Under-ice amphipods in the Greenland Sea and Fram Strait (Arctic): environmental controls and seasonal patterns below the pack ice

During three "Polarstern" cruises to the ice-covered Greenland Sea (spring 1997, summer 1994, autumn 1995) studies on the under-ice habitat (morphology, hydrography, ice-algal biomass) and on the macrofaunal, autochthonous under-ice amphipods (species diversity, abundance) were carried out in order to describe environmental controls and seasonal patterns in this community. In spring, the ice underside was rather smooth and whitish, while in summer melting structures and sloughed-off ice-algal threads were observed, in autumn detritus clumps accumulated in depressions at the ice underside. Only in summer, a thin layer of warm (up to -0.6°C) and less saline (as low as S=6.3) water was found at the ice-water interface above Polar Water. Integrated ice-algal biomass was highest during autumn (2.6 mg chl a m^-2) and lowest during summer (1.2 mg chl a m^-2). Four species of under-ice amphipods occurred in spring and summer (Apherusa glacialis, Onisimus glacialis, O. nanseni, Gammarus wilkitzkii), but only the last species was observed at the ice underside in autumn. A. glacialis and G. wilkitzkii were equally abundant in spring; A. glacialis dominated in summer. The highest total abundance of amphipods occurred during summer (31.9 ind. m^-2), compared to lower abundances in spring and autumn (5.3 and 1.1 ind. m^-2, respectively). A factor analysis revealed seasonal patterns in the data set, which mainly influenced A. glacialis, and species-specific relations between several environmental factors and the distribution of under-ice amphipods. Abundance of A. glacialis was closely related to the under-ice hydrography and ice-algal biomass, whereas the other amphipod species were more influenced by the under-ice morphology. It is therefore stated that the observed thinning of the Arctic sea ice and the resulting increased meltwater input and change in morphology of floes will have a profoundly adverse effect on the under-ice amphipods.     

Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l^-1 (190,000 m^-2) for D. glacialis and 660 l^-1 (901,000 m^-2) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r=0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 µg l^-1). The population structures of each species suggested very different life-history strategies. All developmental stages of D. glacialis were isolated from the ice cores, including females with egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. antarctica were composed primarily of nauplii (>99%), consistent with past observations of a synchronised life cycle for this species. The strong coupling of the developmental cycle of P. antarctica to the growth and decay of sea ice suggests that local extinctions might occur in areas where ice break-out is unpredictable.     

Annual population development and production by Calanus finmarchicus, C. glacialis and C. hyperboreus in Disko Bay, western Greenland

The populations of the copepod species Calanus finmarchicus, C. glacialis and C. hyperboreus were investigated in Disko Bay during a 14-month period in 1996-1997. The three species were predominant in the copepod community. The biomass reached a maximum at the beginning of June (127 mg C m^-3). From the end of July until the end of April the following year, the biomass was <1-6 mg C m^-3. All three species showed seasonal ontogenetic migration. The spring ascent for all three species was just prior to or in association with the break-up of sea ice and the development of the spring bloom, whereas descent occurred over a larger time span during summer. The main overwintering stages were CV for C. finmarchicus, CIV and CV for C. glacialis and C. hyperboreus. Peak abundance of juvenile copepodites, representing the new generation, was in August for C. finmarchicus, in July for C. glacialis and in May/June for C. hyperboreus. From the timing of reproduction and the population development, the life cycles were deduced to be 1 year for C. finmarchicus and at least 2 years for C. glacialis and C. hyperboreus. Secondary production and potential grazing impact of the Calanus community were estimated by two methods based on specific egg-production rates and temperature-dependent production. The Calanus community was not able to control the primary producers during the spring bloom but probably did during post-bloom. The estimates also indicated that grazing on ciliates and heterotrophic dinoflagellates contributes as an essential food source in the post-bloom period.     

Cross-shelf variation in calanoid copepod production during summer 1996 off the Oregon coast, USA

The fecundity of nine species of adult female calanoid copepods, and molting rates for copepodite stages of Calanus marshallae were measured in 24 h shipboard incubations from samples taken during the upwelling season off the Oregon coast. Hydrographic and chlorophyll measurements were made at approximately 300 stations, and living zooplankton were collected at 36 stations on the continental shelf (<150 m depth) and 37 stations offshore of the shelf (>150 m depth) for experimental work. In our experiments, maximum egg production rates (EPR) were observed only for Calanus pacificus and Pseudocalanus mimus, 65.7 and 3.9 eggs fem^-1 day^-1 respectively, about 95% of the maximum rates known from published laboratory observations. EPR of all other copepod species (e.g., C. marshallae, Acartia longiremis and Eucalanus californicus) ranged from 3% to 65% of maximum published rates. Fecundity was not significantly related to body weight or temperature, but was significantly correlated with chlorophyll a concentration for all species except Paracalanus parvus and A. longiremis. Copepod biomass and production in on-shelf waters was dominated by female P. mimus and C. marshallae, accounting for 93% of the adult biomass (3.1 mg C m^-3) and 81% of the adult production (0.19 mg C m^-3 day^-1). Biomass in the off-shelf environment was dominated by female E. californicus, P. mimus, and C. pacificus, accounting for 95% of the adult biomass (2.2 mg C m^-3) and 95% of the adult production (0.08 mg C m^-3 day^-1). Copepodite (C1-C5) production was estimated to be 2.1 mg C m^-3 day^-1 (on-shelf waters) and 1.2 mg C m^-3 day^-1 (off-shelf water). Total adult + juvenile production averaged 2.3 mg C m^-3 day^-1 (on-shelf waters) and 1.3 mg C m^-3 day^-1 (off-shelf waters). We compared our measured female weight-specific growth rates to those predicted from the empirical models of copepod growth rates of Huntley and Lopez [Am Nat (1992) 140:201-242] and Hirst and Lampitt [Mar Biol (1998) 132:247-257]. Most of our measured values were lower than those predicted from the equation of Huntley and Lopez. We found good agreement with Hirst and Lampitt for growth rates <0.10 day^-1 but found that their empirical equations underestimated growth at rates >0.10 day^-1. The mismatch with Hirst and Lampitt resulted because some of our species were growing at maximum rates whereas their composite empirical equations predict "global" averages that do not represent maximum growth rates.     

Acoustical identification of the concentration layer of a copepod species,Calanus euxinus

Swimming trajectories of Calanus euxinus Hulsemann in the Black Sea were studied using an echosounder at 120 and 200 kHz. C. euxinus were acoustically discriminated with respect to vertical migration and swimming speed, according to dissolved oxygen (DO) concentration and the timing of migrations. Species became torpid in water with DO values <0.5 mg l^у. The time spent swimming under DO conditions between 2 and 5 mg l^у was insignificant, and varied greatly from the 10% to 25% of total time spent swimming under normoxic conditions (5-10 mg l^у). C. euxinus formed a concentration layer in the water of 1-3 m thickness. Upward migration was completed in about 3.5 h, starting 2.5 h before and ending 1 h after sunset (average rate: 0.95 cm s^у) in summer. Species ascended discretely from the suboxic to the lower boundry of the cold intermediate layer (CIL) at 0.82 cm s^у, and passed up the CIL and thermocline fast (2.3 cm s^у). Downward migration took less time (2 h), starting ~1 h before and ending ~1 h after sunrise. Swimming speed within the thermocline and CIL was 2.7 cm s^у; copepods subsequently returned to daylight depth at a sinking speed of 0.57 cm s^у. Total time for C. euxinus to settle to their nocturnal depth layer was about 5 h.     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.     

Feeding biology and carbon budget of the sediment-burrowing heart urchin Brissopsis lyrifera (Echinoidea: Spatangoida)

This study of the burrowing heart urchin Brissopsis lyrifera includes measurements on feeding and food selection, ingestion rate, absorption efficiency, ventilation and respiration. B. lyrifera regulated feeding depth, ingestion rate and absorption efficiency in relation to food source. When food was added to the top layer of sediment, B. lyrifera burrowed closer to the surface and ingested mainly enriched surface material, whereas it burrowed deeper and ingested deep-seated sediment in the controls. In non-enriched sediment, the feeding rate was 0.04 g sediment DW h^-1 ind.^-1, and, in macro- and microalga-enriched sediment the feeding rate was 0.06 and 0.08 g sediment DW h^-1 ind.^-1, respectively. Absorption efficiency of TOC was about 43% in non-enriched sediment and in microalga-enriched sediment, but was significantly lower (34%) in macroalga-enriched sediment. Absorption efficiency of N varied between 48% and 55%, and was independent of food source. B. lyrifera feed selectively, enriching the gut TOC content about 2-fold and N content about 2.5-fold. The C/N ratio was therefore lower within the gut compared to the surrounding sediment, while the faecal C/N ratio was almost equal to the surrounding sediment. The faeces were, however, slightly richer in TOC and N compared to the surrounding sediment. For 3-5 cm long B. lyrifera, water current rate varied between 4 and 24 ml water h^-1, with a mean of 11 ml h^-1. Mean respiration rate was 205 µl O₂ h^-1 ind.^-1. The water current rate was not sufficient for B. lyrifera to sustain itself by filter feeding only. However, organic-rich particles from the surface are suggested to be an important contribution to the diet. A carbon budget was calculated for B. lyrifera from measured values of consumption, absorption efficiency and respiration, in order to estimate annual production of B. lyrifera. Compared to literature values, growth was overestimated about tenfold in the budget. A large proportion of the absorbed carbon was suggested to leave the animal as dissolved carbon, through mucus production or through anaerobic pathways, either by the heart urchin or by micro-organisms in the gut.     

Distribution of the alien ctenophore <Emphasis Type="Italic">Mnemiopsis</Emphasis><Emphasis Type="Italic">leidyi</Emphasis> in the Caspian Sea in August 2001

In this study, spatial and vertical distributions of the invasive ctenophore Mnemiopsis leidyi in the Caspian Sea were evaluated by using data collected at 41 stations during the August 2001 cruise. A comparison of data from different depths revealed that M. leidyi were generally confined to surface waters. The maximum size of the ctenophore was only 41-45 mm, and the bulk of individuals (85.5%) were <10 mm in length. The average and maximum biomasses of M. leidyi were calculated as 120 and 351 g wet weight m^-2, respectively. Whilst highest biomasses were observed in the western and central Middle Caspian Sea, hot spot areas of reproduction were present along the coasts of the western Caspian Sea, with abundance values of up to 2285 ind. m^-2. The impact of such high densities of M. leidyi is expected to be significant for the pelagic ecosystem of the Caspian Sea.     

Biodiversity and species–environment relationships of the demersal fish assemblage at the Great Meteor Seamount (subtropical NE Atlantic), sampled by different trawls

Quantitative data collected with different bottom trawls at the Great Meteor Seamount (subtropical NE Atlantic, 30°N; 28.5°W) in 1967, 1970 and 1998 are compared. Bootstrap estimates of total catch per unit effort increased from 6.96 and 10.8 ind. m^-1 h^-1 in 1967 and 1970, respectively, to 583.98 ind. m^-1 h^-1 in 1998. Gear effects and an effect of gear over time accounted for 47.1% and 20% of species variability. Further significant factors were time of day and habitat, while season was not significant. A total of 43 species was collected. Including supplementary species information, a grand total of 46 species was found associated with the Great Meteor Seamount. Diversity was higher in 1967 and 1970 (Shannon's diversity: H'=2.5 and 1.6) than in 1998 (H'=0.9). Species-environment relationships are discussed in terms of a sound-scattering layer-interception hypothesis, i.e. utilisation of prey from a diurnally moving sound-scattering layer for the bentho-pelagic community. This is probably augmented by concentration effects in a circular current around the seamount (Taylor-column). Long-term changes are discussed with respect to a decrease in biodiversity due to considerable increases in Macroramphosus scolopax and Capros aper. In 1998, the increase of abundance of Trachurus picturatus and the respective decreases for genuine benthic species were likely to have been caused by a change of gear.     

Respiration and lipid content of the Arctic copepod<Emphasis Type="Italic"> Calanus hyperboreus</Emphasis> overwintering 1 m above the seafloor at 2,300 m water depth in the Fram Strait

In August 2000 high concentrations of the dominant herbivorous copepod Calanus hyperboreus were detected in the Arctic Fram Strait, west of Spitsbergen, 1 m above the seafloor at 2,290 m water depth. Individuals from that layer were sampled by a hyper-benthic net attached to the frame of an epi-benthic sledge. For comparison, the vertical distribution of C. hyperboreus in the water column was studied simultaneously by a multiple opening/closing net haul from 2,250 m depth to the surface. Maximum abundance was found close to the surface with 6.6 and 10.0 ind. m^?3 at 0–50 m and 50–100 m depth, respectively. However, the major fraction of the population (>40%) occurred between 1,000 and 1,500 m depth. In the deepest layer (2,000–2,250 m) abundance measured 2.2 ind. m^?3 and was twice as high as between 100 and 1,000 m depth. In comparison to individuals from surface waters, copepods from the hyper-benthic layer were torpid and did not react to mechanical stimuli. Stage CV copepodids and females from the deep sample contained 4–10% less lipid and showed significantly reduced respiration rates of 0.24 and 0.26 ml O₂ h^?1 g^?1 dry mass (DM) as compared to surface samples (0.49 and 0.43 ml O₂ h^?1 g^?1 DM). All these observations indicate that the hyper-benthic part of the population had already started a dormant overwintering phase at great depth. Based on the lipid deposits and energy demands, the potential maximum duration of the non-feeding dormant phase was estimated at 76–110 days for females and at 98–137 days for CV copepodids, depending on what indispensable minimum lipid content was assumed. In any case, the estimated times could not meet the necessary requirements for a starvation period of >6 months until the next phytoplankton bloom in the following spring. The ecological implications of these results are discussed with respect to the life cycle and eco-physiological adaptations of C. hyperboreus to its high-Arctic habitat.     

Role of echinoderms in benthic remineralization in the Chukchi Sea

The role of large, epibenthic organisms in carbon cycling at high latitudes is difficult to assess using standard ship-board collection techniques. We used a remotely operated vehicle equipped with video imaging to examine the distribution and abundance of epibenthic organisms in the northeast Chukchi Sea during June 1998. At each of 11 sites, we collected between 25 and 50 images from a minimum of 20 min of video. We observed 15 different epibenthic taxa, with the echinoderms (Ophiura sarsia, O. maculata, Ophiopholis aculeata, Stegophiura nodosa, and Echinarachinus parma) overwhelmingly dominating the epibenthos. Echinoderm density was highly variable, ranging from 0.2 to 256.6 individuals m^-2 (median=16.3), and echinoderm biomass varied between <0.5 and 4,988 mg C m^-2 (median=737). The highest biomass of ophiuroids recorded (3,388 mg C m^-2) is 30% higher than the highest previously reported from an Arctic shelf. Using a relationship between biomass and respiration developed for deep-sea organisms living at cold temperatures, we estimated respiration rates from <0.1 to 15.0 mg C m^-2 day^-1 (median=1.9). Respiration rates measured on board were several orders of magnitude higher than those obtained from the predictive equation. Further work is needed to assess echinoderm respiration rates accurately under in situ conditions. Even with calculated minimal values for respiration rates, a comparison of epifaunal and infaunal respiration at four stations revealed that echinoderm respiration accounted for as much as 25% of total respiration. High epifaunal respiration rates and biomass values are likely supported by high concentrations of particulate organic carbon carried by Bering Sea water flowing through the eastern Chukchi Sea. Our observations support observations from the Eurasian Arctic that echinoderms dominate the epibenthos of Arctic shelves and that the role of these organisms in carbon remineralization must be considered if we are to generate accurate models of carbon cycling in the Arctic.     

Significance of food type for growth of ephyrae Aurelia aurita (Scyphozoa)

We studied growth of newly released Aurelia aurita ephyra larvae fed five different food types, including a large-sized copepod, a phytoflagellate, and suspended POM (particulate organic matter) made from bivalve meat. Experiments were run at saturated food concentration in two different temperatures over 10 days. The effect of small differences in temperature was inconsistent and interacted with the effect of food type, which, in turn, was highly significant. A low average growth rate (4-9% day^-1) was shown when feeding on the large-sized copepod Calanus finmarchicus (80 µg AFDW individual^-1), in spite of an extremely high daily ration of up to 1500% of body AFDW. When feeding on the cryptophyte Rhodomonas baltica (ca. 8 µm cell diameter), the ephyrae showed an average growth rate over the 10 day experiment of 7-11%, but with a considerably higher growth rate during the first days. Suspended POM generated an average growth rate of 7-9% day^-1, whereas fresh bivalve meat, manually placed into the stomach of the ephyra, gave an average growth rate of 12-14% day^-1. Artemia nauplii (ca. 3 µg AFDW individual^-1), used as a general reference, resulted in higher growth rates than any of the other food types (17-31% day^-1). We conclude that A. aurita ephyrae can capture and feed on phytoplankton, large copepods, and POM; that phytoplankton might be of nutritive significance early in development; and that the high quantity of large-sized copepods ingested is inefficiently converted to growth during early development. POM is a potential food source because of the ability of the ephyrae to encounter and ingest it, although concentration, size distribution, and nutritional composition of natural POM probably constrain its effect on growth.     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

Morphology and sinking velocities of fecal pellets of copepod, molluscan, euphausiid, and salp taxa in the northeastern tropical Atlantic

Morphological characteristics and sinking velocities of naturally occurring fecal pellets of copepods, euphausiids, salps, and pelagic mollusks collected in the northeastern tropical Atlantic were investigated during the period of May-June 1992. The fecal pellets of copepods and euphausiids were cylindrical and distinguished only by their size. Those of salps were, in general, rectangular, and slight differences were noted according to the species. The fecal pellets of the molluscan pteropod Clio sp. were conical, while those of the molluscan heteropod Carinaria sp. were spiral. The sinking velocities ranged from 26.5 to 159.5 m day^-1 for copepod fecal pellets, from 16.1 to 341.1 m day^-1 for euphausiid pellets, from 43.5 to 1167.6 m day^-1 for salps' pellets (Cyclosalpa affinis, Salpa fusiformis, Iasis zonaria, and two unidentified species), from 65 to 205.7 m day^-1 for Clio sp. pellets, and from 120.3 to 646.4 m day^-1 for Carinaria sp. fecal pellets. The measured sinking velocities were compared with estimates predicted using the equations of Komar et al. (1981; Limnol Oceanogr 26:172-180), Stokes' law, and Newton's second law, using either a constant density of fecal pellets (1.22 g cm^-3) or densities estimated with the three different equations. When a constant density was used, the three equations overestimated the sinking velocities; Stokes' law resulted in the largest overestimation, and Newton's second law, the smallest. At the taxa level, the overestimation was greatest for euphausiid 1 fecal pellets and smallest for copepod fecal pellets. When the three equations were used to estimate fecal pellet density, the density estimated using the equation of Komar et al. was the greatest, and that using Stokes' law, the smallest, resulting in over- and underestimation of sinking velocities, respectively. Newton's second law resulted in an intermediate density and gave the closest estimate of sinking velocities. We propose that measurement of sinking velocities of a portion of the fecal pellets might guide in choosing an appropriate equation to be used for a reasonable interpretation of vertical mass flux.     

Some quantitative aspects of seagrass ecology in a coastal lagoon of Mauritius

Seagrass distribution was recorded by snorkel dives on a grid of stations in the waterfront of Club Méditerranée at Mon Choisy-Trou Aux Biches lagoon (NW Mauritius) and subsequently mapped using SURFER 6 computer software. Above-ground (AG) and below-ground (BG) standing biomass in terms of dry weight (DW) and ash-free dry weight (AFDW) as well as shoot density and shoot length were monitored monthly from June1997 to May1998 in a mixed stand of Halodule uninervis and Syringodium isoetifolium (dominant) at a shallow, nearshore station in the lagoon. Measurements of physical and chemical parameters [water temperature, current speed, salinity, pH, dissolved oxygen (DO), nitrate and phosphate concentrations] were made simultaneously, as well as at a reference station (ORE) outside the coral reef. The bottom sediment was analysed for grain size and type composition. Variation patterns were examined and statistical correlations drawn to relate plant performance to the environmental variables measured. The SURFER 6 programme generated a satisfactory contour map of seagrass distribution in the lagoon with a cover range of 0-60%. The densest patches occurred adjacent to the shoreline experiencing weaker water currents (3-13 cm s^-1) rather than near the reef (5-35 cm s^-1), where seagrasses were absent. Sand (0.063-2 mm grain size) constituted 97.2% and 77.6% of the nearshore and near-reef sediment, respectively. The dominant grain types were derived from corals (about 80%) and mollusc shells (about 14%). The recorded range of total standing biomass for H. uninervis was 243.1-468.2 g DW m^-2 (326.9ᇛ.7 g) or 71.7-141.2 g AFDW m^-2 (96.8ᆨ.1 g) and for S. isoetifolium it was 271.7-758 g DW m^-2 (460.4끯.1 g) or 119-220.5 g AFDW m^-2 (155.1ᆮ.5 g), with a maximum biomass increase during September-December in both species. AG:BG biomass ratios were generally <1 and approximated 1 during the warmest months of December-February only. Mean shoot density (1,077-4,364 shoots m^-2 in the overall range of 998-4,428 shoots m^-2) and mean shoot length (10.9-20.8 cm in the overall range of 7-31 cm) in S. isoetifolium were higher than in H. uninervis (1,732-4,137 shoots m^-2 in the overall range of 1,522-4,327 shoots m^-2 and 7.9-13.7 cm in the overall range of 6-20 cm, respectively). Temperature showed strong positive correlations with total AFDW biomass of both species (r=0.755, P<0.01 for H. uninervis; r=0.679, P<0.02 for S. isoetifolium) and with DO (r=0.925, P<0.01). High DO levels (10.7-11.2 mg l^-1) coincided with optimum standing biomass at 27.2°C. Correlations were also strong with shoot density (r=0.881, P<0.01 for H. uninervis; r=0.952, P<0.01 for S. isoetifolium) and shoot length (r=0.752, P<0.01 for H. uninervis; r=0.797, P<0.01 for S. isoetifolium). Under optimal environmental conditions, nutrient inputs from surface run-off or underground freshwater seepage in the lagoon due to heavy rainfall may boost up seagrass biomass, as suggested by positive significant correlations between phosphate levels and AG AFDW biomass (r=0.63, P<0.05 for H. uninervis; r=0.65, P<0.05 for S. isoetifolium) and shoot density (r=0.6, P<0.05 for H. uninervis; r=0.687, P<0.02 for S. isoetifolium). The results generated in this study suggest local seagrass standing biomass is comparable to that reported in monospecific stands from elsewhere. Anthropogenic activities increasingly draw down the resilience of the seagrass beds around Mauritius, and preventative measures are indispensable to achieve coastal ecological stability.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Population dynamics and secondary production of the wedge clam <Emphasis Type="Italic">Donax</Emphasis><Emphasis Type="Italic">hanleyanus</Emphasis> (Bivalvia: Donacidae) on a high-energy,subtropical beach of Brazil

The population biology of Donax hanleyanus (Philippi, 1845) (Bivalvia: Donacidae) was studied by monthly sampling from June 1998 through June 2000, at Restinga da Marambaia Beach, Brazil. Two transects were established and divided into ten strata parallel to the waterline, and five replicates were taken with a 0.04 m² sampler in each stratum. The highest densities of D. hanleyanus were recorded in winter (September 1998, July 1999) and autumn (April 2000). A stratified distribution was observed: recruits were found mainly in the middle swash zone, while juveniles and adults occurred across the tidal gradient up to the retention zone. Mortality rates did not differ significantly between years. Annual production (in ash-free dry mass) ranged from 0.76 g AFDM m^-2 year^-1 (1998) to 3.67 g AFDM m^-2 year^-1 (1999), while the production-to-biomass (P/B) ratio varied from 1.45 to 1.59. Life span was ca. 17 months. Of all variables tested, only two, one biological and one physical, seem to have influenced the population dynamics of this species. A significant negative correlation between the densities of the suspension-feeders D. hanleyanus and Emerita brasiliensis (Crustacea: Decapoda) indicated possible intraguild competition, and there was a significant exponential correlation between beach slope and the mean across-shore position of D. hanleyanus. Data compiled from literature suggests a latitudinal gradient in population parameters, with mortality and renewal rate (P/B) of different species of Donax increasing and life span decreasing from temperate to tropical regions. Other factors influencing population dynamics, such as food availability, and the contributions of individual and community biomass to energy and nutrient cycling are also discussed.     

Distribution of gelatinous macrozooplankton in the southern Black Sea during 1996–1999

In this study, spatial and temporal distributions of three gelatinous macrozooplankton, the scyphozoan Aurelia aurita and the ctenophores Pleurobrachia pileus and Mnemiopsis leidyi, were evaluated using the data obtained from seven southern Black Sea cruises carried out from 1996 to 1999. A comparison of nets used for the sampling of gelatinous macrozooplankton illustrated the superiority of the Hensen net with a larger mesh size (300 µm) over the Nansen net (112 µm). P. pileus was mainly concentrated below the mixed layer, while A. aurita and M. leidyi were generally confined to surface waters. Horizontally, P. pileus was associated with deep, offshore waters. P. pileus also displayed the least interannual variation of the three species. Biomass of A. aurita and M. leidyi varied substantially seasonally and inter-annually. In September 1999, the average biomass of M. leidyi was at its lowest level (12 g wet weight m^-2) since its explosive development at the end of the 1980s. This was associated with the appearance of the new predatory ctenophore Beroe ovata, which appeared in the Black Sea at the end of the 1990s.     

Energy budgets and feeding rates of Coryphaenoides acrolepis and C. armatus

This study develops energy budgets and estimates feeding rates for two macrourid fishes, Coryphaenoides acrolepis, dominant in the bathyal eastern North Pacific, and the abyssal cosmopolitan species, Coryphaenoides armatus. Daily energy expenditure by C. acrolepis was nearly twice that of C. armatus. C. acrolepis allocated nearly equal amounts of energy to metabolism and growth. Once sexual maturity was reached reproduction became the dominant energetic cost. Either these costs are necessary to retain adequate numbers of eggs and larvae on the continental slopes, or this fish does not reproduce on an annual basis and the calculated costs are an overestimate. C. armatus allocated relatively more energy to metabolism than growth. It may be semelparous, and this strategy would be of great energetic savings in its food-poor but stable environment. Individual daily ration for C. acrolepis decreased from 0.31% to 0.07% of body weight (BW) and for C. armatus from 0.12% to 0.02% BW with increasing fish length. These rates are substantially lower than those for fishes living in cold waters on the continental shelves. The population feeding rates for C. acrolepis ranged from 0.8 to 15 kg km^-2 day^-1 and for C. armatus from 5 to 2,800 g km^-2 day^-1. The scavenging behaviour of C. acrolepis was used to investigate the role of carrion as a food supply to the deep-sea benthos. It was estimated that the carrion eaten by C. acrolepis is equivalent to 0.04 mg C m^-2 day^-1 or only 0.2-0.4% of the average small particulate flux. Carrion consumption is important for scavengers like C. acrolepis, but it is not an important component of the carbon flux into the deep-sea benthic environment.     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.