Showing papers on "Benthic zone published in 1988"

Denitrification in freshwater and coastal marine ecosystems: Ecological and geochemical significance

Abstract: Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 µmol N m−2 h−1, with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near-ambient conditions range from 2 to 171 µmol N m−2 h−1. Denitrification rates in river and stream sediments range from 0 to 345 µmol N m−2 h−1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 µmol N liter−1 d−1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments. The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N2 fluxes accounted for 76–100% of the sediment-water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N2O fluxes were always small compared to N, fluxes. The loss of nitrogen via denitrification exceeds the input of nitrogen via N2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured. Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.

Deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation

Abstract: The degree of similarity of the ∂13C records of the planktonic foraminiferal species N. pachyderma and of the benthic foraminiferal genus Cibicides in the high-latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle. Whereas continuous formation of deep water is recognized in the southern ocean, the Norwegian Sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation. However, deep water formed in the north Atlantic south of the Norwegian Sea during the last climatic cycle as early as isotope substage 5d, and this area was also the only active northern source during stages 4–2. A detailed reconstruction of the geographic distribution of ∂13C in benthic foraminifera in the Atlantic Ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean, whereas the Glacial North Atlantic Deep Water cannot be traced south of 40°N. At shallower depth an oxygenated 13C rich Intermediate Water mass extended from 45°N to 15°S. In the Pacific Ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700–2600 m and is best explained by stronger formation of Intermediate Water in high northern latitudes.

A response by benthic Foraminifera to the deposition of phytodetritus in the deep sea

Abstract: A recent major discovery has been the rapid sedimentation of phytodetritus to the deep-sea floor1–3 Although benthic mega-faunal invertebrates appear to seek out this relatively fresh food source1,4, and its seasonal arrival on the sea floor may synchronize reproduction in some echinoderms5, a convincing response by ben-thic organisms to phytodetritus has not been demonstrated3. Here I present evidence that certain small benthic Foraminifera (within the meiofaunal size-range) react dramatically to the presence of phytodetritus. Fresh aggregates of this material harbour abundant, low-diversity populations of these protists. The three commonest species are usually poorly represented in the more diverse assemblages inhabiting the underlying sediment. These findings suggest that some deep-sea benthic Foraminifera, like their shallow-water relatives6–8, are specialist feeders that bloom opportunistically when the appropriate food (phytodetritus and associated micro-organisms) becomes available, while others remain unaffected by the organic influx.

Morphotype patterns of Norwegian Sea deep-sea benthic foraminifera and ecological implications

Abstract: Deep-sea benthic foraminifera from Norwegian Sea surface sediments are classified into morphotypes on the basis of test shape and nature of test coiling and show distinct patterns with water depth. The morphotype data are used to determine microhabitat patterns of the foraminifera, which are suggested to be related to the organic-carbon content of the surficial deep-sea sediments.

Nitrogen cycling in coastal marine environments

Abstract: Pelagic Nitrogen Cycling: Primary Productivity and Pelagic Nitrogen Cycling Pelagic Primary Production in Nearshore Waters Microfauna in Pelagic Food Chains Benthic Nitrogen Cycling: Benthic Primary Production and Oxygen Profiles Benthic Nitrogen Fixation Amino Acids and Amines in Marine Particulate Material and Sediments Distribution and Metabolism of Quaternary Amines in Marine Sediments Benthic Mineralization and Bacterial Production Nitrogen in Benthic Food Chains Nitrification in Estuarine and Coastal Marine Sediments Nitrate Reduction and Denitrification in Marine Sediments Benthic Fauna and Biogeochemical Processes in Marine Sediements: The Role of Burrow Structures Benthic Fauna and Biogeochemical Processes in Marine Sediments: Microbial Activities and Fluxes Models of Nitrogen Cycling: Modelling Benthic Nitrogen Cycling in Temperate Coastal Ecosystems Nitrogen Models at the Community Level: Plant-Animal-Microbe Interactions Nitrogen Biogeochemistry and Modelling of Carmarthen Bay.

Interpreting benthic oxygen levels in mudrocks: A new approach

Abstract: Quantified paleoecology and gamma-ray spectrometry have been applied in the analysis of the Kimmeridge Clay, a highly organic-rich British Jurassic mudrock. Decreasing benthic oxygen trends are reflected in decreasing species richness and dominance-diversity values. Similarly, the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen. The calculation of authigenic uranium values from data gathered by gamma-ray spectrometry shows enrichment in more oxygen-deficient environments. The good correlation between the independently derived paleoecological and authigenic U data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks.

Methods for collection and analysis of aquatic biological and microbiological samples

Abstract: Chapter A4 contains methods used by the US Geological Survey to collect, preserve, and analyze waters to determine their biological and microbiological properties. Part 1 consists of detailed descriptions of more than 45 individual methods, including those for bacteria, phytoplankton, zooplankton, seston, periphyton, macrophytes, benthic invertebrates, fish and other vertebrates, cellular contents, productivity, and bioassays; Part 2 consists of a glossary; and Part 3 is a list of taxonomic references.

Pelagic-benthic coupling on the shelf of the northern Bering and Chukchi Seas. I. Food supply source and benthic bio-mass

Abstract: The shelf waters of the northern Bering and Chukchi Seas are ice-covered for 7 mo of the year, but despite thls harsh environment, they are characterized by high benthic biomass. Coupling between water column primary production and the benthos was investigated in summers 1984 to 1986 by measurements of sediment characteristics in relation to those of the water column. Low surface sediment C/N ratios (5.8 to 7.6) suggested a higher quality, nitrogen-rich marine carbon supply to the benthos in the highly productive (ca 250 to 300 g C m-' yr-') Bering Shelf-Anadyr Water (BSAW) compared to lower quality, higher C/N ratios (7 7 to 14 0) in sediment under the less productive (ca 50 g C m-' yr-l) Alaska Coastal Water (ACW). Stable carbon isotope ratios suggested a manne origin for organic matter in BSAW compared to a mixture of manne and terrestrial input in ACW. Mean benthic biomass was significantly different between water locations, with mean benthic biomass decreasing from 20.2 g C m-* under BSAW to 6.3 g C m-2 under ACW Summer benthic biomass remained seasonally constant for the 3 yr. Benthic communities underlying BSAW received a high quality marine food supply on a regular basis interannually, while those in ACW received an interannually variable amount of temgenous organic matter in addition to marine organic matter. We conclude that the quality and quantity of organic carbon deposited to the benthos directly influence benthic biomass.

Physical energy inputs and the comparative ecology of lake and marine ecosystems

Abstract: Although freshwater and marine systems both receive light and heat energy from the sun and are mixed by the wind, only marine systems receive additional mechanical energy from the tide. This input is very small relative to the flux of solar energy but may exceed that from wind. Some obvious physical consequences of this additional energy input include the development of intertidal habitats, the presence of stronger currents, and more vigorous vertical mixing. It is argued that these (and perhaps other) consequences lead to coastal marine ecosystems which differ in a number of important ways from temperate lakes. There is some evidence that coastal marine systems generally maintain a larger standing crop of benthic animals and that these fauna are more effective in mixing the bottom sediments. As a result of better sediment mixing (and perhaps warmer and better oxygenated bottom water), organic matter deposited on the bottom of coastal marine areas may be more completely metabolized and less C, N, and P retained than in lake sediments. Materials that are more tightly bound to particles, like many metals, may behave similarly in lake and marine sediments. Although many lakes are strong sinks for nutrients and metals, marine bays and estuaries may bc much less effective in retaining nutrients. A major consequence of the input of tidal energy appears to be a more intensive yield of fish from marinc systems compared with temperate lakes. The data suggest that this more intense yield is not due to the size or interconnection of marine areas or to higher primary production. Rather, the efficiency of transfer of primary production to fish appears to be greater. Tropical lakes appear more like marine systems in this regard, and this may be related to lower thermal stability and more efficient wind energy transfer because of a small Coriolis effect at low latitudes.

Benthic foraminifera from the Adriatic Sea : principles of phenotypic variation

Abstract: The distribution and morphology of the benthic foraminifera in the Adriatic Sea appear to be strongly dependent on two primary controlling environmental parameters, oxygen concentration and food availability. These factors are both governed by the runoff from the Po and other Italian rivers, and by the subsequent distribution of the fluvial discharge products by the system of surface currents. The area most strongly influenced by runoff products, which is characterized by ample food availability and low oxygen concentrations at the bottom, is a strip parallel to the Italian coast at a water depth of between 20 and 60 m. In the present study we describe in detail the changes in the benthic foraminiferal faunas along transects perpendicular to this zone. The faunas along these transects react essentially in two different ways to the changes in environmental conditions. On the one hand we see qualitative and quantitative changes in the composition of the faunas, and on the other hand a number of species show a high degree of ecophenotypic variation. Statistical analysis of the faunal patterns resulted in the recognition of eight faunal clusters. One of these clusters, occurring in the near-shore zone, is consisting of obviously reworked specimens. The other seven clusters are separated on the basis of differences in the tolerances for stressed conditions and in the preferences for particular feeding niches of the various taxa. In the centre of the zone influenced by run-off products the faunas are dominated by a cluster with Bulimina marginata forma denudata and Nonionella turgida. Obviously the taxa of this cluster have the greatest tolerance for the sometimes stressed conditions found in this area. In the taxa Ammonia parkinsoniana, Elphidium granosum and Elphidium poeyanum, comparable successions of three fundamental morphologies are demonstrated. Compact morphotypes are typical for the near-shore, relatively nutrient-poor and oxygen-rich environment, in which there is a minimum influence of runoff products. In the somewhat deeper, food-enriched environment with lower oxygen concentrations, inflated morphologies occur. Still deeper, in the central part of the strip influenced by run-off products, where oxygen-poor conditions occur, these types are found with supplementary characteristics. The study of the variation in Bulimina marginata suggests that in this taxon the morphology reflects the life position of the animal. On the basis of the distribution of different morphologies in this and in similar species, it is concluded that the degree of niche separation in benthic foraminifera is dependent on the thickness of the oxygen-containing layer. If this thickness is minimal, the inbenthic microhabitat is ruled out, and only taxa adapted to an epibenthic microhabitat are found. In the final chapter some possibilities to apply the results of our study are discussed. It is argued that the bathymetrical distribution of benthic foraminifera in the Adriatic Sea is largely influenced by variations in productivity. Although the Recent situation can be described in great detail, one should be careful about using these data for estimating paleo-depths in other basins and/or other time-slices. Far more promising is the potential use of the present results for the interpretation of benthic foraminiferal faunas from deltaic environments in terms of productivity. It is demonstrated that not only variations in the productivity itself can be distilled, but also the source of such variations, such as variations in run-off, changes in circulation patterns and sea level fluctuations. It is suggested that benthic foraminifera should be used more often in pollution studies, because they can provide an accurate description of the distribution patterns of pollution phenomena in the bottom environments.

Microbial ecosystems of Antarctica

Abstract: Preface 1. Introduction 2. Snow and ice ecosystems 3. Sea-ice ecosystems 4. The marginal-ice zone 5. The open ocean 6. Benthic marine environments 7. Lakes and streams 8. Soil ecosystems 9. Lithic ecosystems: the rock environments 10. Microbial strategies in Antarctica 11. Microbes and humans in Antarctica Glossary Appendixes References Index.

Food, Predation Risk, and Microhabitat Selection in a Marsh Fish Assemblage

Abstract: Fishes moving onto the surface of a tidal freshwater marsh from an adjacent stream were sampled with flume nets in spring through autumn for 2 yr. Significantly higher numbers were found at sites adjacent to shallow-sloped depositional banks than at sites adjacent to deeper, steeper sloped erosional banks of the stream. Marsh surface features appeared similar, so explanations for this apparent habitat selection were sought in the subtidal portion of the stream. To determine whether the relative abundance of benthic invertebrate prey differed in the two environments, litter bags were placed in the subtidal for 4 wk. Despite higher numbers of invertebrates in erosional zones, both the wet biomass per sample and the mean wet biomass per organism were significantly greater in depositional sites. To test the hypothesis that invertebrate food is more available to fishes in the depositional subtidal, banded killifish (Fundulus diaphanus) were placed in feeding enclosures in both depositional and erosional subtidal habitats in two streams. These fish obtained significantly more food in the shallow depositional subtidal. To test the hypothesis that predation pressure differs with subtidal geomorphology, mummichogs (Fundulus heteroclitus) were tethered on the first half of the rising tide in both depositional and erosional environments in three creeks. Significantly more mum- michogs disappeared from tethers and were presumed taken on erosional banks. We conclude that two mechanisms operate in the stream subtidal to give rise to the greater fish abundance at marsh surface sites adjacent to depositional areas. At low tide, when small fishes are confined to creek channels, they select shallow depositional habitats where (1) the availability of benthic invertebrate prey is greatest and (2) predator pressure is less. As the tide rises and inundates the marsh surface, these small fishes seek shelter on the marsh surface adjacent to their preferred low-tide refuge.

Seasonality in East Africa's coastal waters

Abstract: A review of existing literature and data on seasonal patterns in East Africa's coastal waters indicates distinct seasonality in physical, chemical and biological oceanographic parameters. Seasonal patterns are dictated by the behavior of the Inter-Tropical Convergence Zone (ITCZ) which creates 2 distinct seasons the northeast and southeast monsoons. SE monsoon (March to October) meteorological parameters are characterized by high cloud cover, rainfall, river discharge, terrestrial runoff and wind energy while solar insolation and temperatures are low; SE monsoon oceanographic parameters are characterized by cool water, a deep thermocline, high water-column mixing and wave energy, fast currents, low s a h ~ t y and high phosphorus. These parameters are reversed during the NE monsoon. Nitrogen availability and planktonic primary productivity are high along the Somali coast and estuarine and river discharge areas during the southeast monsoons due to nutrient upwelling and terrestrial runoff. In near-shore waters off Tanzania, nitrogen fixation is the major source of nitrogen and is highest during NE monsoons when the water column is stable. Coral reef benthic algal biomass and diversity is greatest during the SE monsoons. Fish catch and reproduction are highest during NE monsoons in Kenya and Tanzania. Transition periods between monsoons may also be important times in determining productivity and reproduction.

Bacterial productivity and microbial biomass in tropical mangrove sediments.

Abstract: Bacterial productivity ((3)H-thymidine incorporation into DNA) and intertidal microbenthic communities were examined within five mangrove estuaries along the tropical northeastern coast of Australia. Bacteria in mangrove surface sediments (0-2 cm depth) were enumerated by epifluorescence microscopy and were more abundant (mean and range: 1.1(0.02-3.6)×10(11) cells·g DW(-1)) and productive (mean: 1.6 gC·m(-2)· d(-1)) compared to bacterial populations in most other benthic environments. Specific growth rates (¯x=1.1) ranged from 0.2-5.5 d(-1), with highest rates of growth in austral spring and summer. Highest bacterial numbers occurred in winter (June-August) in estuaries along the Cape York peninsula north of Hinchinbrook Island and were significantly different among intertidal zones and estuaries. Protozoa (10(5)-10(6)·m(-2), pheopigments (0.0-24.1μg·gDW(-1)) and bacterial productivity (0.2-5.1 gC·m(-2)·d(-1)) exhibited significant seasonality with maximum densities and production in austral spring and summer. Algal biomass (chlorophylla) was low (mean: 1.6μg·gDW(-1)) compared to other intertidal sediments because of low light intensity under the dense forest canopy, especially in the mid-intertidal zone. Partial correlation analysis and a study of possible tidal effects suggest that microbial biomass and bacterial growth in tropical intertidal sediments are regulated primarily by physicochemical factors and by tidal flushing and exposure. High microbial biomass and very high rates of bacterial productivity coupled with low densities of meiofaunal and macroinfaunal consumers observed in earlier studies suggest that microbes may be a sink for carbon in intertidal sediments of tropical mangrove estuaries.

Analysis of the structure of decapod crustacean assemblages off the Catalan coast (North-West Mediterranean)

Abstract: We sampled the communities of decapod crustaceans inhabiting the depth zone between 3 and 871 m off the Catalan coast (North-West Mediterranean) from June 1981 to June 1983. The 185 samples comprised 90 species differing widely in their depth distributions. Multivariate analysis revealed four distinct faunistic assemblages, (1) littoral communities over sandy bottoms, (2) shelf communities over terrigenous muds, (3) upper-slope communitics, and (4) lower-slope or bathyal communities. The brachyuran crab Liocarcinus depurator is the most abundant species of the shelf assemblage, although L. vernalis dominates over the shallow sandy bottoms of the shelf. The dominant species of the upper-slope assemblage are nektobenthic species (Solenocera membranacea, Plesionika heterocarpus, Processa canaliculata), pelagic species (Pasiphaea sivado, Sergestes arcticus), and benthic species (Macropipus tuberculatus, Munida intermedia, Nephrops norvegicus). Aristeus antennatus comprise most of the biomass of the lower-slope community, which supports a greater diversity than the other assemblages. The main assemblages appear to be related to different hydrological characteristics, the extent of seasonal fluctuations, and to the changes in sediment structure associated with changes in the steepness of the bottom.

Differential response of benthic macrofauna and meiofauna to hypoxia in the Gullmar Fjord basin

Abstract: The response of benthic macro-and meiofauna to severe hypoxia was studied in the deep basis (115 m) of Gullmar Fjord, western Sweden. Abundances and bionasses of the faunal taxa, the redox-potential of the sediment, and the temperature, salinity and dissolved oxygen in the bottom water were recorded over the period 1977 to 1981. In the winter of 1979/80, when a depressed oxygen level of 0.21 ml l-1 was recorded, the macrofaunal component of the fauna disappeared. The ensuing recolonization, with an initial peak of opportunistic capitellid polychaetes, proceeded slowly and the pre-collapse community was not reestablished within 1 1/2 yr after the hypoxia. In contrast, the permanent meiofauna exhibited no clear signs of being affected by the hypoxia. In the temporary meiofauna, polychaetes seemed to be negatively affected. The finding of a differential response of macro-and meiofauna confirms previous studies which indicate that macrofauna in general is more sensitive than meiofauna to low oxygen concentrations.

Cenozoic ice-volume history: Development of a composite oxygen isotope record

Abstract: To view Cenozoic ice-volume and ocean-temperature history, a composite δ 18 O time series was constructed for tropical shallow-dwelling planktonic foraminifers and for deep-water benthic foraminifers. We conclude that Cenozoic benthic δ 18 O primarily reflect considerable deep-ocean temperature variation and that tropical planktonic δ 18 O afford the best record of Cenozoic ice volume. The planktonic composite indicates the presence of a significant ice budget for the past 40 m.y. The difference between the benthic and planktonic δ 18 O composites suggests that low-latitude production of warm saline bottom water dominated Tertiary thermohaline circulation.

Marine eutrophication induced oxygen deficiency: Effects on soft bottom Fauna, Western Sweden

Abstract: Marine eutrophication is studied in a multidisciplinary project in the Kattegat, western Sweden. Input of nutrients has increased several fold during this century; nitrogen ≈ 6 times, phosphrorus ≈ 10 times. This has caused elevated concentrations of nutrients in the water and increased primary production. Some adverse effects at the bottom have been noted annually from about August through October in the 1980's, e.g. fish catches have dropped and mortalities have been reported for fish and benthic animals including Nephrops noruegicus. In this paper we evaluate the effects on benthic macrofauna which are suggested to be caused mainly by oxygen deficiency occurring especially around the depth ofa strong halocline (≈ 15m), but also along a bottom transect down to 57 m. At some sites the benthic communities were impoverished, and at others only some species seemed to be especially sensitive, e.g. the bivalve Abra alba and the brittle star Amphiura filijormis. In shallow < 10 m) exp.sed waters an an...

An evaluation of the ABC-method (abundance/biomass comparison) as applied to macrozoobenthic communities living on tidal flats in the Dutch Wadden Sea

Abstract: The ABC-method proposed by Warwick (1986) for detecting pollution effects on marine zoobenthic communities, was tested on distribution patterns of numbers and biomass among species in macrozoobenthos samples taken annually for 13 yr in a uniform way at 15 tidal-flat stations in the western part of the Dutch Wadden Sea. Along the margins of the tidal-flat area studied, where exposition to either drainage or water movements is extreme, the k-dominance plots for numbers were generally situated above those for biomass. In these areas values for biomass and species richness were relatively low and a few small-sized species such asCorophium volutator andHydrobia ulvae were very numerous. In the central part of the area, where environmental conditions are less severe, values for biomass and species richness were higher. Biomass was dominated by large adults ofMya arenaria, Mytilus edulis and/orArenicola marina. However, in only a restricted part of this area were plots for numbers consistently below biomass plots. In most of the central area high numbers ofH. ulvae occasionally occurred, causing plots for numbers to be situated above those for biomass during such periods. There appears to be no reason to interprete a high abundance ofH. ulvae as a sign of pollution or any other kind of disturbance or stress. It is concluded that the ABC method cannot be applied to tidal flat communities without reference to long-term and spatial series of control samples. In areas where such smallsized and sometimes numerous species asH. ulvae occur in strongly fluctuating numbers, the method appears useless for assessing the pollution status of a benthic community.

The annual cycle of sedimentation in Saanich inlet, British Columbia: implications for the interpretation of diatom fossil assemblages

Abstract: Data derived from monthly sediment traps at three depths near the head and mouth of Saanich Inlet yield a detailed record of the seasonal cycle of production and vertical flux to the sediments. The material is primarily composed of diatom frustules and silt-to-clay-sized lithic fragments, with dinoflagellates and naked algae present in summer samples. Diatoms dominate from April to September while clastics dominate from October to March. Particles occur primarily in loose flocs; pellets are common in early winter and in summer material, and consist of an indiscriminant mixture of the same particles seen in co-occurring flocs. The seasonal succession of taxa is similar at both sites and is transported to the sediment with little modification by dissolution. Carbon flux is a poor indicator or productivity, due to the strong effect of recycling in the surface waters during spring and uncertainty as to the source of the carbon accumulating in the traps. Although diatom flux is an adequate indicator of primary production, transforming each taxon to equivalent cell volume yields a more accurate picture of the seasonal cycle of production. Relative abundance (taxon percentage) data can be as useful as, and in some cases more accurate than, absolute flux (number per unit time), while number-per-gram may be highly misleading as an estimator of production. Lateral advection and benthic resuspension affect monthly data and annual valve-flux data, but appear to have no net effect upon annual percentage data.

Optical properties of benthic photosynthetic communities: fiber-optic studies of cyanobacterial mats.

Abstract: A fiber-optic microphobe was used to analyze the spectral light gradients in benthic cyanobacterial mats with 50-micrometer depth resolution and 10-nm spectral resolution. Microcoleus chthononplastes mats were collected from hypersaline, coastal ponds at Guerrero Negro, Baja California. Gradients of spectral radiance, L, were measured at different angles through the mats and the spherically integrated scalar irradiance, Eo, was calculated. Maximal spectral light attenuation was found at the absorption peaks for the dominant photosynthetic pigments: chlorophyll a at 430 and 670 nm, carotenoids at 450-500 nm, phycocyanin at 620 nm, and bacteriochlorophyll a at 800-900 nm. Scattered light had a marked spectral effect on the scalar irradiance which near the mat surface reached up to 190% of the incident irradiance. The spherically integrated irradiance thus differed strongly from the incident irradiance both in total intensity and in spectral composition. These basic optical properties are important for the understanding of photosynthesis and light harvesting in benthic and epiphytic communities.

Tidal jets, nutrient upwelling and their influence on the productivity of the alga Halimeda in the Ribbon Reefs, Great Barrier Reef

Abstract: A field experiment was carried out to study water circulation and benthic biological productivity near a passage through the Ribbon Reefs in the northern Great Barrier Reef of Australia. The currents through the passage were phase-locked with the tide. During rising tides, strong currents through the passage generated localized upwelling on the upper continental slope, enriching the depleted surface waters in nutrients, particularly nitrate and phosphate. Simultaneously, on the shelf side of the passage, a tidal jet-vortex pair system developed, which separated from the Ribbon Reefs so that the coral reefs themselves received little of the upwelled water. This was propagated as a bottom-trapped layer towards the meadows of the calcareous alga Halimeda situated several kilometres inshore of the reefs. Halimeda can accumulate nutrients, particularly nitrate, from the relatively low concentrations available from the upwelling events. The quantity of nitrogen upwelled was more than sufficient to supply the total nitrogen requirements of the Halimeda vegetation. A tidal jet also formed, offshore from the reef passage, during falling tides and the coral-covered offshore side of the Ribbon Reefs may be sustained by the subsequent vertical turbulent entrainment into the jet of deep, nutrient-rich water immediately offshore from the reef passages. These processes require a continuous barrier reef with only narrow passages several kilometers apart. Numerical models successfully reproduce the observations of jet-driven upwelling and of the dynamics of the tidal jet-vortex pair system. The model predictions are very sensitive to the details of the bathymetry of reef passages. As such data are presently unavailable, it is not yet possible to use these models to calculate the jet-driven nutrient upwelling for the whole Great Barrier Reef.

Recovery of benthic macroinvertebrate and epilithic communities following a large flood, in an unstable, braided, New Zealand river

Abstract: Benthic macroinvertebrates and small stones were collected from a riffle in the Ashley River, North Canterbury, New Zealand, on 12 occasions within a 132‐day period following a particularly large flood in 1986. Despite the occurrence of smaller floods during this period, benthic macroinvertebrate communities and stone surface organic layers recovered rapidly. Mean concentration of stone surface organic carbon increased from 0.23 g/m2 to 1.01 g/m2 stone surface in the first 23 days after the flood and chlorophyll a concentration increased from 0.13 mg/m2 (day 3) to 9.2 mg/m2 by day 132. Minor floods during the recovery period had little effect on organic layer biomass. Mean faunal density increased from 230/m2 to 7920/m2 during this time and taxon richness from 7 to 21 per 5 benthic samples. Immediately after the flood and throughout the study period the fauna was dominated by larvae of Deleatidium (Ephemeroptera: Leptophlebiidae), Hydora (Coleoptera: Elmidae), and Chironomidae. Re‐establishment o...

The response of benthic foraminifera to productivity cycles in the eastern equatorial Pacific: Faunal and geochemical constraints on glacial bottom water oxygen levels

Abstract: The Stage 2 glacial maximum in eastern equatorial Pacific Ocean sediments is characterized by a pronounced increase in the organic carbon content which has been attributed either to increased upwelling and higher production along the equatorial divergence during the last glacial period or to enhanced preservation as a consequence of a lower concentration of dissolved O2 in glacial bottom water. Several lines of evidence are presented, based on data from two gravity cores collected from the central Panama Basin, which support the former interpretation. First, both the abundance and the size of benthic foraminifera of the genus Uvigerina increase in Stage 2, coincident with the increase in organic carbon concentration, most probably because of the greater availability of organic detritus during the glacial period. The increase in foraminiferal size and associated greater requirement of oxygen for respiration argues against the probability that oxygen was substantially depleted in the upper centimeter where Uvigerina spp. are thought to live. Second, the number of meiofaunal faecal pellets increases markedly during the Stage 2 maximum, indicating a more populous and active infauna, which again argues against oxygen depletion. Third, there is no significant decrease in the iodine:organic carbon (I:Corg) ratio in the Stage 2 sediments. Because iodine is depleted relative to carbon in anoxic basins but enriched in association with organic matter in oxic environments, the absence of a depletion in the I:Corg ratio in Stage 2 is evidence that bottom water remained oxygen- replete during the last glacial. Fourth, molybdenum is not enriched in sediments deposited in Stage 2; the presence of anoxic or dysaerobic bottom water or shallow pore waters during that time would have been recorded by the addition of Mo to the sediments via coprecipitation with authigenic iron sulphides. The combined faunal and geochemical data are consistent with the notion that Panama Basin deep waters remained oxygen-replete during the last glacial maximum. It is concluded that the late Stage 2 carbon enrichment in the eastern equatorial Pacific more probably reflects higher productivity rather than enhanced preservation.