Pelagic Ecosystem Structure Research Articles

Water column structure defines vertical habitat of twelve pelagic predators in the South Atlantic

Abstract Quantifying vertical distributions of pelagic predators elucidates pelagic ecosystem structure and informs fisheries management. In the tropical South Atlantic Ocean, the recently designated large-scale marine protected area around Ascension Island hosts diverse pelagic predators for which basin-specific vertical habitat information is minimal or absent. We used pop-up satellite archival tags to analyse vertical habitat use in 12 species (bigeye tuna Thunnus obesus, blue marlin Makaira nigricans, blue shark Prionace glauca, dolphinfish Coryphaena hippurus, Galapagos shark Carcharhinus galapagensis, oceanic whitetip Carcharhinus longimanus, sailfish Istiophorus albicans, silky shark Carcharhinus falciformis, swordfish Xiphias gladius, tiger shark Galeocerdo cuvier, wahoo Acanthocybium solandri, and yellowfin tuna Thunnus albacares) and quantify parameters (temperature, dissolved oxygen, diel cycles, lunar phase) known to constrain vertical movements. Predator depth distributions varied widely, and classification trees grouped predators into four clades: (i) primarily epipelagic; (ii) partial thermocline use; (iii) oscillatory diving with thermocline/sub-thermocline use; and (iv) extensive use of sub-thermocline waters. Vertical habitat differences were linked to thermal physiology and foraging ecology, and species-specific physical constraints from other ocean basins were largely conserved in the South Atlantic. Water column features defined species-specific depth distributions, which can inform fisheries practices and bycatch risk assessments and population estimates.

Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.

Migratory Zooplankton Excreta and Its Influence on Prokaryotic Communities

Particulate organic matter (POM) (fecal pellets) from zooplankton has been demonstrated to be an important nutrient source for the pelagic prokaryotic community. Significantly less is known about the chemical composition of the dissolved organic matter (DOM) produced by these eukaryotes and its influence on pelagic ecosystem structure. Zooplankton migrators, which daily transport surface-derived compounds to depth, may act as important vectors of limiting nutrients for mesopelagic microbial communities. In this role, zooplankton may increase the DOM remineralization rate by heterotrophic prokaryotes through the creation of nutrient rich “hot spots” that could potentially increase niche diversity. To explore these interactions, we collected the migratory copepodPleuromamma xiphiasfrom the northwestern Sargasso Sea and sampled its excreta after 12–16 h of incubation. We measured bulk dissolved organic carbon (DOC), dissolved free amino acids (DFAA) via high performance liquid chromatography and dissolved targeted metabolites via quantitative mass spectrometry (UPLC-ESI-MSMS) to quantify organic zooplankton excreta production and characterize its composition. We observed production of labile DOM, including amino acids, vitamins, and nucleosides. Additionally, we harvested a portion of the excreta and subsequently used it as the growth medium for mesopelagic (200 m) bacterioplankton dilution cultures. In zooplankton excreta treatments we observed a four-fold increase in bacterioplankton cell densities that reached stationary growth phase after five days of dark incubation. Analyses of 16S rRNA gene amplicons suggested a shift from oligotrophs typical of open ocean and mesopelagic prokaryotic communities to more copiotrophic bacterial lineages in the presence of zooplankton excreta. These results support the hypothesis that zooplankton and prokaryotes are engaged in complex and indirect ecological interactions, broadening our understanding of the microbial loop.

Response of size and trophic structure of zooplankton community to marine environmental conditions in the northern South China Sea in winter

Abstract The South China Sea (SCS) is a semi-enclosed marginal sea. And in the northern SCS (NSCS), the Pearl River plume, mesoscale eddies and Kuroshio intrusion may influence the structure of pelagic ecosystems. Here, based on mesozooplankton samples collected in the NSCS from December 2014 to January 2015, spatial variations of mesozooplankton biomass, abundance, normalized biomass size spectra (NBSS), size structure and trophic structure were analyzed to study the response of mesozooplankton community to the influence of highly variable oceanographic environment. High biomass, abundance and intercept of NBSS for the mesozooplankton community were observed in highly productive regions (net primary production > 400 mg C m−2 day−1), such as river plume and cold eddy. No consistent spatial variation was observed for the slope of NBSS, probably because of highly dynamic marine environment in the NSCS. For the trophic structure of mesozooplankton community, the ratio of total chaetognaths biovolume to herbivores/omnivores biovolume (RTCH) was significantly high in river plume and Kuroshio intrusion. The high RTCH indicated unstable trophic structures, which may be due to extreme high proportion of smaller size individuals supported by discharge of river and transport of warm water species from the tropical western Pacific by Kuroshio intrusion, respectively.

Why are biotic iron pools uniform across high‐ and low‐iron pelagic ecosystems?

AbstractDissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here we compare mixed‐layer biotic iron inventories from a low‐iron (~0.06 nmol L−1) subantarctic (FeCycle study) and a seasonally high‐iron (~0.6 nmol L−1) subtropical (FeCycle II study) site. Both studies were quasi‐Lagrangian, and had multi‐day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100 ± 30 pmol L−1) for low‐ and high‐iron waters, despite a tenfold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80 ± 24 pmol L−1, largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed twofold to threefold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site‐specific differences in iron recycling efficiencies (which provide 20–50% and 90% of total iron supply in high‐ and low‐iron waters, respectively) help offset the differences in new iron inputs between low‐ and high‐iron sites. Future parameterization of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.

Variation and Assimilation of Arctic Riverine Sestonin the Pelagic Food Web of the Mackenzie River Delta and Beaufort Sea Transition Zone

On-going shifts in the arctic climate and landscape have the potential to increase terrestrial particulate organic matter (POM) transported to the Mackenzie delta and coastal Beaufort shelf. If increased terrestrial/freshwater material exports to the coastal zone can be assimilated by the pelagic food web, then this represents a new pool of carbon for the regional food web and could result in a shift in the nature of carbon supporting pelagic productivity. Analysis of the δ13C of POM from the riverine, estuarine, and marine (shelf) zones of the Mackenzie delta shows that the signatures of terrestrial/freshwater carbon does extend onto the shelf, although it diminishes with distance from the Mackenzie River. δ13C values of calanoid copepods, mysid shrimp, and two species of Amphipods varied depending on their environment (river, estuary, or marine) and local POM δ13C signature. From this, we conclude that exported terrestrial POM can be a subsidy for portions of both the estuarine and marine pelagic food web. However because the response of consumer δ13C depended on both species and location, we also conclude that the response to increased riverine POM will be different for estuarine versus marine biota. The implication is that increases in exported terrestrial carbon represent a significant potential shift in coastal pelagic ecosystem structure and function.

Feeding Habits Variability and Trophic Position of Dolphinfish in Waters South of the Baja California Peninsula, Mexico

AbstractUsing data from analyses of stomach contents and stable isotopes, we examined high‐resolution variations in the feeding habits and trophic position of Dolphinfish Coryphaena hippurus during 2005–2007. In total, 418 specimens were collected off the southern Baja California peninsula, Mexico. Based on analysis of stomach contents (% index of relative importance [%IRI]), Dolphinfish consumed mainly epipelagic prey, with the red crab Pleuroncodes planipes as the most abundant prey species. The diet was highly similar between sexes but differed between size‐classes: Dolphinfish smaller than 65 cm FL fed mainly on Pacific Sardine Sardinops sagax caeruleus (%IRI = 30), whereas those larger than 110 cm FL fed mainly on jumbo squid Dosidicus gigas (%IRI = 45) in addition to red crabs. Dietary differences between the smallest and largest specimens were mainly related to changes in morphology and spatial stratification by size (i.e., to avoid cannibalism). Trophic positions estimated by both methods indicated that the Dolphinfish is a secondary to tertiary carnivore with a high degree of trophic plasticity, and thus it plays different trophic roles within the area. Elevated variability in δ15N and Shannon–Wiener diversity index values on an annual scale revealed that Dolphinfish are opportunistic predators. Interannual diet variation related to the availability and abundance of prey species can be explained by changes in environmental conditions due to climate anomalies associated with El Niño–Southern Oscillation events. We highlight that the Dolphinfish is a good indicator of changes occurring in the food web structure of pelagic ecosystems. The observed variations in feeding habits and trophic position are critical to understanding the ecology and role of Dolphinfish in marine ecosystems by providing knowledge on feeding locations, seasonal prey utilization, and prey availability and allowing predictions of possible ecological responses to environmental change.

Estimation of phytoplankton size fractions based on spectral features of remote sensing ocean color data

AbstractThrough its influence on the structure of pelagic ecosystems, phytoplankton size distribution (pico‐, nano‐, and micro‐plankton) is believed to play a key role in “the biological pump.” In this paper, an algorithm is proposed to estimate phytoplankton size fractions (PSF) for micro‐, nano‐, and pico‐plankton (fm,fn, andfp, respectively) from the spectral features of remote‐sensing data. From remote‐sensing reflectance spectrum (Rrs(λ)), the algorithm constructs four types of spectral features: a normalizedRrs(λ), band ratios, continuum‐removed spectra, and spectral curvatures. Using support vector machine recursive feature elimination, the algorithm ranks the constructed spectral features andRrs(λ) according to their sensitivities to PSF which is then regressed against the sensitive spectral features through support vector regression. The algorithm is validated with (1) simulatedRrs(λ) and PSF, and (2)Rrs(λ) obtained by Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) and PSF determined from High‐Performance Liquid Chromatography (HPLC) pigments. The validation results show the overall effectiveness of the algorithm in estimating PSF, withR2of (1) 0.938 (fm) for the simulated SeaWiFS data set; and (2) 0.617 (fm), 0.475 (fn), and 0.587 (fp) for the SeaWiFS satellite data set. The validation results also indicate that continuum‐removed spectra and spectral curvatures are the dominant spectral features sensitive to PSF with their wavelengths mainly centered on the pigment‐absorption domain. Global spatial distributions offm,fn, andfpwere mapped with monthly SeaWiFS images. Overall, their biogeographical distributions are consistent with our current understanding that pico‐plankton account for a large proportion of total phytoplankton biomass in oligotrophic regions, nano‐plankton in transitional areas, and micro‐plankton in high‐productivity regions.

Control of plankton seasonal succession by adaptive grazing

The ecological succession of phytoplankton communities in temperate seas is characterized by the dominance of nonmotile diatoms during spring and motile flagellates during summer, a pattern often linked to the seasonal variation in the physical environment and nutrient availability. We focus on the effects of adaptive zooplankton grazing behavior on the seasonal succession of temperate plankton communities in an idealized community model consisting of a zooplankton grazer and two phytoplankton species, one motile and the other nonmotile. The grazer can switch between ambush feeding on motile cells or feeding‐current feeding on nonmotile cells. The feeding‐current behavior imposes an additional mortality risk on the grazer, whereas ambush feeding benefits from small‐scale fluid turbulence. Grazer‐phytoplankton feeding interactions are forced by light and turbulence and the grazer adopts the feeding behavior that optimizes its fitness. The adaptive grazing model predicts essential features of the seasonal plankton succession reported from temperate seas, including the vertical distribution and seasonal variation in the relative abundance of motile and nonmotile phytoplankton and the seasonal variation in grazer abundance. Adaptive grazing behavior, in addition to nutrient and mixing regimes, can promote characteristic changes in the seasonal structure of phytoplankton community observed in nature.

Parasite communities indicate effects of cross-shelf distributions, but not mesoscale oceanographic features on northern California Current mid-trophic food web

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 454:19-36 (2012) - DOI: https://doi.org/10.3354/meps09654 Parasite communities indicate effects of cross-shelf distributions, but not mesoscale oceanographic features on northern California Current mid-trophic food web Kym C. Jacobson1,*, Rebecca Baldwin2, Douglas C. Reese3 1Newport Field Station, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanographic and Atmospheric Administration, 2030 S.E. Marine Science Dr., Newport, Oregon 97365, USA 2Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada 3Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon, 97331, USA *Email: kym.jacobson@noaa.gov ABSTRACT: Mesoscale physical oceanographic features, such as jets and eddies, can influence the structure of marine ecosystems. We used trophically transmitted parasite communities of pelagic fishes in the northern California Current to examine effects of physical oceanographic features on pelagic ecosystem structure. We tested the hypotheses that (1) oceanographic features associated with a coastal promontory, Cape Blanco, Oregon (USA), produced a faunal break resulting in different pelagic ecosystems north and south of the cape, and that (2) the use of biological hotspots in the area by pelagic nekton is reflected in the trophic interactions of mid- and upper trophic level fishes. We recovered 19 taxa of trophically transmitted parasites from 10 common pelagic fish species caught between Newport, Oregon, and Crescent City, California. Non-metric multidimensional scaling of parasite communities reflected a trophic structure among these fish species; results were similar to published diet studies. We found no evidence in the trophically transmitted parasites of spatial differences between the pelagic ecosystems north or south of Cape Blanco, or within versus outside of the biological hotspots. However, we found significant cross shelf differences in parasite communities. Therefore, Cape Blanco does not seem to be a strong faunal boundary, rather the strongest influence is cross-shelf transport associated with coastal upwelling. KEY WORDS: Marine parasites · Pelagic fish · Cape Blanco · Hotspot · Inshore−offshore Full text in pdf format PreviousNextCite this article as: Jacobson KC, Baldwin R, Reese DC (2012) Parasite communities indicate effects of cross-shelf distributions, but not mesoscale oceanographic features on northern California Current mid-trophic food web. Mar Ecol Prog Ser 454:19-36. https://doi.org/10.3354/meps09654 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 454. Online publication date: May 21, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Correction: Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific

Correction: Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific

Hydrographic structure and zooplankton abundance and diversity off Paita, northern Peru (1994 to 2004) — ENSO effects, trends and changes

The objective of the present study was to verify possible spatial, seasonal, and inter-annual changes in the zooplankton off Paita (northern Peru), an upwelling area located closely to the limits of cold Humboldt Current and warm Equatorial Surface Waters. Zooplankton was sampled at subsurface on 53 occasions from August 1994 to December 2004 at four stations located 2 to 30 km offshore with a WP-2 net (300 µm). Extremely high surface water temperatures combined with low salinities were observed during the 1997/98 El Niño up to 29.0 °C) and in April 2002 (up to 25.0 °C). Temperatures more than 2 °C above monthly average were also observed in October 1994, in April 2000, and in November 2004. Significant trends were observed for oxygen concentration (increase) and several horizontal and vertical gradients. Among the copepods (72% of all individuals), the most abundant species were Paracalanus parvus (28%), Acartia tonsa (26%), and Calanus sp. (10%). The strong 1997–98 El Niño (EN) event led to drastic changes in species composition that were reversed during the 1998–99 La Niña (LN) event. Community parameters such as total abundance, diversity, equitability and species richness displayed marked variations associated with the 1997–98 EN and long-term trends. Long-term trends were significant for several vertical and horizontal temperature and oxygen gradients, indicating an increase in upwelling intensity at the shelf during the study period. 10-year-trends were also significant for total zooplankton abundance (increase) and community evenness ( J, decline). Our data confirmed the importance of the weak EN in 2002/03 for the study region. Within the trend of increasing zooplankton abundance, a sharp step or shift was observed from 1999 to 2000. When using sequential t-tests to detect shifts in ( x + 1) transformed abundance data, a significant rupture was found between the last sampling in 1999 and the first sampling in 2000. Also, a substantial decrease in diel variability occurred after 1999, probably due to changes in vertical migration patterns. The considerable increase in zooplankton abundance over the study period, the ENSO effect, and the 1999–2000 transition are discussed with regard to synchronicity with other zooplankton time series. The present study contributes with the first evidence from an important area located in the Humboldt Current for synchronous trends and changes that were previously observed elsewhere in the Pacific. Our results demonstrated the importance of long-term zooplankton monitoring studies in upwelling areas, and confirms the idea of dramatic changes in pelagic ecosystem structure occurring in the East Pacific.

Phytoplankton community structure and nitrogen nutrition in Leeuwin Current and coastal waters off the Gascoyne region of Western Australia

Abstract Within the coastal waters of the eastern Indian Ocean adjacent to Western Australia, we tested the hypothesis that regenerated production (and, by inference, the microbial food web) would predominate in oligotrophic Leeuwin Current (LC) and offshore (OS) surface waters. Conversely, we expected that new production would be more important within the ∼5 times more productive shelf countercurrents (Ningaloo and Capes Currents; NCC mean±SD) compared to the DCM (2.7±2.3 nmol L−1 h−1). However, in LCO p=0.05). These values represent conservative estimates for the region due to methodological difficulties encountered with nitrogen uptake experiments in these oligotrophic waters. In contrast with the distinct community composition between different water types, mean estimates of the f-ratio were similar across sampling depths and water types: 0.17±0.07 at the surface and 0.16±0.06 at the DCM of shelf countercurrent waters, and 0.14±0.05 at the surface and 0.14±0.09 at the DCM of LC&OS waters. These results demonstrate the importance of ammonium-based production in both oligotrophic LC&OS waters and the more productive upwelling-influenced countercurrents. They also highlight the utility of chemotaxonomic methods in studying pelagic ecosystem structure along the Gascoyne region of Western Australia.

Physical fluxes, pelagic ecosystem structure, and larval fish survival in Hauraki Gulf, New Zealand

The ecosystem supporting snapper (Pagrus auratus) larvae was studied during three spring–summer (November–January 1985–1988) spawning seasons in Hauraki Gulf, New Zealand. Upwelling-favourable winds caused more incursion of shelf water into the Gulf in 1985–1986 and 1986–1987 than in 1987–1988, but in the first two seasons, the winds were relatively weak. Stronger winds in 1987–1988 drove greater vertical diffusivity and correlated with greater mixed-layer primary biomass and productivity. Effects of vertical mixing appeared to dominate horizontal incursion of upwelled shelf water in supporting upper water column productivity. The more productive 1987–1988 season had greater abundances of nauplii, copepodites, adult copepods, cladocerans, chaetognaths, hydromedusae, decapod larvae, and numerous larval fish taxa (including snapper). There was much higher survival of snapper between the late-stage egg and post-first-feeding larval stages in 1987–1988, which correlated spatially and temporally with high larval prey densities. Neither higher egg production, fewer predators, nor less horizontal advection accounted for these strong larval snapper cohorts. We hypothesize that larval competence improved within the superior larval feeding environment, reducing predatory losses. The ecosystem response to wind mixing may partially explain the correlation of sea temperatures with recruitment, previously observed for the Hauraki Gulf snapper stock.

Calibration and validation of a one-dimensional complex marine biogeochemical flux model in different areas of the northern Adriatic shelf

Abstract. In this paper we show results from numerical simulations carried out with a complex biogeochemical fluxes model coupled with a one-dimensional high-resolution hydrodynamical model and implemented at three different locations of the northern Adriatic shelf. One location is directly affected by the Po River influence, one has more open-sea characteristics and one is located in the Gulf of Trieste with an intermediate behavior; emphasis is put on the comparison with observations and on the functioning of the northern Adriatic ecosystem in the three areas. The work has been performed in a climatological context and has to be considered as preliminary to the development of three-dimensional numerical simulations. Biogeochemical model parameterizations have been ameliorated with a detailed description of bacterial substrate utilization associated with the quality of the dissolved organic matter (DOM), in order to improve the models capability in capturing the observed DOM dynamics in the basin. The coupled model has been calibrated and validated at the three locations by means of climatological data sets. Results show satisfactory model behavior in simulating local seasonal dynamics in the limit of the available boundary conditions and the one-dimensional implementation. Comparisons with available measurements of primary and bacterial production and bacterial abundances have been performed in all locations. Model simulated rates and bacterial dynamics are in the same order of magnitude of observations and show a qualitatively correct time evolution. The importance of temperature as a factor controlling bacteria efficiency is investigated with sensitivity experiments on the model parameterizations. The different model behavior and pelagic ecosystem structure developed by the model at the three locations can be attributed to the local hydrodynamical features and interactions with external inputs of nutrients. The onset of the winter/spring bloom in the climatological simulations is primarily driven by local stratification conditions. During summer the major carbon-transfer pathway developed by the model is the microbial web at all the sites, indicating that a large fraction of organic matter is processed through bacteria during productive periods, as suggested by field observations. The site directly influenced by riverine inputs differs from the others, showing a more alternate shifting among trophic pathways. Applying the conceptual scheme proposed by Legendre and Rassoulzadegan (Ophelia, 41, 153-172, 1995), it can be recognized as a herbivorous spring phase tightly followed by a microbial loop development, a summer microbial web phase and a multivorous trophic web pattern during autumn with a subsequent recovery of microbial processes. Results are discussed in terms of regime shifting from transient to stable water column conditions.Key words. Oceanography: general (continental shelf processes; numerical modelling) – Oceanography: biological and chemical (biochemistry and food chains)

Status, trends and the future of the marine pelagic ecosystem

Globally, humans impact environments and ecosystems faster than they become aware of their effects. The marine pelagic ecosystem includes a tremendously large and diverse environment, which might accordingly be considered to be resilient to externally forced changes, whether from humans or climate. This review considers that general hypothesis by pursuing two objectives. The first is to document the current status of and recent anthropogenic impacts on the marine pelagic ecosystem, with emphasis on the epipelagic zone (0-200 m) where organisms are concentrated and human impacts have been greatest. It shows that humans have proven capable of assuming the role of top carnivore in pelagic ecosystems where living resources are attractive and financially amenable to exploitation, and that overexploitation is the rule under such circumstances. Other anthropogenic activities associated with changes in various marine pelagic ecosystems, such as increased diseases, mortalities, extinctions, habitat invasions, and species replacements, function as sentinels and indicate that portions of the pelagic ecosystem are under considerable stress. It is argued that, without attention, these problems can be expected to worsen up to the year 2025 and beyond. In addition to a comprehensive evaluation of status and trends relating to conservation of the marine pelagic ecosystem, a second major objective is to evaluate whether current paradigms of ecosystem function are sufficient to improve the ability of the scientific community to predict future changes and to recommend relevant management strategies. This review differs from previous ones by proposing that current conceptual models have failed to provide the basis for accurately predicting patterns and features of pelagic communities, notably why specific organisms occur where and when they do. It is argued that predation pressure is shaped by natural selection in the sea as on land, and that it influences organism behaviour, life history strategy and morphology, all of which determine marine pelagic ecosystem structure, and therefore should be used to interpret function. From this perspective, attempting to understand present patterns and predict the future of marine pelagic ecosystems, without understanding the intertwined roles of evolution and predation in forging contemporary pelagic communities, is a hopeless endeavour. It is proposed that both perspectives, resource availability and predation pressure, be incorporated into a revised paradigm of pelagic ecosystem structure and function, a necessity if policies are to predict anthropogenic impacts and environmental conservation is to be effective.

The Southern Ocean Iron RElease Experiment (SOIREE)—introduction and summary

Abstract This volume is dedicated to the Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron fertilisation experiment performed in the polar waters of the Southern Ocean. SOIREE was an interdisciplinary study involving participants from six countries, and took place in February 1999 south of the Polar Front in the Australasian-Pacific sector of the Southern Ocean. Approximately 3800 kg of acidified FeSO4.7H2O and 165 g of the tracer sulphur hexafluoride (SF6) were added to a 65-m deep surface mixed layer over an area of ∼50 km2. Initially, mean dissolved iron concentrations were ∼2.7 nM, but decreased to ambient levels within days, requiring subsequent additions of 1550–1750 kg of acidified FeSO4.7H2O on days 3, 5 and 7 of the experiment. During the 13-day site occupation there were iron-mediated increases in phytoplankton growth rates, with marked increases in chlorophyll a (up to 2 μg l−1) and production rates (up to 1.3 g C m−2 d−1). These resulted in subsequent changes in the pelagic ecosystem structure, and in the cycling of carbon, silica and sulphur, such as a 10% drawdown of surface CO2. The SOIREE bloom persisted for >40 days following our departure from the site, as observed via SeaWiFS remotely sensed observations of Ocean Colour. Papers in this volume report in detail on aspects of this study, from the oceanographic setting of the experiment to a modelling simulation of the SOIREE bloom. A CD-ROM accompanies this volume and contains the main SOIREE datasets and ancillary information including the pre-experiment ‘desktop’ database study for site-selection, and satellite images of the SOIREE bloom.

Pelagic ecosystem structure and functioning in the subtropical front region east of New Zealand in austral winter and spring 1993

Data are synthesized on biomass and fluxes of components of the pelagic food web, downward particulate flux, and associated physical and chemical oceanographic parameters in austral winter and spring in the Subtropical Front (STF) region east of New Zealand. All four food web types (sensu Legendre and Rassoulzadegan, Ophelia, 41, 153-172, 1995) are represented in this region. The STF food web is classified as 'multivorous' in spring and 'herbivorous'/'multivorous' in winter, even though grazing was dominated by microzooplankton. Based on relatively few data over each season, STF community biomass was dominated by large phytoplankton cells (>20 µm) in both seasons. Post- bloom conditions were sampled in spring and highest fucoxanthin vertical fluxes were recorded at this time. A phytoplankton sedimentation event may have occurred just after our mid-October sampling. Subtropical (ST) waters exhibited pronounced seasonality in all plankton compartments. The ST food web is classified as 'multivorous' in both winter and spring. Organic flux in sediment traps was highest in ST water relative to subantarctic (SA) and STF waters. Although the ST data coverage was limited, high spring algal specific growth rates, the large change in total community biomass between winter and spring, and small seasonal differences in early winter and spring macronutrient concentrations suggest that the structure and functioning of this ecosystem depended on the macronutrients entrained during deep winter mixing. Algal populations in SA high-nitrate-low-chlorophyll waters (HNLC) were dominated by small cells. Heterotrophic bacteria dominated community biomass in SA waters in winter and equalled phytoplankton biomass in spring. In contrast to Legendre and Rassoulzadegan's classification of the subarctic Pacific HNLC food web as 'multivorous', the SA food web is classified as 'microbial' tending towards 'microbial loop' in winter (i.e. there was greater carbon uptake by bacteria than phytoplankton) and 'microbial' in spring. Chlorophyll (Chl) a levels in spring were similar to those in winter, but because of a doubling of the C:Chl ratio, algal biomass (as carbon) was 2-fold higher in SA waters in spring, compared with winter. This seasonal increase in algal carbon biomass in spring occurred in spite of a >5-fold increase from winter to spring in microzooplankton biomass, challenging the notion of grazer control of algal biomass. Our findings are compared with Russian data collected in the South Pacific STF region in summer 1985. Aspects of ecosystem func- tioning in the STF region east of New Zealand may hold for other longitudes in the Southern Ocean, although it is likely that some characteristics are regional in nature.

Pelagic ecosystem structure: Heterotrophic compartments of the plankton and their relationship to plankton size fractions 1

Pelagic ecosystem structure: Heterotrophic compartments of the plankton and their relationship to plankton size fractions 1