Midwater trawl catches of adolescent and adult anguilliform fishes during the Sargasso Sea Eel Expedition 1979

During the research program on the biology and migration ofAnguilla spp. carried out with F.R.V. Anton Dohrn in 1979, approximately 1300 adolescent and adult anguilliform individuals were caught covering 8 families, 10 genera and 12 species. Observations on each of these species, including horizontal and vertical distributional patterns, are dealt with herein. The appearance of various species in hauls and the absence of adultAnguilla spp. in the catches obtained are discussed.     

Prochlorococcus ecotype abundances in the North Atlantic Ocean as revealed by an improved quantitative PCR method

The cyanobacterium Prochlorococcus numerically dominates the photosynthetic community in the tropical and subtropical regions of the world's oceans. Six evolutionary lineages of Prochlorococcus have been described, and their distinctive physiologies and genomes indicate that these lineages are "ecotypes" and should have different oceanic distributions. Two methods recently developed to quantify these ecotypes in the field, probe hybridization and quantitative PCR (QPCR), have shown that this is indeed the case. To facilitate a global investigation of these ecotypes, we modified our QPCR protocol to significantly increase its speed, sensitivity, and accessibility and validated the method in the western and eastern North Atlantic Ocean. We showed that all six ecotypes had distinct distributions that varied with depth and location, and, with the exception of the deeper waters at the western North Atlantic site, the total Prochlorococcus counts determined by QPCR matched the total counts measured by flow cytometry. Clone library analyses of the deeper western North Atlantic waters revealed ecotypes that are not represented in the culture collections with which the QPCR primers were designed, explaining this discrepancy. Finally, similar patterns of relative ecotype abundance were obtained in QPCR and probe hybridization analyses of the same field samples, which could allow comparisons between studies.     

Water mass-specificity of bacterial communities in the North Atlantic revealed by massively parallel sequencing

Bacterial assemblages from subsurface (100 m depth), meso- (200-1000 m depth) and bathy-pelagic (below 1000 m depth) zones at 10 stations along a North Atlantic Ocean transect from 60°N to 5°S were characterized using massively parallel pyrotag sequencing of the V6 region of the 16S rRNA gene (V6 pyrotags). In a dataset of more than 830,000 pyrotags, we identified 10,780 OTUs of which 52% were singletons. The singletons accounted for less than 2% of the OTU abundance, whereas the 100 and 1000 most abundant OTUs represented 80% and 96% respectively of all recovered OTUs. Non-metric Multi-Dimensional Scaling and Canonical Correspondence Analysis of all the OTUs excluding the singletons revealed a clear clustering of the bacterial communities according to the water masses. More than 80% of the 1000 most abundant OTUs corresponded to Proteobacteria of which 55% were Alphaproteobacteria, mostly composed of the SAR11 cluster. Gammaproteobacteria increased with depth and included a relatively large number of OTUs belonging to Alteromonadales and Oceanospirillales. The bathypelagic zone showed higher taxonomic evenness than the overlying waters, albeit bacterial diversity was remarkably variable. Both abundant and low-abundance OTUs were responsible for the distinct bacterial communities characterizing the major deep-water masses. Taken together, our results reveal that deep-water masses act as bio-oceanographic islands for bacterioplankton leading to water mass-specific bacterial communities in the deep waters of the Atlantic.     

Seasonal succession in the pelagic ecosystem of the North Atlantic and the utilization of nitrogen

Observations during the Joint Global Ocean Flux Study (JGOFS)North Atlantic Bloom Experiment in 1989 are compared with amixed-layer model of phytoplankton seasonal succession in whichthe latitudinal variation of the succession is driven by physicalforcing. In the model, the first phytoplankton groups to growat the end of the winter are those with the fastest intrinsicgrowth rates: the phytoflagellates and picophytoplankton. Theincreases of these groups are soon halted by the rapid growthof microzooplankton and heteroflagellates whereupon they aresucceeded by diatoms, the next fastest growers. With depletionof silicate, the diatom bloom ends and is briefly replaced byphytoflagellates and picophytoplankton, before these in turnare replaced by slower growing dinoflagellates. Differencesin the physical forcing cause the timings and magnitudes ofthese changes to vary with latitude. By sampling the model resultsat the times and places of the JGOFS observations, it is shownthat the major changes of populations and nutrients are reproduced,as are many production and grazing rates. The model resultssuggest the nature of nutrient utilization in the region. Whilenitrate and silicate are both reduced to low values at 47N,only silicate is depleted at 60N. Nitrate is not depleted atnorthern latitudes due to the greater depth of the mixed layer,more intense vertical mixing and the shorter season, so thatphytoplankton have more nutrients to utilize and a shorter timein which to do it. Phytoflagellates and picophytoplankton areunable to utilize all the inorganic nitrogen because of thegrazing by the micrograzers, and diatoms because of silicatedepletion. Dinoflagellates are slow growing and only have sufficienttime to deplete the nitrogen at low latitudes. There is no needto invoke limitation by a trace nutrient, such as iron, to reproducethe events in the NE Atlantic during 1989.     

Notes on feeding ecology of macrourid fishes from the mid-Atlantic Ridge, North Atlantic

At least 16 macrourid fishes (Gadiformes, Macrouridae) inhabit the mid-Atlantic Ridge of the North Atlantic. To investigate the roles of macrourids in the food-web and compare feeding patterns of the most frequent co-occurring species, diet information on five of the species were described and compared. Pelagic and benthopelagic copepods were the most numerous prey but did not contribute much on a weight basis. Cephalopods were by far the most important prey of the small grenadiers, while shrimps and fish became increasingly significant with increasing size. Previous studies from other areas have also found pelagic prey to be important, but in contrast to this study, cephalopods were more important on the mid-Atlantic Ridge than in continental margin locations.     

The elemental composition of swimbladders in some deepwater fishes of the North Atlantic

The first data on the elemental contents of the swimbladder wall of three deepwater fish species from the North Atlantic (the snubnosed spiny eel Notacanthus chemnitzii, the Kaup’s arrowtooth eel Synaphobranchus kaupii, and the roughhead grenadier Macrourus berglax) are presented. It is supposed that survival of specimens of the first two species upon their ascent from great depths that subject them to sharp changes of pressure is connected with the properties of the durable and flexible swimbladder walls with increased contents of some metals (aluminum, copper, iron, mercury, potassium, manganese, chromium, and cadmium). Adaptation to sharp changes of pressure in N. chemnitzii and S. kaupii probably develops during ontogeny, during the transition from habitation in the epipelagic zone during the larval stage to the deepwater habitat of the adult fish.     

In-stream behaviour of threatened fishes and their food organisms based on remote video monitoring

Can remote underwater video be used to investigate the in-stream behaviour of small fishes and decapods? Diel activity of two threatened freshwater fishes (Macquaria australasica and Gadopsis bispinosus), a palaemonid prawn and an atyid shrimp, was established from remote underwater video in a pool of an upland stream in the current study. Decapods and large fishes (>5 cm TL) were nocturnal, whereas, small fishes (<5 cm TL) were diurnal. The suitability of using underwater video to quantify short-term (seconds) behavioural interaction among individuals was also demonstrated, with early-juvenile-phase Gadopsis bispinosus exhibiting interference competition on 35% of occasions when two or more individuals were observed. This study demonstrates that remote underwater video is useful for observing the in-stream behaviour of threatened freshwater fauna where other techniques are not viable, and presents sub-sampling of video as a means of reducing video processing time in assessing fish diel activity patterns.     

Inputs,losses and transformations of nitrogen and phosphorus in the pelagic North Atlantic Ocean

The North Atlantic Ocean receives the largest allochthonous supplies of nitrogen of any ocean basin because of the close proximity of industrialized nations. In this paper, we describe the major standing stocks, fluxes and transformations of nitrogen (N) and phosphorus (P) in the pelagic regions of the North Atlantic, as one part of a larger effort to understand the entire N and P budgets in the North Atlantic Ocean, its watersheds and overlying atmosphere. The primary focus is on nitrogen, however, we consider both nitrogen and phosphorus because of the close inter-relationship between the N and P cycles in the ocean. The oceanic standing stocks of N and P are orders of magnitude larger than the annual amount transported off continents or deposited from the atmosphere. Atmospheric deposition can have an impact on oceanic nitrogen cycling at locations near the coasts where atmospheric sources are large, or in the centers of the highly stratified gyres where little nitrate is supplied to the surface by vertical mixing of the ocean. All of the reactive nitrogen transported to the coasts in rivers is denitrified or buried in the estuaries or on the continental shelves and an oceanic source of nitrate of 0.7–0.95 × 10¹² moles NO₃^–1 y^–1 is required to supply the remainder of the shelf denitrification (Nixon et al., this volume). The horizontal fluxes of nitrate caused by the ocean circulation are both large and uncertain. Even the sign of the transport across the equator is uncertain and this precludes a conclusion on whether the North Atlantic Ocean as a whole is a net source or sink of nitrate. We identify a source of nitrate of 3.7–6.4 × 10¹² moles NO₃^– y^–1 within the main thermocline of the Sargasso Sea that we infer is caused by nitrogen fixation. This nitrate source may explain the nitrate divergence observed by Rintoul & Wunsch (1991) in the mid-latitude gyre. The magnitude of nitrogen fixation inferred from this nitrate source would exceed previous estimates of global nitrogen fixation. Nitrogen fixation requires substantial quantities of iron as a micro-nutrient and the calculated iron requirement is comparable to the rates supplied by the deposition of iron associated with Saharan dust. Interannual variability in dust inputs is large and could cause comparable signals in the nitrogen fixation rate. The balance of the fluxes across the basin boundaries suggest that the total stocks of nitrate and phosphate in the North Atlantic may be increasing on time-scales of centuries. Some of the imbalance is related to the inferred nitrogen fixation in the gyre and the atmospheric deposition of nitrogen, both of which may be influenced by human activities. However, the fluxes of dissolved organic nutrients are almost completely unknown and they have the potential to alter our perception of the overall mass balance of the North Atlantic Ocean.     

Identification of pelagic eggs of marine fishes by rearing method

Pelagic fish eggs are usually fixed by formalin solution just after collection in the field. Most of these eggs are difficult to be identified to species, because only limited characters are available for identification and considerable changes in structures take place with fixation. In this study, species names of formalin fixed eggs were estimated by comparing their egg diameters and the diameter and number of oil globules with those of the fresh ones identified by the rearing method. Materials were collected in Wakasa Bay in May, August, and October, 1979–1982. Fresh eggs were classified into 40 species or types by the rearing method, but fixed eggs were divided into only 24 types. Comparison of fresh and fixed eggs revealed that 7 out of 24 types of fixed eggs were referable to species, and 13 types were composed of species groups. The rearing method is surely a useful step in indentifying species names and/or species types of fixed eggs.     

On the origin of the North Atlantic redfish of the genus Sebastes (Scorpaenidae), as revealed by analysis of their parasite fauna

A comparative analysis of the parasite fauna of the Sebastes redfishes, which inhabit the North Atlantic and the North Pacific, was carried out. The relative youth of the host-parasite relationship in the North Atlantic Sebastes corroborates the hypothesis of their Pacific origin. The species attribution of the ancestor of the North Atlantic Sebastes is suggested.     

On the utilization of food by planktophage fishes

Making some plausible assumptions about the over-all mechanism of food catching and consumption by fishes and evaluating in the light of those assumptions some available experimental data, it is possible to calculate from those data the variation of several important factors with the concentration of food. The factors considered are: total rate of metabolism, total diurnal energy expenditure in the process of feeding, average number of hours per day during which the fish feeds, average length of path traveled by a fish per day, and the so-called “energetic coefficient of growth.” A possible relation with the work of N. Rashevsky (Bull. Math. Biophysics,20, 299–308, 1959) is discussed.     

New plagioporines (Digenea: Opecoelidae) from deep-sea fishes of the North Atlantic Ocean

Three new species of digenean are described from demersal fishes taken in deep waters of the northwestern and northeastern Atlantic Ocean: Podocotyle schistotesticulata n. sp. from Antimora rostrata; P. harrisae n. sp. from Coryphaenoides (Lionurus) carapinus; and Gaevskajatrema halosauropsi n. sp. from Halosauropsis macrochir. The status of the nominal species of the genus Podocotyle Dujardin, 1845 is presented in annotated summary, along with a tabulation of the distinctive characteristics of recognised species of the genus. Gaevskajatrema Gibson & Bray, 1982 is discussed and its diagnosis amended.     

Vital role of drift algae in the life history of the pelagic harpacticoid Parathalestris croni in the northern North Atlantic

The harpacticoid copepod Parathalestris croni, a widespreadpelagic species of the northern North Atlantic, was found tobe closely associated with seaweed originating in the intertidalzone floating on the surface of the sea around Iceland. Theabundance of the copepod increased with distance from the shoreand it showed a special affinity for the brown alga Ascophyllumnodosum. The copepods in the macroalgal clumps were mostly ovigerousfemales, and the seaweed clearly serves a nest function. Theeggs were deposited on the seaweed where the nauplii, whichare unable to swim, crawl on the algae until they metamorphoseinto the first copepodite stage (CI). The CI copepodites leavethe floating weed, and the juvenile stages may eventually dispersewidely in the water column until floating weed is colonized,probably by adults or juveniles in the last one or two copepoditestagea Parathalestris croni is one of a group of vety few speciesknown or suspected to be dependent for survival on the extremelyunpredictable habitat of floating weed in the northern NorthAtlantic.     

Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC—potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L^?1), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.