Organisation in the pelagic ecosystem

A continuous, steady-state theory has been developed for the abundance of organisms in the pelagic ecosystem as a function of their body weight. It is based on accepted relationships for the weight-dependence of metabolism and growth, in a context where individual organisms are assigned to one of a series of size classes for which the nominal weights increase in a geometric progression. Analysis of the biomass flow in such a representation leads to the conclusion that, in the steady state, the total biomass in any given size class decreases in a regular manner with increasing size. Explicitly,b(w ₂)/b(w ₁)~(w ₂/w ₁)^0.22, whereb(w ₂) andb(w ₁) are the total biomasses in the size classes characterised by weightsw ₂ andw ₁, respectively. The exponent (–0.22) represents a balance between catabolism and anabolism, based on published reviews concerning the revelant parameters. This result agrees favourably with data collected by other workers in the subtropical oceans. The theory can be used to draw conclusions about the functional dynamics of the pelagic ecosystem, such as community respiration and rate of biomass flow.     

Breeding pelagic copepods

Summary A cultivation technique has been developed to breed marine pelagic copepods for experimental purposes. Heterotrophic dinoflagellates occurring in the copepod cultures prevent the cultures from fouling due to sedimentation of autotrophic algae, administered daily from continuous cultures. The dinoflagellates keep themselves well in suspension, consume any excess of algae and dead organic material, and, importantly, appear to be an excellent food for the copepods (KLEIN BRETELER, 1980).At presentPseudocalanus elongatus, Acartia clausi, Centropages hamatus andTemora longicornis have been bred through 10, 11, 33, and 35 filial generations, respectively, and are still breeding continuously. Growth rate and isochronal development were dependent on both the concentration of food and the presence of dinoflagellates.     

Deep pelagic biology

The deep pelagic habitat is a vast volume of cold, dark water where food is scarce and bioluminescence is the principal source of light and communication. Understanding the adaptations that allow animals to successfully inhabit this daunting realm has been a difficult challenge because investigators have had to conduct their work remotely. Research in the deep water column is going through an essential transformation from indirect to direct methods as undersea vehicles provide unprecedented access, new capabilities, and new perspectives. Traditional methods have accurately documented the meso- and macro-scale zoogeographic patterns of micronekton and zooplankton, as well as their distribution and migration patterns in the vertical plane. The new in situ technologies have enabled advances in studies of behavior, physiology, and in particular, the role of gelatinous animals in deep pelagic ecology. These discoveries reveal a deep-water fauna that is complex and diverse and still very poorly known.     

The living pelagic resources of the Americas

Abstract

Annual catches of pelagic organisms, excluding marine mammals, from the Atlantic and Pacific waters surrounding the Americas for the period 1969–1971 averaged 17, 000, 000 metric tons. Catches of the pelagic organisms from the Pacific alone were nearly 14, 000, 000 tons. However, considering the recent drop in the catches of the Peruvian anchovy, the data for 1972 and 1973 when compiled, will show a drastic decline in the catches of pelagic species. Catches from American waters are compared with those from other waters and their relationship to the area and population of the Americas is examined. The potential maximum catch of pelagic organisms currently utilized is projected to 26, 000, 000 tons, whereas if the markets and fishing technology could be developed for non‐conventional species, then it is estimated that the catch could be increased to 42, 000, 000 tons. Rational utilization of the pelagic resources of the Americas necessitates conservational and managerial measures. This in turn calls for international agreements and/or other international arrangements. This paper was presented at the Inter‐American meeting on “Science and Man in the Americas”; in 1973 at Mexico City.     

Computational visual ecology in the pelagic realm

Visual performance and visual interactions in pelagic animals are notoriously hard to investigate because of our restricted access to the habitat. The pelagic visual world is also dramatically different from benthic or terrestrial habitats, and our intuition is less helpful in understanding vision in unfamiliar environments. Here, we develop a computational approach to investigate visual ecology in the pelagic realm. Using information on eye size, key retinal properties, optical properties of the water and radiance, we develop expressions for calculating the visual range for detection of important types of pelagic targets. We also briefly apply the computations to a number of central questions in pelagic visual ecology, such as the relationship between eye size and visual performance, the maximum depth at which daylight is useful for vision, visual range relations between prey and predators, counter-illumination and the importance of various aspects of retinal physiology. We also argue that our present addition to computational visual ecology can be developed further, and that a computational approach offers plenty of unused potential for investigations of visual ecology in both aquatic and terrestrial habitats.     

The food web in the pelagic environment

Summary 1. The study of grazing along ecological successions helps in understanding how the mechanisms of transfer of energy evolve and provides a measure of its effectiveness in the different situations. Phytoplankton populations show regular changes along succession. Average size of cells and relative abundance of mobile organisms increase, and productivity or rate of multiplication slows down; there are also changes in the chemical composition, exemplified in the plant pigments by an absolute and relative decrease of chlorophyll a.2. Along a succession, animals are offered different kinds of food, and the resulting selection produces a shift in the composition of zooplankton populations. The speed at which phytoplankton succession proceeds is a very important factor, and grazing may be effective in the regulation of such speed. In a general way, along usual successions, food in the form of small particles, richly suspended in a more or less turbulent environment, is replaced by scarcer food concentrated in bigger units and dispersed in a more organized (stratified) environment.3. Indiscriminate filter feeders are at an advantage in the first stages of succession, but it can be shown that, given the usual properties of food organisms in later stages, it pays to adopt a more selective and hunting behaviour and to concentrate more and more on bigger prey. The effectiveness of such adaptation depends on the distribution of food in size classes and also on its mobility or other clues that prey organisms can offer, and, in general, on the predictability of their distribution. Divergence between microphagous passive filterers and macrophagous hunters must be rapid. Distribution of populations of copepods along time and in relation with phytoplankton distribution sustain such views.4. In general, in later stages of succession, total transfer of energy may be lower, buts its efficiency seems to be regularly improved. Similar considerations could be extended to the discussion of energy transfer between other, superior, trophic levels.

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Das Nahrungsnetz in der pelagischen Umwelt

Kurzfassung Vergleichende Studien über die Organisation des Nahrungsnetzes während verschiedener Stadien von Planktonsukzessionen können Aufschlüsse liefern über Gesetzmäßigkeiten der Energietransformation. Von dieser Annahme ausgehend, werden zunächst verschiedene Stadien von Phytoplanktonsukzessionen beschrieben und dann an Hand von Beispielen aus der Literatur und theoretischen Erwägungen allgemeinere Gesetzmäßigkeiten abgeleitet. Phytoplanktonpopulationen verändern sich im Verlaufe von Sukzessionen in ganz bestimmter Weise: Die Durchschnittgröße der Zellen und die relative Häufigkeit von Organismen mit Eigenbewegung nehmen zu, während Produktivität oder Zellteilungsrate abnehmen; ferner kommt es zu Veränderungen in der chemischen Zusammensetzung, etwa zu einer Abnahme der Menge an Chlorophyll. Im Verlauf der Sukzessionen verändert sich das Nahrungsangebot für die Zooplankter; dieser Umstand führt zu Verschiebungen in der Komposition der Zooplanktonpopulationen. Die Geschwindigkeit, mit welcher Phytoplanktonsukzessionen fortschreiten, ist ein sehr wichtiger Faktor; grazing ist hier möglicherweise als Geschwindigkeitsregulativ wirksam. Ganz allgemein wird im Verlauf gewöhnlicher Sukzessionen Nahrung, welche in Form von kleinen dispers suspendierten Partikeln in einem mehr oder minder turbulenten Wasserkörper vorliegt, ersetzt durch geringere Nahrungsmengen, welche in größeren Partikeln konzentriert sind und in einer stärker organisierten (stratifizierten) Umwelt vorkommen. In den ersten Sukzessionsstadien sind unspezifische Filtrierer im Vorteil; in späteren Stadien werden jedoch räuberische Formen und solche, die sich auf immer größere Nahrungsorganismen konzentrieren, bevorzugt. In den späteren Sukzessiontadien ist die Gesamtmenge der transferierten Energie möglicherweise geringer, aber der Nutzeffekt scheint größer zu sein.

     

Colonization of the pelagic realm by calanoid copepods

The evolution of calanoid copepods probably extends back into the mid-Paleozoic. Environmental change from the Paleozoic through to the Tertiary is reviewed. Turbidity, water clarity, oxygen, pelagic primary production, and tectonically induced changes in the morphology of the oceans are probably all important drivers of calanoid evolution and their invasion of the pelagic realm. Current views of the phylogeny of the Calanoida are presented as well as a review of some recent work on metabolic potential, female genital system, and nervous system. It is hypothesized that ancestors of the Arietelloidea and Diaptomoidea invaded the water column in the Devonian at a similar time to the Ostracoda and that the ancestors of the Calanoidea–Clausocalanoidea, with their myelinated axons, arose in the Permian during the major deep ventilation of the ocean. Present day distributions of some Diaptomidae, Centropagidae, and Calanidae suggest that these families successfully came through the Jurassic/Cretaceous expansion of the oxygen minimum zone and the K-T boundary event. Some Arietelloidea and Clausocalanoidea became secondarily benthic and may have survived the K-T boundary event in this environment. It is postulated that some Clausocalanoidea reinvaded the water column (e.g. Clausocalanidae, Aetideidae, Phaennidae, Scolecitrichidae) in the Tertiary after finding refuge in deep, low oxygen water away from the sea surface. It is possible that these hypotheses may be testable using genetic information in the near future.     

Ecology of pelagic larvae and juveniles of the genusSebastes

Synopsis Pelagic larvae and juveniles of the genusSebastes are widely distributed in the continental shelf and slope waters of subarctic to temperate oceans, with greatest abundance in the Northern Hemisphere. We review the ecology and distribution of these planktonic and micronektonic life stages in relation to oceanographic conditions. Special attention is paid to the west coast of North America, where abundance of larvae from samples collected during 1951–1981 is described. After transformation, the pelagic juveniles are widely distributed, often at great distances from benthic adult habitats. These stages are most frequently distributed in either midwater or near-surface habitats; this dichotomy may require different strategies for successful recruitment.     

Cryptic and conspicuous coloration in the pelagic environment

Despite the importance of cryptic and conspicuous coloration in pelagic ecosystems, few researchers have investigated the optimal reflectance spectra for either trait. In this study, the underwater radiance distribution in tropical oceanic water was modelled using measured inherent optical properties and radiative transfer calculations. The modelled light field was then used to predict the reflectance spectra that resulted in minimal or maximal object contrast as a function of depth, viewing angle, azimuth and solar elevation. The results matched commonly observed trends in the coloration of many pelagic organisms and showed that optimal coloration for either crypticity or conspicuity is a complex function of the parameters examined. The effects of viewing angle and depth were substantial and non-intuitive, showing that red coloration is most cryptic at depth. The effects of viewing azimuth were less significant and the effects of solar elevation were minor. White coloration and black coloration were equally cryptic/conspicuous when viewed from below. Although conspicuous objects viewed from below had the lowest contrast when viewed from a short distance, they had the longest sighting distances. The contrast of maximally conspicuous objects viewed from short distances was greatest at wavelengths displaced from the wavelength of maximum light penetration.     

Field experiments on the optical orientation of pelagic rotifers

Avoidance of shore by pelagic rotifers is considered to be the result of an optical orientation. Field experiments show that the spatial light distribution in the shore region determines the preferred direction of migration. The behaviour of Eudiaptomus gracilis was tested in comparison to that of rotifers.This publication is dedicated to Pater Dr. Josef Donner on the occasion of his 70th birthday     

The evolutionary origin of the durophagous pelagic stingray ecomorph

Studies of the origin of evolutionary novelties (novel traits, feeding modes, behaviours, ecological niches, etc.) have considered a number of taxa experimenting with new body plans, allowing them to occupy new habitats and exploit new trophic resources. In the marine realm, colonization of pelagic environments by marine fishes occurred recurrently through time. Stingrays (Myliobatiformes) are a diverse clade of batoid fishes commonly known to possess venomous tail stings. Current hypotheses suggest that stingrays experimented with a transition from a benthic to a pelagic/benthopelagic habitat coupled with a transition from a non-durophagous diet to extreme durophagy. However, there is no study detailing macroevolutionary patterns to understand how and when habitat shift and feeding specialization arose along their evolutionary history. A new exquisitely preserved fossil stingray from the Eocene Konservat-Lagerstätte of Bolca (Italy) exhibits a unique mosaic of plesiomorphic features of the rajobenthic ecomorph, and derived traits of aquilopelagic taxa, that helps to clarify the evolutionary origin of durophagy and pelagic lifestyle in stingrays. A scenario of early evolution of the aquilopelagic ecomorph is proposed based on new data, and the possible adaptive meaning of the observed evolutionary changes is discussed. The body plan of †Dasyomyliobatis thomyorkei gen. et sp. nov. is intermediate between the rajobenthic and more derived aquilopelagic stingrays, supporting its stem phylogenetic position and the hypothesis that the aquilopelagic body plan arose in association with the evolution of durophagy and pelagic lifestyle from a benthic, soft-prey feeder ancestor.     

Predator-prey relationships within the pelagic community of Neusiedler See

Neusiedler See, a shallow alkaline lake, has become increasingly eutrophic; this enrichment improved the nutritive situation of the herbivorous zooplankton leading to a higher standing stock. A multiple regression analysis of the long-term development of the crustacean plankton indicates that abiotic factors (i.e. wind, temperature) have the most important impact on the community in spring and autumn, biotic factors (i.e. food, predation) during the summer months. Currently an invertebrate (Leptodora kindti) and two planktivorous fish (Pelecus cultratus and Alburnus alburnus) control the population development of Diaphanosoma mongolianum during summer. L. kindti predation acts on immature stages, whereas the fishes consume adult stages. L. kindti densities of 100 to 200 ind. × m^-3 affect the prey population to a negligible extent; densities between 300 and 500 ind. × m^-3 result in elimination rates of 5% to >40% of the juvenile Diaphanosoma stock. The impacts by the invertebrate predator are pronounced but short-term events. Juvenile and underyearling fish eliminate 4–13% of the zooplankton in the open lake and 5–33% inshore; their predation pressure acts on all younger stages of the crustaceans. Planktivorous fish older than 0 + concentrate on the large food items (adult stages of the crustaceans). A. alburnus is able to consume 6–16% of the zooplankton standing stock during summer. P. cultratus eliminates about 1–49% of D. mongolianum, 1–4% of A. spinosus and 1–31% of L. kindti. From May until August the position of the dominant predator changes, beginning with juvenile fish which are then followed by P. cultratus and then by A. alburnus. Predation by L. kindti is of importance in July and August; at that time it causes suppression of the juveniles of D. mongolianum, but, on the other hand, the older stages of L. kindti are endangered themselves by the predatory impact of planktivorous fish.     

High-resolution vertical profiles of pelagic tunicates

The objectives of our research were to obtain high-resolutionvertical profiles of the abundance of pelagic tunicates andto relate their distribution to physical and biological variables.The abundance of doholids (Tunicata: Thaliacea) from near thesurface to near the seafloor was determined by obtaining continuousvideotape recording from a descending submersible. These verticalprofiles, accompanied by temperature and fluorescence measurements,revealed (i) pronounced changes in dololiolid concentrationwithin 2 m depth intervals, and (ii) two general modes of verticaldistribution, one characterized by a peak in abundance in thethennocline and the second by a more or less even distributionthroughout much of the water column. This in situ approach canprovide estimates of zooplankton abundance in near real-time     

Marine pelagic ecosystems: the west Antarctic Peninsula

The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 2 degrees C increase in the annual mean temperature and a 6 degrees C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.6 degrees C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in icedependent Adélie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along the WAP and the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response.     

Particle selectivity of pelagic tintinnid agglutination

Scanning electron microscopic observation reveals that several agglutinated, pelagic tintinnid taxa apparently possess a capability of incorporating specific mineral grains, such as monospecific coccoliths, for their loricae. Since the ocean water generally contains other coccolith taxa, as well as diatoms and clays, it seems possible that the tintinnids can single out a particular coccolithophore species from the variety of suspended matter for their loricae-building.     

Quantitative ichnology of modern pelagic deposits in the abyssal Atlantic

Quantitative sedimentologic aspects of bioturbation were investigated in a series of deep-sea box cores, which were collected over a broad latitudinal range (35°N to 30°S) and bathymetric range (1.4 to 5.7 km) in the central Atlantic Ocean. All cores were thoroughly bioturbated, and numerous generations of burrowing could be discerned in many instances. Preservation of individually distinct burrows ranged from poor to excellent, and eight ichnogenera plus several unnamed biogenic structures were recognized.The distribution of calcium carbonate, organic carbon, selected species of planktic foraminifera and sediment grain size were examined in several cores. These measurements indicated that the surficial Mixed Layer, which ranged from 3 to 10 cm thick, was statistically more homogeneous than the underlying Transition Layer. This relationship is seen most clearly with respect to lateral and vertical variations in the calcium carbonate content of the cores. Bioturbation of abyssal pelagic deposits appears to be a two-phase process consisting of (a) homogenization of the upper few centimeters (i.e., the Mixed Layer) by shallow-burrowing meiofauna, and (b) heterogeneous mixing of the underlying strata (i.e., the Transition Layer) by relatively deep burrowers. The heterogeneously mixed Transition Layer becomes the preserved trace fossil record; under normal circumstances the homogenized Mixed Layer is not preserved.     

A technique for determining the distribution of pelagic fish larvae

A technique is described for studying small scale distributional patterns of fish larvae in the pelagic region. A plankton net is mounted beneath the bows of a small boat, the contents of the net are emptied continually by a suction pump. This system has advantages over previous techniques as it allows a large number of small discrete samples to be taken over a wide area in a short space of time. Preliminary results showed the population of perch larvae in Llyn Tegid is extremely clumped and the overall abundance of the larvae decreases as samples are taken progressively further away from the south-west end of the lake.     

Complex wasp-waist regulation of pelagic ecosystems in the Pacific Ocean

‘Wasp-waist’ control of marine ecosystems is driven by a combination of top-down and bottom-up forcing by a few abundant short-lived species occupying intermediate trophic levels that form a narrow ‘waist’ through which energy flow from low to high trophic levels is controlled. It has been assumed that wasp-waist control occurs primarily in highly productive and species-poor systems (e.g. upwelling regions). Two large, species-rich, pelagic ecosystems in the relatively oligotrophic eastern and western Pacific Ocean also show wasp-waist-like structure, in that short-lived and fast-growing cephalopods and fishes at intermediate trophic levels comprise the vast majority of the biomass. Possible forcing dynamics of these systems were examined using ecosystem models by altering the biomass of phytoplankton (bottom-up forcing), large pelagic predators (top-down forcing), and intermediate ‘wasp-waist’ functional groups independently and observing how these changes propagated throughout the ecosystem. The largest effects were seen when altering the biomass of mid trophic-level epipelagic and mesopelagic fishes, where dramatic trophic cascades occurred both upward and downward in the system. We conclude that the high productivity and standing biomass of animals at intermediate trophic levels has a strong top-down influence on the abundance of primary producers. Furthermore, their importance as prey for large predators results in bottom-up controls on populations at higher trophic levels. We show that these tropical pelagic ecosystems possess a complex structure whereby several waist groups and alternate trophic pathways from primary producers to apex predators can cause unpredictable effects when the biomasses of particular functional groups are altered. Such models highlight the possible structuring mechanisms in pelagic systems, which have implications for fisheries that exploit these wasp-waist groups, such as squid fisheries, as well as for fisheries of top predators such as tunas and billfishes that prey upon wasp-waist species.