Pelagic Systems Research Articles

貧酸素海域の生態系評価を目的とした内湾複合生態系モデル"ZAPPAI(雑俳)"の開発と適用 : 干潟創生、浚渫・覆砂、流入負荷削減施策に対する東京湾生態系の自律的応答と赤潮に対する耐性

貧酸素海域の生態系評価を目的とした内湾複合生態系モデル"ZAPPAI(雑俳)"の開発と適用 : 干潟創生、浚渫・覆砂、流入負荷削減施策に対する東京湾生態系の自律的応答と赤潮に対する耐性

First remains of Lepisosteiformes actinopterygians from the Jurassic of Monte Nerone area (Umbria-Marche Apennine, Italy)

Since the early nineteenth century, the structural high of Mt. Nerone in the Umbria-Marche Sabina Domain (UMS – Central/Northern Apennines, Italy) attracted scholars from all over Europe due to the wealth of fossil fauna preserved in a stunningly well-exposed Mesozoic sedimentary succession. Several geopalaeontological studies were focused on the abundant and diverse invertebrate fauna, whereas contributions dealing with Mesozoic vertebrates were to date virtually lacking. Recently, the first material referable to hybodont sharks, consisting of an articulated crushing dentition, was described from the area and referred to Asteracanthus cf. A. magnus. In this contribution, we report the first evidence of ginglymodians actinopterygians from the Upper Jurassic of Mt. Nerone. The material is represented by seven highly tritorial isolated teeth collected from three classic fossiliferous localities of the area (i.e. Pian del Sasso, Fosso Pisciarello, I Ranchi). The general morphology of the material under study allow us to conservatively refer the teeth to the Neoginglymodi, a clade formed by Lepisosteiformes and Semionotiformes. The occurrence of durophagous organisms, to date represented by hybodont sharks and lepisosteiformes fishes, reveals interesting palaeoecological scenarios characterizing the pelagic carbonate platform-basin system of Mt. Nerone, which were most likely triggered by large-scale geodynamic processes. The complex submarine palaeotopography, inheritance of the Western Tethys Early Jurassic rifting, aroused the establishment of new infaunal and epifaunal communities opening up unexplored trophic niches for durophagous predators. The particular geodynamic setting of the UMS Domain, consisting of predominantly interconnected structural highs and lows, and the relative evolution of a diverse invertebrate fauna characterized by terebratulid, ostreids, limids, brachiopods, crustaceans, and gastropods, attracted both hybodontids anddurophagous ginglymodians in the Tethyan Realm, influencing their dispersal during the Late Jurassic.

Zooplankton selectivity and nutritional value of phytoplankton influences a rich variety of dynamics in a plankton population model.

Mathematical modeling may be an excellent tool to analyze and explain complex biological phenomena. In this paper, we use a mathematical model to reveal various interesting dynamical features of phytoplankton-zooplankton interaction and attempt to explain the reason for contrasting dynamics shown by different laboratory and field experiments. Our study shows that the phytoplankton-zooplankton interaction in a pelagic system is very complex and the plankton dynamics, including the bloom phenomenon, strongly depends on the selective predation of zooplankton and the nutritional value of phytoplankton. The study supports the existing hypothesis that decoupling at the plant-animal interface may occur due to strong fish predation on zooplankton. In addition, we argue that decoupling of the food chain may also occur under low to intermediate nutrient inflow if zooplankton feeds on phytoplankton having lower nutritional value. It is also shown that nutrient enrichment can destabilize an otherwise stable system if zooplankton feeds on highly nutritious prey, but unable to destabilize the system if zooplankton feeds on low-nutritious prey. This may be one possible explanation to the longstanding question: Why do some experiments show the paradox of enrichment and others do not?

Quantitative assessment of the environmental risk due to climate change-driven coastline recession: A case study in Trincomalee coastal area, Sri Lanka

Climate change may exacerbate the environmental damage due to coastal hazards and increase associated risks. This would result in degradation of coastal wetlands, especially in developing countries due to the lack of sufficient resilience to coastal hazards such as inundation and coastal erosion. Environmental damages will lead to a decrease in services provided by the coastal wetland ecosystems that contribute to human wellbeing. To provide better insights on the little known issue of quantifying potential climate change-driven environmental risk, this article presents a stepwise approach to quantify coastline recession-driven risk associated with the tourism service provided by the Trincomalee beaches, dunes and pelagic system (Indian Ocean) along the East coast of Sri Lanka in 2110. To achieve this, here we first estimate the loss value of the tourism service due to sea level rise (SLR) and storm induced erosion in 2110, by using economic valuation techniques followed by a scenario-based approach. This is followed by the quantification of the environmental risk value by combining the result of the aforementioned loss value with the exceedance probability of coastal erosion derived from a prior study. Results show a medium environmental risk value ranging from 0 to 11,000 US$/Ha of beach area due to complete beach loss by 2110. This indicates that SLR and storm induced erosion in 2110 is not likely to pose a very high environmental risk associated with the tourism service of ecosystems in Trincomalee coasts. The approach presented in this study can be directly applied in other coastal areas of interest to gain a better understanding of the likely costs of climate change driven environmental risk, which is an emerging topic in coastal zone management.

Estimating contributions of pelagic and benthic pathways to consumer production in coupled marine food webs.

Pelagic and benthic systems usually interact, but their dynamics and production rates differ. Such differences influence the distribution, reproductive cycles, growth rates, stability and productivity of the consumers they support. Consumer preferences for, and dependence on, pelagic or benthic production are governed by the availability of these sources of production and consumer life history, distribution, habitat, behavioural ecology, ontogenetic stage and morphology. Diet studies may demonstrate the extent to which consumers feed on prey in pelagic or benthic environments. But they do not discriminate benthic production directly supported by phytoplankton from benthic production recycled through detrital pathways. The former will track the dynamics of phytoplankton production more closely than the latter. We develop and apply a new analytical method that uses carbon (C) and sulphur (S) natural abundance stable isotope data to assess the relative contribution of pelagic and benthic pathways to fish consumer production. For 13 species of fish that dominate community biomass in the northern North Sea (estimated >90% of total biomass), relative modal use of pelagic pathways ranged from <25% to >85%. Use of both C and S isotopes as opposed to just C reduced uncertainty in relative modal use estimates. Temporal comparisons of relative modal use of pelagic and benthic pathways revealed similar ranking of species dependency over 4years, but annual variation in relative modal use within species was typically 10%-40%. For the total fish consumer biomass in the study region, the C and S method linked approximately 70% and 30% of biomass to pelagic and benthic pathways, respectively. As well as providing a new method to define consumers' links to pelagic and benthic pathways, our results demonstrate that a substantial proportion of fish biomass, and by inference production, in the northern North Sea is supported by production that has passed through transformations on the seabed.

An ecosystem model for estimating shellfish production carrying capacity in bottom culture systems

A production model has been developed to estimate scallop bottom culture carrying capacity. The model coupled pelagic system, scallop energetics and population dynamics with hydrodynamics. The methodology considers temperature and particle organic carbon (POC) were used to determine the carrying capacity through scallop growth, production and economic benefit. The model was applied to the Zhangzidao culture area in the North Yellow Sea, the largest marine ranching of Japanese scallops Patinopecten yessoensis in China. The studied area was divided into four boxes according to the temperature and depth gradients. The results showed that the model could successfully capture the main characteristics of the observed environmental variables, POC concentrations and biological behaviour of the cultured scallops. The food availability showed seasonal variations. The low temperature had also a great effect on the growth of the scallops in the south area during period of January to July. In contrary, the high temperature would cause high mortality rate of the scallops in the north area in September. Based on the production and profit, the optimal seeding densities were estimated to be 82,500, 105,000, 82,500, 60,000 in. ha−1, respectively in the Boxes1, 2, 3 and 4. There is the potential to increase the seeding density in the northeast area (Box 2), while the southwest area has already exceeded its capacity. The successful application of the model suggests that it could be used as a management tool for estimating the production carrying capacity in the bottom culture systems.

Stoichiometric food chain model on discrete time scale.

Stoichiometry-based models can yield many new insights into producer - grazer systems. Many interesting results can be obtained from models continuous in time. There raises the question of how robust the model predictions are to time discretization. A discrete stoichiometric food-chain model is analyzed and compared with a corresponding continuous model. Theoretical and numerical results show that the discrete and continuous models have many properties in common but differences also exist. Stoichiometric impacts of producer nutritional quality also persist in the discrete system. Both types of models can exhibit qualitatively different behaviors with the same parameter sets. Discretization enlarges the parameter ranges for the existence of chaotic dynamics. Our results suggest that the stoichiometric mechanisms are robust to time discretization and the nutritional quality of the producer can have dramatic and counterintuitive impacts on population dynamics, which agrees with the existing analysis of pelagic systems.

First report of vampyrellid predator-prey dynamics in a marine system.

We report for the first time the in situ dynamics of a vampyrellid in a marine system. A high sampling frequency (twice-weekly) was applied in a tropical eutrophic lagoon (Rio de Janeiro, Brazil) for 5 years (2012-2016). The vampyrellid Hyalodiscus sp. specifically fed on the diatom Chaetoceros minimus during a short time window (~3 months), although the prey was intermittently detected as the dominant phytoplanktonic species over a longer period (~1 year). A classic Lotka-Volterra predator-prey dynamic was observed between the two partners, with a significant modification of the short-term oscillations of the prey. Specific abiotic preferences (i.e., relatively low temperature, intermediate salinity, and stratified conditions) associated with prey availability seemed to define this narrow temporal window of occurrence. Our results suggest that vampyrellids can be ecologically relevant in marine pelagic systems, with their impact on planktonic dynamics strongly depending on complex interactions between both biotic and abiotic factors.

Big fish eat small fish: implications for food chain length?

Food chains in the pelagic zones of oceans and lakes are longer than in terrestrial ecosystems. The perception of the pelagic food web has become increasingly complex by progressing from a linear food chain (phytoplankton — crustacean zooplankton — planktivorous fish — predatory fish) to a food web because of an increasing appreciation of microbial trophic pathways, side-tracks by gelatinous zooplankton and a high prevalence of omnivory. The range of predator:prey size ratios by far exceeds the traditionally assumed range of 10:1 to 100:1, from almost equal length to 105:1. The size ratios between primary consumers and top predators are 3½ orders of magnitude bigger in pelagic than in terrestrial food webs. Comparisons between different pelagic ecosystems support ecosystem size as an important factor regulating the maximal trophic level, while energy limitation of the number of trophic levels is less well supported. An almost 1:1 relationship between ingestion by predators and prey mortality and a better chemical match between primary producer and herbivore biomass are further distinctive features of the pelagic food web whose role in explaining the higher number of trophic levels in pelagic systems needs further examination.

In situ observations lead to the discovery of the large ctenophore Kiyohimea usagi (Lobata: Eurhamphaeidae) in the eastern tropical Atlantic.

We report on the first records of Kiyohimea usagi Matsumoto Robison 1992 (Ctenophora; Eurhamphaeidae) in the Atlantic Ocean. This large, fragile ctenophore cannot be captured by nets, and can only be studied in its natural habitat, the pelagic ocean. In the eastern Atlantic, in the Cape Verde region, in situ observations were obtained using the manned submersible JAGO and a towed pelagic observation system. Between 2015 and 2018 we documented 10 individuals which were encountered between 47-590 m depth. A description of the gastrovascular canal system is provided and potential feeding behavior is discussed. Our study confirms how in situ observations in the poorly explored pelagic realm will lead to the discovery of relatively large and previously undocumented fauna.

Hide and seek in the Bay of Biscay—a functional investigation of marine megafauna and small pelagic fish interactions

AbstractPrey and predator distributions influence one another. Understanding the scale and the orientation of predator–prey spatial correlations is crucial in foraging ecology. Growing evidence suggests that predator–prey interactions are more constrained by functional characteristics of both the predator and the prey. Unfortunately, in marine pelagic systems, the scale and orientation of spatial correlations between predators and prey have been only little explored from a functional point of view. We tested the existence of fine-scale association between predators and fish functional groups. Visual predator sightings and acoustic fish records were collected synchronously during oceanographic surveys from 2004 to 2014. Prey biomass was integrated by nautical miles and split into four size classes (&amp;lt;10 cm; 10–20 cm; 20–30 cm; &amp;gt;30 cm) and two depth layers (surface, deep). We computed the relative biomass by prey size and depth category from 0 to 12 nm around predator sightings to determine the predators’ proximity to local prey biomass. Two cetaceans (common, bottlenose dolphins) and three seabirds (northern gannets, auks, northern fulmars) were studied. No association was found in fulmars, indicating they probably do not feed on considered fishes in the area. Gannets and auks were positively correlated with local prey biomass for sizes &amp;lt;20 cm at both depth layers. Significant negative relationships were found between common dolphins and prey size classes &amp;lt;20 cm at both depth layers, and between bottlenose dolphins and all size ranges at the deeper layer. Our results suggest that the fine-scale spatial overlap of predator and prey is influenced by their functional traits, and that prey exhibit predator avoidance behaviour in presence of swimming predators but not of flying ones.

A Model‐Based Analysis of Physical and Biogeochemical Controls on Carbon Exchange in the Upper Water Column, Sea Ice, and Atmosphere in a Seasonally Ice‐Covered Arctic Strait

AbstractIn this study, we consider a 1‐D model incorporating both sea ice and pelagic systems in order to assess the importance of various processes on the vertical transport and exchange of carbon in the seasonally ice‐covered marine Arctic. The model includes a coupled ice‐ocean ecosystem, a parameterization of ikaite precipitation and dissolution, a formulation for ice‐air carbon exchange, and a formulation for brine rejection and freshwater dilution of dissolved inorganic carbon (DIC) and total alkalinity (TA) associated with ice growth and melt. Sensitivity analyses illustrate that (1) the pelagic ecosystem accounts for more than half of the net ocean carbon uptake, but ice algae have little effect on the air‐sea exchange in the standard run; (2) inclusion of ikaite precipitation and dissolution do not strongly affect the net ocean carbon uptake for concentrations within the observed range but can become important for larger concentrations; (3) varying DIC and TA in the ice by equal amounts, or varying brine deposition depth, does not affect the net ocean carbon uptake, because the coincident changes in TA and DIC concentrations at the sea surface serve to counteract one another with respect to sea surface pCO2; and (4) the proportions of carbon released to the water column (versus to the atmosphere) during ice growth and melt are important quantities to constrain in order to determine the contribution of the combined ice‐ocean system to oceanic uptake of atmospheric carbon.

Dependency of Antarctic zooplankton species on ice algae-produced carbon suggests a sea ice-driven pelagic ecosystem during winter.

How the abundant pelagic life of the Southern Ocean survives winter darkness, when the sea is covered by pack ice and phytoplankton production is nearly zero, is poorly understood. Ice-associated ("sympagic") microalgae could serve as a high-quality carbon source during winter, but their significance in the food web is so far unquantified. To better understand the importance of ice algae-produced carbon for the overwintering of Antarctic organisms, we investigated fatty acid (FA) and stable isotope compositions of 10 zooplankton species, and their potential sympagic and pelagic carbon sources. FA-specific carbon stable isotope compositions were used in stable isotope mixing models to quantify the contribution of ice algae-produced carbon (αIce ) to the body carbon of each species. Mean αIce estimates ranged from 4% to 67%, with large variations between species and depending on the FA used for the modelling. Integrating the αIce estimates from all models, the sympagic amphipod Eusirus laticarpus was the most dependent on ice algal carbon (αIce : 54%-67%), and the salp Salpa thompsoni showed the least dependency on ice algal carbon (αIce : 8%-40%). Differences in αIce estimates between FAs associated with short-term vs. long-term lipid pools suggested an increasing importance of ice algal carbon for many species as the winter season progressed. In the abundant winter-active copepod Calanus propinquus, mean αIce reached more than 50% in late winter. The trophic carbon flux from ice algae into this copepod was between 3 and 5mgCm-2 day-1 . This indicates that copepods and other ice-dependent zooplankton species transfer significant amounts of carbon from ice algae into the pelagic system, where it fuels the food web, the biological carbon pump and elemental cycling. Understanding the role of ice algae-produced carbon in these processes will be the key to predictions of the impact of future sea ice decline on Antarctic ecosystem functioning.

Accounting for activity respiration results in realistic trophic transfer efficiencies in allometric trophic network (ATN) models

Allometric trophic network (ATN) models offer high flexibility and scalability while minimizing the number of parameters and have been successfully applied to investigate complex food web dynamics and their influence on food web diversity and stability. However, the realism of ATN model energetics has never been assessed in detail, despite their critical influence on dynamic biomass and production patterns. Here, we compare the energetics of the currently established original ATN model, considering only biomass-dependent basal respiration, to an extended ATN model version, considering both basal and assimilation-dependent activity respiration. The latter is crucial in particular for unicellular and invertebrate organisms which dominate the metabolism of pelagic and soil food webs. Based on metabolic scaling laws, we show that the extended ATN version reflects the energy transfer through a chain of four trophic levels of unicellular and invertebrate organisms more realistically than the original ATN version. Depending on the strength of top-down control, the original ATN model yields trophic transfer efficiencies up to 71% at either the third or the fourth trophic level, which considerably exceeds any realistic values. In contrast, the extended ATN version yields realistic trophic transfer efficiencies ≤ 30% at all trophic levels, in accordance with both physiological considerations and empirical evidence from pelagic systems. Our results imply that accounting for activity respiration is essential for consistently implementing the metabolic theory of ecology in ATN models and for improving their quantitative predictions, which makes them more powerful tools for investigating the dynamics of complex natural communities.

Why the damselfish Chromis chromis is a key species in the Mediterranean rocky littoral-a quantitative perspective.

The damselfish Chromis chromis is typically the most abundant fish species in the rocky littoral environment of the Mediterranean Sea, where it feeds in huge shoals on incoming zooplankton and rests near the seabed each night. Research has shown that large populations of C. chromis play a fundamental role by transferring carbon, nitrogen and phosphorus directly from the pelagic system to the littoral in the form of solid and liquid wastes and therefore that this species acts as a vital conduit for nutrients. In addition, C. chromis represents a major prey item for predatory fish and seabirds, it has been shown to exert significant predation pressure on coastal zooplankton populations, but it is also a major consumer of fish eggs and hence may have a strong influence on the dynamics of other fish species. In this review paper the contention is that C. chromis represents a key species in the Mediterranean Sea rocky littoral. This view is supported by quantitative evidence that has been amassed over a number of years, but is collated here for the first time. Chromis chromis is of only minor importance to commercial fisheries (it is usually discarded), but trends in population abundance of this species can be indicative of levels of fishing pressure on other commercial species. It has been demonstrated that where predator numbers are able to recover from intensive fishing (e.g.in marine protected areas; MPAs), this is often mirrored by lower densities of C. chromis. Several papers have championed C. chromis as a possible indicator species for human disturbance. As well as being directly and indirectly affected by fishing, the species is also sensitive to a number of other anthropogenic pressures, most notably poor water quality and noise pollution.

How biophysical interactions associated with sub- and mesoscale structures and migration behavior affect planktonic larvae of the spiny lobster in the Juan Fernández Ridge: A modeling approach

The Juan Fernández Ridge (JFR) is a chain of topographical elevations in the eastern South Pacific (∼33–35°S, 76–81.5°W). Rich in endemic marine species, this ridge is frequently affected by the arrival of mesoscale eddies originating in the coastal upwelling zone off central-southern Chile. The impacts of these interactions on the structure and dynamics of the JFR pelagic system have, however, not been addressed yet. The present model-based study is focused on the coupled influence of mesoscale-submesoscale processes and biological behavior (i.e., diel vertical migration) on the horizontal distribution of planktonic larvae of the spiny lobster (Jasus frontalis) around the JFR waters. Two case studies were selected from a hydrodynamic Regional Ocean Modeling System to characterize mesoscale and submesoscale structures and an Individual-based model (IBM) to simulate diel vertical migration (DVM) and its impact on the horizontal distribution and the patchiness level. DVM behavior of these larvae has not been clearly characterized, therefore, three types of vertical mechanisms were assessed on the IBM: (1) no migration (LG), (2) a short migration (0–50 m depth, DVM1), and (3) a long migration (10–200 m depth, DVM2). The influence of physical properties (eddy kinetic energy, stretching deformation and divergence) on larval aggregation within meso and submesoscale features was quantified. The patchiness index assessed for mesoscale and submesoscale structures showed higher values in the mesoscale than in the submesoscale. However, submesoscale structures revealed a higher accumulation of particles by unit of area. Both vertical migration mechanisms produced larger patchiness indices compared to the no migration experiment. DVM2 was the one that showed by far the largest aggregation of almost all the aggregation zones. Larval concentrations were highest in the submesoscale structures; these zones were characterized by low eddy kinetic energy, negative stretching deformation, and slight convergence. Stretching deformation flow appeared to be triggered by the eddy-eddy interactions and the Robinson Island barrier effect, and it likely promotes the aggregation of the spiny lobster larvae in the Juan Fernández system. These results highlighted the importance of the coupled effect of physical (mesoscale and submesoscale oceanographic features) and biological processes (DVM) in the generation of larval patchiness and concentration of spiny lobster larvae around the JFR, which could be key for their survival and retention in those waters.

Plankton dynamics across the freshwater, transitional and marine research sites of the LTER-Italy Network. Patterns, fluctuations, drivers

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems.This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity.Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.

Terrestrial fatty acids as tracers of finfish aquaculture waste in the marine environment

AbstractWaste from open‐cage aquaculture flows directly into the marine environment from uneaten feeds, faecal material and dissolved nutrients. Sustainable management outcomes are regularly based on the dispersal patterns of the waste, with biochemical tracing a key tool in understanding the footprint of aquaculture. We examined the use of fatty acid (FA) analysis to trace aquaculture waste for this purpose, with the aim of identifying specific biomarkers for environmental applications, as well as identifying challenges that are regularly encountered. Overall, the widespread use of terrestrial‐based oils in the production of marine aquaculture feeds has increased the use of FA biomarkers to trace aquaculture waste across benthic and pelagic systems, in vertebrates, invertebrates and environmental samples such as sediment and seston. A combination of linoleic acid (LA), oleic acid (OA) and α‐linolenic acid (ALA), which are dominant C18 FA in terrestrial seed and animal oils, is the most commonly used biomarkers, along with overall shifts in FA profile related to diet. A challenge regularly encountered, particularly in lower trophic species, was the capacity of an organism to biomodify or selectively spare certain dietary FA, which can mask assimilation of aquaculture‐derived FA. In these species, controlled feeding studies are needed to properly understand FA metabolism and to ensure correct interpretation of FA data collected from wild samples. Overall, as FA biomarkers have the capacity to link aquaculture waste in the environment with potential effect, they are a valuable and increasingly applied tool for examining the overall influence of aquaculture in marine ecosystems.

Plankton community respiration and bacterial metabolism in a North Atlantic Shelf Sea during spring bloom development (April 2015)

Spring phytoplankton blooms are important events in Shelf Sea pelagic systems as the increase in carbon production results in increased food availability for higher trophic levels and the export of carbon to deeper waters and the sea-floor. It is usually accepted that the increase in phytoplankton abundance and production is followed by an increase in plankton respiration. However, this expectation is derived from field studies with a low temporal sampling resolution (5–15 days). In this study we have measured the time course of plankton abundance, gross primary production, plankton community respiration, respiration of the plankton size classes (>0.8 µm and 0.2–0.8 µm) and bacterial production at ≤5 day intervals during April 2015 in order to examine the phasing of plankton autotrophic and heterotrophic processes. Euphotic depth-integrated plankton community respiration increased five-fold (from 22 ± 4 mmol O2 m−2 d−1 on 4th April to 119 ± 4 mmol O2 m−2 d−1 on 15th April) at the same time as gross primary production also increased five-fold, (from 114 ± 5 to 613 ± 28 mmol C m−2 d−1). Bacterial production began to increase during the development of the bloom, but did not reach its maximum until 5 days after the peak in primary production and plankton respiration. The increase in plankton community respiration was driven by an increase in the respiration attributable to the >0.8 µm size fraction of the plankton community (which would include phytoplankton, microzooplankton and particle attached bacteria). Euphotic depth-integrated respiration of the 0.2–0.8 µm size fraction (predominantly free living bacteria) decreased and then remained relatively constant (16 ± 3 – 11 ± 1 mmol O2 m−2 d−1) between the first day of sampling (4th April) and the days following the peak in chlorophyll-a (20th and 25th April). Recent locally synthesized organic carbon was more than sufficient to fulfil the bacterial carbon requirement in the euphotic zone during this productive period. Changes in bacterial growth efficiencies (BGE, the ratio of bacterial production to bacterial carbon demand) were driven by changes in bacterial production rates increasing from <30 ± 14% on 4th April to 51 ± 11% on 25th of April. This study therefore shows a concurrent rather than a phased increase in primary production and community respiration attributable to cells >0.8 µm during the development of the spring bloom, followed 5 days later by a peak in bacterial production. In addition, the size fractionated respiration rates and high growth efficiencies suggest that free living bacteria are not the major producers of CO2 before, during and a few days after this shelf sea spring phytoplankton bloom.

Transfer of lipid molecules and polycyclic aromatic hydrocarbons to open marine waters by dense water cascading events

Settling particles were collected by a set of moored sediment traps deployed during one year in the western Gulf of Lion along Cap de Creus and Lacaze-Duthiers submarine canyons and on the adjacent southern open slope. These traps collected particles during periods of pelagic settling and also during events of deep water flushing by dense shelf water cascading (DSWC). Analyses of lipid biomarkers (n-alkanes, n-alkan-1-ols, sterols and C37-C38 alkenones) and polycyclic aromatic hydrocarbons (PAHs) showed much higher transfer of terrestrial lipids and PAHs to open deep waters during DSWC than in the absence of cascading. The area of highest lateral fluxes was mostly located at 1000m depth but also at 1500m depth and extended along the canyons and to the adjacent slope. Higher fluxes were observed near the bottom (30m above bottom; mab) than at intermediate waters (500 mab) which is consistent with the formation and sinking of dense water over the continental shelf, and its transport through the canyons towards the continental slope and deep basin. DSWC involved the highest settling fluxes of terrestrial lipids and PAHs ever described in marine continental slopes and the pelagic domain, as illustrated by peak values of C23-C33 odd carbon numbered alkanes (405ngm−2d−1), C22-C32 even carbon numbered alkan-1-ols (850ngm−2d−1), β-sitosterol+sitostanol (4800ngm−2d−1) and PAHs (55µgm−2d−1). The algal lipids also showed higher transfer to deep waters during DSWC but to a lower extent than the terrigenous compounds. However, the C37-C38 alkenones constituted an exception and their settling fluxes were not influenced by DSWC. The lack of influence of the DSWC on the C37-C38 alkenone settling is consistent with absence of haptophyte algal inputs from the continental shelf and reinforces the reliability of these molecules for palaeothermometry and palaeoproductivity measurements in pelagic systems.