Particulate DNA in subtropical oceanic and estuarine planktonic environments

Particulate DNA was measured in estuarine, coastal, and oligotrophic oceanic environments near the southwest coast of Florida and in the Gulf of Mexico. Particulate organic carbon and nitrogen (POC and PON), chlorophyll a, bacterial direct counts, DNA, and bacterial activity as determined by thymidine incorporation all showed a high degree of intercorrelation. Normalization of the data for offshore/onshore similarities by dividing by POC yielded significant correlations only for DNA, direct counts, and bacterial activity. Most (70–99%) of the particulate DNA in offshore samples was in the 0.2- to 1-m fraction, while DNA in nearshore and estuarine samples was associated with larger particles. Cellular DNA contents obtained by dividing DNA by direct counts in the 0.2- to 1-m fraction were in the range of reported bacterial genome weights. However, DNA nitrogen comprised a greater proportion of the PON than reported for microorganisms in culture. Collectively, these results suggest that (1) most of the particulate DNA in oceanic environments is contained in bacterioplankton; (2) DNA is a significant proportion of the cell biomass, possibly due to growth under nutrientlimiting conditions.     

Diel vertical migration and feeding of copepods at an oceanic site near South Georgia

Twelve Longhurst Hardy Plankton Recorder (LHPR) profiles were taken over a 16 h period in January 1990, in order to study feeding of four copepod species at an Antarctic oceanic site near South Georgia. Vertical distributions of their life stages, as well as those of dominant competitors and predators, are described in relation to the feeding cycles of Calanoides acutus CV, Calanus simillimus CV, Calanus propinquus CV and Rhincalanus gigas CIII, CV and CVI. Comparisons with vertical ring-net catches, which were used for concomitant gutevacuation experiments, demonstrated the suitability of the LHPR for these fine-scale studies. Planktonic predators, with the exception of the diel migrant Themisto gaudichaudii, resided deeper than the herbivores. During the day and around midnight, when feeding rates were low, species and stages reached their maximum vertical separation. At these times, new generation copepodites of the four species lived progressively deeper and the overwintered generation (i.e., R. gigas Stages CIV, CV, CVI) were progressively shallower. During the afternoon or evening (depending on species), all stages older than CII, as well as Euphausia frigida and T. gaudichaudii, migrated upwards, to amass in the surface mixed layer. Feeding was restricted to darkness, although R. gigas commenced several hours before dusk. In detail their migration and feeding differed widely, with combinations of unimodal and apparent bimodal cycles. As a whole, the results suggest that (1) feeding could occur during sinking as well as during upward migrations, (2) upward migrations were not always associated with feeding increases, and (3) individuals appeared to descend after filling their guts.     

Feeding rates and diel vertical migration of copepods near South Georgia: comparison of shelf and oceanic sites

Seventeen Longhurst Hardy Plankton Recorder profiles were taken over a diel cycle in January 1990 to study the feeding of four major copepods over the South Georgia shelf. Ontogenetic changes in vertical migration were followed and feeding cycles determined by gut fluorometry for Calanoides acutus Stage CV, Calanus sinillimus CV and CVI, C. propinquus CV and Rhincalanus gigas CV and CVI. In common with a neighbouring oceanic site visited two weeks later and reported elsewhere, all four species had a diel cycle of feeding and migration. The vertical distributions of C. simillimus (all stages), R. gigas (nauplii) and Euphausia frigida (postlarvae) were similar at both sites, the night being spent within the chlorophyll maximum at 15 to 30 m. However, the biomass dominants, C. acutus and R. gigas, dwelt below the chlorophyll maximum, about 30 m deeper than their oceanic counterparts. Unlike the oceanic site, feeding at the shelf site was not restricted to darkness, but increased 6 to 10 h before nightfall and finished at dawn; the intervening period coincided with sinking and digestion. Daylight feeding may have been induced by the shorter night, lower light levels or greater food requirements at the shelf site, despite planktonic predators being over three times more abundant. Daily ration estimates for R. gigas at both sites were only 2% body carbon per day. These low values contrast with its smaller competirors, whose rations were in the range 5.6 to 27%.     

Vertical distribution,diurnal and seasonal migration of copepods in Saronic Bay,Greece

The problem of vertical distribution and the movements of copepods was studied from several plankton samples collected by vertical hauls in the Saronic Bay, Greece during two cruises (8 to 17 August and 21 November to 1 December, 1969). The species, whose vertical movements were analyzed, were divided into 3 groups: (1) Those which perform diurnal vertical migration (generally psychrophilic species found in summer samples in deeper layers and in reduced numbers). In autumn, the number of specimens is generally increased and many individuals reach the surface at night. From surface hauls it is known that these species abound in night surface hauls during the cold period. (2) Those species which execute a seasonal vertical migration. These, too are, in general, psychrophilic and found in summer in the deep water layers. In autumn, the population of the upper layers increases. From surface hauls it is known that these species abound in day and night surface samples. (3) Copepods which remained at the surface layer in both seasons of our cruises. These species are thermophilic and are absent from surface hauls during the cold period.     

Vertical distributions of eggs,nauplii and copepodites of Calanus helgolandicus (Copepoda: Crustacea) in the Celtic Sea

The vertical distributions of eggs, nauplii, copepodites and adults of Calanus helgolandicus (Claus) from five oblique plankton-net hauls taken in May (1980), March and September (1981) and January (1982) at a site in the shelf sea to the south-west of the United Kingdom are described. The water depth is approximately 95 m and becomes thermally stratified during the summer months when a thermocline of ∼6 C° develops. In early spring when the water column was isothermal (∼8 °C), the development of the eggs and nauplii took place below 60 m and a single ontogenetic migration was observed between Nauplius VI (NVI) and Copepodite I (CI). As the temperature of the water increased, this migration occurred in progressively earlier naupliar stages. The eggs were distributed throughout the water column in the profile taken in early May when a 1 C° thermocline occurred between 30 to 40 m. The majority of the NI to NIV stages occurred below 40 m, with the ontogenetic migration taking place in the NIV stage; the NV and NVI stages were found above the thermocline. In September, the eggs were again distributed throughout the water column (101 490 m^-2), with a maximum number of >4 500 m^-3 occurring in the surface to 5 m depth interval. Nauplius I and II were found at all depths, demonstrating that hatching occurred throughout the water column. The ontogenetic migration in these late-summer profiles took place between the NII and NIII stages, the remainder of the nauplii being found above the thermocline in the top 20 m. This is the first time that an ontogenetic migration, similar to the developmental ascent observed in the naupliar stages of the euphausiid Euphausia superba in the deep ocean, has been shown for a copepod nauplius.     

Substrate-dependent bacterivory by intertidal benthic copepods

The trophic importance of bacteria to harpacticoid copepods in intertidal areas remains poorly understood, and so do the mechanisms of bacterial feeding. It is, for instance, unclear whether harpacticoids directly target bacterial cells or merely co-ingest them with substrates to which bacterial cells may be attached. Here, we investigate bacterial uptake and substrate requirement for four mud intertidal species (Microarthridion littorale, Platychelipus littoralis, Delavalia palustris and Nannopus palustris) by means of ¹³C-labeled bacteria and biomarker fatty acids (FA). Bacterial uptake strongly depended on grazing on a primary food source but bacterial ingestion rates were low, and no clear indication of copepods directly targeting bacteria was found. Delavalia was the only species that accumulated bacteria-derived FA and gained in polyunsaturated FA (PUFA) probably through bioconversion of bacteria-derived FA. In general, however, our results suggest that bacteria represent a relatively minor and low-quality food for intertidal harpacticoid copepods.     

Respiration and excretion by oceanic salps

Rates of oxygen consumption and ammonium nitrogen excretion were measured on the solitary and/or aggregate generations of ten species of oceanic salps collected by SCUBA divers during cruises in the Atlantic Ocean (1982–1985). Species that were visibly more active had higher metabolic rates than did less active species. Rates were 1.5 to 2 times lower and O:N ratios were lower when salps were held before incubation than when incubation began at the time of collection. Respiration rate showed a better relationship to length than to weight, suggesting that metabolic activity may be connected mainly with swimming. O:N ratios were between 13 and 28 for most species and generations, but higher and more variable in Pegea spp. Exretion of urea was low or undetectable. Rates of metabolic demand (turnover) ranged from 9.7 to 99% body carbon d^-1 and 6.4 to 55.6% body nitrogen d^-1.Contribution No. 5988 from the Woods Hole Oceanographic Institution and No. 412 from the Allan Hancock Foundation     

Influences of abundance,behavior, species composition,and ontogenetic stage on active emergence of meiobenthic copepods in subtropical habitats

The emergence of meiobenthic copepods in subtidal sand and seagrass sites in Tampa Bay, Florida (USA), was investigated on nine dates from March 1983 to August 1984. Numbers of emerging copepods were dependent on habitat, date and diel sampling period. Greater numbers typically emerged in seagrass sites, and differences between habitats may be related to lower copepod abundances in sand sites. Both copepod abundance and behavior significantly affected numbers emerging among dates. An increase in emergent behavior alone resulted in consistently greater numbers emerging during postsunset periods. Over 30 species in 15 families were found to emerge. Total numbers of emerging copepods were affected by the sporadic presence and seasonal behavior of certain species. Numbers of the predominant speciesParadactylopodia brevicornis, Tisbe furcata, Parategastes sp.,Mesochra pygmea, Laophontid sp., andHarpacticus sp. emerging depended on differences in both species abundance and behavior. The emergent behavior ofParategastes sp.,Harpacticus sp., andT. furcata also was influenced by total copepod densities in the sediment. Juvenile copepods constituted only a small proportion of the total numbers emerging. The consistent postsunset entry of between 10³ to 10⁵ copepods m^?2 into the water column in subtidal sand and seagrass habitats will contribute to the increased probability of copepod dispersal and reassortment of the benthic community, while providing a pathway for benthic-pelagic exchange.     

Vertical distribution,abundance and community structure of oncaeid copepods in the Oyashio region,western subarctic Pacific

The vertical distribution, abundance and community structure of oncaeid copepods were investigated in the Oyashio region, western subarctic Pacific. Samples were collected with a 0.10 mm mesh closing type net from five discrete layers down to a maximum depth of 2,000 m in September and December 1996 and in April and October 1997. The copepods were widely distributed from epipelagic to bathypelagic zones, and showed prominent peaks of abundance above the thermocline and/or between 250 and 1,000 m depth. Standing stocks of total oncaeid copepods ranged from 1.5 to 2.5×10⁵ inds m^–2 at 0–2,000 m in the water column, which are the same order of magnitudes as those reported in tropical, subtropical and polar regions by previous workers. A total of 38 species and two forms belonging to the genera Oncaea, Triconia, Spinoncaea, Conaea and Epicalymma, and two provisionally classified species of the family Oncaeidae were identified in this study. Of these, 14 species have already been recorded from the eastern subarctic Pacific. Several warm-water species were also found in December 1996 and/or October 1997, when the effect of warm-core rings originating from the Kuroshio Current was evident in the epipelagic zone. Dominant species throughout the whole water column were T. borealis, T. canadensis, O. grossa, O. parila, O. rimula, O. lacinia, Epicalymma spp. and Oncaea sp. A, and these eight species together always made up about half of the total oncaeid numbers. Community analysis revealed that species composition in epipelagic and upper mesopelagic zones varied temporally due to the changes of hydrographic conditions, whereas those in lower mesopelagic and bathypelagic zones were relatively stable, reflecting the constancy of deep environments.Communicated by O. Kinne, Oldendorf/Luhe     

Vertical distributions and photosynthetic action spectre of two oceanic picophytopianlcters,Prochlorococcus marinus andSynechococcus sp.

Two picophytoplankters,Prochlorococcus marinus andSynechococcus sp., were isolated from the bottom of the euphotic zone (150 m depth) in the western Pacifie Ocean. The concentration ofP. marinus at this depth was more than 10⁴ cells ml^–1 while that ofSynechococcus sp. was less than 10² cells ml^–1. TheP. marinus isolate has a high divinyl-chlorophylla:b ratio similar to that of the Mediterranean strain, while theSynechococcus sp. isolate is of the phycourobilinrich type. The growth rate ofP. marinus was higher thanSynechococcus sp. when both were cultured under weak blue-green to blue-violet light (ca. 2 E m^–2 s^–1). While the chlorophyll-specific absorption spectra showed higher values inSynechococcus sp., the photosynthetic action spectre revealed thatP. marinus was able to use blue-violet light, whereasSynechococcus sp. was able to use blue-green light, more efficiently for photosynthesis. The photosynthetic quantum yield ofP. marinus was higher than that ofSynechococcus sp. at any wavelength between 400 and 700 nm. The calculated in situ photosynthesis rates per Gell volume forP. marinus were estimated to be higher than forSynechococcus sp. at 50 and 150 m depth. These results indicate thatP. marinus photosynthetically surpassesSynechococcus sp. in the blue-light-rieh environment of the oceanic euphotic zone. This may be why the former predominates at depths in temperate to tropical open ocean waters.     

Vertical distribution,population structure and life cycles of four oncaeid copepods in the Oyashio region,western subarctic Pacific

Vertical distribution and population structure of four dominant oncaeid copepods (Triconia borealis, Triconia canadensis, Oncaea grossa and Oncaea parila) were investigated in the Oyashio region, western subarctic Pacific. Seasonal samples were collected with 0.06 mm mesh nets from five discrete layers between the surface and 2,000 m depth at seven occasions (March, May, June, August and October 2002, December 2003 and February 2004). The depth of occurrence of major populations of each species differed by species; the surface–250 m for T. borealis, 250–1,000 m for T. canadensis, 250–500 m for O. grossa and 500–1,000 m for O. parila. The ontogenetic vertical migration characterized by deeper occurrence of early and late copepodid stages, and shallower occurrence of middle copepodid stages was observed in T. canadensis and O. parila. Of the four oncaeid copepods, almost all copepodid stages occurred throughout the study period, suggesting that their reproduction continues throughout the year in the region. Nevertheless, a clear developmental sequence of stage-to-stage was traced for T. canadensis and O. grossa copepodids, implying their generation time to be 1 year. For T. borealis and O. parila copepodids, no clear seasonal succession was observed thus estimation of their generation time was uncertain. The present comprehensive results of vertical distribution and life cycle features for T. borealis, T. canadensis, O. grossa and O. parila are compared with the few published data on oncaeid species distributing in high latitude seas.     

Microbial food partitioning by three species of benthic copepods

Using radioactively labeled bacteria and photoautotrophs in undisturbed sediment cores, we show that three cooccurring species of benthic copepods feed on different microbial food sources in their natural environment. Specifically, Thompsonula hyaenae feeds on photoautotrophs, Halicyclops coulli feeds on bacteria, and Zausodes arenicolus feeds on both photoautotrophs and bacteria. Species of benthic copepods feed differently from one another in the field, and meiofaunal species' distributions could be influenced by distributions of their preferred microbial food.     

Material dispersion by oceanic internal waves

Internal gravity waves that are generated in the open ocean have a universal frequency spectrum, called Garrett–Munk spectrum. By initializing internal waves that satisfy the Garrett–Munk spectrum in a non-hydrostatic numerical model, we investigate the material dispersion produced by these internal waves. Three numerical experiments are designed: Exp.-1 uses a linearly stratified fluid, Exp.-2 has an upper mixed layer, and Exp.-3 incorporates a circular front into the upper mixed layer. Resorting to neutrally buoyant particles, we investigate the dispersion in terms of metrics of the relative dispersion and finite-scale Lyapunov exponent (FSLE). Exp.-1 shows that the dispersion regime produced by these internal waves is between ballistic and diffusive based on relative dispersion, and is however ballistic according to FSLE. The maximum FSLE at scales of 100 m is about 5 day\(^{-1}\), which is comparable to that calculated using ocean drifters. Exp.-2 demonstrates that internal waves can generate flows and material dispersion in an upper mixed layer. However, when mixed layer eddies are present, as in Exp.-3, the dispersion in the mixed layer is controlled by the eddies. In addition, we show that inertial oscillations do not affect the relative dispersion, but impact FSLE at scales of inertial oscillations.     

Grazing by adult estuarine calanoid copepods of the Chesapeake Bay

Grazing by adult female Eurytemora affinis, Acartia tonsa and A. clausi on natural distributions of particles from the Chesapeake Bay has been investigated. During the course of a year's sampling, a wide variety of particle size-biomass distributions were observed as seasonal shifts in detritus, and over 150 algal species occurred. These distributions were grouped into 5 basic types in the analyses of feeding. All three species demonstrated similar capabilities for feeding over a broad range of particle size with selection (higher filtering rates) on larger particles and on biomass peaks. Feeding on multiple-peak distributions resulted in strong selection or tracking of each biomass peak with reduced filtering rates between peaks. Evidence is presented which suggests that the copepods first feed on large particles and then successively switch to biomass peaks of the smaller size categories. Comparisons of the feeding behavior of Eurytemora affinis and the Acartia species showing that the Acartia species have greater capabilities for taking large particles may be associated with modifications of their mouth parts for raptorial feeding. The results suggest considerable flexibility in copepod feeding behavior which cannot be explained solely by the mechanism of a fixed sieve.     

Grazing of phytoplankton by copepods in eastern Antarctic coastal waters

Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke‐out. Phytoplankton biomass increased again from mid‐ to late‐February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break‐out, whereas larval polychaetes and ctenophores dominated after ice break‐out. Oncaeacurvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C␣m⁻³␣d⁻¹) was recorded on 21 December when O.␣curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m⁻³ d⁻¹); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February (⋍5 mg C␣m⁻³␣d⁻¹). The main species responsible for grazing during this period were O. curvata, Oithonasimilis, Calanoidesacutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity. Received: 11 October 1996 / Accepted: 22 October 1996     

Zooplankton growth rates: the influence of size and resources in tropical marine copepodites

Growth rates were determined for copepodites of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total copepodite development time was as short as 4 to 5 d. Mean instantaneous growth rates (g) ranged from as high as 1.2 d⁻¹ to as low as 0.1 d⁻¹. In general, cyclopoid copepods appeared to grow more slowly than calanoids of the same size. Enhancement of resources by nutrient addition caused a 32% increase in growth rates in experiments from a mesotrophic site, but only a 17% increase at a more eutrophic site. Additionally, copepodites at both sites showed faster development and generally larger size at stage in response to nutrient addition. Growth rates were positively related to chlorophyll concentration in the >2 μm size-fraction. A significant relationship of growth rate to body size (r ² = 0.45) emerged across a wide range of trophic status, but it was confounded with resource availability. It appears that growth in tropical copepod copepodites may be frequently limited by resources in a size-dependent manner. Received: 30 May 1997 / Accepted: 13 May 1998     

Suspension-feeding by herbivorous calanoid copepods: A cinematographic study

Many planktonic calanoid copepods are commonly described as filter-feeders. Direct observations using high-speed micro-cinematography indicate that these animals are suspension-feeders. They capture and handle the food particles not passively according to size and shape but, in most cases, actively using sensory inputs for detection, motivation to capture, and ingestion.     

Gut pigment accumulation and destruction by arctic copepods in vitro and in situ

The results presented here were obtained at six locations during three cruises in 1985 (off the coast of Labrador), 1986 (at the eastern end of Viscount Melbourne Sound) and 1988 (off the coast of Labrador). In situ chlorophyll maximum concentrations were >7 gl^-1 at depths of between 0 and 30 m in all sampling areas. In feeding experiments copepods attained higher gut pigment concentrations the longer they had been previously starved and higher concentrations when fed in the dark than when fed in the light. Community ingestion rates calculated from changes in particulate chlorophyll were higher than estimates derived from gut pigment data except when copepods had been starved for 24 h. Differences between estimates by the two methods suggested pigment destruction. In feeding experiments pigment: biogenic silica ratios in food and faecal pellets suggested that the length of starvation period affected the degree of pigment destruction differently at different stations and that feeding in the light greatly increased pigment destruction. A comparison of pigment: silica ratios in the water column, and in faecal pellets collected from copepods which had fed there, suggested that pigment destruction may occur in situ sometimes and that the degree to which it occurs may be affected by feeding history, light, diel feeding behaviour and species composition.     

Vertical and horizontal movements of striped marlin (Tetrapturus audax) near the Hawaiian Islands,determined by ultrasonic telemetry,with simultaneous measurement of oceanic currents

We measured the vertical and horizontal movements of striped marlin (Tetrapturus audax) off the leeward coast of the Island of Hawaii between 20 November and 18 December 1992 while simultaneously gathering data on water temperature and oceanic currents. Fish movements were monitored by ultrasonic depth-sensitive transmitters, depth-temperature profiles by an expendable bathythermograph system, and oceanic current patterns by an acoustic Doppler current profiler. Like Indo-Pacific blue marlin (Makaira mazara), striped marlin near Hawaii spend >85% of their time in the mixed layer (i.e., above 90 m depth). The maximum depth for striped marlin appears to be limited by water temperatures 8 C° colder than the mixed layer, rather than by an absolute lower temperature. We also found that the horizontal displacements of some striped marlin can be strongly influenced by currents.     

Food selection by copepods: discrimination on the basis of food quality

The copepod Acartia tonsa displayed nearly two-fold higher ingestion rates on faster-growing cells of the diatom Thalassiosira weissflogii compared to ingestion rates on slower-growing cells of that species at the same cell concentration. Ingestion rates on slow-growing cells were also enhanced by the addition of cell-free aliquots of algal exudate to the experimental feeding chambers. In addition, the faster-growing algal cells were selectively ingested by the copepod when the two cell types were mixed together in different proportions, indicating that physiological differences between growing cells are a critical factor in the food detection/selection process of zooplankton. Consideration of cell carbon, nitrogen, and protein composition suggests that the copepods are maximizing nitrogenous ingestion (total protein and/or nitrogen). Selectivity for cells with higher protein content results in a higher daily protein ration, even if the selection process results in a decreased rate of ingestion in mixtures of cell types.