine aspects of total level-bottom sediment community processes on the seafloor. Baguley et al. (2008) labeled sediment bacteria with 14C and made them available to free-living nematode populations in small, repressurized incubation chambers. The results were inconclusive. There was little evidence that nematodes rely to any degree on bacterial cells as a food source. However, total microbial heterotrophic uptake of a14C labeled mixture of dissolved free amino acids was used to determine microbial uptake rates in combination with production of14C carbon dioxide and utilization of 3-H thymidine to determine respiration and growth rates simultaneously (Deming and Carpenter 2008). A free-falling benthic lander was used to implant incubation chambers on the seafloor to measure total SCOC (Figure7.63). The secondary production of the
Figure 7.74. Nonmetric multidimensional scaling (MDS) on intersample Sørensen’s similarities of pooled demersal fish data (from Wei et al. 2012b). The distances between samples represent dissimilarities in species composition. (a) Symbol sizes are relative water depth, withsmall circles being very shallow on the right and very deep on the left;colorsindicate four depth intervals with equivalent numbers of samples. (b) Symbol sizes show relative depth, andcolorsindicate three studies of different sampling times.
Figure 7.75. Thex-axis of the nonmetric multidimensional scaling (MDS1) plotted against (a) depth and (b) total macrofaunal biomass, where MDS1 represents species composition of demersal fishes in multivariate space. The trend lines show the MDS1 as smooth spline functions of depth or macrofaunal biomass (from Wei et al.2012b).
Figure 7.76. Organic carbon budget for deep-sea bottom biota; * refers to “total living biomass” on and in the sea floor (microbes, meiofauna, macrofauna, and megafauna (from Rowe et al.2008b;
republished with permission of Elsevier Science and Technology Journals, permission conveyed through Copyright Clearance Center, Inc.).
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dominant amphipod Ampelisca mississippiana was estimated in the head of the Mississippi Trough using size frequencies in the population (Soliman and Rowe2008), but it is rare that such rates can be measured in deep water because growth is slow, organisms are small, and numerous samples are required over time.
All of the stock and process data collected above during the DGoMB 2000–2002 survey have been incorporated into a model of presumed food webs at four deep locations: the Mississippi Trough head, at mid-slope depths, in the lower slope/abyssal iron stone region and on the abyssal plain (Rowe et al.2008b) (Figure7.77). Processes are driven by the input of POC as estimated from the SCOC regression equation (Figure7.63) and model-estimated input inferred from satellite-determined surface chlorophyllaestimates (Biggs et al.2008). This POC input to the organic carbon pool (Morse and Beazley 2008) is then divided up into five biological size categories (bacteria, meiofauna, macrofauna, megafauna, and fishes) using
Figure 7.77. Four food web carbon budgets at depths 0.4 km (upper left), 1.5 km (upper right), 2.6 km (lower left), and 3.6 km (lower right) (from Rowe et al.2008b; republished with permission of Elsevier Science and Technology Journals, permission conveyed through Copyright Clearance Center, Inc.), in mg C m2for theboxesand mg C m2day1for thearrows.
carbon as the basic model currency. The habitats at four depths are pictured: Mississippi Trough, mid-slope, iron stone area on the Mississippi Fan, and the abyssal plain, with standing stocks and total carbon flow decreasing exponentially as depth increases.
In the original rendition, most of the organic carbon was recycled by the bacteria, but a more recent assessment of the original rates in Deming and Carpenter (2008) led to a considerable downward revision of the microbial component (Rowe and Deming 2011) (Fig- ure7.78) because the microbes consume dissolved organic carbon (DOC) and not POC. The POC must be released into a dissolved form (DOC) before it is accessible to the bacteria. The authors suggest that this remobilization is done through “messy feeding” by motile inverte- brates, viruses, or exoenzymes produced by the bacteria. How the bacterial assemblage as a whole would respond to an oil spill or free methane remains to be seen.
Table7.5, accompanied by Figure7.79, is a simplified summary of quantitative information on the major stocks and the fluxes or transfers between those stocks in the deep Gulf of Mexico as gleaned from the reviews in the above sections. This carbon cycle would require about 33 mg new N/m2/day for the organic matter production by photosynthesis. The sources of this could be rain, dust, mixing up through the nutricline by storms, recycling from the zooplankton and
Figure 7.78. Model of carbon cycling by seafloor bacteria in relation to transformations from POC, by invertebrates, into DOC, thus reducing the role of bacteria in the processes (redrawn from Rowe and Deming2011; reprinted by permission of Taylor & Francis Ltd.). The units are mg organic C/m2 for the stocks (boxes) and mg organic C/m2/day for the fluxes (arrows).
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zooplankton and mid-water fishes)
carcasses or feces) Deepwater scavengersc 1,200 (poorly known, low
concentrations but integrated over 2.7 km of
water column)
12d(consumed over the deep water column, most
lost to respiration)
3–5 (transferred as particulate matter or aggregates sinking to the
bottom) Seafloor communitiese 1,660 (mostly inactive
microbes), 3–3.7 km depth
3–5 (rain of particles from above)
0.2 (long-term burial)
The five listed stocks are represented in Fig.7.79. Respiration is not explicit
aEl Sayed (1972) and Biggs et al. (2008)
bHopkins (1982) and Hopkins and Baird (1977)
cEstimated from Sutton et al. (2008) from the Atlantic Ridge
dModified from Del Giorgio and Williams (2005)
eRowe et al. (2008b)
Figure 7.79. Simplified relationship between surface-produced organic matter and its routes to the deep ocean floor biota (modified from Rowe2013).
microbiota, and nitrogen fixation by the species complex Trichodesmium. The major loss of organic matter from each heterotrophic stock is respiration, but that is not explicit in the budget. Even so, considerable carbon dioxide is produced over the deepwater column as the organic material that sinks into it is metabolized. The deep consumers in the water column are obscure deepwater scavengers. Although present in very low concentrations, this stock is integrated over a water column of about 2.5 km (1.6 mi). While this rendition represents the extreme deep abyssal plain of the Gulf of Mexico at a 3.2–3.7 km (2–2.3 mi) depth, at lesser depths up the continental slope, more particulate matter would reach the seafloor resulting in higher biomass, as is the case.
The effects of new or alien organic matter are not immediately apparent. Large plant detritus such asThalassia, Zostera orSargassumis probably of some importance. Carcasses may be as well. How fossil organics such as oil or gas would be incorporated into such a carbon budget is not as yet known.