Karen F. Wishner

Marine snow studies in the Northeast Atlantic Ocean: distribution, composition and role as a food source for migrating plankton

During a 25 d Lagrangian study in May and June 1990 in the Northeast Atlantic Ocean, marine snow aggregates were collected using a novel water bottle, and the composition was determined microscopically. The aggregates contained a characteristic signature of a matrix of bacteria, cyanobacteria and autotrophic picoplankton with inter alia inclusions of the tintiniid Dictyocysta elegans and large pennate diatoms. The concentration of bacteria and cyanobacteria was much greater on the aggregates than when free-living by factors of 100 to 6000 and 3000 to 2 500 000, respectively, depending on depth. Various species of crustacean plankton and micronekton were collected, and the faecal pellets produced after capture were examined. These often contained the marine snow signature, indicating that these organisms had been consuming marine snow. In some cases, marine snow material appeared to dominate the diet. This implies a food-chain short cut wherby material, normally too small to be consumed by the mesozooplankton, and considered to constitute the diet of the microplankton can become part of the diet of organisms higher in the food-chain. The micronekton was dominated by the amphipod Themisto compressa, whose pellets also contained the marine snow signature. Shipboard incubation experiments with this species indicated that (1) it does consume marine snow, and (2) its gut-passage time is sufficiently long for material it has eaten in the upper water to be defecated at its day-time depth of several hundred meters. Plankton and micronekton were collected with nets to examine their vertical distribution and diel migration and to put into context the significance of the flux of material in the guts of migrants. “Gut flux” for the T. compressa population was calculated to be up to 2% of the flux measured simultaneously by drifting sediment traps and <5% when all migrants are considered. The in situ abundance and distribution of marine snow aggregates (>0.6 mm) was examined photographically. A sharp concentration peak was usually encountered in the depth range 40 to 80 m which was not associated with peaks of in situ fluorescence or attenuation but was just below or at the base of the upper mixed layer. The feeding behaviour of zooplankton and nekton may influence these concentration gradients to a considerable extent, and hence affect the flux due to passive settling of marine snow aggregates.

Control of deep-sea benthic community structure by oxygen and organic-matter gradients in the eastern Pacific Ocean

At boundaries of oxygen minimum zones, bathyal faunas experience steep gradients in oxygen and organic matter availability. The present study compares changes in microbial, meiofaunal, macrofaunal and megafaunal benthic assemblages along these gradients on Volcano 7, a 2.3Km high seamount in the eastern tropical Pacific

Pelagic and benthic ecology of the lower interface of the Eastern Tropical Pacific oxygen minimum zone

The distributions of pelagic and benthic fauna were examined in relation to the lower boundary of the oxygen minimum zone (OMZ) on and near Volcano 7, a seamount that penetrates this feature in the Eastern Tropical Pacific. Although the broad, pronounced OMZ in this region is an effective barrier for most zooplankton, zooplankton abundances, zooplankton feeding rates, and ambient suspended particulate organic carbon (POC) peaked sharply in the lower OMZ (about 740–800 m), in association with the minimum oxygen concentration and the increasing oxygen levels just below it. Zooplankton in the lower OMZ were also larger in size, and the pelagic community included some very abundant, possibly opportunistic, species. Elevated POC and scatter in the light transmission data suggested the existence of a thin, particle-rich, and carbon-rich pelagic layer at the base of the OMZ. Gut contents of planktonic detrifvores implied opportunistic ingestion of bacterial aggregates, possibly from this layer. Benthic megafaunal abundances on the seamount, which extended up to 730 m, peaked at about 800 m. There was a consistent vertical progression in the depth of first occurrence of different megafaunal taxa in this depth range, similar to intertidal zonation. Although the vertical gradients of temperature, salinity, and oxygen were gradual at the lower OMZ interface (in contrast to the upper OMZ interface at the thermocline), temporal variability in oxygen levels due to internal wave-induced vertical excursions of the OMZ may produce the distinct zonation in the benthic fauna. The characteristics of the lower OMZ interface result from biological interactions with the chemical and organic matter gradients of the OMZ. Most zooplankton are probably excluded physiologically from pronounced OMZs. The zooplankton abundance peak at the lower interface of the OMZ occurs where oxygen becomes sufficiently high to permit the zooplankton to utilize the high concentrations of organic particles that have descended through the OMZ relatively unaltered because of low metazoan abundance. A similar scenario applies to megabenthic distributions. Plankton layers and a potential short food chain (bacteria to zooplankton) at OMZ interfaces suggest intense utilization and modification of organic material, localized within a thin midwater depth zone. This could be a potentially significant filter for organic material sinking to the deep-sea floor.

Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

Abstract The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the highest and lowest concentrations of plant biomass in the world’s ocean. During the US JGOFS Process Study in the Arabian Sea (September 1994–January 1996), the absolute maximum in biomass of epipelagic zooplankton in the entire study was observed during the Southwest Monsoon season inshore of the Findlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monsoon, as was the strongest onshore–offshore gradient in biomass. Lowest biomass throughout the study was observed at the most offshore station (S15), outside the direct influence of the monsoon forcing. The greatest day/night contrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exceeding nighttime in the Southwest Monsoon season. The diel vertical migration patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of zooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppress migration. Based on the distribution of biomass, we hypothesize that inshore of the Findlater Jet, zooplankton grazing on phytoplankton is the dominant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous, conditions that result in reduction of the carbon flux mediated by the zooplankton. Predation by mesopelagic fish, primarily myctophids, may equal daily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integrated, may have evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Abundance, distribution and population structure of the copepod Calanus finmarchicus in a springtime right whale feeding area in the southwestern Gulf of Maine

Abstract Springtime aggregations of the planktivorous right whale ( Eubalaena glacialis ) occur in the northern Great South Channel region of the southwestern Gulf of Maine, where they feed upon dense concentrations of the copepod Calanus finmarchicus . This association was studied during the multidisciplinary South Channel Ocean Productivity Experiment (SCOPEX) in 1988 and 1989. The spatial and temporal variability of the abundance, geographic distribution, and population structure of these copepods were analyzed using data from 99 vertically-stratified or horizontally-sequenced MOCNESS plankton tows. Higher water column abundances and higher relative proportion of older copepod lifestages occurred near feeding whales compared to sites without whales, but total water column copepod biomass and Calanus abundance did not always differ between these types of locations. This suggests that the whales seek out aggregations of older copepod lifestages rather than simply the most dense aggregations. Other factors (and perhaps an element of chance) may influence which specific patches, among all patches potentially suitable in terms of copepod abundance and age composition, the whales utilize at a particular time. The times and locations of the highest Calansus water column abundances varied between years, as did the presence of feeding whales, probably because of year-to-year differences in the springtime temperature cycle and current strength. A temporal progression of lifestages occurred within the region in both years during the roughly 3-week duration of each survey, indicative of a growing rather than a diapausing population, at least up to the copepodite 4 (C4) stage. Due in part to a delay in the springtime warming in 1989 compared to 1988, the copepod development cycle, which is largely driven by in situ temperature, was delayed about 1–2 weeks in 1989. Peak abundances of younger Calanus were found in the northwestern part of the region each year, whereas peak abundances of older Calanus were found in the southwestern and northeastern part. This was probably due to the advection of maturing copepods by the regional circulation, especially the near-surface current associated with the movement of the low-salinity surface plume which forms each spring off Cape Cod. The copepod development cycle occurs within a moving frame of reference (i.e. the water itself); thus, peak abundances of the older copepods (those fed on by the whales) occurred later in the spring and further downstream in 1989 (when there were colder springtime temperatures and faster currents) than in 1988 (when the springtime temperatures were warmer and currents slower). Maximum Calanus abundances and biomass and water-colum abundances of older copepodite stages were significantly higher (about double) in 1989 than in 1988, both in the region as a whole and at sites where whales were feeding. Maximum concentrations from the MOCNESS tows were 13,300 m −3 in 1988 and 30,800 m −3 in 1989; however, a thin, visibly-red surface patch of Calanus , sampled in 1989 by a bucket, had a concentration of 331,000 m −3 . Copepods were also more aggregated in the vertical (i.e. more highly concentrated at the depth of maximum abundance) in 1989 than in 1988, and samples from whale-feeding areas were more homogeneous in composition (higher proportion of Calanus relative to all zooplankton) in 1989. At smaller spatial and temporal scales, abundances varied by a factor of 1–890 X in samples from horizontal tows spanning about 0.5–1.5 km and by a factor of 1–50 X over 24 h in the same geographic location in whale-feeding areas. Some of this variability was probably due to advection by the semidiurnal tidal currents. Near feeding whales, the copepod spatial distribution was patchy on small scales (with an estimated mean patch “size” of about 500 m), but the patchiness varied in texture interannually. Copepod abundances were much lower in early spring (March 1988) than in later spring (May 1988), with the March population structure dominated by adult females and the May population dominated by copepodite 4 and 5 stages (C4 and C5).

Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount :1. General trends

The distribution of zooplankton in the oxygen minimum zone (OMZ) of the eastern tropical Pacific and near a seamount was investigated. The oxygen minimum layer appeared to be an important factor influencing the vertical distribution of zooplankton taxa below the thermocline. The maximum zooplankton in terms of biomass and numbers was in the thermocline zone, with a secondary maximum in the depth zone that included the lower OMZ interface (600-1000 m). Most taxonomic groups showed this secondary peak in abundance in the lower OMZ interface depth zone and minimum abundances above it. The secondary maximum in zooplankton at the lower OMZ interface seems to be unique to OMZ regions. Only larvaceans and mollusks did not appear to be strongly influenced by the minimum concentrations of oxygen. Four vertical distribution patterns characteristic of different taxonomic groups may be indicative of their different tolerances to minimum oxygen concentrations. The physical intrusion of the seamount (summit at 730 m depth) did not cause major changes in the distribution of pelagic zooplankton. Significant biological and physical differences between locations above and away from the seamount were found only within the upper OMZ (100-300 m).

Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount: 2. Vertical distribution of copepods

Abstract The abundance and vertical distribution (0-1230 m) of copepods were studied in the eastern tropical Pacific near the seamount Volcano 7 to examine the influence of the extreme oxygen minimum zone (OMZ). Maximum zooplankton biomass and copepod abundance were in the thermocline zone. A secondary peak in biomass and copepod abundance was evident between 600 and 1000 m, which included the depth of the lower interface of the OMZ. This prominent secondary peak in zooplankton is a feature unique to OMZ regions. There were four general trends of vertical distribution of copepod abundance. These trends appeared to be related to the oxygen concentration and gradients. The most common vertical distribution was a pattern of maximum abundance in the mixed layer and thermocline zones, with a secondary maximum in the zone of the lower OMZ interface (600-1000 m). Clausocalanus spp., Oncaea , spp., Euchaeta spp., Oithona spp. and Corycaeus spp. showed this trend. Low oxygen concentration did not appear to restrict these groups, since they were present throughout the OMZ. The second vertical distributional pattern was vertical migration between the thermocline and the OMZ. Pleuromamma robusta showed this pattern, with maximum abundance at night in the thermocline zone and during the day in the core of the OMZ. In addition, there was a secondary maximum of abundance at the lower OMZ interface zone. The third type of distribution was shown by copepods that were abundant in the upper OMZ and at the lower OMZ interface zones. Eucalanus inermis, Haloptilusparalongicirrus and Heterostylites longicornis were dominant copepod species that exhibited this pattern. They were either absent from the mixed layer or at similar abundances in the mixed layer and upper OMZ. The fourth pattern was shown by copepod species that live primarily above the OMZ day and night. The majority of the species appeared to be tolerant of the extreme low oxygen concentrations. Rhincalanus spp. was the dominant copepod that was excluded by low oxygen concentrations. In general, species also found in other OMZ regions showed similar distributions in this study, indicating that low oxygen is a major controlling factor. Some vertical niche separation among congeneric species was indicated for Eucalanidae, Metridiidae and Augaptilidae.

Feeding ecology of benthopelagic zooplankton on an eastern tropical Pacific seamount

The gut contents of dominant deep-sea benthic boundary layer zooplankton (primarily copepods and mysids) and the vacuole contents of phaeodarians collected and preserved in situ at four depths (from 724 to 3112 m) on an eastern tropical Pacific seamount (Volcano 7; 13°23′N, 102°27′W) between 23 November and 4 December 1988 were analyzed using transmission electron microscopy (TEM). Suspended, and sinking plus resuspended particles, were quantitatively sampled to characterize potential food sources. The broad oxygen minimum characteristic of this region intersects the summit of the seamount and affects the feeding ecology of these organisms. Several copepods and mysids and an amphipod contained guts packed with what appeared to be gram-positive bacteria, an unusual finding. We hypothesize that the source of these bacteria-like bodies was a mat or aggregate that originated in the oxygen minimum or at its upper or lower interface. The presence of the bacteria-like bodies in 43 to 100% of the particlefeeding zooplankton that were sectioned and that had gut contents, suggests that the bacteria-like bodies are an important food source. The diverse gut and vacuole contents of other detritivores were similar among depths. Particles and microorganisms from the depths were also similar. This finding can be explained by the rapid sinking of particles and aggregates from surface waters and their relatively intact transit through the broad oxygen minimum with its reduced populations of zooplankton. The presence of algal cells in guts and vacuoles of benthic boundary-layer zooplankton suggests that these zooplankton select particles of recent surface origin for consumption. The presence of the guts filled with bacteria-like bodies shows that some deep-sea copepods and mysids that are normally generalist feeders can specialize opportunistically. The similarity of gut contents of crustaceans and vacuole contents of phaeodarians suggests that these two very different groups of particle feeders utilize similar food sources in the deep sea.

Acoustic scattering from zooplankton and micronekton in relation to a whale feeding site near Georges Bank and Cape Cod

Abstract This research was part of the South Channel Ocean Productivity Experiment (SCOPEX), a multidisciplinary study to investigate the biological and physical processes associated with the very high annual springtime abundance of right whales ( Eubalaena glacialis ) in the Great South Channel off New England. Right whales appear to gather there in the spring because of the increased abundance of aggregations of their principal prey, the copepod Calanus finmarchicus . Observations of hydroacoustic scattering were made in relation to the hydrography, whale distributions, and other biological measurements in the vicinity of the Great South Channel during May 1986, March, April and May of 1988, and May and June of 1989. Copepods were detected (at 200 kHz) as a near-surface layer with strong diel changes. In 1989, a second frequency (120 kHz) was used to discriminate between copepod layers (which the 120 kHz detected only weakly) and other targets (which both frequencies detected). Acoustically distinct layers of zooplankton and micronekton were observed, which were often correlated in time and space with the copepod layers. Quantitative estimates derived from the acoustic data indicate that the abundance of zooplankton varied from 1–5 g wet weight m −3 to 18–25 g wet weight m −3 which correlates well with the abundances observed from MOCNESS tows. The acoustic data revealed a complex diel migration of two layers in addition to the copepods. Euphausiids (predominantly Meganyctiphanes sp.) were found in a layer above the bottom, and a mid-water layer may have been due to sand lance ( Ammodytes americanus ). The observed biological phenomena appeared to be related to the complex hydrography of the region. A surface thermal front existed at the northern entrance to the channel in 1988 and 1989, with colder vertically mixed water to the south and warmer stratified water to the north. A Fast Fourier Transform analysis for spectral composition and autocovariance shows (a) strong contrasts in the spectral density across one frontal feature (predominantly a salinity front) as opposed to away from the front, and (b) significant differences between those areas where a whale moved more rapidly (presumably searching for food) and where it spent more time (presumably or observably feeding). The behavior of whales, in particular the right whale, can be shown to be related to the spatial scales and abundance of their prey by the use of hydroacoustic estimates of target distribution and abundance.

In situ filtering and ingestion rates of deep-sea benthic boundary-layer zooplankton in the Santa Catalina Basin

In situ rates of filtration, particulate ingestion, and carbon ingestion of deep-sea benthic boundary-layer zooplankton were determined in December 1984 in the Santa Catalina Basin, at 1 300 m depth in the California Borderland, by a short-term radioisotope-incorporation technique. Zooplankton were collected at 1 or 50 m above the bottom with an opening-closing net system on a submersible, and incubated at depth with labelled amino acids in special cod-end chambers. Concentrations of particulate material and particulate organic carbon in the ambient water were also measured. The zooplankton had a median weight-specific filtration rate of 12.4 ml (mg dr. wt)-1 h-1 and a median carbon ingestion rate of 5.4 μg C (mg dr. wt)-1 h-1. Filtration rates were not significantly different from those in similar experiments in the north Atlantic at 2 175 m depth or Narragansett Bay in the winter, although the medians of the deep-sea experiments were lower than for the Bay. In the Santa Catalina Basin, rates from experiments at 1 m above the bottom in more turbid water were not significantly different from those at 50 m above the bottom in clearer water. These deep-sea benthic boundary-layer zooplankton may have the potential to respond to food pulses, and their relatively high ingestion rates suggest that they could have significant effects on particulate, chemical, and bacterial processes in the near-bottom water column.