Effects of cadmium on Pontoporeia affinis (Crustacea: Amphipoda) in laboratory soft-bottom microcosms

Laboratory soft bottom microcosms, containing sediment with natural populations of meio-and microfauna and the amphipod Pontoporeia affinis Lindström, were used to investigate chronic effects of cadmium on a typical Baltic Sea benthic community. Temperature, salinity, organic content of the sediment and the life cycles of the amphipods in laboratory control microcosms were in good agreement with conditions in the Baltic Sea. In Experiment 1, P. affinis was exposed to four assayed cadmium concentrations, 0 (control), 6.3, 41 and 127 g Cd·1^-1, for 265 d. At the lowest cadmium concentration (6.3 g Cd·1^-1), the sediment contained about 700 times, and P. affinis about 1 700 times, the water concentration at the end of the experiments. In microcosms without P. affinis, the sediments accumulated less cadmium. Effects on P. affinis embryogenesis were detected after 105 d of exposure in all tested cadmium concentrations. Mortality of juveniles was significantly increased after 265 d of exposure in mean assayed concentrations of 41 and 127 g Cd·1^-1. In Experiment 2, P. affinis was exposed to cadmium in assayed concentrations, 0, 6.5 and 46 g Cd·1^-1 for 460 d. At the lowest water concentration (6.5 g Cd·1^-1), the prolonged exposure did not increase the sediment concentration, which indicates a steady state, while concentrations in P. affinis reached about 3 500 times the lowest water concentration after 460 d. Juvenile P. affinis matured sexually and reproduced normally in all microcosms, but their offspring died in all microcosms with added Cd. This occurred at water concentrations not uncommon in polluted marine areas. The effects are serious in comparison to those found in conventional short-term laboratory experiments with the same concentrations and demonstrate the importance of long-term experiments in ecotoxicology.     

Corophium multisetosum (Amphipoda: Corophiidae) in Canal de Mira,Portugal: Some factors that affect its distribution

The population ofCorophium multisetosum (L.) from Canal de Mira, Ria de Aveiro, Portugal, was sampled seasonally from December 1985 to September 1986, as part of a larger survey of the benthic invertebrate macrofauna. Its distribution along the channel exposes the species to a range of salinities from freshwater to above 30. A principal components analysis, using the physical and chemical parameters of the sediment as variables and sampling stations as operational taxonomic units, indicated that abundance is negatively correlated with salinity, depth, and the occurrence of sediments rich in particles below 125µm and rich in organic matter. Abundance is positively correlated with temperature. The distribution of the species does not seem to be affected by the occurrence of sediment grades between 125 and 1 000µm. Paired-choice salinity experiments indicated thatC. multisetosum prefers salinities within the range 2.5 to 10. In multi-choice experiments concerning sediment grade, amphipods did not show any significant preference within the 125 to 500µm range, although the 125µm grade was chosen less frequently. The influence of temperature on the overall distribution ofC. multisetosum is discussed.     

Relation of particle-size spectrum and food abundance to particle selectivity in the mud snail Hydrobia totteni (Prosobranchia: Hydrobiidae)

We studied the relationship between particle-size selectivity and variable particle-size distribution in the American mud snail Hydrobia totteni Morrison (Prosobranchia: Hydrobiidae) collected in the summer of 1979 and 1982, from saltmarsh mudflats at Flax Pond, Old Field, New York, USA. Using individual size fractions of native sediment, snails fed fastest on intermediate-sized particles (41 to 63 m); this pattern was related to diatom abundance, which was similarly greatest on the intermediate particle-size classes. These results conform to another study, which found a quite different relationship between feeding rate and particle size, but a similarly strong correlation between particle size and diatom abundance. Snails were fed a range of particle-size distributions of glass beads. As median particle size increased, preference shifted towards smaller particles. This shift cannot be explained by feeding rates determined from individual particle-size classes. The most reasonable alternative model is a shift towards feeding on fine particles that occur among the coarser particles that are not ingested. Our data suggest that diatom growth on certain particle sizes is as important to particle selectivity as the particle sizes themselves. Particle size preference cannot be extrapolated from studies of feeding rates on individual size classes since selectivity is qualitatively different in mixtures of various size classes.     

Experimental analysis of byssus thread production by Mytilus edulis and Modiolus modiolus in sediments

Laboratory experiments have been conducted on byssus thread production by two species of mussel, Mytilus edulis L. and Modiolus modiolus (L.), representing the epibyssate and endobyssate species of the Mytilidae, respectively. Mussels were placed in seven particle sizes of sediment ranging from 50 m to 16 mm for 12 d. The number of byssus threads per mussel, length of threads, number of threads per particle and size of pads were then recorded. Modiolus modiolus (endobyssate) produced more threads than Mytilus edulis (epibyssate). M. edulis produced most threads in the size range 2 to 16 mm and Modiolus modiolus in the size range 500 to 1000 m. M. modiolus produced longer threads than Mytilus edulis. Both species produced longer threads in particle sizes finer than 2 mm. M. edulis produced smaller pads than Modiolus modiolus. Both species produced larger pads in sediments coarser than 1 000 m. Results were also analysed in terms of attachment units (a thread attached to one or more particles, or a particle to which more than one thread is attached). The thread:particle ratio of the attachment units ranged from ca. 1:10 in the finest sediments to ca. 10:1 in the coarsest sediments. Mytilus edulis attached more threads to single particles (72% of attachment units) than Modiolus modiolus (37% of attachment units). M. modiolus had a wider spread of ratios in the finer particle sizes than Mytilus edulis. The significance of our results are discussed in relation to the ecology and palaeoecology of epibyssate and endobysste species.     

Gulf of Mexico hydrocarbon seep communities

Sediment and water samples were collected by submersible in September 1986 at 16 locations on the carbonate cap overlying a conical diapir, which was formed by the upward migration of oil and gas through a subsurface fault on the continental slope off Louisiana, USA (27°47N; 91°30.4W). The biological community at the site was photographed quantitatively with still and video cameras. Rigorous spatial sampling indices were maintained so that variation in chemical parameters and in the abundance of photographed organisms could be estimated within the bounds of the study site. Concentrations of extractable organic material (EOM) ranged from 0.24 to 119.26 in the sediment samples, while methane concentrations in the water samples were from 0.037 to 66.474 M. The visible biological community was predominantly composed of the chemosynthetic tube worms (Vestimentifera) Lamellibrachia sp. and Escarpia sp., and an undescribed, methane-oxidizing mussel (Mytilidae: Bathymodiolus-like), as well as diverse non-chemosynthetic organisms. The ranked abundance of tube worms was significantly correlated (p<0.05) with the concentration of EOM in the sediment samples, while the abundance of mussels was significantly correlated (p<0.05) with the concentration of methane in the water samples. Tube worms and mussels both occurred in dense clusters; however, the clusters of mussels had a more restricted distribution within the study site than did clusters of tube worms. Both organisms were most abundant in the vicinity of the subsurface fault.     

Potential effects of a resuspension event on the vertical distribution of copepod eggs in the sea bed: a laboratory simulation

Adult female Acartia tonsa and Labidocera aestiva were obtained from inshore marine waters near Turkey Point, Florida (29°54.5N;84°31W) during April and May 1991. For each species, eggs spawned overnight in the laboratory were collected, mixed with sediment, and added to a plastic tube (26 to 29 cm longx5 cm diam) to yield a 2 cm-thick layer. This tube was connected to a 245 cm-long tube, seawater was added and the contents were thoroughly mixed by turning the assembly end-over-end several times. The vertical distribution of eggs in the sediments following resuspension varied according to the grain size of the sediments. For sediments of 125 m particle-diameter, 68 to 72%, and 70 to 73% of the eggs of A. tonsa and L. aestiva, respectively, occurred in the upper 4 mm.For sediments 63 to <125 m particle-diameter, 34 to 36% of the eggs of A. tonsa and L. aestiva, respectively, occurred in the upper 4 mm. For sediments of <63 m particle-diameter, egg distribution was uniform for both species throughout the 2 cm layer. The results indicate that the vertical distribution of eggs in the sea bed following a resuspension event should be highly dependent upon the grain-size composition of the sediments.     

Patterns of micro- and meiofaunal abundance in marine sediments,measured with the adenosine triphosphate assay

Adenosine triphosphate (ATP) measurements are used to determine vertical and seasonal distributions of microorganisms and meiofauna in sediments from a 14 m-deep mud bottom in central Long Island Sound on 12 sampling dates from April 1975 to October 1976. Below the topmost 1 cm of sediment, ATP measurements can be useful in estimating and comparing standing stocks of microorganisms and meiofauna. In the top 1 cm, however, large quantities of newly settled bivalves (Yoldia limatula, Nucula annulata, and Mulinia lateralis) and juvenile polychaetes (Owenia fusiformis) in summer and fall months account for total ATP concentrations. The ATP content of individual meiofauna ranges from 1.97 ng individual copepod nauplius^-1 to 190.7±60 ng individual M. lateralis ^-1. In general, the total ATP content of individual polychaetes and bivalves is much higher than that of individuals of other groups. However, on a mg ATP per g wet or dry tissue basis, the ATP content of micro- and meiofaunal taxa are not significantly different. In addition to providing a means for comparing micro- and meiofaunal standing stocks, ATP measurements permit examination of the relative contribution of different meiofauna to the total living biomass of meiofauna in sediments. Total sediment ATP concentrations are greatest in the top 1 cm at all seasons, and decrease with increasing depth in the sediment. Annual concentrations in the topmost centimeter average 4.22 g g dry sediment^-1 and range seasonally from 1.09 to 7.64 g g dry sediment^-1. At a depth of 10 cm, values average 0.16 and range from 0.019 to 0.35 g g dry sediment^-1. High ATP concentrations in surface sediment reflect high concentrations of microorganisms and meiofauna at the sediment-water interface. The top 2 cm of sediment contain 71% of all meiofauna, with 41% occurring in the topmost cm. In general, densities are lowest in the winter and highest during the spring and summer, averaging 490 individuals 10 cm^-2, and varying from 87 to 1366 individuals 10 cm^-2. Because of wide variation in recruitment patterns of the benthos in Long Island Sound, the extremes of the range in meiofaunal densities can be observed in the same month in two different years. In order to monitor and compare standing stocks of organisms less than 1 mm in size in sediments, the ATP assay can save hours of processing time compared with alternate methods such as direct counts.     

Effect of the spionid polychaeteBoccardia syrtis on the distribution and survival of juvenileMacomona liliana (Bivalvia: Tellinacea)

The tellinid bivalveMacomona liliana (Iredale) occurs at relatively low densities on a sandbank (Te Tau bank) in Manukau Harbour, New Zealand. Te Tau bank is dominated by polychaetes, including the tube-building spionidBoccardia syrtis (Rainer). JuvenileM. liliana are known to disperse as post-settlement juveniles by byssus-drifting. Laboratory experiments were conducted to assess whether the low abundance ofM. liliana on Te Tau bank was due to the presence ofB. syrtis, or to some reaction to the sediment itself. In particular, we, examined how juvenileM. liliana survived after exposure to Te Tau bank sediments with and without the spionid tube-mat for 1 mo in still-water conditions, and how they responded when given a choice of different sediment/tube-mat treatments in moving water. JuvenileM. liliana did not appear to have a strong aversion to settling and burrowing in sediments from Te Tau bank. Sediments without the tube-mat did not adversely affect the survivorship of the bivalves, but survival was significantly lower amongst theB. syrtis tube-mat. In the presence of a current, the juvenile bivalves settled in Te Tau bank sediments without a tube-mat, but avoided settling amongst liveB. syrtis. An artificial tube-mat enhanced settlement. Avoidance of liveB. syrtis appears to be an avoidance of the worms themselves rather than a response to the physical presence of their tubes.     

Particle-size selection by Pseudopolydora paucibranchiata (Polychaeta: Spionidae) in suspension feeding and in deposit feeding: influences of ontogeny and flow speed

Pseudopolydora paucibranchiata Okuda suspension feeds and deposit feeds at the sediment-water interface, where it is exposed to a variety of particles differing in physical characteristics and nutritional value. In flume experiments (conducted in August 1994 and May 1995) with two sizes of either suspended or deposited beads, I measured particle-size selection separately in each feeding mode. The same influences of palp width and of ambient flow speed were observed in each mode. At velocities 0.74 cm s^-1 there were no relationships between palp width and the proportion of gut contents composed of large beads. At velocities 1.8 cm s^-1 worms with narrower palps ingested relatively fewer large beads (and more small beads) than did worms with wider palps. Palp width and body length were linearly related, and results were similar when analyzed with body length as the independent variable. As flow speed increased, selectivity changed in a worm-size-specific manner: worms with a palp width -1. Assuming that in the field (1) particle size is the principle criterion for selection, and (2) the amount of digestible food component in deposited and suspended particles, respectively, is related to particle surface area and volume, I hypothesize that changes in selectivity as velocity rises can cause juveniles to experience a decreasing profitability of suspension feeding and a simultaneously increasing profitability of deposit feeding. Juveniles could maintain a diet of high food value despite flow variations by adjusting the proportion of time they spend suspension feeding relative to deposit feeding.     

Functional feeding morphology of the euphausiid Nyctiphanes australis

The structure of the feeding basket, mandible and stomach armature of the krill Nyctiphanes australis (G. O. Sars, 1883) was examined by scanning electron microscope with the aim of predicting its diet. N. australis were collected during February 1986 and October 1986 to February 1987 in Otago Harbour, New Zealand (45° 50 S; 170° 37 E). Predictions based on the functional morphology were tested by examining stomach contents with SEM. Intersetule distances of the feeding basket (1 to 7.5 m) are finer than in other krill species, suggesting that N. australis can efficiently collect nanoplankton-sized particles (2 to 20 m). The mandibular edge index (0.74) matched the edge index of Meganyctiphanes norvegica. This indicates, in contrast to the fine feeding-basket setulation, that N. australis has a mandible resembling that of predominantly carnivorous krill species. The ratio of mandibular palp length to mandible width is 3.2±0.2, or relatively longer than the elongate palp thought to be indicative of herbivorous habit in Euphausia superba. The fine structure of the molar shows specialized surfaces which differ from those of other krill species. The internal armature of the stomach is heavily spinose, as is common in krill of herbivorous habit. Layers of various-shaped spines at differing densities were observed. The functional morphology suggests that N. australis is an opportunistic omnivore. The stomach contents tentatively support this prediction, containing fragments of phytoplankton and detrital material. However, stomach contents were generally amorphous, making identification of dietary components in the field difficult.     

Stromatolitoid microbial nodules from Bermuda —a special micro habitat for meiofauna

In Bermuda, stromatolitoid microbial nodules are found in dense groups in seagrass beds and on subtidal sandy or rocky bottoms. They develop between January and the beginning of August and may reach 15 cm in diameter and 6 cm in height. Nodules grow on top of the sediment surface and consist of convex interwoven mats of the cyanobacterium Phormidium corium and fine sediment particles trapped in these mats. Nodules were studied in the field and laboratory (in 1989–1992) with respect to their structure, microbial community, sediment chemistry and associated meiofauna. Vertical profiles taken with microelectrodes showed steep gradients of oxygen and sulfide. While free oxygen was only detected in the upper 2 mm, the sulfide concentrations increased with depth and reached maximal 1250 moll^-1 inside the nodules. On the basis of microscopic observations and sediment chemistry, vertical sections through a nodule reveal four distinct layers: (A) an oxic green surface layer of growing cyanobacteria mats, (B) an anoxic yellow-white laminated layer of sediment particles, mucus and unpigmented cyanobacteria sheaths, (C) an anoxic sulfidic (sulfide <50 moll^-1) interface where colour changes from yellow to gray and (D) an anoxic sulfidic (sulfide can increase to 1250 moll^-1) gray core composed of sediment and decomposing cyanobacteria. The following meiofauna taxa were found in the nodules: Ciliata, Turbellaria, Gnathostomulida, Gastrotricha, Nematoda, Kinorhyncha, Polychaeta, Ostracoda and Harpacticoida. The distribution of the meiofauna was significantly different (p<0.05) within the four layers. The highest density of individuals was found in the sulfidic interface and core. Nematodes represented the dominant group in general. Thiobiotic organisms, such as the Gnathostomulida, Solenofilomorphidae, and Stilbonematinae were primarily found in the anoxic sulfidic layers of the nodules.     

Contribution ofSynechococcus spp. to size-fractioned primary productivity in three water masses in the Northwest Atlantic Ocean

The distribution of phycoerythrin-richSynechococcus spp. relative to eukaryotic algae and the contribution ofSynechococcus spp. toin situ primary production were compared at a neritic front, in warm-core eddy 84-E, and at Wilkinson's Basin, during a cruise to the Northwest Atlantic Ocean in July/August 1984. Immunofluorescence analyses ofSynechococcus strains demonstrated the restricted distribution of the tropical oceanic serogroup to the warm-core eddy, while strains of the neritic serogroup and those labelled by antiserum directed against a motile strain, were abundant in all three water masses. Although the majority ofSynechococcus spp. cells were observed in the 0.6 to 1 m fraction, an increasing proportion of the totalSynechococcus spp. cells were found in the 1 to 5 m fraction as nitrate concentrations increased near the base of the thermocline. From immunofluorescence analyses, we determined that the increasing proportion of largerSynechococcus spp. cells at depth was not the result of a change in strain composition, and may therefore be associated with increasing cell volume due to the enhanced nutrient supply. The contribution of the different size fractions to the total standing crop of chlorophyll and thein situ rate of photosynthesis was distincty different for the three water masses. At the neritic front, the larger photoautotrophs of the 1 to 5 m and >5 m fractions were the major contributors to chlorophyll concentrations and primary production.Synechococcus spp. appeared to provide only 6% of the dawn-to-duskin situ primary production at the neritic front. In modified Sargasso water in the warm-core eddy,Synechococcus spp. contributed 25% to thein situ rate of integrated primary production. In this warm-core eddy, the 0.2 to 0.6 m fraction made a major contribution to the standing crop of chlorophyll and primary production that equalled or exceeded that of the larger sze categories. Furthermore, at the bottom of the euphotic layer, eukaryotes numerically dominated the 0.2 to 0.6 m fraction, which contributed 61% of the primary productivity. At Wilkinson's Basin, theSynechococcus spp.-dominated 0.6 to 1.0 m fraction made the greatest contribution to the standing crop of chlorophyll an primary production, while smaller photoautotrophs (0.2 to 0.6 m) accounted for little of the chlorophyll or photosynthetic rates measured over the euphotic layer. Largest numbers ofSynechococcus spp. (2.9x10⁸ cells l^-1) occurred at the 18% isolume, coincident with a shoulder in the chlorophyll fluorescence profile and the site of maximumin situ primary productivity. At Wilkinson's Basin,Synechococcus spp. contributed 46% to thein situ photosynthesis integrated over the water-column.     

Impact of the polychaete Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon fluoranthene

Polychaetes belonging to the genus Capitella are often present in high numbers in organic-rich sediments polluted with, e.g., oil components, and Capitella spp. may have a great impact on the biogeochemistry of these sediments. We examined the influence of Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon, fluoranthene. Capitella sp. I were added to microcosms (10 000 ind m⁻²) and the impact of a pulse-sedimentation of fluoranthene-contaminated sediment (3 mm layer) was studied for a period of 12 d after sedimentation. The sediment oxygen uptake and total sediment metabolism (TCO₂ production) increased in cores with worms (71 to 131%), whereas the anaerobic activity, measured as sulfate reduction rate 12 d after sedimentation, was lower compared to cores without worms. The effect of fluoranthene on sulfate reduction was most pronounced in the presence of worms, with a 34% reduction versus 16% in cores without worms. The reduced sulfur pools in cores with worms were smaller than in cores without worms, suggesting that the reduced anaerobic activity was caused by increased oxidation of the sediment, which may favor O₂ and other electron-acceptors (e.g. NO₃ ⁻, Fe³⁺, Mn⁴⁺) in organic matter decomposition. The sediment oxygen uptake and TCO₂ production did not show significant changes due to fluoranthene treatment, indicating that these parameters were either less sensitive to fluoranthene stress or recovered more rapidly (i.e. within 48 h) than sulfate reduction rates. Bioturbation by Capitella sp. I altered the depth profile of fluoranthene such that fluoranthene was found in deeper sediment layers (down to 2 cm) where diffusional loss and microbial breakdown probably are reduced relative to surface layers. In cores without worms, fluoranthene was found down to 1 cm, with 75% remaining in the upper 5 mm. Received: 5 December 1996 / Accepted: 11 February 1997     

Population dynamics and secondary production in an estuarine population of Caulleriella caputesocis (Polychaeta: Cirratulidae)

Between July 1978 and March 1980, the distribution, population dynamics and secondary production of Caulleriella caputesocis St. Joseph, 1864, in Southampton Water, South England, were investigated. The distribution of the polychaete was related to amount of silt and copper-content of the sediment, the highest densities occurring in sediment containing 60 to 100% silt and less than 50 ppm copper. Breeding occurred at a low level throughout the year, usually when the worms were over one year old, with peak breeding between May and September. C. caputesocis in Southampton Water is monotelic, and oocytes measured 120 to 160 m in diameter at spawning. Unshed oocytes were resorbed. Annual secondary production varied between 0.01 and 5.90 g C m^-2 yr^-1 and P:B ratio of the species ranged between 0.84 and 7.09.     

Silicon and the ecology of marine plankton diatoms. II. Silicate-uptake kinetics in five diatom species

The variation of the rate of silicate uptake with varying silicate concentration in the medium was investigated in short-term experiments with the following marine diatom species:Skeletonema costatum, Thalassiosira pseudonana, T. decipiens, Ditylum brightwellii, andLicmophora sp. The uptake conformed to Michaelis-Menten kinetics only after a correction had been made for reactive silicate that apparently could not be utilized by the diatoms. The magnitude of this correction was in the range of 0.3 to 1.3 g-at Si/l. Mean values of the half-saturation constant of silicate uptake were calculated for the different species. The lowest value was found inS. costatum (0.80 g-at Si/l) and the highest inT. decipiens (3.37 g-at Si/l). Growth limitation by low silicate concentrations could be a cause of species succession in marine plankton-diatom blooms.     

Impacts of copepods on marine seston,and resulting effects on<Emphasis Type="Italic"> Calanus finmarchicus</Emphasis> RNA:DNA ratios in mesocosm experiments

We investigated the impact of copepods on the seston community in a mesocosm set-up, and assessed how the changes in food quantity, quality and size affected the condition of the grazers, by measuring the RNA:DNA ratios in different developmental stages of Calanus finmarchicus. Manipulated copepod densities did not affect the particulate carbon concentration in the mesocosms. On the other hand, chlorophyll a content increased with higher copepod densities, and increasing densities had a positive effect on seston food quality in the mesocosms, measured as C:N ratios and 3:6 fatty acid ratios. These food quality indicators were significantly correlated to the nutritional status of C. finmarchicus. In contrast to our expectations, these results suggest a lower copepod growth potential on higher quality food. However, in concordance with earlier studies, we found that when copepods were in high densities the large particles (>1000 µm³) decreased and that the smaller particles (<1000 µm³) increased in number. These patterns were closely linked to the condition of C. finmarchicus, which were of better condition (RNA:DNA ratios) with increasing biovolumes of large particles, and, conversely, lower RNA:DNA ratios with increasing biovolumes of smaller particles. Consequentially, the selective grazing by copepods stimulated increased biovolumes of smaller plankton, and this increase was responsible for the increased food quality, in terms of C:N and 3:6 ratios. Thus, we conclude that the decreasing growth potentials of C. finmarchicus were a result of a decrease of favourably sized food particles, induced by copepod grazing.Communicated by O. Kinne, Oldendorf/Luhe     

Stable isotope study of sedimentary carbon utilization by Capitella spp.: effects of two carbon sources and geochemical conditions during their diagenesis

Two experiments were conducted to determine the effect of the type of sediment organic matter and geochemical conditions during diagenesis on the stable carbon isotope ratio ¹³C of the deposit-feeding polychaete worms Capitella spp. Laboratory experiments showed ¹³C values of-13.5 to-13.9%. for worms grown on fresh and aged kelp, Macrocystis pyrifera. Field experiments on worms in the Santa Barbara Channel, California (USA), in 1986 revealed more negative ¹³C values (to-19%.) when oil and kelp were added to sediments. The more negative values suggest the incorporation of oil carbon into worm tissues, supporting previous indications of the importance of hydrocarbons in food webs near oil seeps.     

The common jellyfish Aurelia aurita: standing stock,excretion and nutrient regeneration in the Kiel Bight,Western Baltic

The population dynamics, ammonia and inorganic phosphate excretion, and nutrient regeneration of the common jellyfish Aurelia aurita was investigated from 1982 to 1984 in the Kiel Bight, western Baltic Sea. During summer 1982, medusae abundance ranged between 14 and 23 individuals 100 m^-3, biomass was estimated at about 5 g C 100 m^-3 and the mean final diameter of individuals was 22 cm. Abundance, based on numbers, in 1983 and 1984 was an order of magnitude lower; biomass was less than 2 g C 100 m^-3 and jellyfish grew to 30 cm. During the summers of 1983 and 1984, A. aurita biomass constituted roughly 40% of that of the total zooplankton>200 m. In 1982, for which zooplankton data were lacking, it was assumed that medusae biomass was greater than that of all other zooplankton groups. Total ammonia excretion ranged between 6.5 and 36 mol h^-1 individual^-1, whereas inorganic phosphate release was 1.4 to 5.7 mol h^-1 individual^-1. Allometric equations were calculated and exponents of 0.93 for NH₄–N release and 0.87 for PO₄–P excretion were determined. Nitrogen and phosphorus turnover rates were 5.4 and 14.6% d^-1, respectively. In 1982, the medusae population released 1 100 mol NH₄–N m^-2 d^-1, about 11% of the nitrogen requirements of the phytoplankton. The inorganic phosphate excretion (150 mol m^-2 d^-1) sustained 23% of the nutrient demands of the primary producers. In the other two years the nutrient cycling of the medusae was much less important, and satisfied only 3 to 6% of the nutrient demands. It is suggested that in some years A. aurita is the second most important source of regenerated nutrients in Kiel Bight, next to sediment.     

Influence of the mineralogical composition on microbial activities in marine sediments: an experimental approach

We investigated the influence of the mineralogical composition of marine sediments on bacterial activity in experimental microcosms. Calcite and quartz were added to natural marine sediments and microbial response in terms of total bacterial abundance and biomass, β-D-glucosidase exo-enzymatic activity and bacterial incorporation of a radio-labelled (³H-leucine) substrate were investigated for a period of one month. We report here that after 15 days the mineralogical composition of the sediment (calcite vs. quartz) had an impact on bacterial abundance and activity (reduced for ca 15% and 56%, respectively). However, such impact was mitigated or even disappeared in high organic nutrient conditions.     

Nitrogenous excretion by the sediment-living bivalve Nucula tenuis from the Oslofjord,Norway

The resting rate of ammonia excretion for the sediment living bivalve Nucula tenuis (Montagu) was found to be 38.8 gN mg^-1 dw h^-1×10^-4 in August and November 1985 in the Oslofjord. The excretion rate of experimental individuals was 37% higher when placed in artificial glass bead sediment. The regression between dry weight and excretion was logN excretion=1.338+1.192 log x, where excretion is gN individual^-1 h^-1×10^-4 and log x=mg dry weight.