Andrew J Gooday

Responses by benthic organisms to inputs of organic material to the ocean floor: a review

Most of the photosynthetically produced organic material reaching the ocean-floor is transported as settling particles, among which larger particles such as faecal pellets and macroaggregates (marine snow) are particularly important. Recent studies in the northeastern Atlantic have demonstrated that macroaggregates originating from the euphotic zone settle at a rate of approximately 100-150 m d-1 to form a deposit (phytodetritus) on the sediment surface. Bacteria and protozoa (flagellates and foraminifers) rapidly colonize and multiply on phytodetritus, while large deposit feeding animals ingest it. Other inputs, for example Sargassum, wood and vertebrate carcasses, also evoke a rapid response by benthic organisms. However, the taxa that respond depend on the form of the organic material. The intermittent or seasonally pulsed nature of phytodetritus and many other inputs regulate the population dynamics and reproductive cycles of some responding species. These are often opportunists that are able to utilize ephemeral food resources and, therefore, undergo rapid fluctuations in population density. In addition, the patchy distribution of much of the organic material deposited on the ocean-floor probably plays a major role in structuring deep-sea benthic ecosystems.

Phytodetritus on the deep-sea floor in a central oceanic region of the Northeast Atlantic

In a midoceanic region of the northeast Atlantic, patches of freshly deposited phytodetritus were discovered on the sea floor at a 4500 m depth in July/August 1986. The color of phytodetritus was variable and was obviously related to the degree of degradation. Microscopic analyses showed the presence of planktonic organisms from the euphotic zone, e.g., cyanobacteria, small chlorophytes, diatoms, coccolithophorids, silicoflagellates, dinoflagellates, tintinnids, radiolarians, and foraminifers. Additionally, crustacean exuviae and a great number of small fecal pellets, “minipellets,” were found. Although bacteria were abundant in phytodetritus, their number was not as high as in the sediment. Phytodetrital aggregates also contained a considerable number of benthic organisms such as nematodes and special assemblages of benthic foraminifers. Pigment analyses and the high content of particulate organic carbon indicated that the phytodetritus was relatively undegraded. Concentrations of proteins, carbohydrates, chloroplastic pigments, total adenylates, and bacteria were found to be significantly higher in sediment surface samples when phytodetritus was present than in equivalent samples collected at the same stations in early spring prior to phytodetritus deposition. Only the electron transport system activity showed no significant difference between the two sets of samples, which may be caused by physiological stress during sampling (decompression, warming). The chemical data of phytodetritus samples displayed a great variability indicative of the heterogeneous nature of the detrital material. The gut contents of various megafauna (holothurians, asteroids, sipunculids, and actiniarians) included phytodetritus showing that the detrital material is utilized as a food source by a wide range of benthic organisms. Our data suggest that the detrital material is partly rapidly consumed and remineralized at the sediment surface and partly incorporated into the sediment. Incubations of phytodetritus under simulated in situ conditions and determination of the biological oxygen demand under surface water conditions showed that part of its organic matter can be biologically utilized. Based on the measured standing stock of phytodetritus, it is estimated that 0.3–3% of spring primary production sedimented to the deep-sea floor. Modes of aggregate formation in the surface waters, their sedimentation, and distribution on the seabed are discussed.

Possible Roles for Xenophyophores in Deep-Sea Carbon Cycling

This paper examines the involvement of xenophyophores. a group of large (<1–25 cm). agglutinating protozoans, in cycling of organic matter on the deep-sea floor. It is suggested that test volumes are usually < 1% protoplasm, and that even where xenopbyophores are abundant, the plasm contributes relatively little biomass to benthic communities. Thus, they are thought to be relatively unimportant as respirers of carbon or as prey, except to specialized predators. However, xenophyophores may have the potential to take up DOM, and could thereby playa role in carbon transformation. The possibility also exists that the copious quantities of fecal material sequestered within xenophyophore tests may be sites of enhanced microbial activity (Tendal, 1979), and as such, could provide food for metazoan test inhabitants and other deposit-feeding taxa in the deep sea. Ideas concerning DOM uptake and microbial enhancement require verification before their importance can be considered. Evidence that xenophyophores enhance deposition of fine particles comes from flow visualization, excess 234Th profiles, and photographic observations. Reticulate and folded tests are believed to act like small, passive particle traps. Increased deposition of organic matter associated with xenophyophore tests is one possible explanation for elevated densities of metazoan fauna associated with tests and surrounding sediments. The activities of xenophyophores and associated biota generate local hotspots of carbon deposition, mineralization, and perhaps burial. Xenophyophores are a significant source of heterogeneity on the sea floor, at the scale of individual tests (ems) and of population patches (kms).

Physical reworking by near-bottom flow alters the metazoan meiofauna of Fieberling Guyot (northeast Pacific)

Although much of the deep sea is physically tranquil, some regions experience near-bottom flows that rework the surficial sediment. During periods of physical reworking, animals in the reworked layer risk being suspended, which can have both positive and negative effects. Reworking can also change the sediment in ecologically important ways, so the fauna of reworked sites should differ from that of quiescent locations. We combined data from two reworked, bathyal sites on the summit of Fieberling Guyot (32°27.631′N, 127°49.489′W; 32°27.581′N, 127°47.839′W) and compared the results with those of more tranquil sites. We tested for differences in the following parameters, which seemed likely to be sensitive to the direct or indirect effects of reworking: (1) the vertical distribution of the meiofauna in the sea bed, (2) the relative abundance of surface-living harpacticoids, (3) the proportion of the fauna consisting of interstitial harpacticoids, (4) the ratio of harpacticoids to nematodes. We found that the vertical distributions of harpacticoid copepods, ostracods, and kinorhynchs were deeper on Fieberling. In addition, the relative abundance of surface-living harpacticoids was less, the proportion of interstitial harpacticoids was greater, and the ratio of harpacticoids to nematodes was greater on Fieberling. These differences between Fieberling and the comparison sites suggest that physical reworking affects deep-sea meiofauna and indicate the nature of some of the effects.

Microforaminifera — Perspective on a Neglected Group of Foraminifera

Because of their small size, <100 µm, microforaminifera were usually overlooked by protozoologists, marine biologists and micropaleontologists. Karl Grells pioneering work on this group, in the 1950s, demonstrated its great potential for the study of the biology of foraminifera. However, only recently has the scanning electron microscope given us the means to study the tests of these minute foraminifera. They are a very diverse and abundant group. In this paper we present a general overview of various aspects of microforaminifera, including detailed description of the family Rotaliellidae, and some perspectives for future studies.