Shelf Food Webs Research Articles

Biodiversity and functioning of mesozooplankton in a changing Ross Sea

Literature on Ross Sea zooplankton is limited, although it is the most productive system and has the highest biomass of phytoplankton in the Southern Ocean. Energy transfer within the food web and response of continental shelf food web to climate change depend on the knowledge of the density and distribution of zooplankton. We evaluated its density, composition, spatial distribution and their potential relations with environmental factors and specific water mass presence. Particular attention was given to copepods. Mesozooplankton samples were collected between 14 January and February 11, 2017 from 14 stations in the western Ross Sea and Terra Nova Bay, while other micronekton samplings were completed at 6 stations. Results highlighted three pools of stations: one inside Terra Nova Bay with the highest densities, one on the platform with lower mean density, and the third represented by a N–S transept at 175°E with the lowest mean density. This partitioning of the region fit with the grouping of stations according to a ternary plot based on the different percentages of water masses in each station and reflected the similarity of zooplankton. The presence of specific water masses and their contribution in the upper 200 m influences zooplankton biodiversity and density. A phylogenetic tree of the zooplankton was constructed to analyse the distance among the observed taxa. The area located near the coast exhibited the highest degree of phylogenetic overdispersion. In coastal waters, typical copepod species constituted the main part of the neritic zooplankton of Terra Nova Bay and Ross Sea. Oithona similis was the most abundant species both in coastal and offshore sectors, followed by pteropods, polychaetes and euphausiids.

The impact of advection on a Subarctic fjord food web dominated by the copepod Calanus finmarchicus

Fjord and shelf food webs are frequently supplemented by the advection of external biomass, which in high-latitude seas often comes in the form of lipid-rich copepods that can support a wide range of fish species, including Northeast Arctic cod (Gadus morhua). A seasonal match or mismatch at the lower trophic levels (phytoplankton and zooplankton) is central in determining how much energy and biomass is available for higher trophic levels (fish). Here, we quantify the inflow of the copepod Calanus finmarchicus into the Vestfjorden fjord system using high-resolution measurements of ocean currents and zooplankton (laser optical plankton counter). We evaluate a spatio-temporal match/mismatch between the phytoplankton bloom and Calanus and assess the input of advected copeods at the lower trophic level fjord and shelf food web based on an integrative approach employing stable isotope analyses (C, N), fatty acid trophic marker analyses, and biovolume spectrum analyses. Our results suggest two different sources of the Calanus population in the fjord/shelf system: one fraction overwintered locally and started ascending early to feed on the phytoplankton bloom that peaked around April 11. The other fraction had only recently (end of April) been and still was being advected from the oceanic overwintering habitats. Ca. 119 g C/s of Calanus were advected into the fjord, comparable to the biomass of Calanus advected into an Arctic fjord, and the mesozooplankton community was dominated by the copepod. The fjord food web was tightly coupled between the phytoplankton spring bloom, the local part of the Calanus population (trophic level 1.8–2.4) and cod larvae (high levels of wax esters). On the shelf, our results suggest that the impact of advected Calanus in the food web is at its starting point (low trophic level, large difference of δ13C of POM and Calanus). We highlight important factors that can contribute to the successful spawning of Northeast Arctic cod: an extended phytoplankton bloom that can support both locally and advected Calanus, which in turn can supply the essential nauplii prey for first-feeding cod larvae.

Unveiling the wasp-waist structure of the Falkland shelf ecosystem: the role of Doryteuthis gahi as a keystone species and its trophic influences

Abstract The Falkland Shelf is a highly productive ecosystem in the Southwest Atlantic Ocean. It is characterized by upwelling oceanographic dynamics and displays a wasp-waist structure, with few intermediate trophic-level species and many top predators that migrate on the shelf for feeding. One of these resident intermediate trophic-level species, the Patagonian longfin-squid Doryteuthis gahi, is abundant and plays an important role in the ecosystem. We used two methods to estimate the trophic structure of the Falkland Shelf food web, focusing on the trophic niche of D. gahi and its impacts on other species and functional groups to highlight the importance of D. gahi in the ecosystem. First, stable isotope measurements served to calculate trophic levels based on an established nitrogen baseline. Second, an Ecopath model was built to corroborate trophic levels derived from stable isotopes and inform about trophic interactions of D. gahi with other functional groups. The results of both methods placed D. gahi in the centre of the ecosystem with a trophic level of ~ 3. The Ecopath model predicted high impacts and therefore a high keystoneness for both seasonal cohorts of D. gahi. Our results show that the Falkland Shelf is not only controlled by species feeding at the top and the bottom of the trophic chain. The importance of species feeding at the third trophic level (e.g. D. gahi and Patagonotothen ramsayi) and observed architecture of energy flows confirm the ecosystem's wasp-waist structure with middle-out control mechanisms at play.

TrophicCS: Spatialized trophic data of the Celtic Sea continental shelf food web.

Understanding the dynamics of species interactions for food (prey-predator, competition for resources) and the functioning of trophic networks (dependence on trophic pathways, food chain flows, etc.) has become a thriving ecological research field in recent decades. This empirical knowledge is then used to develop population and ecosystem modeling approaches to support ecosystem-based management. The TrophicCS data set offers spatialized trophic information on a large spatial scale (the entire Celtic Sea continental shelf and upper slope) for a wide range of species. It combines ingested prey (gut content analysis) and a more integrated indicator of food sources (stable isotope analysis). A total of 1337 samples of large epifaunal invertebrates (bivalve mollusks and decapod crustaceans), zooplankton, fish, and cephalopods, corresponding to 111 taxa (94% determined at the species level), were collected and analyzed for stable isotope analysis of their carbon and nitrogen content. Samples were collected between 2014 and 2016, mostly during the month of November and between 57 and 516 m depth. Sample size varied between taxa (from 1 to 52), with 98 taxa having at least three samples. The gut contents of 1027 fish belonging to 10 commercially important species: black anglerfish (Lophius budegassa), white anglerfish (Lophius piscatorius), blue whiting (Micromesistius poutassou), cod (Gadus morhua), haddock (Melanogrammus aeglefinus), hake (Merluccius merluccius), megrim (Lepidorhombus whiffiagonis), plaice (Pleuronectes platessa), sole (Solea solea), and whiting (Merlangius merlangus) were analyzed. Sampling occurred in November 2014 and 2015. The gut content data set contains the occurrence of prey in gut, identified to the lowest taxonomic level possible. No prey were assigned for 274 empty gut contents. To consider potential ontogenetic diet changes, a large size range was sampled for each species. The TrophicCS data set was used to improve understanding of trophic relationships and ecosystem functioning in the Celtic Sea. Data are released under a CC-BY-NC-SA license, and please cite this paper when reusing the data.

Investigating the dynamics of methylmercury bioaccumulation in the Beaufort Sea shelf food web: a modeling perspective.

High levels of methylmercury (MeHg) have been reported in Arctic marine biota, posing health risks to wildlife and human beings. Although MeHg concentrations of some Arctic species have been monitored for decades, the key environmental and ecological factors driving temporal trends of MeHg are largely unclear. We develop an ecosystem-based MeHg bioaccumulation model for the Beaufort Sea shelf (BSS) using the Ecotracer module of Ecopath with Ecosim, and apply the model to explore how MeHg toxicokinetics and food web trophodynamics affect bioaccumulation in the BSS food web. We show that a food web model with complex trophodynamics and relatively simple MeHg model parametrization can capture the observed biomagnification pattern of the BSS. While both benthic and pelagic production are important for transferring MeHg to fish and marine mammals, simulations suggest that benthic organisms are primarily responsible for driving the high trophic magnification factor in the BSS. We illustrate ways of combining empirical observations and modelling experiments to generate hypotheses about factors affecting food web bioaccumulation, including the MeHg elimination rate, trophodynamics, and species migration behavior. The results indicate that population dynamics rather than MeHg elimination may determine population-wide concentrations for fish and lower trophic level organisms, and cause large differences in concentrations between species at similar trophic levels. This research presents a new tool and lays the groundwork for future research to assess the pathways of global environmental changes in MeHg bioaccumulation in Arctic ecosystems in the past and the future.

Food web structure in relation to environmental drivers across a continental shelf ecosystem

AbstractQuantification of the physical and biological factors that influence the spatial structuring of food webs is central to inform effective resource management. We used baseline‐corrected stable isotope ratios (δ13C and δ15N) of 63 invertebrate and fish to investigate food web structure across a continental shelf gradient—the Celtic Sea Shelf in the Northeast Atlantic Ocean. Hierarchical clustering on δ13C and δ15N showed that the shelf food web is characterized by four trophic levels with trophic groups spread across pelagic and benthic trophic pathways. Four biomass‐weighted isotopic diversity metrics provided indicators on the status of the system, showing a relatively complex food web with high trophic redundancy at intermediate trophic levels suggesting resilience to disturbances. Two sets of statistical models, at the community scale and for each trophic group, identified five distinct trophic assemblages associated with different chlorophyll a concentrations, water depth, and bottom temperature. A cold, vertically mixed‐water assemblage over the outer shelf comprised the largest habitat and most diverse assemblage, highlighting the importance of cold productive conditions in the Celtic Sea. Trophic group model results were used to generate spatial area predictions to compare functioning of groups using isotopic overlap (similarity and nestedness) metrics. Isotopic niche area was larger (spanning two trophic levels) in shallow habitats, but not in habitats underlying high primary production or nutrient‐rich water masses, suggesting stronger benthic‐pelagic trophic coupling in inner shelf habitats. Results suggest that depth and intensity of pelagic production are major drivers of trophic structure and functioning of Celtic Sea communities.

Global Systematic Review of Methodological Approaches to Analyze Coastal Shelf Food Webs

The study of marine coastal food webs has a central role in the application of the integrated ecosystem approach for marine management. Changes in food webs caused by natural or anthropic drivers can lead to dramatic shifts in the overall structure and function of coastal marine ecosystems and deterioration of their services. The present review investigates the methodological approaches employed for the assessment of coastal shelf food webs at a global scale and highlights existing gaps and limitations. Out of 1652 published articles that initially met our search criteria, 880 passed the initial screening and 493 were found relevant and were fully analysed. The information extracted included the spatiotemporal coverage of the studies; the main methodological approaches utilized for the assessment of population state variables (i.e. biomass, size, abundance) and trophic levels; the biotic components and driving factors considered; indices used to describe the structure and functioning of coastal food webs; and main knowledge gaps. Results showed that most studies have been conducted at a subnational level, mostly in the Temperate Northern Atlantic marine realm. Overall, 54% of the studies provided quantitative information on food web structure. The most common methodological approach utilized was modelling (40%), followed by non-experimental-based correlations (30%), and natural or manipulative experiments (14%). Information on population state variables was provided by 69% of the studies, while 42% employed some of the following trophic level determination techniques: stable isotopes, gut contents, fatty acids, and molecular analysis, which were either combined or used in isolation. Specific natural or human drivers were incorporated in 76% of the studies, with fishing being the most common driver. Modelling approaches included multiple indices to describe food web attributes and/or the structure and functioning of coastal shelf ecosystems. Despite the great progress achieved through the development of new tools and techniques, food web analysis still suffers from important knowledge gaps and limitations of the methodological approaches, which are extensively discussed. The present review establishes a useful knowledge base to provide guidance for future research and assessments on coastal shelf food webs, and to support ecosystem-based management.

The occurrence and ecology of Renilla spp. over the southwest Atlantic continental shelf (28–34ºS)

Sea pansies are marine colonial cnidarians of the genus Renilla (Pennatulacea: Renillidae) and play a significant ecological role in the continental shelf food webs. Renilla species are endemic to the Americas and most have been reported in the southwest Atlantic. Austral winter expeditions were undertaken at random isobaths over the Pelotas Basin shelf, southernmost Brazil (28–34ºS), to report the current occurrence of Renilla spp. and the key factors controlling their distribution, as well as to infer the ecological interactions between sea pansies and other marine species. Renilla muelleri, R. musaica, and the endemic R. tentaculata were recorded over this shelf area. Their spatial distribution, as determined by a multivariate canonical correspondence analysis, was mainly associated with environmental parameters, such as bottom water temperature, salinity, and substrate composition. A high abundance of small individuals (both recruit and juvenile colonies) suggests the occurrence of recruitment events for some species. Renilla populations over this southwest Atlantic shelf can be negatively affected by ecological interactions occurring among sand dollars, and benthic and pelagic predators, in addition to human activities.

Sipunculan fauna in the Pacific Arctic region: a significant component of benthic infaunal communities

The Sipuncula show low species diversity in the Arctic Ocean, but they often occur in very high numbers and biomass, and can dominate shelf benthic communities. We describe here the patterns of sipunculan diversity, abundance, biomass, and species distribution in the Pacific Arctic, and the relation of those patterns to controlling environmental factors. Five species (including two subspecies) belonging to two families and three genera are known to be present in Pacific-influenced Arctic waters. Although sipunculans were found at only about 30% of sampled stations, they constituted, if present, up to about 10% of the total macrobenthic abundance, and over 80% of the total infaunal biomass, particularly in many locations in the Chukchi Sea. The mean sipunculan abundance was 24.5 ± 155.9 individuals m−2, with a maximum of 1745 individuals m−2. The mean sipunculan biomass was 14.0 ± 73.8 g wet weight m−2, with a maximum of 698.7 g wet weight m−2 in the northwest Chukchi Sea. The sipunculan abundances and biomass observed were related to latitude, depth, and sediment characteristics. The populations observed should be considered significant and characteristic components of the Pacific Arctic shallow shelf systems and food webs. This conclusion proves to be true also for other Arctic shelf seas like the Barents Sea, and deep Arctic ecosystems.

Climate change and polar range expansions: Could cuttlefish cross the Arctic?

Climate change can have major effects on the distribution of species. In marine ecosystems, the cold waters of the Arctic have restricted warmer water species from crossing between Eurasia and North America. However, with Arctic waters becoming warmer, various marine species have expanded their distribution. Cuttlefish are fast-growing, voracious predators and are absent in American waters. The European cuttlefish Sepia officinalis is the most northerly distributed cuttlefish, with potential to expand its range and cross to the American continent, potentially causing changes in shelf food webs. Climate model predictions suggest that the S. officinalis could potentially reach American shores, by 2300 via the north Atlantic with medium mitigation of greenhouse gas concentrations; we predict that adult dispersal of cuttlefish across the Atlantic sector would require a migration distance of over 1400 km at depths below 200 m and temperatures above 7 °C (temperature below which cuttlefish can not maintain routine metabolic processes physiologically). For temperatures above 9.5 °C (temperature above which cuttlefish can grow), 2500 km would be required, and such conditions will possibly exist by the year 2300. If they reach American shores they could have large impacts on coastal marine ecosystems, due to their wide diet (e.g. diet covers many shallow-water crustacean and fish species) and its potential as prey, and due to their short life-history strategy of “live fast, die young”.

High density of ice krill (Euphausia crystallorophias) in the Amundsen sea coastal polynya, Antarctica

High densities of ice krill Euphausia crystallorophias were observed along six acoustic transects within the Amundsen Sea Coastal Polynya, Antarctica. Two-frequency acoustic backscatter data was examined in the austral summers of January 2011 and February 2012. A dB identification window (Sv120−38) identified ice krill dominating the acoustic backscatter. The density of ice krill, calculated with the stochastic distorted-wave born approximation model, ranged between 4.5 and 30g wet mass m−2 for each transect (a mean of 16g wet mass m−2 for all transects), these high values are an order of magnitude higher than recorded previously in the Ross Sea Polynya. High densities were detected along the ice shelf and near the boundary between pack ice and coastal polynya, and we postulate that these could be important habitats for ice krill. The high densities observed along the transects make ice krill a potentially important, but poorly known contributor to these high-latitude shelf food webs.

A toolbox to evaluate data reliability for whole-ecosystem models: Application on the Bay of Biscay continental shelf food-web model

Ecosystem models are always simplifications of reality and as such their application for ecosystem-based management requires standard validation. Here, the “DataReli” toolbox is proposed to evaluate the quality of the data used during the construction of ecosystem models, their coherence across trophic levels, and whether data limitations prevent the model long-term applications. This toolbox is the combination of three operational and complementary analyses: (i) the pedigree index to determine to what extent a model was calibrated on data of local origin; (ii) the graphical analysis known as PREBAL to assess whether a model respects some basic ecological and fisheries principles; and (iii) a sensitivity analysis to evaluate the robustness of model predictions to small variations in input data. The toolbox is delivered to potential users with main generic recommendations on how interpreting results conjointly and on which decisions to make about parameters’ revisions or model uses’ restrictions. (i) Corrections of parameters should be preferentially envisaged when modelling data-rich environments. (ii) For those models with an overall pedigree index above 0.4, a closer look at the pedigree routine, i.e. values by parameters and compartments, and the PREBAL analysis would help to prioritize parameters needing improvement. (ii)’ For Ecopath models of no overall acceptable quality (overall pedigree index <0.4), we recommend stopping the DataReli procedure at this point. (iii) In terms of sensitivity analysis, marked responses of model predictions to small variations in the input values must preferentially lead to restrictions in the model applications compared to corrections of parameter estimates. A concrete application of the “DataReli” toolbox to the pre-existing Ecopath model of the Bay of Biscay continental shelf food web is presented. For the present case study, the general level of input data reliability is considered as satisfying with regard to the model applications.

An assessment of the trophic structure of the Bay of Biscay continental shelf food web: Comparing estimates derived from an ecosystem model and isotopic data

Comparing outputs of ecosystem models with estimates derived from experimental and observational approaches is important in creating valuable feedback for model construction, analyses and validation. Stable isotopes and mass-balanced trophic models are well-known and widely used as approximations to describe the structure of food webs, but their consistency has not been properly established as attempts to compare these methods remain scarce. Model construction is a data-consuming step, meaning independent sets for validation are rare. Trophic linkages in the French continental shelf of the Bay of Biscay food webs were recently investigated using both methodologies. Trophic levels for mono-specific compartments representing small pelagic fish and marine mammals and multi-species functional groups corresponding to demersal fish and cephalopods, derived from modelling, were compared with trophic levels calculated from independent carbon and nitrogen isotope ratios. Estimates of the trophic niche width of those species, or groups of species, were compared between these two approaches as well. A significant and close-to-one positive (rSpearman2=0.72, n=16, p<0.0001) correlation was found between trophic levels estimated by Ecopath modelling and those derived from isotopic signatures. Differences between estimates were particularly low for mono-specific compartments. No clear relationship existed between indices of trophic niche width derived from both methods. Given the wide recognition of trophic levels as a useful concept in ecosystem-based fisheries management, propositions were made to further combine these two approaches.

Trophodynamics and organic matter assimilation pathways in the northeast Chukchi Sea, Alaska

We analyzed trophic linkages in the northeast Chukchi Sea shelf based on the stable carbon and nitrogen isotopic analysis of 39 species collected in 2009 and 2010. To decipher organic matter assimilation pathways, benthic fauna were first categorized into nine trophic guilds based on their physical location in the seabed (epibenthic, surface, or subsurface), feeding mode (suspension feeder, deposit feeder, predator, or scavenger), and food source (suspended particulate organic matter, phytoplankton, zooplankton, sediment, microflora, meiofauna, or macrofauna). A discriminant function analysis (DFA) determined that feeding modes were predicted by stable isotope values at an overall classification success rate of 42%, although classification success of each individual guild varied from 0 to 66%. In some instances, stable isotopes classified trophic guilds incorrectly more often than correctly, suggesting high trophic redundancy in the system. A striking pattern was observed where the δ13C values of individuals in some trophic guilds, ranging from about −23 to −17‰, were substantially more 13C-enriched than representative end-members, which includes phytoplankton (−24.0‰), suspended particulate organic matter (−24.3‰), and bulk sediment organic matter (−23.3‰). In contrast, near-seafloor suspended particulate organic matter was significantly 13C-enriched compared to suspended particulate organic matter of near surface waters (p<0.05), and bulk sedimentary organic matter was more 13C-enriched than overlying phytoplankton at seven of 10 stations. This suggests the presence of an unmeasured 13C-enriched end-member that is a product of biogeochemical alteration and reworking by the sediment microbial community. Although the microbial community is difficult to quantify using bulk stable isotope analytical techniques, these results indicate it cannot be overlooked as a critical component and avenue through which large amounts of reduced carbon are assimilated by a rich and diverse arctic shelf food web.

Presence and distribution of serotonin in the stomach of the Antarctic silverfish Pleuragramma antarcticum

Serotonin is a signal molecule with a wide range of functions in vertebrates. In Antarctic fishes, the serotonergic system has been studied in the brain, revealing differences from temperate fishes related to the long-term cold adaptation. To date, little is known regarding the peripheral nervous system, and no information is available for the stomach. In the present work, we contribute to fill the gaps by investigating the presence and the immunohistochemical distribution of serotonin in the stomach of the Antarctic silverfish Pleuragramma antarcticum, a cold-adapted key species of the Southern Ocean shelf food web. The main aim was to investigate the serotonergic system at the gastric level, in order to reveal possible peculiarities related to long-term cold adaptation, similar to the ones seen in the central nervous system of Antarctic fishes. Serotonin immunoreactivity was detected in the pyloric and cardiac mucosa of P. antarcticum stomach with immunopositive cells in the pyloric and cardiac surface epithelia and in the tubular glands. No immunopositive fibers and neuronal cell bodies were found. Our results highlight that the serotonin distribution pattern at the gastric level is similar to that described in temperate teleosts. This finding suggests that in P. antarcticum, long-term adaptations to the Antarctic condition do not affect the serotonergic system at the gastric level. In addition, our data constitute the baseline information for further investigations aimed at clarifying the effects of short-term temperature variations on the gastric serotonergic system of Antarctic species in the frame of the global climate change.

Amphipod-supported food web: Themisto gaudichaudii, a key food resource for fishes in the southern Patagonian Shelf

The trophic role of the hyperiid amphipod Themisto gaudichaudii in the southern Patagonian shelf food web was assessed from the analysis of stomach contents of the local fish assemblage. A total of 461 trawl samples were collected during seven seasonal cruises. A total of 17 out of 38 fish species were found to ingest T. gaudichaudii. This amphipod was a main prey item in five of these species, showing high values of alimentary index: Seriolella porosa (99.9%), Macruronus magellanicus (68.8%), Micromesistius australis (59.1%), Patagonotothen ramsayi (48.6%), and Merluccius hubbsi (10.9%). The contribution of T. gaudichaudii, in weight, to their summer diet was 60%, on average. This contribution was minimal in winter and maximal in summer. Fisheries studies have indicated that these five species, mainly M. magellanicus, account for almost 85% of the fish biomass in the area. Although the remaining 15% did not feed heavily on T. gaudichaudii, they are known to prey on the main hyperiid predators. Our study shows that T. gaudichaudii contributes greatly, both directly and indirectly, to supporting the fish community. We thus proposed that T. gaudichaudii plays a key role as a “wasp-waist” species in the sub-Antarctic region, similar to that of krill in Antarctic waters, channeling the energy flow and enabling a short and efficient food chain.

Lower trophic levels and detrital biomass control the Bay of Biscay continental shelf food web: Implications for ecosystem management

The Bay of Biscay (North-East Atlantic) has long been subjected to intense direct and indirect human activities that lead to the excessive degradation and sometimes overexploitation of natural resources. Fisheries management is gradually moving away from single-species assessments to more holistic, multi-species approaches that better respond to the reality of ecosystem processes. Quantitative modelling methods such as Ecopath with Ecosim can be useful tools for planning, implementing and evaluating ecosystem-based fisheries management strategies. The aim of this study was therefore to model the energy fluxes within the food web of this highly pressured ecosystem and to extract practical information required in the diagnosis of ecosystem state/health. A well-described model comprising 30 living and two non-living compartments was successfully constructed with data of local origin, for the Bay of Biscay continental shelf. The same level of aggregation was applied to primary producers, mid-trophic-levels and top-predators boxes. The model was even more general as it encompassed the entire continuum of marine habitats, from benthic to pelagic domains. Output values for most ecosystem attributes indicated a relatively mature and stable ecosystem, with a large proportion of its energy flow originating from detritus. Ecological network analysis also provided evidence that bottom-up processes play a significant role in the population dynamics of upper-trophic-levels and in the global structuring of this marine ecosystem. Finally, a novel metric based on ecosystem production depicted an ecosystem not far from being overexploited. This finding being not entirely consistent over indicators, further analyses based on dynamic simulations are required.

Squid as a resource shared by fish and humans on the Falkland Islands’ shelf

Nektonic squids are the dominant group in the Falklands Islands’ fishery representing >75% of the total annual catch. Among these squids, the benthopelagic Loligo gahi is an important link between zooplankton and fish in the shelf food web. It is preyed upon by diverse and numerous demersal and benthopelagic fishes, penguins and marine mammals, and is harvested by fishermen. Pelagic squids, Illex argentinus, Onykia ingens, and Gonatus antarcticus though seasonally very abundant, are scarce in fish diet because of the absence of specialised pelagic predators. This ecological niche is shared between humans (targeting Illex) and penguins (targeting Onykia and Gonatus). Martialia hyadesi though was abundant in some years in the past, it has now virtually disappeared from the shelf ecosystem. The biomass produced by L. gahi remains in the Falkland waters, whereas that of pelagic squid is transported out of the shelf either to the northern Patagonian slope ( I. argentinus) or to deeper waters around the islands ( O. ingens and G. antarcticus). Generally squids occupy the ecological niche of epipelagic fish on the Falkland shelf and slope.

Contribution of phytoplankton and benthic microalgae to inner shelf sediments of the north-central Gulf of Mexico

Marine sediment may contain both settled phytoplankton and benthic microalgae (BMA). In river-dominated, shallow continental shelf systems, spatial, and temporal heterogeneity in sediment type and water-column characteristics (e.g., turbidity and primary productivity) may promote spatial variation in the relative contribution of these two sources to the sediment organic matter pool available to benthic consumers. Here we use photosynthetic pigment analysis and microscopic examination of sediment microalgae to investigate how the biomass, composition, and degradation state of sediment-associated microalgae vary along the Louisiana (USA) inner shelf, a region strongly influenced by the Mississippi River. Three sandy shoals and surrounding muddy sediments with depths ranging from 4 to 20 m were sampled in April, August, and October 2007. Pigment composition suggested that sediment microalgae were primarily diatoms at all locations. We found no significant differences in sediment chlorophyll a concentrations (8–77 mg m −2) at the shoal and off-shoal stations. Epipelic pennate diatoms (considered indicative of BMA) made up a significantly greater proportion of sediment diatoms at sandy (50–98%) compared to more silty off-shoal stations (16–56%). The percentage of centric diatoms (indicators of settled phytoplankton) in the sediment was highest in August. Sediment total pheopigment concentrations on sandy stations (<20 mg m −2) were significantly lower than concentrations at nearby muddy stations (>40 mg m −2), suggesting differences in sediment microalgal degradation state. These observations suggest that BMA predominate in shallow sandy sediments and that phytodetritus predominates at muddy stations. Our results also suggest that the relative proportion of phytodetritus in the benthos was highest where phytoplankton biomass in the overlying water was greatest, independent of sediment type. The high biomass of BMA found on shoals suggests that benthic primary production on sandy sediments represents a potentially significant local source of sediment microalgal carbon that may be utilized by benthic consumers in continental shelf food webs.

Sociometric analysis of the role of penaeids in the continental shelf food web off Veracruz, Mexico based on by-catch

Continental shelf macrofauna are impacted by trawling and associated by-catch activities. These activities have been predicted to change the macrofaunal community structure and the resulting food web. Four food webs (three seasonal and one pooled) were constructed using shrimp by-catch samples from 21 surveys from 1991 to 1994. Network sociometric analyses were performed on the webs in order to explore structural characteristics focusing on the role of penaeids. The number of nodes varied from 43 to 73. The out-degree centrality value for the combined web was the highest. For the penaeid node, the windy season centrality was the highest (47%) and the rainy season the lowest (11%). The number of cutpoints by season was: windy four, dry two, and three for the combined web. Using the lambda sets analysis; penaeid is a key node for the structural cohesion of the trophic network. The rainy season food web is the most homogeneous and penaeid is an important node such that its elimination would cause a major structural and functional disruption to the network. These modeling exercises suggest over-fishing by trawlers would impact the continental shelf community structure and thus significantly change the existing trophic relationships.