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.     

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.     

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.     

Alternative feeding mechanisms in megalopae of the blue crab Callinectes sapidus

The apparent mismatch between the energy requirements for planktotrophic growth and prey availability has long been paradoxical. One hypothesis to explain this paradox is that planktotrophic larvae display plasticity in feeding mechanisms in response to variable prey types and concentrations. This hypothesis was tested by videotaping megalopae of the brachyuran crab Callinectes sapidus Rathbun feeding on various-sized prey. Frame-by-frame analysis of the videotapes indicated that C. sapidus megalopae used both raptorial and suspension feeding to capture prey while in the water column. Raptorial feeding was used to capture macro-zooplankton, including copepods. The swimming form of suspension feeding was based on a modified fling-and-clap mechanism using the chelipeds. Suspension feeding while at rest utilized a weak current generated by the mouthparts to direct prey to the mouth. Both suspension-feeding mechanisms resulted in the efficient capture of rotifer-sized particles. The energy/handling time ratios for all three feeding mechanisms are very similar (E/H range 0.016-0.019 µg C s^-1) for the natural prey tested. These results support the hypothesis that feeding in brachyuran larvae is plastic and includes mechanisms of both raptorial and suspension feeding. The ability to suspension feed at rest is adaptive, since megalopae use selective tidal transport to re-invade an estuary and may spend up to 18 h day^-1 clinging to a benthic substrate. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0781-1.     

Arm regeneration in the field in Ophiocoma echinata (Echinodermata: Ophiuroidea): effects on body composition and its potential role in a reef food web

Eighteen percent and 47% of two populations of Ophiocoma echinata in the Florida Keys were undergoing regeneration. An individual would take approximately 720 days to completely regenerate three arms. Regeneration of three arms had a greater effect on gonad production in females than in males, and reduced storage material in the stomach of both. Regeneration in O. echinata was estimated to be 0.07 kJ m^-2 day^-1, which could provide the equivalent of 0.07% of primary production on a reef to higher trophic levels per day.     

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.     

Studies on pigments,proteins and photosynthetic rates in some mangroves and mangrove associates from Bhitarkanika,Orissa

Pigment contents, proteins and net photosynthesis were investigated in fully developed leaf of 1-year-old seedlings of six mangroves (Bruguiera gymnorrhiza, Rhizophora apiculata) and mangrove associates (Caesalpinia bonduc, Cerbera manghas, Derris heterophylla, Thespesia populnea), collected from Bhitarkanika, located on the east coast of India. Large variations in the photosynthetic rates (P_N) among the six species were observed, ranging from 10.16 µmol CO₂ m^-2 s^-1 in C. bonduc to 15.28 µmol CO₂ m^-2 s^-1 in R. apiculata. The total leaf protein content ranged from 12.09 mg g^-1 dry wt in T. populnea to 51.89 mg g^-1 dry wt in B. gymnorrhiza. The chlorophyll a/b ratio was typically about 3.0 in all the studied species, except C. bonduc (2.8). Photosynthetic rates and chl a/b ratio in the leaves were found to be correlated. Analysis of chlorophyll and xanthophyll spectra suggested: (1) variations in different forms and amounts of carotenes as well as xanthophylls and (2) the presence of high amounts of near-UV-absorbing substances in leaves, particularly in the two mangroves (B. gymnorrhiza, R. apiculata) and a mangrove associate (T. populnea), which appears to be an adaptive feature. Estimation of the chl a/b ratios in isolated thylakoids yielded a low value of 1.8 for R. apiculata and >2.6 for other species. The total protein/chlorophyll ratios in thylakoids varied considerably from 3.14 (D. heterophylla) to 10.88 (T. populnea) among the mangrove associates and from 16.09 to 18.88 between the members of the Rhizophoraceae. The chlorophyll/carotenoid ratios in thylakoids of the six species were more or less similar. The absorption spectra for washed thylakoids of C. manghas and D. heterophylla exhibited absorption characteristics typical for C₃-plant thylakoids. However, thylakoids isolated from R. apiculata, B. gymnorrhiza, C. bonduc and T. populnea exhibited an unusual increase in absorption in the blue region (380-410 nm) of the absorption spectrum. The presence of high-absorbing (in the short-wavelength, near-UV region) pigments appears to be closely associated with the thylakoids in R. apiculata and T. populnea. Our results, therefore, suggest a wide range of variation, not only in protein and pigment contents of photosynthetic tissues, but also in the spectral characteristics and composition of the pigments in mangrove species. An understanding of the nature of these pigments in mangroves and their associates, under their natural conditions and especially in relation to eco-physiological adaptations, is necessary, not only in relation to conservation, but also to allow propagation under different salinity conditions.     

Ostracod species-specific utilisation of sediment detritus and newly settled cyanobacteria, Aphanizomenon sp., in the Baltic Sea: evidence from stable carbon isotopes

The three meiobenthic ostracod species Candona neglecta, Paracyprideis fennica and Heterocyprideis sorbyana are commonly encountered in the deep (20-40 m) soft bottoms of the Baltic proper, and may contribute more to the total meiobenthic biomass than any other group. Experimental data indicate substantial differences in their utilisation of settling phytodetritus, with C. neglecta able to exploit newly settled organic material to a larger extent than P. fennica and H. sorbyana. Ostracod species composition, as well as ostracod and sediment carbon isotope contents, were studied in the field from an area with local differences in potential food resources. The study was performed during the late period of summer blooms of the cyanobacteria Aphanizomenon sp. Results showed that when all samples were taken together, adult C. neglecta (-22.4‰) was significantly more depleted in '¹³C than adult P. fennica (-21.0‰) and H. sorbyana (-20.3‰), indicating differences in food selection among the species. The flocculent sediment layer had, in all instances, lighter carbon ratios than did the lower layers. This trend was mirrored in most cases in the slight enrichment in C. neglecta compared to generally greater enrichment in the other two species. Carbon signatures of C. neglecta also varied significantly between stations, indicating that this species fed on different resources depending on location. Juvenile C. neglecta were far more depleted in '¹³C than adults and reflected the carbon signature of the cyanobacteria Aphanizomenon sp. The latter is known to be the most '¹³C-depleted phytoplankton member in the area.     

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.     

Differences in life-cycle traits of<Emphasis Type="Italic"> Calanus finmarchicus</Emphasis> originating from 60°N and 69°N,when reared in mesocosms at 69°N

Two different Calanus finmarchicus (Gunnerus) cohorts originating from 60°N (Bergen) and 69°N (Tromsø) were investigated in equal environmental conditions to study their different physiological responses to the same environment. A two-plus-two-bag mesocosm study was carried out between March and July 1998, in Håkøybotn, Tromsø, in order to determine development and mortality rates of the two parallel cohorts of C. finmarchicus. For practical reasons, the cohort from Bergen was incubated 10 days earlier than the Tromsø cohort. Consequently, they were exposed to elevated food conditions as compared to the Tromsø cohort. A high initial mortality among the Bergen cohort could clearly be ascribed, by genetic discrimination, to "contamination" with C. helgolandicus. After this initial mass mortality, the mortality was constantly 0.03-0.04 day^у. In cohorts starting from naupliar stage I, there was no significant difference in development or growth, the median development time (NI-CIV) differing by only 7 days (~6%). The difference in development time can be explained to a large extent (~4 days) by temperature differences. This is substantiated with model simulations using a physiological model developed for C. finmarchicus. There was a time lag in incubation between the two cohorts, resulting in elevated temperature during incubation of the Tromsø cohort. A fraction of both cohorts differentiated sexually at stage CV, with males differentiating before females. Females from both cohorts produced eggs, but specific egg production differed significantly (P>0.001, t-test). This was supported by elevated RNA:DNA ratio in females from the Bergen cohort. Both cohorts demonstrated quite similar development and physiological growth rates and, consequently, are considered to belong to the same genetic population inhabiting the Norwegian Shelf. The study demonstrates that C. finmarchicus is capable of adaptation to changes in environment and, thereby, demonstrates a significant physiological plasticity.     

A comparison of catches of fishes and invertebrates by two light trap designs, in tropical NW Australia

Light traps were deployed in two sampling programs. In the first, small and large traps were released to drift with the current at stations along a cross-shelf transect on the NW Shelf off the coast of Western Australia. In the second program, pairs of small and large traps were deployed on moorings 150 m off the coastline. The composition and size-frequency distributions of catches of fishes in small and large traps were similar for both modes of deployment. In drifting traps, nearly 78% of this catch was composed of reef fishes, and these were collected in significantly greater numbers by the small design than by large traps (9.51 vs. 5.84 individuals h^-1, respectively). Nine taxa (amphipods, mysids, crab megalopae, copepods, cumaceans, isopods, caridean shrimps, polychaetes and the euphausiid, Pseudeuphausia latifrons) accounted for 99% of the total catch of invertebrates by drifting traps. Of these, catches of amphipods, copepods, cumaceans and P. latifrons were greater in large traps than in small traps (3,134 vs. 1,687 h^-1, 1,018 vs. 214 h^-1, 551 vs. 165 h^-1 and 74 vs. 9 individuals h^-1, respectively). In contrast, crab megalopae were more abundant in catches by small traps than by large traps (3,134 vs. 1,687 individuals h^-1, respectively). The catch rate of fishes in moored traps was higher than in drifting traps (105 vs. 20 fishes h^-1) and was dominated by baitfishes (86% of total catch). Reef fishes were also captured in greater numbers by small traps than by the large design (10.17 vs. 4.4 individuals h^-1) in this mode of deployment. Despite these differences in catch rates, multivariate analysis showed that cross-shelf patterns in catches of fishes and invertebrates were mapped equally well by both trap designs. Variation in the efficiency of trap designs thus appears to be small when compared to changes in the composition and abundance of zooplankton assemblages that occur at scales of tens of kilometers.     

Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

During the "International Arctic Ocean Expedition 1991" (20 August-21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m^-3 in the surface layer (0-50 m) to <25 ind. m^-3 below 200 m depth. The small copepods Oithona similis and Microcalanus pygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaea borealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0ǂ.3 g DM m^-2. Dry mass in the upper 50 m measured 20.9 mg m^-3 and was dominated by Calanus hyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridia longa, Microcalanus pygmaeus and Pareuchaeta spp., the decapod Hymenodora glacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m^-3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.     

Below-ground nitrogen cycling in relation to net canopy production in mangrove forests of southern Thailand

Rates of accumulation, transformation and availability of sediment nitrogen in four mangrove forests of different age and type in southern Thailand were examined in relation to forest net canopy production. Net ammonification (range: 0.3-2.3 mmol N m^-2 day^-1), nitrification (range: 0-0.7 mmol N m^-2 day^-1) and nitrogen fixation (range: 0-0.6 mmol N m^-2 day^-1) in surface sediments equated to <10% of canopy nitrogen demand (range: 7.5-32 mmol N m^-2 day^-1). By mass balance, we estimated that most of the nitrogen required for tree growth must be derived from root-associated nitrogen fixation and/or mineralisation processes occurring possibly to the maximum depth of live root penetration (75-100 cm). Denitrification, nitrification, rainfall and tidal exchange were comparatively small components of sediment nitrogen flow. Denitrification (range: 0-3.8 mmol N m^-2 day^-1) removed 3-6% of total nitrogen input at three Rhizophora forests, but removed 23% of total nitrogen input in a high-intertidal Ceriops forest. Nitrogen burial ranged from 4% to 12% of total nitrogen input, with the greatest burial rates in two forests receiving the least tidal inundation. Inputs of nitrogen to the forests were rapid (range: 11-37 mmol N m^-2 day^-1), likely originating from upstream sources such as agricultural and industrial lands, sewage and shrimp ponds. Our results indicate that ~70% to 90% of the nitrogen supplied to the forest floor is shunted via the ammonium pool to trees to sustain the rapid rates of net canopy production measured in these forests. Differences in plant-sediment nitrogen relations between the forests appeared to be a function of the interaction between intertidal position and stand age.     

Diel movement patterns of the Hawaiian stingray,<Emphasis Type="Italic"> Dasyatis lata</Emphasis>: implications for ecological interactions between sympatric elasmobranch species

The Hawaiian stingray, Dasyatis lata, is a common benthic elasmobranch in nearshore Hawaiian waters. Acoustic telemetry was used to track the movements of seven rays in Kaneohe Bay, Oahu, Hawaii. Rays were tracked continuously over 31-74 h periods. Geographical movements were analyzed to determine space utilization and rate of movement. Rays were found to utilize significantly larger activity spaces at night (0.83ǂ.70 km²) (mean-SD) than during the day (0.12ǂ.15 km²). Mean total activity space for rays tracked was 1.32ǂ.75 km². Rates of movement were also significantly higher at night (0.34ǂ.30 km h^-1) than during the day (0.15ǂ.22 km h^-1). Average straight-line swimming speed was 0.64ǂ.16 km h^-1, with a maximum observed swimming speed of 1.9 km h^-1. Tidal stage had no effect on rate of movement. Comparison with previously published data on juvenile scalloped hammerhead sharks, Sphyrna lewini, in Kaneohe Bay revealed a high degree of overlap in habitat use and time of activity, suggesting possible ecological interactions between these two species.     

Abundance, growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the Oyashio region, western subarctic Pacific

Abundance and life-cycle features of the mesopelagic hyperiid amphipod Primno abyssalis (formerly P. macropa) in the Oyashio region, western subarctic Pacific, were investigated using samples collected between July 1996 and July 1998. P. abyssalis was collected throughout the entire survey period, with abundance peaks occurring in spring to autumn. While all maturity stages of males and females were observed throughout the study period, the peak reproduction season was in summer. Instar analysis based on the segment number of the pleopod rami indicated that hatched juveniles molted 10 times before becoming adult males and 13 times before becoming adult females. Judging from the dry and ash-free dry weights of each instar, males and females continued to feed throughout the final instar stage. Based on cohort analysis of seasonal samples and laboratory observations on molting frequencies, growth in body length of P. abyssalis was linear with time, and estimated generation lengths were 2.3-3.8 years for females and 1.4-1.9 years for males. Brood size of females ranged from 66 to 337 and increased with increasing female body length. Lifetime fecundity, calculated as the sum of six successive broods, was 1,004. Compared with P. abyssalis in the southern Sea of Japan, those in the Oyashio region have a larger number of adult instars (six versus five for females, three 3 vs one for males), a lower growth rate (0.014 mm day^-1 vs 0.021 mm day^-1), and mature earlier (instar 13 vs instar 15 for females; instar 10 vs instar 11 for males). These characteristics are considered to be advantageous life-history traits to counteract higher niche competition within the mesopelagic community and higher predation pressure by mesopelagic fishes in the Oyashio region than in the Sea of Japan.     

Effects of exposure to toxic and non-toxic dinoflagellates on oxygen consumption and locomotion in stage 1 larvae of the crabs Cancer oregonensis and C. magister

Sublethal effects on larval crabs upon exposure to toxic dinoflagellates were examined in the laboratory in early 1999. Specifically, oxygen consumption rates and geotaxis responses were determined for stage 1 larvae of the crabs Cancer oregonensis (Dana) and C. magister Dana that were exposed to non-toxic (Alexandrium tamarense, strain 115) or toxic (A. fundyense, strain 1719) dinoflagellates or to freshly hatched nauplii of the brine shrimp Artemia sp. In C. oregonensis, larvae exposed to the toxic dinoflagellate showed reduced rates of oxygen consumption compared to those exposed to non-toxic dinoflagellates or brine shrimp nauplii. Larvae exposed to a filtrate of the non-toxic dinoflagellate showed no change in oxygen consumption, but a reduced rate when exposed to filtrate from the toxic alga at densities >5᎒² cells ml^-1. In C. magister, larvae exposed to the non-toxic A. tamarense or the toxic A. fundyense had reduced oxygen consumption rates. Larvae exposed to filtrates of non-toxic and toxic dinoflagellates had no change in oxygen consumption. In geotaxis tests, C. oregonensis larvae exposed for 1 day to the toxic A. fundyense reduced their level of locomotion compared to those exposed to non-toxic A. tamarense or to brine shrimp nauplii. C. magister larvae showed no change in activity after a 1-day exposure to the toxic A. fundyense. After a 4-day exposure to A. fundyense, C. magister larvae had much reduced locomotion. Reduced locomotory activity in larvae exposed to toxic algae is consistent with the changes in oxygen consumption rates. Responding to exposure to toxic algae by reducing locomotion may affect vertical migration in these negatively buoyant crab larvae, resulting in sinking below a toxic alga bloom, at least temporarily.     

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.     

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.     

Interception and dispersal of artificial food falls by scavenging fishes in the abyssal Northeast Atlantic: early-season observations prior to annual deposition of phytodetritus

The hypothesis that the behaviour of deep-sea scavenging fishes is influenced by seasonal input of organic matter from the ocean surface was investigated by observing responses to baits placed on the sea floor at 4800 m depth in the NE Atlantic (48°50′N; 16°30′W) during spring (April 1994). Data from the present study are compared with those from previous studies of the same location made in summer 1989. The first fishes to arrive at baits were the grenadier Coryphaenoides (Nematonurus) armatus and the eel Histiobranchus bathybius, after delays of 28 and 29 min, respectively; these results are not significantly different from those of summer 1989. Similarly, other indices of activity (staying time and swimming speed) showed no evidence of differences between years/seasons. However, the rate of radial dispersal of bait (0.009 m s⁻¹) by C. (N.) armatus was much slower than in all previous studies. A change in the size distribution of C. (N.) armatus to smaller individuals in spring 1994 was also evident. It is suggested that the fish on the abyssal plain may not comprise a steady-state population and that major episodic or seasonal migrations may occur. Received: 18 October 1996 / Accepted: 20 December 1996     

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.