Trophic Levels Of Marine Ecosystems Research Articles

Nuclear Pollution and Its Effects on Marine Ecosystems

Nuclear pollution is a severe environmental problem deriving mainly from industrial activities and can pose extremely negative affect to plants and animal bodies. Nuclear waste also has extreme negative effect on human bodies and can cause various diseases including skin disease, infant deficiency and cancer. There are few studies about the effect of nuclear waste to food webs and how it affects different trophic levels within the ecosystems. This study explains the effect of nuclear pollution on the marine ecosystems and focuses on the transportation of the pollutants through food webs as well as the sufferance of species in different trophic levels. It is concluded that nuclear pollutants can experience biomagnification within food webs and are transported to higher trophic levels by their food consumption. In addition, planktons, fish and seals are representative species of different trophic levels in marine food webs, and these species all suffer from nuclear contamination after nuclear pollutants enter their bodies through biomagnification. Future studies can focus on the process of energy transportation within food webs, such as whether the absorption of energy would be less efficient by higher trophic levels after consuming organisms with nuclear pollutants inside comparing to organisms with no nuclear pollutants. This study propose a explanation of respective effects of different trophic levels in marine ecosystems by nuclear pollutions, which can call on attention to the extreme negative effect of nuclear pollution to marine ecosystems.

Environmental DNA in the Kuroshio reveals environment-dependent distribution of economically important small pelagic fish

IntroductionSmall pelagic fishes constitute large proportions of fisheries and are important components linking lower and higher trophic levels in marine ecosystems. Many small pelagic fishes in the Northwest Pacific spawn upstream in the Kuroshio and spend their juvenile stage in the Kuroshio Front area, indicating that the Kuroshio Current system impacts their stock fluctuations. However, the distribution of these fish relative to the Kuroshio has not been determined due to dynamic spatio-temporal fluctuations of the system. Here, the recent development of environmental DNA (eDNA) monitoring enabled us to investigate the distribution patterns of four economically important small pelagic fishes (Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus, chub mackerel Scomber japonicus, and blue mackerel Scomber australasicus) in the Kuroshio Current system. MethodsThe influence of environmental factors, such as sea water temperature, salinity, oxygen concentration, chlorophyll-a concentration, and prey fish on the occurrence and quantity of target fish eDNA was analyzed using generalized additive models. In addition, the detection (presence) of target fish eDNA were compared between the offshore and inshore side areas of the Kuroshio axis.ResultsSea water temperature showed important effect, especially on the distribution of Japanese sardine and Japanese anchovy, whereas the distribution pattern of chub mackerel and blue mackerel was greatly influenced by the eDNA quantity of Japanese sardine and Japanese anchovy (especially potential prey fish: Japanese anchovy). In addition, we found that the four target fish species could be observed in areas on the inshore side or around the Kuroshio axis, while they were hardly found on the offshore side.ConclusionBased on eDNA data, we succeeded in revealing detailed spatial distribution patterns of small pelagic fishes in the Kuroshio Current system and hypothesized predator–prey relationships influence their distribution in small pelagic fish communities.

Dietary niche partitioning in Early Jurassic ichthyosaurs from Strawberry Bank.

Jurassic ichthyosaurs dominated upper trophic levels of marine ecosystems. Many species coexisted alongside each another, and it is uncertain whether they competed for the same array of food or divided dietary resources, each specializing in different kinds of prey. Here, we test whether feeding differences existed between species, applying finite element analysis to ichthyosaurs for the first time. We examine two juvenile ichthyosaur specimens, referred to Hauffiopteryx typicus and Stenopterygius triscissus, from the Strawberry Bank Lagerstätte, a shallow marine environment from the Early Jurassic of southern England (Toarcian, ~183 Ma). Snout and cranial robusticity differ between the species, with S. triscissus having a more robust snout and cranium and specializing in slow biting of hard prey, and H. typicus with its slender snout specializing in fast, but weaker bites on fast-moving, but soft prey. The two species did not differ in muscle forces, but stress distributions varied in the nasal area, reflecting differences when biting at different points along the tooth row: the more robustly snouted Stenopterygius resisted increases or shifts in stress distribution when the bite point was shifted from the posterior to the mid-point of the tooth row, but the slender-snouted Hauffiopteryx showed shifts and increases in stress distributions between these two bite points. The differences in cranial morphology, dentition and inferred stresses between the two species suggest adaptations for dietary niche partitioning.

Accounting for a nonlinear functional response when estimating prey dynamics using predator diet data

Abstract Forage fish species are key in the transfer of energy from lower to upper trophic levels in marine ecosystems. Therefore, understanding their population dynamics, including population levels, is crucial for understanding productivity and the regulation of marine food webs. However, many forage fishes are poorly sampled by bottom trawl surveys, leading to poor estimates of their abundance. These estimates can be improved by using predator stomach contents as an additional sampling strategy; however, nonlinear relationships between prey abundance and predator consumption (i.e. the functional response) may bias stomach data as well. Using predator stomach contents and bottom trawl survey data, this study aimed to minimize this bias by developing a model to estimate prey dynamics and account for the predator functional response. This model was tested using a series of simulations and applied to a case study of northern sand lance Ammodytes dubius on the Grand Bank, Newfoundland, Canada. The simulations revealed that when predators consumed prey following a nonlinear functional response, our model outperformed a classical model (the model adopted by most studies) that assumed a linear functional response. In the case study, we estimated the relative abundance of sand lance from 1995 to 2018, which exhibited oscillatory dynamics with a period of approximately 7 years. Our results demonstrate that our model is capable of more accurately estimating the abundance of data‐limited prey populations, which contributes to a better understanding of food web dynamics.

Feeding Ecology of Three Euphausiid Species in the North Pacific Ocean Inferred From 18S V9 Metabarcoding and Stable Isotope Analysis

Euphausiids are abundant micronekton and important links between higher and lower trophic levels in marine ecosystems; however, their detailed diets cannot be fully understood by conventional microscopy, especially in subtropical areas. Here, we report the euphausiid community structure in the California Current (CC) area and the eastern/western North Pacific subtropical gyre (ESG and WSG) and detail the feeding ecology of the dominant species (Euphausia pacifica, E. brevis, and E. hemigibba) in each region using a combined approach of gut content analysis via 18S V9 metabarcoding and stable carbon and nitrogen isotope analysis. A pronounced omnivorous feeding of all studied euphausiid species was supported by both methods: phytoplanktonic taxonomic groups (Dinophyta, Stramenopiles, and Archaeplastida), Copepoda, and Hydrozoa were detected in the gut contents; all the three euphausiid species displayed an intermediate trophic position between the net plankton (0.2–1.0 mm) and the myctophid fish (15.2–85.5 mm). However, Hydrozoa found in euphausiid gut contents likely derived from a potential cod-end feeding, based on isotope analysis. E. pacifica in the CC province ingested more autotrophic prey, including pelagophyte and green algae, due to a greater abundance of Stramenopiles and Archaeplastida in shallow layers of CC water. On the other hand, non-autotrophic prey such as mixotrophic Kareniaceae dinoflagellates, Pontellidae and Clausocalanidae copepods, and Sphaerozoidae rhizarian contributed more to the diets of E. brevis and E. hemigibba because of a lower chlorophyll a concentration or potentially a scarcity of autotrophic prey availability in ESG and WSG. The feeding patterns of dominant euphausiid species conducting filter feeding were thus largely determined by phytoplankton prey availability in the environments. Dietary difference across three species was also indicated by stable isotope analysis, with a lower mean trophic level of E. pacifica (2.32) than E. brevis (2.48) and E. hemigibba (2.57). These results verify direct trophic interactions between euphausiids and primary production and suggest that the omnivorous feeding habit is a favorable character for dominant Euphausia species.

Dietary Shifts and Risks of Artifact Ingestion for Argentine Shortfin Squid Illex argentinus in the Southwest Atlantic

Cephalopods play an important role in both ecology and fisheries. Variations in the dietary compositions of squids with large populations further promote interactions between different trophic levels in marine ecosystems. Moreover, due to marine pollution, squids are also at risks of artifact ingestion, and there is limited understanding about the influence of artifacts on the feeding behavior of squids. We examined 296 stomachs of the Argentine shortfin squid Illex argentinus collected through commercial catches across the Southwest Atlantic from February to April of 2018 and 2019 to establish the monthly dietary compositions and risks of artifact ingestion. The results supported typical observations that the Argentine shortfin squids switched from a diet dominated by crustaceans at small sizes to consuming larger prey, predominantly fishes and/or cephalopods, at large sizes during their growth and southward migration. Significantly higher consumption of fishes was observed in recent years compared with that observed in other studies since 1992. The ingested artifacts examined were composed of plastic and non-plastic materials. Artifacts were observed in 19.9% of the total number of stomachs, with 20.5% of the empty stomachs containing artifact remains. The results indicate that although the main dietary compositions of the Argentine shortfin squid maintain the taxonomic groups of prey compositions, the dietary structure, i.e., composition percentage, is varying. The detection of artifacts suggests that environmental monitoring is needed in this region in order to manage and conserve the squid and safeguard aquatic food safety.

Use of Sediqualsoft® to Determine the Toxicity of Cigarette Butts to Marine Species: A Weather Simulation Test

Cigarette butts (CBs) are among the dominant constituents of marine and beach litter. Few studies have been conducted, and the environmental effects of CBs on marine species are still poorly understood. This study aims to evaluate the ecotoxicological effects on marine organisms of both classic and electronic CBs. Three representative species of different trophic levels in marine ecosystems (Aliivibrio fischeri, bacteria; Phaeodactylum tricornutum, algae, primary producers; Paracentrotus lividus, echinoderms, consumers) were tested. The effects of natural ageing of CBs due to exposure to atmospheric conditions (natural sunlight vs. simulated rain) and for different times (1 vs. 2 weeks) were evaluated. The results were weighted together to obtain a synthetic hazard level to the environment (Class of Hazard) from Sediqualsoft®. Classic CBs (CCBs) performed the worst and posed a mild to moderate risk compared to electronic CBs (absent Class of Hazard). Smoked classic CBs posed a higher environmental risk than unsmoked. The highest risk was produced by classic CBs after one week of exposure in dry weather. Echinoderms and the body size reduction in normo-formed (72 h) plutei were shown to be the more sensitive organism and endpoint, respectively. We recommend the use of Sediqualsoft® software for risk assessment studies of sediments contaminated with contaminants of various types, especially in conjunction with a weight of evidence approach (WOE).

Minor effects of dietary methylmercury on growth and reproduction of the sheepshead minnow Cyprinodon variegatus and toxicity to their offspring

Methylmercury (MeHg) is a neurotoxic compound that is found in virtually all fish and biomagnifies in aquatic food webs. Although MeHg concentrations in marine and estuarine fish are often elevated, the impacts of MeHg on marine and estuarine fish have largely been understudied. To evaluate the impact of dietary MeHg on marine fish reproduction and effects on their offspring, female juvenile sheepshead minnows (Cyprinodon variegatus) at three months of age were experimentally exposed to MeHg-contaminated diets for two months and then paired with Hg-free males for spawning. Egg production, hatching success of embryos, time to hatching, survival of larvae, growth of larvae and swimming behavior of larvae were determined. Selenium (Se) was also measured and Se/Hg molar ratios were calculated to assess whether Se reduced MeHg toxicity. MeHg had no significant impact on fish reproduction or on survival and growth of larvae. Larvae produced by MeHg-exposed mothers had concentrations of Hg about 1 ppm (dry wt), or about 12% of that in the muscle of their mothers and consistently displayed 6–15% increased swimming speed relative to controls; the ecological significance of this moderate effect on swimming speed requires further study. The Se/Hg molar ratios in these fish, which were >1 in controls (adults and larvae) and MeHg-exposed larvae but <1 in Hg-exposed adults, did not correlate with MeHg effects. The sheepshead minnow, at a low trophic level, appears to have a high tolerance of MeHg; however, it can pass MeHg to higher trophic levels in marine ecosystems where upper level predators have MeHg concentrations sometimes exceeding US FDA safety limits of 1 ppm wet wt.

Hatchetfishes (Stomiiformes: Sternoptychidae) biodiversity, trophic ecology, vertical niche partitioning and functional roles in the western Tropical Atlantic

Species of the family Sternoptychidae (hatchetfishes) occur worldwide and play critical roles by sequestering carbon, recycling nutrients, and acting as a key trophic link between epipelagic primary consumers and higher trophic levels in marine ecosystems. Nevertheless, basic knowledge on their ecology is still lacking and their functional ecology remains understudied with respect to composition, organization, functions and environment interactions. Here we integrated comprehensive information collected in the western Tropical Atlantic on the diversity, abundance, distribution and trophic ecology of hatchetfishes, including physicochemical features of their habitats and extensive carbon and nitrogen stable isotope data on its main prey groups. On this basis we defined five functional groups of hatchetfishes with different diet preference, isotopic composition, and vertical abundance peaks and reveal a possible high resource partitioning. Additionally, these species might have a different feeding tie chronology. Hence, hatchetfishes segregate in different ecological groups responding differently to environmental constraints including oxygen concentration and presenting diverse functional roles. As deep-sea species that migrate to epipelagic waters, hatchetfishes may play a key role in the transfer of sub-surface photoassimilated carbon to deeper waters, a pathway through which the effects of climate change at the surface are transferred to the deep ocean. Moreover, as consumers of gelatinous organisms, these species convert “gelatinous energy” into “fish energy” readily usable by higher trophic levels, including endangered and commercially important species. This is a crucial trophic relationship that has been historically underestimated due to methodology limitations (e.g., quickly digested gelatinous organisms were probably underestimated in previous studies, based solely on stomach contents). Considering in ecosystem models this trophic relationship, as well as the functional organization of hatchetfishes, is important to properly answer key ecological questions including resource use, carbon transportation, and influence of mesopelagic community in climate change process.

Feeding ecology of Sardina pilchardus considering co-occurring small pelagic fish in the eastern Adriatic Sea

Small pelagic fish with their significant biomass are connecting plankton production and higher trophic levels in marine ecosystems. The aim of this paper was to analyse feeding habits of sardine Sardina pilchardus (Walbaum, 1792), regarding the season and the spatial area, but also considering trophodynamics of other co-occurring small pelagic fish: anchovy Engraulis encrasicolus (Linnaeus, 1758), round sardinella Sardinella aurita (Valenciennes, 1847), chub mackerel Scomber japonicus (Houttuyn, 1782) and horse mackerel Trachurus trachurus (Linnaeus, 1758) in the eastern Adriatic Sea. Sardine revealed copepods (average contribution 30%), decapod larvae (18%), mysids (15%) and copepod eggs (9%) as their main source of energy. Seasonal oscillations of the sardine fullness index were determined, with highest values in the autumn and spring (%Jr = 0.71 and 0.66, respectively). Spatial analysis showed no significant difference in the diet of fish caught in offshore and coastal waters (ANOSIM, Global r = 0.034; p > 0.05). The ANOSIM test showed a significant difference in prey composition among five investigated species (Global r = 0.081, p < 0.05). The greatest difference in diet was observed between sardine and chub mackerel, as well as between sardine and horse mackerel. While copepods dominated in the sardine, anchovy and round sardinella stomach, horse mackerel and chub mackerel showed selectivity for larger prey as euphausiids, which dominated in terms of numbers, while the most frequent prey were decapod larvae. However, calanoid copepods, fish larvae, decapod larvae, decapod larvae–megalopa stage, copepod eggs, amphipods and isopods were present in the diet of all small pelagic fish. Interspecies diet overlap, besides being significant for marine food web modelling, is particularly important for future fishery management, since investigations in this field are still mostly based on a single-species diet approach.

Lipid-rich and protein-poor carbon allocation patterns of phytoplankton in the northern Chukchi Sea, 2011

The carbon allocations of phytoplankton into different photosynthetic end products (lipids, LMWM, polysaccharides, and proteins) were determined to understand physiological conditions of phytoplankton in the northern Chukchi Sea during the Korean Arctic expedition, 2011, using the 13C isotope tracer technique. The carbon allocation rates of lipids, LMWM, polysaccharides, and proteins were 0.00009–0.00062 h−1, 0.00001–0.00049 h−1, 0.00001–0.00025 h−1, and 0.00001–0.00062 h−1 within the euphotic depths from surface to 1% light depths during our cruise period, respectively. Significant relationships between protein production rates and chlorophyll a concentrations (large and total) were found in this study. Moreover, we found a significant negative relationship between lipid production rates and ammonium concentrations. These relationships match well with the previous results for environmental/physiological conditions for phytoplankton growth. Overall, phytoplankton allocated more photosynthetic carbon into lipids (42.5 ± 17.7%) whereas relatively lower to proteins (20.4 ± 15.5%) in this study. The lipid-rich and protein-poor allocation patterns in this study suggest that phytoplankton in the northern Chukchi Sea were in a stationary growth phase under nutrient deficient condition based on biological and environmental conditions observed during our study period. Based on comparison with the previous studies in the northern Bering Sea and southern Chukchi Sea, we found that the photosynthetic carbon allocation patterns depending on physiological status of phytoplankton under the different growth and/or nutrient conditions could be largely vary at different regions in the Arctic Ocean. More intensive research on the physiological status of phytoplankton is further required to determine how phytoplankton response to the changing environmental conditions and consequently how they impact on higher trophic levels in marine ecosystems in the Arctic Ocean.

Dynamics of particulate and dissolved organic and inorganic phosphorus during the peak and declining phase of an iron-induced phytoplankton bloom in the eastern subarctic Pacific

Abstract Phosphorus (P) is an essential element for all organisms and thus the P cycle plays a key role in determining the dynamics of lower trophic levels in marine ecosystems. P in seawater occurs conceptually in particulate and dissolved organic and inorganic (POP, PIP, DOP, and DIP, respectively) pools and clarification of the dynamics in these P pools is the basis to assess the biogeochemical cycle of P. Despite its importance, behaviors of each P pool with phytoplankton dynamics have not been fully examined. We measured the four operationally defined P pools (POPop, PIPop, DOPop, and SRP) during an iron-induced phytoplankton bloom (as part of the subarctic ecosystem response to iron enrichment study (SERIES)) in the eastern subarctic Pacific in summer 2002. During our observations of the iron-enriched patch from day 15 to day 26 after the iron infusion, chlorophyll-a concentration in the surface layer decreased from 6.3 to 1.2 μg L− 1, indicating the peak through decline phase of the phytoplankton bloom. At the bloom peak, P was partitioned into POPop, PIPop, and DOPop in proportions of 60, 27, and 13%, respectively. While chlorophyll-a and POPop showed similar temporal variations during the declining phase, PIPop showed a different peak timing with a 2 day delay compared to POPop, resulting in a rapid change in the relative proportion of PIPop to total particulate P (TPP = POPop + PIPop) at the peak (25%) and during the declining phase of the bloom (50%). A part of POPop was replaced by PIPop just after slowing down of phytoplankton growth. This process may have a significant role in the subsequent regeneration of P. We conclude that measurement of TPP alone is insufficient to show the interaction between P and phytoplankton dynamics and fractionation of TPP into POPop and PIPop provides useful insights to clarify the biogeochemical cycle of P.

Ecotoxicity and genotoxicity of cadmium in different marine trophic levels

Cadmium ecotoxicity and genotoxicity was assessed in three representative species of different trophic levels of marine ecosystems – the calanoid copepod Acartia tonsa, the decapod shrimp, Palaemon varians and the pleuronectiform fish Solea senegalensis. Ecotoxicity endpoints assessed in this study were adult survival, hatching success and larval development ratio (LDR) for A. tonsa, survival of the first larval stage (zoea I) and post-larvae of P. varians, egg and larvae survival, as well as the presence of malformations in the larval stage of S. senegalensis. In vivo genotoxicity was assessed on adult A. tonsa, the larval and postlarval stage of P. varians and newly hatched larvae of S. senegalensis using the comet assay. Results showed that the highest sensitivity to cadmium is displayed by A. tonsa, with the most sensitive endpoint being the LDR of nauplii to copepodites. Sole eggs displayed the highest tolerance to cadmium compared to the other endpoints evaluated for all tested species. Recorded cadmium toxicity was (by increasing order): S. senegalensis eggs < P. varians post-larvae < P. varians zoea I < S. senegalensis larvae < A. tonsa eggs < A. tonsa LDR. DNA damage to all species exposed to cadmium increased with increasing concentrations. Overall, understanding cadmium chemical speciation is paramount to reliably evaluate the effects of this metal in marine ecosystems. Cadmium is genotoxic to all three species tested and therefore may differentially impact individuals and populations of marine taxa. As A. tonsa was the most sensitive species and occupies a lower trophic level, it is likely that cadmium contamination may trigger bottom-up cascading effects in marine trophic interactions.

PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies

Abstract. PISCES-v2 (Pelagic Interactions Scheme for Carbon and Ecosystem Studies volume 2) is a biogeochemical model which simulates the lower trophic levels of marine ecosystems (phytoplankton, microzooplankton and mesozooplankton) and the biogeochemical cycles of carbon and of the main nutrients (P, N, Fe, and Si). The model is intended to be used for both regional and global configurations at high or low spatial resolutions as well as for short-term (seasonal, interannual) and long-term (climate change, paleoceanography) analyses. There are 24 prognostic variables (tracers) including two phytoplankton compartments (diatoms and nanophytoplankton), two zooplankton size classes (microzooplankton and mesozooplankton) and a description of the carbonate chemistry. Formulations in PISCES-v2 are based on a mixed Monod–quota formalism. On the one hand, stoichiometry of C / N / P is fixed and growth rate of phytoplankton is limited by the external availability in N, P and Si. On the other hand, the iron and silicon quotas are variable and the growth rate of phytoplankton is limited by the internal availability in Fe. Various parameterizations can be activated in PISCES-v2, setting, for instance, the complexity of iron chemistry or the description of particulate organic materials. So far, PISCES-v2 has been coupled to the Nucleus for European Modelling of the Ocean (NEMO) and Regional Ocean Modeling System (ROMS) systems. A full description of PISCES-v2 and of its optional functionalities is provided here. The results of a quasi-steady-state simulation are presented and evaluated against diverse observational and satellite-derived data. Finally, some of the new functionalities of PISCES-v2 are tested in a series of sensitivity experiments.

Modeling photosynthesis in sea ice‐covered waters

AbstractThe lower trophic levels of marine ecosystems play a critical role in the Earth System mediating fluxes of carbon to the ocean interior. Many of the functional relationships describing biological rate processes, such as primary productivity, in marine ecosystem models are nonlinear functions of environmental state variables. As a result of nonlinearity, rate processes computed from mean fields at coarse resolution will differ from similar computations that incorporate small‐scale heterogeneity. Here we examine how subgrid‐scale variability in sea ice thickness impacts simulated net primary productivity (NPP) in a configuration of the Community Earth System Model (CESM). CESM simulates a subgrid‐scale ice thickness distribution and computes shortwave penetration independently for each ice thickness category. However, the default model formulation uses grid‐cell mean irradiance to compute NPP. We demonstrate that accounting for subgrid‐scale shortwave heterogeneity by computing light limitation terms under each ice category then averaging the result is a more accurate invocation of the photosynthesis equations. Moreover, this change delays seasonal bloom onset and increases interannual variability in NPP in the sea ice zone in the model. The new treatment reduces annual production by about 32% in the Arctic and 19% in the Antarctic. Our results highlight the importance of considering heterogeneity in physical fields when integrating nonlinear biogeochemical reactions.

Metal mobility and bioaccumulation differences at lower trophic levels in marine ecosystems dominated bySargassumspecies

Concentrations of cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn) and iron (Fe) were measured in three macroalgal species,Codium simulans,Sargassum sinicolaandGracilaria pachydermatica, and in the sea slugElysia diomedea, living in marine ecosystems dominated bySargassumspecies and located near exploited phosphorite deposits. Metal concentrations in macroalgae and sea slugs were significantly different among sites. The highest concentrations of Cd recorded inC. simulansandS. sinicola, as well as inE. diomedeawere recorded at the site closest to the phosphorite deposit. In general, Cd and Zn concentrations inE. diomedeawere higher than those recorded in specimens ofC. simulans,S. sinicolaandG. pachydermatica. In contrast, Pb, Cu, and Fe concentrations inE. diomedeawere lower than or similar to those in macroalgae from all sampled sites. The information generated contributes to the knowledge about potential mobility and metal bioaccumulation at lower trophic levels in marine eco systems.

Variation in assimilation efficiencies of dominant Neocalanus and Eucalanus copepods in the subarctic Pacific: Consequences for population structure models

The assimilation efficiency of zooplankton is an essential parameter required to estimate energy transfer to higher trophic levels in marine ecosystems. However, little information is available for large oceanic copepods, especially the Neocalanus and Eucalanus species dominant in the subarctic Pacific. In this study, the assimilation efficiencies of the C5 stages of Neocalanus cristatus, Neocalanus flemingeri and Eucalanus bungii were evaluated using eight phytoplankton species as food. The average assimilation efficiencies of N. cristatus, N. flemingeri and E. bungii ranged between 45 and 66%, 44 and 66% and 34 and 65%, respectively. The assimilation efficiency was highly variable depending on the food phytoplankton species. In all species, the assimilation efficiency showed a significant negative relationship with the ash content of the phytoplankton (r2=0.79–0.87, p<0.001). The assimilation efficiency of large-body sized N. cristatus for large-sized diatoms was higher than for the other copepod species. In population models of N. cristatus, changes in assimilation efficiency affect the growth and survival rates of the population. The Lagrangian ensemble model (LEM) for N. cristatus showed that, for assimilation efficiencies less than 57%, the population could not be maintained. Because variations in assimilation efficiency may have significant effects on the copepod population, their variability should be incorporated into marine ecosystem models in the future.

Multi-level trophic cascades in a heavily exploited open marine ecosystem

Anthropogenic disturbances intertwined with climatic changes can have a large impact on the upper trophic levels of marine ecosystems, which may cascade down the food web. So far it has been difficult to demonstrate multi-level trophic cascades in pelagic marine environments. Using field data collected during a 33-year period, we show for the first time a four-level community-wide trophic cascade in the open Baltic Sea. The dramatic reduction of the cod (Gadus morhua) population directly affected its main prey, the zooplanktivorous sprat (Sprattus sprattus), and indirectly the summer biomass of zooplankton and phytoplankton (top-down processes). Bottom-up processes and climate-hydrological forces had a weaker influence on sprat and zooplankton, whereas phytoplankton variation was explained solely by top-down mechanisms. Our results suggest that in order to dampen the occasionally harmful algal blooms of the Baltic, effort should be addressed not only to control anthropogenic nutrient inputs but also to preserve structure and functioning of higher trophic levels.

Examining the 'global spreading hypothesis' using graph theory

A persistent view, called the 'global spreading hypothesis', maintains that the frequency, magnitude and geographical extent of harmful algal blooms have increased in recent decades. These putative trends have been variously attributed to nutrient enrichment, climate change and inadequate databases. Biotic explanations for these trends have not generally been taken into account, but the recent tendencies of fisheries to graduate from predation to extermination are likely to have profound effects at all trophic levels of marine ecosystems, and need to be explored in this context. This study examines a model pelagic foodweb from the perspective of graph theory, and calculates the changes in the positional importance of phytoplankton in the interaction network. The positional importance of algae in foodwebs is quantified and community-wide effects on how their network position can change are demonstrated.

Linking the foraging performance of a marine predator to local prey abundance

Summary Knowledge of the functional response of predators to prey densities conditions our understanding of food webs. Such links are still poorly understood within the higher trophic levels of marine ecosystems. We present the first field study recording the foraging effort and foraging yield of a seabird (the Great Cormorant, Phalacrocorax carbo) as well as the abundance and quality of prey within its foraging area. We confirm that Great Cormorants foraging off West‐Greenland show the highest foraging performance recorded for a marine predator (between 17 and 41 g fish caught per minute underwater). Former work suggests that such high foraging yield should be based upon the exploitation of extremely profitable prey patches. Contrary to this hypothesis, average prey abundances estimated within the foraging areas of the cormorants were low (0·03–0·09 prey m−2, depending on methods), as was the average calorific value of the prey items (4·2 kJ g−1). Our study suggests that Great Cormorants remain highly successful predators even when exploiting modest prey resources. These findings have implications for our understanding of predator–prey relationships, and for the management of Great Cormorant populations.