The occurrence of non-native tubenose goby Proterorhinus semilunaris in the pelagic 0+ year fish assemblage of a central European reservoir

In July 2008, early juvenile tubenose goby Proterorhinus semilunaris were found in nocturnal pelagic waters of the Vranov Reservoir, Czech Republic. Presence of benthic-living prey in the guts of these fish suggested migration between benthic and pelagic habitats.     

The Origin and Initial Rise of Pelagic Cephalopods in the Ordovician

Background

During the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of todays open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.

Methodology/Principal Findings

We reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.

Conclusions/Significance

The exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.     

Biofilm and Diatom Succession on Polyethylene (PE) and Biodegradable Plastic Bags in Two Marine Habitats: Early Signs of Degradation in the Pelagic and Benthic Zone?

The production of biodegradable plastic is increasing. Given the augmented littering of these products an increasing input into the sea is expected. Previous laboratory experiments have shown that degradation of plastic starts within days to weeks. Little is known about the early composition and activity of biofilms found on biodegradable and conventional plastic debris and its correlation to degradation in the marine environment. In this study we investigated the early formation of biofilms on plastic shopper bags and its consequences for the degradation of plastic. Samples of polyethylene and biodegradable plastic were tested in the Mediterranean Sea for 15 and 33 days. The samples were distributed equally to a shallow benthic (sedimentary seafloor at 6 m water depth) and a pelagic habitat (3 m water depth) to compare the impact of these different environments on fouling and degradation. The amount of biofilm increased on both plastic types and in both habitats. The diatom abundance and diversity differed significantly between the habitats and the plastic types. Diatoms were more abundant on samples from the pelagic zone. We anticipate that specific surface properties of the polymer types induced different biofilm communities on both plastic types. Additionally, different environmental conditions between the benthic and pelagic experimental site such as light intensity and shear forces may have influenced unequal colonisation between these habitats. The oxygen production rate was negative for all samples, indicating that the initial biofilm on marine plastic litter consumes oxygen, regardless of the plastic type or if exposed in the pelagic or the benthic zone. Mechanical tests did not reveal degradation within one month of exposure. However, scanning electron microscopy (SEM) analysis displayed potential signs of degradation on the plastic surface, which differed between both plastic types. This study indicates that the early biofilm formation and composition are affected by the plastic type and habitat. Further, it reveals that already within two weeks biodegradable plastic shows signs of degradation in the benthic and pelagic habitat.     

More rapid shift to a benthic niche in larger Gadus morhua juveniles

Trophic use by Atlantic cod Gadus morhua juveniles was examined early and late in the shift from pelagic to benthic habitats. Changes in the proportion of pelagic copepods, estimates of benthic prey indicated by isotope mixing models and stable‐isotope values between sample periods suggested a gradual shift towards a benthic niche. Values of the trophic proxies, however, changed most markedly in the largest juvenile group, suggesting a more rapid trophic niche shift, and in turn competitive advantage, of larger juveniles.     

Some reflections about the structure of the pelagic zone of the brackish Lake Grevelingen (SW-Netherlands)

Summary The seasonal succession of the plankton in the marine brackish Lake Grevelingen, a closed sea arm in the S.W.-Netherlands, comprises the initial stagessensu Margalef and is characterized by predominantly small phytoplankton (flagellates, diatoms) and zooplankton (rotifers, tintinnids, copepods), maintaining relatively high levels of production from early spring (February) to late summer (September). The structure of the plankton in the course of seasonal succession is in agreement with the concepts of Margalef.Simplification of the pelagic food web in Lake Grevelingen has occurred as a consequence of the elimination of the tides. Some examples are given in relation to the composition of the phyto- and zooplankton and of its significance. The occurrence of rotifer-dominated zooplankton blooms in early spring is emphasized.Closed sea arms such as Lake Grevelingen, showing the same morphometry as the previous tidal estuary, contain extended shallow areas which influence strongly the pelagic zone. The abundance in the zooplankton of larval stages of several littoral-benthic species demonstrate these influences clearly. The shallows of the lake, occupied by eelgrass beds (Zostera marina) in summer, influence the pelagic zone in several ways: large quantities of detritus are given off after the growing season, sheltered habitats are supplied for small pelagic animals, and eelgrass leaves represent a substrate for epifauna species.Contribution no. 168 of the Delta Institute for Hydrobiological Research.     

Life histories as sampling devices: optimum egg size in pelagic fishes*

In teleosts, egg size is an important determinant of larval size. The latter determines where larvae start feeding in the size spectrum of biomass particles. Egg and larval size also determine the size range of predators exploiting these early ontogenetic stages. Thus, egg size is important in determining the particle size range sampled during ontogeny. However, egg size also largely determines how many can be produced by a female. Optimum egg size for marine, pelagic fishes was estimated using a modification of a model developed by Ware. A numerical analysis using empirical information on size and temperature dependent mortality and growth in pelagic, marine systems suggests that allometry of size-dependent mortality and temperature are significant factors in determining optimum egg size. Allometry of mortality will depend on the frequency of encounters between particles of different sizes. Both empirical and modelling studies of such frequencies in pelagic and other environments are required.     

Is coexistence mediated by microhabitat segregation? An in-depth exploration of a fish invasion

Vendace Coregonus albula has invaded the subarctic Pasvik hydrosystem, northern Norway and Russia, after being translocated from its native Finnish range into Lake Inari (Finland), upstream of Pasvik. The development of the invader and the effects on two native lake communities in the upstream and downstream part of the hydrosystem, respectively, have been monitored since 1991. In the upstream lake location, the invader relegated the native whitefish Coregonus lavaretus from the pelagic habitat within a few years. An expected similar development in the downstream location was, however, not observed. Data are presented on the diet and microhabitat use (vertical pelagic zone) for three types of replicated situation: (1) the invader v. the native species, (2) upstream v . downstream lakes and (3) early and late invasion stages, to explore the outcome of the competitive interactions. The use of different microhabitats available within the deeper pelagic zone in the downstream location was observed to have delayed the negative effects on native whitefish, possibly enabling a persistent coexistence between the two species in the pelagic zone of this lake.     

Climate control on the long-term anomalous changes of zooplankton communities in the Northwestern Mediterranean

In marine ecosystems, pelagic copepods, chaetognaths and jellyfish play a key role in matter and energy flow. While copepods support most food webs and the biological pump of carbon into the deep ocean, chaetognaths and jellyfish may affect the strength of the top-down control upon plankton communities. In this study, we show that the main events in the long-term variability of these functional groups in the Northwestern Mediterranean were tightly linked to changes of climate forcing of the North Atlantic sector. Large-scale climate forcing has altered the pelagic food-web dynamics through changes in biological interactions, competition and predation, leading to substantial changes manifested as bursts or collapses in zooplankton populations, and consequently to a major change ca. 1987. These events become more frequent in the 1980s and the early 1990s in the studied zooplankton functional groups suggesting a shift in the functioning of the pelagic ecosystem. The environmental modifications and the results reported here are therefore, indicators of a regime change pointing to a more regeneration-dominated system in the study area. We suggest a chain of mechanisms, whereby climate variation has modified the long-term dynamics of pelagic copepods, chaetognaths and jellyfish in the Ligurian Sea.     

Resting eggs, seasonal dynamics, and production of Bosmina longispina maritima (P. E. Muller) (Cladocera) in the northern Baltic proper

The seasonal dynamics and production of Bosmina longispina maritimawas studied both in the pelagic zone and bottom sediments ofthe northern Baltic proper in 1979. Resting eggs started tohatch in April–May, but the pelagic zone population didnot show signs of increase until August; this is typical forthe species in this area. Relatively few of those hatched inthe spring and early summer survived the low temperatures andlack of food below the thermocline. The population increasewas started with individuals hatched after the thermal stratificationweakened. From August until October the number of resting eggsin the sediment rose in direct response to their productionin the pelagic zone. A new hatching period began in October–November,but conditions in the pelagic zone were not suitable for theirfurther development. Such mistiming of hatching may, along withother factors, help to explain the large annual variations inthe productivity of B. longispina maritima in the Baltic. Sexualreproduction was found to be of relatively greater importancein the study area than in the areas where Bosmina reaches higherproductivity. This was assumed to be due to more intense selectionpressures in the prevailing conditions.     

Environmental variability, early life-history traits, and survival of new coral reef fish recruits

For benthic marine organisms with complex life cycles, conditionsexperienced by pelagic larvae can influence juvenile survival.Trait-specific selective mortality has been documented in thelaboratory and field, yet our knowledge of the factors contributingto the existence, strength, and consistency of natural selectivemortality is limited. We compiled previously published and unpublisheddata on the common Caribbean coral reef fish, Thalassoma bifasciatum,recruiting to Barbados, West Indies, and the upper Florida Keysto examine how environmental variability during pelagic larvallife influences the distribution of early life-history traitsexhibited by new recruits. We explored how the scope of variabilityin otolith-derived traits such as larval growth, pelagic larvalduration (PLD), size and condition at settlement, and earlyjuvenile growth influences the degree to which mortality ofjuveniles is selective. At both locations, contrasting oceanographicconditions (periodic passage of large low-salinity North BrazilCurrent [NBC] rings near Barbados and seasonal variation inwater temperature at Florida) led to significant differencesin the early life-history traits of recruits. Mortality wasmost frequently selective for the two most variable traits,condition at settlement and early juvenile growth. Environmentalvariability (including variation in predation pressure and stress-inducingconditions) also likely influences juvenile mortality and consequentlythe degree to which selective loss of particular traits occurs.As we begin to better understand the spatial, temporal, andspecies-specific circumstances in which events occurring duringlarval life influence juvenile performance, studies must alsobe extended to examine how these processes are translated intoadult fitness.     

Carbon and nitrogen stable isotopic inventory of the most abundant demersal fish captured by benthic gears in southwestern Iceland (North Atlantic)

Stable isotopes (δ¹³C and δ¹⁵N) were used to examine the origin of organic matter for the most representative demersal species of the SW Icelandic fishery, accounting for over 70% of landings of those species in the North Atlantic. Samples were collected during a 2-week period in early September 2004 from landings and directly during fishing cruises. Stable isotopes showed that particulate organic matter and sedimentary organic matter were at the base of the food web and appeared to fill two different compartments: the pelagic and the benthic. The pelagic realm was composed of only capelin and sandeel; krill and redfish occupied an intermediate position between pelagic and benthic realms; while anglerfish, haddock, cod and ling resulted as the true demersal species while tusk, rays and plaice were strongly linked to the benthic habitat.     

Morphological and functional development of characters associated with settlement in the yellowfin goby, <Emphasis Type="Italic">Acanthogobius flavimanus</Emphasis>

Morphological and functional development of characters from pelagic larval to benthic juvenile periods in the yellowfin goby, Acanthogobius flavimanus, were examined on the basis of 275 specimens (9.4–25.9mm in standard length) collected in shallow water in the Tama River estuary, central Japan. Judging from the development of osteological features, late pelagic juveniles possessed fully developed swimming and feeding abilities. Rapid changes in various characters, including the formation of a loop-shaped gut, increases in condition factor and teeth number, broader upper-field view capability, and extended scaly area and pigmentation development, occurred during the transition from late pelagic to early benthic periods. Most of the changes were associated with structural and functional aspects enabling more efficient use of benthic resources or the avoidance of predators in the benthic habitat.     

The influence of light on the diel vertical migration of young-of-the-year burbot Lota lota in Lake Constance

The diel vertical distribution of young-of-the-year (YOY) burbot Lota lota in the pelagic zone of Lake Constance was compared to light intensity at the surface and to the light intensity at their mean depth. Lota lota larvae inhabited the pelagic zone of Lake Constance from the beginning of May until the end of August. From early June, after the stratification of the water column, fish performed diel vertical migrations (DVM) between the hypolimnion and epilimnion. The amplitude of DVM increased constantly during the summer and reached 70 m by the end of August. Lota lota started their ascent to the surface after sunset and descended into the hypolimnion after sunrise. As the YOY fish grew from May to August, they experienced decreasing diel maximum light intensities: in May and early June L. lota spent the day at light intensities >40 W m⁻², but they never experienced light intensities >0·1 W m⁻² after the end of June. From this time, L. lota experienced the brightest light intensities during dusk and dawn, suggesting feeding opportunities at crepuscular hours. The present study implies, that YOY L. lota in the pelagic zone of Lake Constance increased their DVM amplitude during the summer to counteract a perceived predation risk related to body size and pigmentation.     

Feeding ecology of pelagic fish larvae and juveniles in slope waters of the Gulf of Mexico

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with δ¹³C values of POM depleted by 3–6‰ and δ¹⁵N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean δ¹³C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean δ¹⁵N values ranged 4–5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing δ¹³C or δ¹⁵N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two‐source mixing model suggest approximately an equal contribution of organic matter by both sources (POM = 55%; pelagic Sargassum spp. = 45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a δ¹³C fractionation change of ±0·25‰ or a δ¹⁵N fractionation change of ± 1·0‰ relative to original fractionation values.     

An alternative mode of larval development inScoloplos armiger (O. F. Müller, 1776) (Polychaeta,Orbiniidae)

During a three-year plankton monitoring at the island of Helgoland (German Bight, North Sea), pelagic larvae ofScoloplos armiger (O. F. Müller, 1776) were observed to occur regularly from February to April. Up to the present day, only benthic development within egg cocoons has been described (Anderson, 1959). Planktonic larvae comprise early, ovoid trochophores up to advanced stages with 11 setigers. The main morphological difference between the two larval types is the more numerous differentiation of lateral cilia by the pelagic larvae. Annual alterations in appearance and abundance suggest that reproduction is correlated with water temperature.     

CRETACEOUS PLANKTONIC FORAMINIFERA FROM BORNHOLM AND THEIR ZOOGEOGRAPHIC SIGNIFICANCE

Late Cretaceous formations exposed along the south coast of Bornholm Island, Denmark, contain well preserved assemblages of planktonic foraminifera, which are described and illustrated. Two taxa, Whiteinella baltica and Hedbergella bornholmensis , are described as new. The oldest assemblage, Middle to early Upper Cenomanian in age, is essentially monospecific and indicative of a restricted pelagic environment. The overlying Lower Senonian faunules are diverse, contain large populations which are associated with other pelagic microfossils, and provide a more typical example of Boreal planktonic foraminiferal associations. Taxonomically the assemblages are largely composed of species of Hedbergella, Whiteinella, Archaeoglobigerina, Globigerinelloides , and Heterohelix and double-keeled species of Globotruncana , particularly G. marginata. These species are widely distributed in space and time. Thus Bornholm assemblages have a cosmopolitan aspect, which can be identified in microfaunas from the Western Interior of North America and Alaska. Because of the lower diversity, lack of restricted stratigraphic markers, Lower Senonian correlations between Bornholm, and probably Boreal pelagic microfaunas generally, coeval Tethyan assemblages are less precise than within the Tethys.     

Disappearance of a spring cohort in a population of the dragonet,Repomucenus valenciennei, with spring and autumn spawning peaks in Tokyo Bay, Japan

The occurrence of eggs, pelagic larvae and juveniles, and settled juveniles of the dragonetRepomucenus valenciennei in Tokyo Bay, Japan, were investigated by plankton net and bottom trawl surveys between September 1990 and September 1991. Eggs, and pelagic larvae and juveniles appeared from April to November (spring to autumn), peaking in both spring and autumn. From the temporal pattern of egg and pelagic fish occurrence, and pelagic duration reported elsewhere (ca. one month), settlement could be predicted as occurring from late spring to autumn. However, settled juveniles appeared from August to December, with an abrupt peak in November. Aging from daily increments in the otoliths of settled recruits in 1990 indicated that the latter comprised individuals which had hatched between mid-September and early November (i.e. autumn cohort), implying that individuals which had hatched in spring to summer (April to August) were not recruited. Benthic hypoxia occurs widely in Tokyo Bay, from June to October each year, particularly in the northern part, which is the main nursery area ofR. valenciennei. The timing of dissolved oxygen recovery, and appearance of settled fish coincided closely (i.e. November), indicating that summer hypoxic conditions prevented the settlement of the spring cohort and hence recruitment to the population.     

On the Importance of Planktonic Protozoans in the Eutrophication Process of the Baltic Sea

Analysing the results of various authors recent studies in the pelagic region of the Baltic revealed that protozoan biomass is in the same range or even higher than metazooplankton biomass. The dominant groups of planktonic protozoans are heterotrophic pico- and nanoflagellates (various taxonomic groups), large heterotrophic flagellates (mainly dinoflagellates) and ciliates. Regularly the spring bloom of phytoplankton is accompanied by a maximum of protozoan biomass which declines in early summer as a result of intensive grazing pressure by metazooplankton and changing food conditions. The analysis of results from different stations indicated that biomasses of protozoans increase with an increasing degree of eutrophication. Several trophic levels within the microbial web should be added to the traditional view on the pelagic food web of the Baltic. Our knowledge regarding the quantitative aspect of the microbial matter flux of the Baltic is very limited up to now and complex ecological (and taxonomical) studies using standardized methods including all protozoan components are necessary. Protozoans (various trophic groups and levels), besides bacteria, should be viewed as the metabolically most active heterotrophic component in the pelagic region of the Baltic, their activity should increase with an increasing degree of eutrophication.     

Larval and early post-larval stages in the abbreviated development of Heterosquilla tricarinata (Claus, 1871) (Crustacea, Stomatopoda)

Heterosquilla tricarinata was laboratory-cultured through itscomplete larval development and found to have one propelagicand two pelagic larval stages prior to metamorphosis to thejuvenile. These larval stages, the first juvenile, and relevantportions of the second juvenile stage, are described and figured.Individual larvae do not change in size during intermoult periods.Larvae occurring in the plankton show a progressive decreasein mean size between early (September) and late (November) springtime.Reasons for this are suggested. The first pelagic stage of H.tricarinata is anatomically very advanced in development, andthe number of pelagic stages very few, in comparison with otherknown stomatopod life-histories. Ecological implications ofthis are discussed in relation to the high-latitude distributionof the species. Comparison is made between the final pelagicstage of H. tricarinata and that of its congenor H. brazieri. ^*Permanent address: Zoology Department, University of Queensland,Brisbane, Australia     

Habitat use of arctic charrSalvelinus alpinus in Thingvallavatn,Iceland

Synopsis Habitat use by four morphs of arctic charr,Salvelinus alpinus, was investigated in Thingvallavatn, Iceland, by sampling with pelagic and benthic gill nets. Sampling was done in May/June and August/September. Greatest abundance of fish was recorded in the littoral and epipelagic zone in early autumn. Catches were low in early summer. The four morphs are partly segregated in habitat. Small (SB-) and large benthivorous (LB-) charr have a more restricted spatial distribution than piscivorous (PI-), and especially planktivorous (PL-) charr. Both benthivorous morphs are mainly found in the littoral zone, and occur in largest numbers in stony shallows at depths between 0 and 10 m. PL-charr usually dominates in numbers in all habitats. PI-charr is most abundant in epibenthic habitats, although numbers are always low. All morphs are caught in higher numbers at night than during the day, but the diurnal activity difference is highest among SB-charr. The habitat use by different morphs is as may be expected from their morphology and diets. Within the population of PL-charr, young and small fish are more abundant on the bottom than in the pelagic zone, and there is a surplus of females in the pelagic zone. Along the benthic profile, young, small and immature PL-charr are more abundant in deep than in shallow waters. The results are discussed in relation to food supply, competition and predation. Possible reasons for the occurrence of four arctic charr morphs are also discussed.Contribution from the Thingvallavatn project.