A new apparatus for force measurement in marine bioadhesion

In an experimental study on the effect of parrotfish (probably Scarus taeniurus) grazing on the structure of benthic reef communities, fishes in densities of 0.6 to 1.5 parrotfish per m² or 9 to 17 g wet weight of fish per m² of feeding surface were found to have an optimum effect, resulting in the greatest benthic species richness and biomass on 2-dimensional surfaces. The presence of refuges (3-dimensional habitats), however, has a greater impact on bemthic community structure (number of species and biomass) than does just the density of parrotfish in such an experimental system. Coral recruitment is enhanced by the presence of refuges and, like coralline algae, is more successful under increased grazing pressure. These optimum densities of parrotfishes relate well to observed field densities where, in a collection from a Hawaiian patch reef, there were 1.1 fish or 10.8 g wet weight of parrotfish per square meter of collection area. The success of coralline algae and corals under high grazing pressure may have important consequences for the stability and structure of modern coral reefs.     

Productivity of bacteria and microalgae and the effect of grazing by holothurians in sediments on a coral reef flat

Bacterial productivity in sandy sediments on reef flats at Lizard Island, Great Barrier Reef was determined from the rate of incorporation of tritiated thymidine into DNA. The study was conducted during January 1982 and July 1983. A small diurnal increase occurred in sediments having a dense population of microalgae. Bacterial production was 120 to 370 mg C m^-2 d^-1 in summer on reef flats, which was equivalent to 30–40% of primary production by benthic microalgae. In winter, rates of primary production by benthic microalgae and secondary production by bacteria were about one-half to one-fifth of those in summer. There was much variation in production, due to patchiness in the distribution of benthic microbes, especially microalgae. Doubling times for the bacteria in surface sediment were 1 to 2 d in summer and 4 to 16 d in winter on the reef flats. These high productivity values for bacteria indicated that a net input of organic matter to the sediment was needed to support the growth of bacteria. Sediment bacteria thus have a very important role in transforming organic matter on the reef flats. Grazing by Holothuria atra depressed both primary production and bacterial production. It was estimated that these holothurians ate about 10 to 40% of bacterial carbon produced each day in summer, and thus have an important role in the carbon cycle. Harpacticoid copepods were numerically important components of the benthic meiofaunal community and probably had a significant impact on bacterial density as grazers.     

Benthic community respiration in the N.W. Atlantic Ocean: in situ measurements from 40 to 5200 m

Benthic community respiration was measured in situ at 9 stations along the Gay Head-Bermuda transect from depths of 40 to 5200 m. Three methods were used; bell jar respirometers, grab respirometers, and free vehicle respirometers. Benthic community respiration rates spanned three orders of magnitude, decreasing from 21.5 ml O₂ m^-2 h^-1 at 40 m in November to 0.02 ml O₂ m^-2 h^-1 at 5200 m. Rates decreased two orders of magnitude between 40 and 1800 m and then significantly declined again between the continental rise (3650 m) and the abyssal plain stations. Predictive equations for benthic community respiration along the transect reflect a strong correlation with depth of water. Of lesser significance are the correlations with water temperature, dissolved oxygen, benthic animal biomass, surface primary productivity and sediment organic matter. Calculations show that annual benthic respiration can utilize 1 to 2% of the surface primary productivity. Of the 2 to 7% organic carbon fixed at the surface which supposedly reaches the bottom, only 15 to 29% is utilized by the benthic community at 2200, 3000, and 3650 m. The energy requirements of other biological components of deep-sea benthic communities, such as benthopelagic and macro-epibenthic animals, not included in these measurements, must also be considered in calculating a balance of carbon.Contribution from Scripps Institution of Oceanography.     

Distribution,abundance, and substrate preferences of demersal reef zooplankton at Lizard Island Lagoon,Great Barrier Reef

Demersal zooplankton, those plankton which hide within reef sediments during the day but emerge to swim freely over the reef at night, were sampled quantitatively using emergence traps planced over the substrate at Lizard Island Lagoon, Great Barrier Reef. Densities of zooplankton emerging at night from 6 substrate types (fine, medium, and coarse sand, rubble, living coral and reef rock) and from 5 reef zones (seaward face, reef flat, lagoon, back reef, and sand flat) were determined. A large population of nocturnal plankton including cumaceans, mysids, ostracods, shrimp, isopods, amphipods, crustacean larvae, polychaetes, foraminiferans and copepods are resident members of the reef community at Lizard Island. The mean density of plankton emerging throughout the reef was 2510±388 (standard error) zooplankton/m² of substrate. Biomass averaged 66.2±5.4 mg ash-free dry weight/m² of substrate. Demersal zooplankton exhibited significant preferences for substrate types and reef zones. The highest mean density of zooplankton emerged from coral (11,264±1952 zooplankton/m²) while the lowest emerged from reef rock (840±106 zooplankton/m²). The density of demersal plankton was six times greater on the face than in any other zone, averaging 7900±1501 zooplankton/m². Copepods dominated samples collected over living coral and rubble while foraminiferans, ostracods and decapod larvae were most abundant from sand. Plankton collected with nets at night correlated only qualitatively with plankton collected in emergence traps from the same location. Although abundant, demersal plankton were not numerous enough to meet the metabolic needs of all corals at Lizard Island Lagoon. Demersal plankton appear especially adapted to avoid fish predation. The predator-avoidance strategies of demersal plankton and maintenance of position on the reef are discussed. Our results indicate that much of the zooplankton over coral reefs actually lives on the reef itself and that previous studies using standard net sampling techniques have greatly underestimated plankton abundance over coral reefs.     

Defensive mechanisms and ecology of some tropical holothurians

The ecology, defensive behavior and toxicity of three species of reef flat holothurians (Actinopyga mauritiana, Holothuria atra and Holothuria difficilis) were studied at Eniwetok Atoll, Marshall Islands. The average diurnal population density of H. difficilis ranged from 1.4 to 32 holothurians/900 cm²; resting respiratory rates (0.05 ml O₂/g wet wt/h) were comparable during day and night; nourishment in H. difficilis may be primarily from bacteria and foraminifera in which about 2% of the dry weight of sediment consumed is utilized, and the species probably passes at least 3 g dry wt of sediment/m²/day (>1 kg/m²/year). A fundamental difference in energy flow is suggested: considerably more energy is passed from benthic algae to grazing and browsing fishes to predatory fishes on coral reefs whereas, in extra-tropical latitudes, more energy is shunted from benthic algae to invertebrates to predators. The effects of holothurin leading to death in fishes are irreversible. Holothuria difficilis is best protected from predation. Its body wall is toxic and it can accurately eject Cuvierian tubules, which are also toxic. The discharge of tubules was regulated by a circadian rhythm in May. Studies on holothurians and sponges suggest that many exposed coral reef invertebrates have evolved effective defensive mechanisms in association with high intensity predation.Supported by A.E.C. Contract AT (29-2)-226 with the University of Hawaii.     

Spatial and temporal variation in a soft-bottom fish assemblage in St Vincent Bay,New Caledonia

Monthly trawl surveys were performed in 1989 in North Bay and South Bay of St Vincent (New Caledonia) with both a shrimp trawl and fish trawl to produce a reference standard of the natural variability of an unexploited tropical soft-bottom fish assemblage. A total of 230 species belonging to 62 families were recorded. The mean density and biomass were 0.18 fishes m^-2 and 4.31 g m^-2, respectively. The major variations were explained by spatial factors. Species richness, density and biomass were greater in South Bay (204 species, 0.26 fishes m^-2 and 5.90 g m^-2) than in North Bay (105 species, 0.10 fishes m^-2 and 2.71 g m^-2), 34% of the species being present in both areas. The North Bay assemblage was characterized by four abundant benthic species (Saurida undosquamis, Gerres ovatus, Secutor ruconius and Upeneus moluccensis) and by numerous pelagic species (Carangidae, Sphyraenidae and Scombridae). The South Bay assemblage was characterized by several Mullidae, Bothidae and Balistidae, and by some rare species usually found on coral reefs (Pomacentridae and Chaetodontidae). These differences were induced by the physical and benthic characteristics of the two bays. North Bay was an homogeneous, confined, deposit area with few benthic organisms, whereas the substrate was more heterogeneous and the benthic organisms more diversified and abundant in South Bay, which was connected to the adjacent reef lagoon. Species richness remained stable in time, except in January when a hurricane disturbed the environment. Seasonal tendencies in species composition were evidenced in North Bay, with an autumn-winter structure opposed to a spring-summer structure, and characterized by the relative importance of the major species. No seasonal tendencies were observed in the organization of the South Bay assemblage. Nevertheless, mean density and biomass were at a minimum in summer in both bays; maxima occurred in winter. Biomass was negatively correlated to both temperature and rainfall, and reflected the population variations of the main species, particularly their reproductive migrations. Thus, the soft-bottom fish assemblages were strongly organized spatially in New Caledonia, but remained relatively stable over time.     

Benthic community structure, diversity, and productivity in the shallow Barents Sea bank (Svalbard Bank)

The Barents Sea is among the most productive areas in the world oceans, and its shallow banks exhibit particularly high rates of primary productivity reaching over 300 g C m^?2year^?1. Our study focused on the Svalbard Bank, an important feeding area for fishes and whales. In order to investigate how benthic community structure and benthic secondary production vary across environmental gradients and through time, we sampled across the bank and compared results with a similar study conducted 85 years ago. Considerable variability in community structure and function across bank corresponded with differences in the physical structure of the habitat, including currents, sedimentation regimes and sediment type, and overlying water masses. Despite an intensive scallop fishery and climatic shifts that have taken place since the last survey in the 1920s, benthic community structure was very similar to that from the previous survey, suggesting strong system resilience. Primary and secondary production over shallow banks plays a large role in the Barents Sea and may act as a carbon subsidy to surrounding fish populations, of which many are of commercial importance.     

Precision of the mean and the design of benthos sampling programmes: caution advised

To calculate the number of samples required to estimate population density to a specified precision, a prior estimate of the sample variance is needed. Using data from the freshwater benthic literature, Downing (1979, 1980a) calculated a regression equation to predict sample variance from sampler size and population density. He used predicted sample variance to calculate the number of samples, of a range of sizes, required to estimate a range of population densities to a specified precision. He concludes that massive savings (1300 to 5000%) of total surface area sampled may be achieved by using sample units of small surface area. These conclusions are misleading. The data set used for the regression does not adequately cover the combination of a low-density population sampled by a device of small surface area. The benthic community of Belhaven Bay, East Lothian, Scotland was sampled in 1982 with a 0.1 m² grab and a 0.0018 m² corer, providing 112 sets of replicate data which were used to test the hypothesis that for a specified precision of the mean a considerable saving of total area sampled may be obtained by sampling with a device of small surface area. The benthos of Loch Creran, Argyll, Scotland was sampled with contiguous corer samples on four occasions in 1980 and 1981, providing 234 independent sets of replicate data. Contiguous samples were grouped to form several simulated series of samples of increasing surface area. A sampler of small surface area provided a saving of total area sampled of about 20%. Whether such a small saving is justifiable will depend on the extra field expenses incurred by taking many small samples.     

Benthic response to sedimentation of a spring phytoplankton bloom: Process and budget

The response of benthos to sedimentation of the spring phytoplankton bloom in the Kiel Bight (Western Baltic Sea) is described in terms of biomass (ATP) and activity (heat production and ETS-activity). Input of the bloom (11.5 g C m^-2) over a period from March 25 to April 19, 1980 to the sediment surface was in the form of cells and fresh phytodetritus as indicated by low C/N ratios (7) and high energy charge values (0.78). Benthic microbial activity was immediately stimulated by this input as heat production doubled and the activity of ETS tripled over winter values within 12 d in the absence of a significant increase in ambient temperature. A comparison of the two activity parameters suggests that anaerobic metabolism is more important during the winter (February and March) than after input of the bloom. Meiofauna was not able to take part in the first activity outburst. Benthic ATP-biomass (excluding macrofauna) doubled in late April due to microbial production, and doubled again in early May when meiofauna started reproductive activity. For macrofauna a general statement was not possible, although the sediment surface feeder Macoma baltica commenced a build up of glycogen and lipid resources immediately following bloom input whereas Nephtys ciliata, feeding on sediment and small macrofauna, showed a less pronounced and delayed effect from this input. An energy budget based on heat production measurements was calculated. A daily heat loss of the benthic community of 21.7 KJ m^-2 d^-1 (35.5 KJ m^-2 d^-1) was found, when a depth of 3 cm sediment (5 cm) was assumed. Heat production of macrofauna contributed less than 5% of this activity. The input of the bloom was burned within 21 (13) d. Preliminary estimations for an annual budget suggest that the vertical transport of particulate organic matter via sedimentation can only explain 25% (15%) of the benthic activity in the shallow water ecosystem of the Kiel Bight. This indicates the presence of other sources of organic carbon such as benthic primary production or other transport processes providing carbon to the sediments.Publication No. 384 of the Joint Research Program of Kiel University (Sonderforschungsbereich 95)     

Nutrient cycling between the water column and a marine sediment. I. Organic carbon

The carbon flow through the sediments at a station located in 18.3 m of water off the Scripps Institution of Oceanography, San Diego, California (USA) was determined. The parameters studied [and their mean rates of input (+) or output (-) to the benthos] were macro-detritus (+0.028 gC m^-2day^-1), fallout of particulate debris (+3.3 gC m^-2day^-1), benthic net photosynthesis during the day (-0.06 gC m^-2 daylight period^-1), burial (0 gC m^-2day^-1), benthic respiration at night (-0.28 gC m^-2 night period^-1), and resuspension (-3.0 gC m^-2day^-1). Resuspension of sediment at this station was found to have a controlling effect on the sediment organic carbon content. Benthic photosynthesis was able to provide 79% of the organic carbon required by the benthos for respiration during the daylight hours. A carbon-flow diagram linking together all of the above measurements is presented.     

Cross-shelf distribution of copepods and fish larvae across the central Great Barrier Reef

The distribution of total dry weight of zooplankton, copepod numbers and ichthyoplankton across the outer continental shelf in the central Great Barrier Reef was examined at bi-weekly intervals for three months over summer of 1983. Copepods were sampled (236 m net) within 10 m of the surface and within 10 m of the bottom. Mean densities in surface waters decreased markedly from the mid-shelf to outer shelf and the Coral Sea, but no cross-shelf gradient occurred in the bottom-water. Densities of copepods on the mid-shelf (surface and bottom waters) and in bottom-waters of the outer shelf were typically ca. 400 m^–3. Significantly lower densities (ca. 100 m^–3) occurred in surface waters of the outer shelf, except during outbursts of Acartia australis, when densities in these waters differed little from those elsewhere on the shelf. In oceanic waters, 10 km from the outer shelf station, copepod densities in surface waters were ca. 40 m^–3. Four of the five most abundant copepod taxa in surface waters, Paracalanus spp., Eucalanus crassus, Acrocalanus gracilis and Canthocalanus pauper, tended to be most abundant at the mid-shelf end of the transect. Acartia australis was sporadically very abundant in surface waters of the outer shelf, as was Paracalanus spp. in bottom-water of the outer shelf. An assemblage of Coral Sea species of copepod occurred in bottom-water of the outer shelf during two major intrusions, but not at other times. Densities of all common species varied considerably between cruises. Maximum densities of all common species except A. australis tended to be associated with diatom blooms linked to intrusions but a bloom did not necessarily mean all common species were abundant. Fish larvae included both reef and non-reef taxa, with reef taxa predominating on the outer shelf (approx 2:1 in density of individuals) and non-reef taxa dominating in nearshore samples (approx 2:1). Nine of the ten most abundant taxa analysed showed highly significant variation in numbers among stations and all but one of these also exhibited significant station x cruise interactions. Interactions generally reflected changes in the rank importance of adjacent stations from one cruise to the next or lack of any significant cross-shelf variation on some cruises where overall abundance of the taxa was low.     

Indications from photosynthetic components that iron is a limiting nutrient in primary producers on coral reefs

Because iron is not available generally in oxygenated sea water, it may be a limiting factor in marine primary production. This hypothesis was tested in the context of Davies Reef, Latitude 18°50′S (one of the coral reefs in the central region of the Great Barrier Reef system). Samples were collected for study in the period August, 1980 to March, 1981. Sea water around the reef contained ≦2x10^-6 M Fe, surface sediments from the reef contained 66±26 (1 SD) ppm total Fe, and interstitial water near the surface contained ≧5x10^-7 M Fe. Thus, Fe constituted a trace component of the reef environment, but limited Fe should be available to algae associated with the sediments. Specific biochemical analyses to test the Fe status of benthic photosynthetic organisms were carried out with a common blue-green alga, Phormidium sp., and a ubiquitous symbiotic dinoflagellate, Gymnodinium microadriaticum (zooxanthellae). The blue-green alga contained the electron transport protein, flavodoxin, which is found only in Fe-deficient organisms. Supporting evidence for Fe stress in this organism included chlorosis in the presence of plentiful biliprotein, and very low extractable photosynthetic cytochrome, c-553. The latter observations were shown to be the result of Fe deficiency in laboratory cultures of a blue-green alga, Synechococcus sp. These cultures showed that production of flavodoxin is not a universal response of algae to Fe stress, but that lowered cellular concentrations of Fe-containing proteins involved in photosynthesis probably is universal. The zooxanthellae from a soft coral, Sinularia sp., had three-fold lower total Fe and ferredoxin (an electron transport protein), than the same alga from a clam, Tridacna maxima. Thus, some algae in symbiotic associations may also suffer Fe-deficiency. It was concluded that the degree and extent of Fe-stress in primary producers on a coral reef may influence growth rates, biomass, and distribution of species.     

Stability of sea urchin dominated barren grounds following destructive grazing of kelp in St. Margaret's Bay,Eastern Canada

Kelp regeneration was observed for the first time in St. Margaret's Bay, Nova Scotia, Canada in an area known to have been devoid of macroalgae for several years. The regeneration was destroyed by sea urchins within 10 months. Experimentally induced kelp regeneration met a similar fate under normal grazing pressure. At the lowest sea urchin biomass and density encountered re-establishment of mature kelp stands seems highly unlikely. The sea urchin population required to suppress kelp regeneration is fed by benthic microalgae. Diatoms and other pioneer algal community species were found in the guts of sea urchins. The mean standing crop of benthic microalgae was found to be, 2.2 g C m^-2 and production estimated as ca 15 g C m^-2yr^-1 at 8m depth. Most of the primary production of St. Margaret's Bay has been lost with the disappearing kelp populations.     

Ecology of a Caribbean coral reef. The Porites reef-flat biotope: Part II. Plankton community with evidence for depletion

A quantitative assessment of drifting net plankton crossing a reef-flat biotope was obtained on a Caribbean coral reef. The spatial distribution and abundance of plankton were sampled to provide estimates of the removal of this potential food resource by suspension-feeding populations. Sampling was largely confined to the reef flat and adjacent waters of Laurel Cay, a flourishing coral reef present on the insular shelf off southwestern Puerto Rico. A prior study provides information on the meteorological and hydrographic characteristics of this area. Evidence for plankton accrual was found in the quantitative depletion of qualitatively similar populations sampled downstream of densely populated reef communities. Numerically, the diatom crop was reduced by 91% and zooplankton by 60% in water streaming off the reef. Significant diel and seasonal variations in plankton abundance were obcerved, as well as notable differences in volume flow, the latter closely related to the local wind regime. A time course of net plankton accrual was calculated, taking into account these various factors. During the summer season (July–August), when zooplankton was relatively abundant and water movement over the reef vigorous, the total gain from plankton reached 0.25 gC/m²/day; 75% of this occurred during a 4 h period at sunrise and sunset. Plankton retained on the reef flat in January of February and in September was around 0.1 g C/m²/day. Zooplankton biomass contributed the greatest share, exceeding that of diatoms by a factor of 10 during the day and 42 in the early evening. A mean annual accrual of 0.18 g C/m²/day is equivalent to 4 to 13% of net community metabolism.     

Sedimentation and benthic mineralization of organic detritus in a Norwegian fjord

The supply of particulate material to the sea-bed as well as the oxygen consumption and the redox potential of the sea-bed were measured during a one-year period (1979/1980) at 60 and 90 m depth in the inner part of a west Norwegian fjord, Fanafjorden. At both sites, uniform sedimentation rates of total particulate material (825 and 885 g m^-2 yr^-1, respectively) and particulate inorganic material (576 and 616 g m^-2 yr^-1, respectively) were found. The sedimentation rates of particulate organic carbon (96 and 107 g m^-2 yr^-1, respectively) and particulate organic nitrogen (10 and 12 g m^-2 yr^-1, respectively) were low in winter, higher in summer and autumn, with maxima in May/June, reflecting similar maxima in the phytoplankton biomass in the area, with 6 to 8 wk delay. The oxygen consumption of the sea-floor was lowest in winter/spring and highest in summer. Thirtytwo and 38 g C m^-2 yr^-1 (respiration quotient=0.85) were metabolized by the sediment at 60 and 90 m, respectively. The simultaneous measurements of sedimentation rates and sediment oxygen uptake throughout a whole year demonstrated that the benthic mineralization is governed by the sedimentation over a longer time-scale, but that seasonal imbalances do occur. A box-model of the flux of particulate organic carbon to the sediment surface is presented, and includes the relevant processes and some quantitative estimates.     

Biomass of suspended bacteria over coral reefs

The biomass of bacteria suspended in water flowing over coral reefs at Lizard Island and Yonge Reef (Northern Great Barrier Reef) was estimated by measurement of muramic acid. Values ranged from 20 mg C m^-3 in the open water up to about 60 mg C m^-3 over the reef flat. Direct counts of total numbers of free bacteria were made for comparison. Values of around 2.0x10⁹ cells g^-1 muramic acid showed that there was a good agreement between direct counts and muramic acid content of free bacteria in the open water. In samples containing suspended particulate matter, ratios of direct counts to muramic acid concentration were lower, because bacteria on particles could not be counted. Thus, these ratios were used to indicate the proportions of bacteria attached to particles. Changes in the biomass and numbers of bacteria were determined in water masses identified either by a drogue or fluorescein, as they moved across the reefs. In the zone on the outside of the reef, the number of free bacteria decreased compared to open sea water, but total biomass increased, showing that particulate matter containing bacteria was thrown up into suspension. About 50% of bacteria were attached to particles. Water flowing over the reef flats contained much particulate material with bacteria attached. Bacteria constituted between about 5 and 20% of particulate organic carbon.     

Biomass and trophic structure of the macrobenthos in the south-west lagoon of New Caledonia

The biomass of the macrobenthic communities of the southwest lagoon of New Caledonia is analysed with special reference to tropical soft-bottom assemblages. The samples were obtained in 1986 from 350 Smith-McIntyre grabs as well as by diving over an area of 3 500 m², and covered a wide size-range of benthic organisms. The classification of the biomass data into taxonomic groups revealed the following: (1) a zoobenthic biomass (mean=13 g ash-free dry wt m^-2) that is high compared to values previously published for tropical systems, but fairly low compared to those recorded for medium and high latitudes; (2) a large macrophytic biomass (45% of ash-free dry wt); (3) dominance by weight of sponges, molluscs and cnidarians over the other faunal groups. The distribution of the biomass among the trophic groups revealed the following: (1) the dominance by weight of suspension-feeders, which account for 67% of the total zoobenthic ash-free dry wt; (2) the absence of trophic group amensalism between filter-feeders and deposit-feeders whatever the degree of bottom muddiness; (3) the existence of a relationship between the biomass of surface deposit-feeders and ATP at the surface layer of the sediment, suggesting that this group feeds preferentially on living particles. Differences in trophic structure between the muddy-bottom, greysand and white-sand communities of the lagoon reflect the functional characteristics of each bottom type.     

Energy flow through the benthos in a Scottish Sea Loch

Analysis of material collected in settlement traps at regular intervals over a period of a year gave an estimate of the annual organic input to the bottom sediment of about 28 gC/m² year, which is a little less than one third of the primary production in the overlying water column. The aerobic benthic community metabolism, estimated from in situ respiration measurements, was not significantly different from the carbon input. The rate of release of ammonia from the sediment was also measured in situ and would be sufficient to supply the greater part of the required input of inorganic nitrogen for photosynthesis in the water column.     

Community metabolism of intertidal flats in the Ems-Dollard estuary

To obtain an insight into the flux of carbon through intertidal sediments of the Ems-Dollard estuary, the annual cycles of gross benthic primary production and community respiration were measured at six stations, together with a set of environmental parameters. In a stepwise multiple regression analysis it was shown that temperature alone and temperature plus viable bacteria explained 50 and 70% respectively of the observed variation in community respiration. Other variables, including the rate of primary production and amount of organic carbon in the sediment were less important. The rate of primary production could not be fitted adequately into a multiple regression equation. The annual values of community respiration (177–794 gO₂·m^-2·yr^-1) and primary production (82–628 gO₂·m^-2·yr^-1) were within the range of published values. except for one station in the vicinity of a wastewater outfall, which had an extreme production (average 984 gO₂·m^-2·yr^-1). At four stations, annual community respiration exceeded primary production by 40%. It is concluded that the main carbon flux within the sediment, from CO₂ to benthic primary producers, to benthic consumers and from there to CO₂ again,was completed within a month or so, leaving untouched the large bulk of organic matter within the sediment. Possible effects of wastewater discharges on community metabolism are discussed.Publication No. 43 of the project Biological Research in the Ems-Dollard Estuary     

Spatial distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum in coastal bottoms (Cap de Creus, northwestern Mediterranean Sea)

Current knowledge on the abundance and distribution patterns of different soft coral species is relatively limited when compared to other benthic suspension feeders such as gorgonians and hard coral species. To overcome this scarcity of information, the distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m depth) extent using a remotely operated vehicle. A. acaule was the most abundant species in the study area with highest recorded density of 18 col m^?2 found at depths of 35–45 m in areas that are directly exposed to strong near-bottom currents. Conversely, A. palmatum was only found as scattered solitary colonies at greater depths in soft bottoms, with maximum density of 2.4 col m^?2. Medium and large colonies of A. acaule were preferentially found on sloping and vertical rocky bottoms where they form dense patches. High-density patches of A. acaule were preferentially found on vertical rocky bottom, while isolated colonies were preferentially observed on coralligenous substrata as well as on flat soft and maërl substrates. A. acaule biomass distribution showed highest values between 40 and 45 m depth, and between 60 and 65 m depth. This suggests that deeper populations are formed by colonies that are bigger than the equivalent shallower ones. Although both species are almost genetically identical, ecologically they are very different. For this reason, conservation plans should consider the differential ecological traits shown by these two soft coral species.