Lower Depth Limit Research Articles

Life in deep subsurface

Life extends far deeper into the Earth's subsurface than presumed possible 30 years ago. In the past, it was assumed that life is a surface phenomenon, and that even “hardy prokaryotic types” are not capable of living deeper than tens of meters below the surface [1]. In the 1990s, it became apparent that genetically and metabolically diverse microbial communities existed under highly reducing conditions in the deep subsurface [2]. Today we know that life in the deep subsurface is ubiquitous and comprises a large proportion of the biomass on Earth [3]. Many questions concerning life in the deep remain unanswered. What is the lower depth limit of the deep biosphere? Which energy sources are fueling these communities? How are genetic diversity and functional activity linked to geochemical factors? What we know is that the deep subsurface is an extreme environment and that the microorganisms living here have developed numerous mechanisms to deal with high pressure and temperature, limited energy and nutrient availability, extreme acidity and alkalinity, metal toxicity, and radioactivity [4].

Age- and size-based morphological comparison between the brown alga Sargassum macrocarpum (Heterokonta; Fucales) from different depths at an exposed coast in northern Kyoto, Japan

Although large perennial brown algae are known to show great morphological plasticity responding to specific environmental factors such as depth and wave exposure, there are few reports showing this morphological variability taking into account age or size composition. Here, we conducted age- and size-based morphological comparison between the perennial brown alga Sargassum macrocarpum C. Agardh from their upper depth limit, middle depth, and lower depth limit at an exposed coast in northern Kyoto, Japan. Model II regression was fitted for the relationships between age or stipe diameter (SD) and morphological variables including SD, holdfast weight (HW), number of main branches (MBN), total weight of main branches (MBW), thallus weight (TW), and thallus length (TL) of 30 specimens collected seasonally from each depth zone. The differences between depths in these regression equations were analyzed using SMATR. Although SD, HW, and MBN of the specimens did not differ between depths under both age- and SD-basis, there were significant differences between depths in MBW, TW, and TL, suggesting that the differences in TW between depths are resulted from differences in MBW. Whereas TW at the lower depth limit was lower than that at the upper depth limit or middle depth in many months, TL at the upper depth limit was shorter than that at the middle depth or lower depth limit in some months. These results suggest that S. macrocarpum at shallower depths tend to have short and bushy morphology, whereas those at deeper depths have long and less bushy morphology.

The effect of in situ shading on a Posidonia oceanica meadow situated within a fish farm induced moderately nutrient enriched environment

The aim of this research was to explore the possibility of a successful and balanced integration of fish farming installations into an ecosystem dominated by Posidonia oceanica (L.) Delile species. We selected light, temperature, seabed topography, sediment characteristics, meadow density, bottom coverage, maximum leaf length and lower depth limit as principle components in assessing the influence of the fish farm. All P. oceanica descriptors showed significant correlation with light deprivation effect while sediment organic matter content revealed slightly higher values than normal, increasing with distance from the cages. The results point to a conclusion that in such lightly nutrient enriched ecosystems, the seagrass growth and distribution are principally controlled by the shadow that cages cast on the seabed below, and that when carefully planned, fish farms do not necessarily degrade the health status of the surrounding area, but in fact facilitate a transition into a secondary stable state.

Determining bathymetric distributions of the eelgrass Zostera marina L. in three turbid estuaries on the eastern North Pacific coast

Abstract Improved methods for determining bathymetric distributions of dominant intertidal plants throughout their estuarine ranges are needed. Eelgrass (Zostera marina) is a seagrass native to estuaries of the northeastern Pacific and many other marine regions of the northern hemisphere. The techniques described here employed large-format aerial photography using false color near-infrared film with digital image classification, and the production of digital bathymetric models of shallow estuaries, such as those occurring in turbid waters of the United States Pacific Northwest. Application of geographic information system procedures to Z. marina classifications and estuarine bathymetries yielded intertidal eelgrass bathymetric distributions based on a very large number of data points. Similar bathymetric patterns were obtained for the three estuaries surveyed, and approximately 90% of the classified Z. marina occurred within the depth range -1.0 m to +1.0 m (mean lower low water). Comparison of these depth distributions with ground surveys of Z. marina lower depth limits indicated that the area of undetected subtidal eelgrass constituted <10% of the areal distributions classified (with ≥86% overall accuracy) in each estuary. Zostera marina areal distribution in one estuary was distinctly different from those in the other two systems, which illustrated the potential usefulness of this technique in exploring causal factors for such differences in areal distributions of estuarine intertidal vegetation.

Changes in depth distribution and biomass of sublittoral seaweeds at Helgoland (North Sea) between 1970 and 2005

Recent investigations of the intertidal macrophyto- and zoobenthos of the island of Helgo- land (North Sea) revealed that species composition and spatial extent of communities have changed within the last century. To evaluate the situation in the subtidal, a diving study from the late 1960s was repeated with comparable methods in 2005 and 2006. Along 2 vertical transects, the cover of dominant brown seaweeds, Fucus serratus, Laminaria digitata, L. hyperborea, Saccharina latissima (= Laminaria saccharina), Sargassum muticum and Desmarestia aculeata, was semi-quantitatively assessed to define vegetation zones. Within each zone, all macroalgal species were estimated quantitatively in 3 to 6 random 1 m 2 quadrats. Additionally, a replicated biomass survey was performed at 6 depths (0.5, 2, 4, 6, 8 and 10 m below mean low water spring tide (MLWS)). Comparison of recent and historical data showed some characteristic changes. The previously dominant brown seaweed S. latissima showed a decline in the northern part of the island, but is still present at other sites. S. muticum invaded the kelp forest, but is not dominant within this vegetation. The vertical distribution of L. hyperborea increased, and its lower depth limits as well as those of various understory seaweeds deepened by about 2 to 8 m. Biomass data followed this trend. The maximum biomass of L. hyperborea shifted from 2 m below MLWS in 1967 to 4 m below MLWS in 2005. The overall downward extension of the Laminaria forest is concordant with the increase in water transparency observed around Helgoland since 1975.

A comparative biodiversity study of the associated fauna of perennial fucoids and filamentous algae

Anthropogenic activities worldwide have contributed to vegetation changes in many coastal areas, changes that may in turn affect faunal and algal assemblages in the involved ecosystems. In the northernmost part of the Baltic Sea the salinity is extremely low (3–4) and the only structurally complex alga present is Fucus radicans. Since in this area F. radicans is living at its salinity tolerance limit, it is potentially very sensitive to environmental changes. Any change in salinity could thus alter the overall algal community, changing it to one dominated solely by filamentous algae. To determine the importance of F. radicans to the associated faunal community, we examined differences between the 2 main vegetation types present, i.e., F. radicans and filamentous algae, in the Kronören marine reserve in the northernmost part of the Baltic Sea. A similar study was conducted in the Askö area in the northern Baltic Proper, where the more-investigated Fucus vesiculosus is the only large fucoid present. The biomass of associated fauna was significantly higher in both the F. radicans and F. vesiculosus than in the filamentous algal vegetation at some, but not all, sites. The F. radicans community also displayed a greater diversity of associated fauna in 3 of 5 investigated Kronören sites, whereas no difference in diversity was detected between F. vesiculosus and the filamentous algal vegetations in the Askö sites. Furthermore, the F. radicans community displayed a different faunal community, being the only investigated algal community with a faunal community dominated by K-strategy species, according to abundance–biomass comparison curves. This pattern may be due to the low epiphytic load on these Fucus plants. In contrast, the F. vesiculosus community, as well as the algal communities with no Fucus in both areas, had high biomasses of filamentous algae and an invertebrate fauna dominated by Chironomidae, occurring in great abundance but only with a low biomass. ANOSIM analyses of faunal composition demonstrated a significant difference between the 2 vegetation types in both areas, largely due to greater abundance of Gammarus spp. and Theodoxus fluviatilis in the fucoid vegetation. Differences observed between the F. radicans and filamentous algal vegetation types were generally more pronounced than those between F. vesiculosus and nearby filamentous algal vegetation. These observations may be due to abiotic factors that differ between the 2 investigated areas, factors such as depth distribution, wave action and eutrophication level. This study has demonstrated that the less-investigated F. radicans may be as important as the larger F. vesiculosus for the associated faunal assemblages. At the same time, the limited extent of F. radicans at shallower depths makes F. radicans vegetation potentially more vulnerable to anthropogenic changes, as declines in fucoid vegetation are usually first manifested in populations at their lower depth limits, whereas shallow populations are less affected.

THE DISTRIBUTION AND ABUNDANCE OF DEEPWATER MACROALGAE IN CENTRAL CALIFORNIA

Deepwater macroalgal assemblages (DWAAS) were sampled from 30m to their lower depth limits at 4 locations in central California using enriched air Nitrox and the ROV Ventana. The lowest observed depth limits were 78 m for nongeniculate (encrusting) coralline algae, 58 m for non‐calcified red algae (i.e. Maripelta rotata), 40 m for foliose brown algae (i.e. Desmarestia tabacoides), and 37 m for kelps (i.e. Pleurophycus gardneri). Green algae, such as Derbesia marina and an undescribed deep water species of Codium, were observed at 30m, while large patches of an unknown green or blue‐green algal film of coccoid cells were found from 45 to 48 m. DWAAS zonation was generally similar to that in southern California, with subtle differences in the zonation of foliose red algae and the occurrence of an unknown Chlorophyta or Cyanophyta from 45–49m. Twenty four species of algae (13 Rhodophyta, 11 Phaeophyta) were sampled during the percent cover analysis at 30 m. Nongeniculate corallines varied from 53% to 91% cover depending on location, while geniculate corallines (i.e. Bossiella schmittii) averaged ca. 25% cover. Pleurophycus gardneri, a stipitate kelp previously described as “rare” in central California, was the most abundant brown alga, averaging from 2.68 to 4.8 individuals m−2. Algal abundance (percent cover and density), zonation, and light levels at the lower algal depth limits were similar between sites and locations, suggesting that the environment of deep water algal populations in this region is less variable than that of the shallow water kelp forest communities above them.

Organic flux control on bathymetric zonation of Mediterranean benthic foraminifera

A data set of benthic foraminiferal faunas counted in 138 surface samples from the Mediterranean Sea has been used to investigate whether the bathymetrical distribution of the dominant taxa is controlled by the amount of labile organic matter transported to the sea floor. We find that most of the major taxa show a clear W to E shallowing of their upper or lower depth limit, coinciding with a W to E decrease in the surface water primary production, and in the estimated flux of the labile organic matter to the sea floor. This observation implies that the bathymetrical succession of these taxa is indeed determined by the organic flux. In the western Mediterranean, we find successions from more oligotrophic taxa at greater water depths to more eutrophic taxa in more shallow water. Towards the eastern Mediterranean, most eutrophic taxa tend to become increasingly rare, or even to disappear, whereas the more oligotrophic taxa show a clear shoaling of their depth range. Deep infaunal taxa are mainly limited to the western part of the Mediterranean. This is explained by their dependency on a relatively elevated organic flux, and by the fact that the bacterial stocks on which they feed may become unattainable when the redox front is positioned too deep in the sediment. The close similarity between the flux level controlling our main faunal boundary, and the flux levels coinciding with important faunal changes in other parts of the world ocean, suggests that a flux level of about 2–3 g labile C m −2 y −1 level corresponds to a benthic ecosystem threshold value of global importance.

Thermal tolerances of embryos and planktotrophic larvae of Archaeopneustes hystrix (A. Agassiz) (Spatangoidea) and Stylocidaris lineata (Mortensen) (Cidaroidea), bathyal echinoids from the Bahamian Slope

Thermal tolerances of early embryos and echinopluteus larvae were studied for two common bathyal echinoids, Archaeopneustes hystrix (A. Agassiz) and Stylocidaris lineata Mortensen. Both species have adult distributions centered at a depth of 550 m on the slopes of the Bahamian Islands, though A. hystrix extends to a shallower depth than S. lineata. Early embryos incubated at 4 temperatures in laboratory incubators developed at the same rates as embryos transplanted by submersible to depths having comparable temperatures. Embryos of both species could tolerate temperatures as high as 25°C. Stylocidaris lineata retained this ability during 2-arm and 4-arm echinopluteus stages, but larvae of A. hystrix died at temperatures above 20°C. The lower depth limits of both species and the upper limit of A. hystrix corresponded closely to the thermal tolerances of larvae. Larvae of S. lineata could potentially migrate to the euphotic zone to feed, but larvae of A. hystrix probably must disperse and feed at greater depths.

Morphology, Larval Development, and Distribution of Bathypera feminalba n. sp. (Ascidiacea: Pyuridae), a Deep-Water Ascidian from the Fjords and Sounds of British Columbia

A new species of the ascidian genus Bathypera (Ascidiacea: Pyuridae), B. feminalba, is described from deep-water habitats of Saanich Inlet and Barkley Sound, British Columbia, Canada, with data on the depth distribution, substratum use, reproduction, embryology, and larval development. The species is characterized by hourglass-shaped spicules topped with a single large spine and several smaller spines, a branchial sac with 7 folds per side, and irregular and curved stigmata. B. feminalba spawned in response to light following dark adaptation during the month of June. Development was similar to that of other pyurids. The tadpole larva, the first to be described in this genus, has the same sensory organs found in shallow-water relatives: an ocellus, a statocyst, and 3 large conical adhesive papillae. The ocellus is sensitive to monochromatic light between 475 and 600 nm with peak sensitivity in the blue region of the spectrum. This species occurs below 20-30 m depth, where it occupies vertical cliffs, the sides of boulders, and also smaller cobbles. It shares this habitat with a characteristic assemblage of ascidians, brachiopods, serpulid polychaetes, and sponges, few of which are found in shallow water. On a small scale, the distribution of B. feminalba is aggregated, with the pattern suggesting gregarious larval settlement. At the upper end of its depth range, this species appears to be excluded from surfaces with coralline algae; competition with algae or interactions with some organism associated with the coralline algal community may determine its upper limit. The coast of the northeastern Pacific between southern Alaska and central California has a diverse ascidian fauna of more than 80 known species (Austin 1985). Many species occurring in the intertidal and shallow subtidal zones have been studied extensively; indeed, some species from this region are among the best-known ascidians anywhere. However, the biology of ascidians living beyond scuba depths in this region remains virtually unknown. Between 1978 and 1984 we examined the ascidians of Saanich Inlet and Barkley Sound fjords, using a manned submersible, dredges, and scuba. We found a relatively sharp faunal transition: below about 30 m depth, most but not all of the shallow-water ascidians are replaced by a distinct ascidian assemblage with several previously undescribed or recently described species. This assemblage includes, besides the pyurid Bathypera feminalba n. sp. described in this paper, the pyurid Boltenia polyplacoderma recently described from 91-117 m depth off central California (Lambert 1993); an ascidiid, Rhopalaea cloneyi n. sp. (Vazquez & Young, unpubl.); the corellid Chelyosoma columbianum, which lives in deep water throughout the north Pacific; and the ascidiid Ascidia ceratodes, which defies the pattern predicted by equatorial submergence by living in shallower water in California than in British Columbia. A few species of ascidians characteristic of shallow subtidal habitats, notably Halocynthia igaboja, Styela coriacea, and Ascidia paratropa, also extend into deep water in British Columbia. Ascidians share the deep fjordic habitats with a fauna dominated by brachiopods (Tunnicliffe & Wilson 1988), galatheid crabs (Burd & Brinkhurst 1984), hexactinellid sponges, serpulid polychaetes, and cup corals (reviewed by Levings et al. 1983). The physical and biological factors that impose the upper limit of these organisms remain unknown, though low oxygen levels may determine their lower depth limit in fjords (Tunnicliffe 1981; Burd & Brinkhurst 1984). Ascidians of the genus Bathypera Michaelsen, 1904 are beautiful animals that resemble old-fashioned beaded purses. The beads are minute papillae arranged in a regular pattern and each containing a spiky calcareous spicule (Lowenstam 1989), often shaped like the acanthus-leaf decorations found atop Corinthian columns in ancient Greek architecture (Van Name 1945). The branchial sac has more than 4 plications on each side, and the stigmata are normally This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 04:49:15 UTC All use subject to http://about.jstor.org/terms

Latest Oligocene to Earliest Pliocene Benthic Foraminiferal Biofacies of the Northeastern Gulf of Mexico

We examined uppermost Oligocene to lowermost Pliocene sections from four northeastern Gulf of Mexico boreholes for quantitative benthic foraminiferal faunal changes, stratigraphic ranges, paleobathymetry, organic carbon content, and planktonic foraminiferal relative abundances. The Eureka boreholes provide a depth transect in the De Soto Canyon area from the upper to lower bathyal zone: E68-136 (557m present depth, -600m paleodepth), E66-73 (857m present depth, 860-lOOOm paleodepth), E68-151A (1326m present depth, -1300m paleodepth), and E68-14 1A (1599m present depth, -1600m paleodepth). A number of taxa last appeared in the late Oligocene to early Miocene (Biochrons P22-N5) at E68-136; several of these disappearances constitute global last occurrences. A global benthic foraminiferal taxonomic turnover that began in the latest early Miocene in other parts of the ocean was restricted to the middle Miocene at E68-136 (Biochrons N9-N12), although faunal abundance changes began in late early Miocene Biochron N8. At middle bathyal borehole E66-73, ten taxa last occurred in Biochrons N8-N10, which is consistent with the timing of the taxonomic turnover in the Pacific and Atlantic. Depth-related faunal trends are examined and compared with previously published distributional data, resulting in revised paleobathymetric ranges of 12 taxa. Detailed age-paleodepth reconstructions reveal several stratigraphically and bathymetrically significant predominance biofacies in the northeast Gulf of Mexico: 1) Uvigerina pigmea dominated the middle-upper bathyal late Neogene; 2) Lenticulina spp. dominated the late Oligocenemiddle Miocene bathyal zone; 3) Oridorsalis spp., Gyroidinoides spp., and Globocassidulina subglobosa dominated the late Neogene lower bathyal zone; and 4) Uvigerina proboscidea was important in the late Neogene in the middle to upper bathyal zones. Four distinct bathymetric migrations are mapped, and 34 additional taxa are shown to have distinct paleobathymetric distributions. Planktonic foraminiferal biostratigraphic control allows us to evaluate the stratigraphic usefulness of benthic foraminiferal ranges. We revise the stratigraphic ranges of 12 bathyal benthic foraminiferal taxa, requiring re-correlation of the benthic foraminiferal zonal boundaries of Berggren and Miller (1989). INTRODUCTION Previous studies have shown that one of the largest benthic foraminiferal faunal changes of the Cenozoic occurred throughout the deep sea (>200m) in the late early to middle Miocene (Berggren 1972; Schnitker 1979, 1986; Woodruff and Douglas 1981; Boersma 1986; Thomas 1985, 1986a, 1886b, 1989, 1992; Woodruff 1985; Kurihara and Kennett 1986; Miller and Katz 1987; Thomas and Vincent 1987; Woodruff and Savin 1989; Miller et al. 1992; Thomas 1992; for an alternative view, see Boltovskoy and Boltovskoy 1988; Boltovskoy et al. 1992). These authors documented that major changes in taxonomic composition, percentages, and absolute abundances of benthic foraminifera began in the late early Miocene and culminated in the middle Miocene. Although this event has been well documented at open ocean locations, it remains poorly documented in marginal seas such as the Gulf of Mexico. The Eureka boreholes examined in this study (northeast Gulf of Mexico) yield faunal abundance changes and stratigraphic ranges that reflect this global middle Miocene benthic foraminiferal event. Benthic foraminiferal faunas have been recognized for their potential to assess paleobathymetry (e.g. Natland 1933; Bandy 1960). While Bandy (1960) promoted the concept that benthic foraminifera have distinct upper and lower depth limits, Streeter (1973) and Schnitker (1974) established that deep-water (>200m; bathyal-abyssal) benthic foraminifera are correlated to water mass properties that may vary independently of depth. Numerous subsequent studies have documented that deep-water benthic foraminiferal distributions are associated with physiochemical properties other than depth, and that depth alone does not control the vertical distribution of benthic foraminifera. For example, vertical distributions are correlated with water masses (e.g. Lohmann 1978; Corliss 1979; Schnitker 1979; Murray 1984; for Gulf of Mexico examples, see Pflum and 88? 86? 84? TEXT-FIGURE 1 Eureka bo ehole location map in the Gulf of Mexico. Contours are in meters. micropaleontology, vol. 39, no. 4, pp.367-403, plates 1-6, text-figures 1-32, tables 1-5, 1993 367 This content downloaded from 157.55.39.152 on Sat, 26 Nov 2016 04:11:28 UTC All use subject to http://about.jstor.org/terms M. E. Katz and K. G. Miller: Latest Oligocene to Earliest Pliocene benthicforaminiferal biofacies of the northeastern Gulf of Mexico TABLE 1 Age model parameters Datum Age (Ma) Depth (feet below sea level) E68-136 E66-73 E68-151 FO C. acutus & 5.

Pattern of planktonic bioluminescence in the northern Sargasso Sea: Seasonal and vertical distribution

The vertical structure of bioluminescence potential (BPOT) and flash density (FD) were measured on five cruises to the northern Sargasso Sea in 1987 and 1988. Depth-integrated (0 to 150 m) BPOT did not vary seasonally, remaining within the range 9 to 15 × 1015 photons m−2 in all months sampled. Conversely, depth-integrated FD was significantly higher (> 2 × 105 flashes m−2) during winter (November and March) than during summer (< 9 × 104 flashes m−2 in May and August). The vertical patterns of BPOT and FD were well correlated within a single profile, more highly so in summer than in winter. Despite intracruise variability in the vertical pattern of BPOT and FD, there were clear summer-winter differences in the vertical distribution of BPOT and FD. During winter, BPOT and FD were maximal and relatively uniform throughout the surface mixed layer; for example in November they declined sharply within the thermocline at 130 to 150 m. During summer, BPOT and FD were greatest (12 to 25 × 1013 photons m−3 and 600 to 1 200 flashes m−3, respectively) at subsurface depths. Commonly in summer, the upper depth limit of high BPOT and FD occurred at the base of the surface mixed layer (10 to 40 m) and the lower depth limit was located at the base of the subsurface fluorescence maximum (usually at 100 to 120 m).

Influence of oxygen concentration on the distribution of Mysis relicta Lovén in a eutrophic temperate lake

The lower depth limit of a Mysis population in Crystal Lake, southern Ontario, gradually moved upward through the summer stagnation period, generally following a position just above the 1.0 mg/L dissolved oxygen isopleth. Mysis held in bioassay chambers demonstrated a 16-h LC50 value of 1.0 mg dissolved oxygen/L. Avoidance chamber experiments revealed that Mysis could detect and move away from areas of low dissolved oxygen concentrations. It is suggested that Mysis, which has been used as an indicator of oligotrophic conditions, could survive in a mesotrophic or eutrophic lake that undergoes oxygen depletion in the hypolimnion by moving upward in the water column.

Light level variation in Lake Tutira after transient sediment inflow and its effect on the submersed macrophytes

Abstract After exceptionally heavy rainfalls and associated land movements in July 1985, sediment inflow into Lake Tutira, Hawke's Bay, New Zealand, caused a five‐fold reduction in the depth at which 5% of the surface photosynthetically active radiation remained. Light level reduction was associated with increased nephelometric turbidity caused by the suspended sediment. The decreased light levels, as monitored by measurement of the vertical attenuation coefficient of light in the lake, persisted for about three months and were accompanied by a marked reduction in the lower depth limit of aquatic macrophytes: from 6.7 to 1.8 m for Elodea canadensis, and from 4.6 to 1.8 m for Hydrilla verticillata. As water quality returned to its pre‐storm condition, these species regrew very rapidly in the denuded areas from surviving rhizomes.

A new technique for layered earth magnetotelluric inversion

A new one‐dimensional (1-D) inverse method for layered‐earth interpretation of magnetotelluric (MT) response curves is based on the method of Schmucker (1972). It involves transforming the E over B response into a nondimensional complex logarithm response, computing the partial derivatives from a new algorithm for the logarithm response, and iteratively solving (by damped least squares) for the logarithm of the conductivity contrast between layers. Error bars for the layer conductivities are estimated by a simple application of propagation of errors assuming random and independent response errors. Backus‐Gilbert type smoothing kernels are also computed in order to specify the upper and lower depth limits on the conductivity model and to examine whether the layer conductivities are locally averaged values. The kernels are found to be an important aid in model interpretation and emphasize the fact that layer conductivities are averages. The method is illustrated using artificial and real data.