Larval Concentrations Research Articles

A comparison of spring larval fish assemblages in the Strait of Georgia (British Columbia, Canada) between the early 1980s and late 2000s

The concentration and composition of the larval fish assemblage in the Strait of Georgia (British Columbia, Canada) has changed between the early 1980s (1980 and 1981) and the late 2000s (2007, 2009 and 2010). During both periods, the spring larval fish assemblages were dominated by pelagic species: Clupea pallasi (Pacific herring), Merluccius productus (Pacific hake), Leuroglossus schmidti (northern smoothtongue) and Theragra chalcogramma (walleye Pollock). The average concentration of Merluccius productus, Theragra chalcogramma, Leuroglossus schmidti, and Sebastes spp. declined between the early 1980s and the late 2000s; in contrast, the absolute concentration and proportion of Pleuronectidae and several demersal fish taxa increased in the spring larval assemblage. Examination of the associations between larval fish assemblages and environmental fluctuations suggests that large-scale climate processes are potential contributors to variations in overall larval concentrations of the dominant taxa and assemblage composition in the Strait of Georgia.

Diversity and distribution of early life stages of carangid fishes in the northwestern Mediterranean: responses to environmental drivers

AbstractThe diversity and distribution of the early stages of carangid fishes were investigated in relation to environmental conditions off the Catalan Coast (NW Mediterranean). Data were obtained during four oceanographic cruises conducted in spring–summer 2003 and 2004. A total of 4743 larvae from seven species and five genera were identified. Most were thermophilic species very abundant in the warm waters of the south and eastern Mediterranean. The presence of larvae and juveniles of Caranx crysos, Caranx rhonchus, Seriola dumerili and Trachinotus ovatus in the northwestern Mediterranean is reported for the first time. Except for Trachurus trachurus, which spawns in winter–spring, all species reproduce in spring–summer with a temporal succession in their spawning peaks. On average, the spatial patterns of different species (except T. trachurus) showed common features: a clear preference for spawning in coastal areas, high abundance of larvae in the south warmer zone in relation to the surface productive waters of Ebro river runoff and relatively high larval concentrations south of the thermal front and its associated anticyclonic eddy. The presence of larvae of thermophilic species of carangids in the northwestern Mediterranean indicates successful reproduction and establishment of these species in the cold part of the basin.

A study of the effect of Bacillus thuringiensis serotype H14 (subspecies israelensis) delta endotoxin on Musca larva.

House flies (Musca domestica) are of major public health concern in areas with poor sanitation and hygiene conditions. Biological control through the use of parasitoids and pathogens is one of the alternatives to the use of chemical pesticides for control of insects of public health importance. The effects of the delta endotoxin of Bacillus thuringiensis on house fly larval mortality were studied. Gel filtration and SDS-PAGE methods were used for separation and purification of proteins. Delta endotoxin was incubated with larvae in concentrations of 0.43 mg/mL and 0.27 mg/mL in bioassay tests. The results of this study indicated protein crystal toxicity against larvae of the house fly. A concentration of 0.43 mg/mL of this toxin caused 100% mortality in house fly larvae. The LD50 amount of these toxins was calculated as 125 µg/g. The results of this study suggest that the use of the protein crystal including delta endotoxin of Bacillus thuringiensis serotype H14 is an effective weapon in the biological fight against the house fly.

Inhibited reproductive capacity of Egyptian cotton leaf worm Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) by the chitin synthesis inhibitor Novaluron.

For evaluating the effects of Novaluron on various parameters of the reproductive capacity of Spodoptera littoralis, a concentration range of 1.000.0001ppm was applied on the newly moulted penultimate (5 th ) instar larvae and a concentration range of 0.10-0.0001ppm was applied on the newly moulted last instar larvae. A predominant inhibitory effect of Novaluron was exhibited on the oviposition efficiency since the oviposition rate was seriously regressed in no certain trend, regardless the time of larval treatment and concentration level. Treatment of penultimate or last instar larvae with Novaluron resulted in drastically reduced fecundity in a dose-dependent course. A reducing action of Novaluron was exerted also on fertility after treatment of larvae with different concentration levels, regardless the time of treatment. After treatment of penultimate or last instar larvae with Novaluron, the embryonic development had been generally subjected to a retarding effect since the incubation period was pronouncedly prolonged, regardless the concentration level.

Sympatric Clupeoid Fish Larvae in the Northeastern Mediterranean: Coexistence or Avoidance?

The summer (July 2002 and 2003) larval concentrations of two commercially important clupeoid species (order: Clupeiformes), the European anchovy (Engraulis encrasicolus) and round sardinella (Sardinella aurita), were measured in Kavala Gulf (northern Aegean Sea), in order to examine their distribution patterns. Anchovy was generally most abundant, reaching 13 852 larvae 10 m−2 in 2003 and occurred in more stations compared to round sardinella. Despite the spawning of both species occurring across the entire gulf, the general spatial distribution of their larvae was different, with European anchovy larvae occupying the entire gulf and round sardinella larvae mainly concentrated at offshore stations. The reason proposed for the spatial segregation of their larvae is direct competition for resources. Yet, coexistence of the two species was observed in frontal areas with abundant food resources that prevent competition.

Metamorphosis alters contaminants and chemical tracers in insects: implications for food webs.

Insects are integral to most freshwater and terrestrial food webs, but due to their accumulation of environmental pollutants they are also contaminant vectors that threaten reproduction, development, and survival of consumers. Metamorphosis from larvae to adult can cause large chemical changes in insects, altering contaminant concentrations and fractionation of chemical tracers used to establish contaminant biomagnification in food webs, but no framework exists for predicting and managing these effects. We analyzed data from 39 studies of 68 analytes (stable isotopes and contaminants), and found that metamorphosis effects varied greatly. δ(15)N, widely used to estimate relative trophic position in biomagnification studies, was enriched by ∼ 1‰ during metamorphosis, while δ(13)C used to estimate diet, was similar in larvae and adults. Metals and polycyclic aromatic hydrocarbons (PAHs) were predominantly lost during metamorphosis leading to ∼ 2 to 125-fold higher larval concentrations and higher exposure risks for predators of larvae compared to predators of adults. In contrast, manufactured organic contaminants (such as polychlorinated biphenyls) were retained and concentrated in adults, causing up to ∼ 3-fold higher adult concentrations and higher exposure risks to predators of adult insects. Both food web studies and contaminant management and mitigation strategies need to consider how metamorphosis affects the movement of materials between habitats and ecosystems, with special regard for aquatic-terrestrial linkages.

Numerical simulations of larval transport into a rip‐channeled surf zone

Competent larvae of intertidal invertebrates have to migrate toward shore for settlement; however, their migration through the surf zone is not understood. We investigated larval transport mechanisms at a rip‐channeled beach. Because tracking larvae in the surf zone is infeasible, we used a three‐dimensional biophysical model to simulate the processes. The coupled model consists of a physical module for currents and waves, and a biological module for adding larval traits and behaviors as well as Stokes drift to Lagrangian particles. Model calculations were performed with and without onshore wind forcing. Without wind, wave‐driven onshore streaming occurs in the bottom boundary layer outside the surf zone. With onshore wind, onshore currents occur near the surface. In the surf zone, offshore‐directed rip currents and compensating onshore‐directed currents over shoals are formed in both no‐wind and wind cases. In the biological module, neutral, negative, and positive buoyant particles were released offshore. Additionally, particles either sank in the presence of turbulence or not. Two scenarios achieved successful onshore migration: Negatively buoyant larvae without wind forcing sink in the turbulent bottom boundary layer and are carried onshore by streaming; positively buoyant larvae drift toward shore in wind‐driven surface currents to the surf zone, then sink in the turbulent surf zone and remain near the bottom while transported shoreward. In both cases, the larval concentration is highest in the rip channel, consistent with field data. This successful result is only obtained if turbulence‐dependent sinking behavior and Stokes drift are included in the transport of larvae.

Distributions of invertebrate larvae and phytoplankton in a coastal upwelling system retention zone and peripheral front

Near-coastal retention of larvae affects the ecology of many marine species. In coastal upwelling ecosystems having strong offshore transport, larval ecology is greatly influenced by nearshore retention in bays and in the lee of headlands. Further, frontal dynamics along the periphery of retention zones can drive larval accumulation and transport. The purpose of this study is to examine larval distributions and associated processes across a retention zone and its peripheral front in the coastal upwelling environment of Monterey Bay, California, USA. During fall 2009 an autonomous underwater vehicle (AUV) was used to observe environmental variability at high resolution and acquire targeted water samples. Invertebrate larvae in samples were subsequently identified and quantified using molecular methods. To infer ecological processes we examine larval distributions in relation to environmental processes revealed by the AUV data and the greater regional observing system. As a window into biological–physical interactions that may concentrate motile larvae in convergence zones, we examine more extensive in situ and remote sensing observations that describe distribution patterns of motile phytoplankton. During the 10-day study intensification of upwelling caused flow of cold water into the bay and formation of an upwelling front. Drifter and satellite observations showed retention of near-surface water within the bay inshore of the front, where a bloom of motile phytoplankton intensified. Larval distributions were related to processes inferred at a range of scales. At the scale of the retention zone, dense phytoplankton accumulations indicated concentration of motile plankton in a convergence zone created by flow toward the coast, as well as nutritional support for larvae. At the scale of the front, velocity and water property measurements indicated convergence between cold deep-shelf water transported shoreward along shoaling bathymetry and the overlying warm surface water, influencing plankton accumulation and vertical transport. At the finest scales resolved, aerial photography revealed banded accumulations of dense phytoplankton bloom patches and narrow foam lines, common indicators of small-scale convergence zones and consistent with internal wave processes. Exceptionally high larval concentrations were detected in samples from locations affected by frontal and internal wave dynamics. This study illustrates how autonomous feature recognition and targeted sampling with an AUV, applied within the greater context of multidisciplinary observation across regional to small scales, can advance plankton ecology research.

The effect of mesoscale oceanographic features on the distribution of mussel larvae along the south coast of South Africa

The coupling of benthic and pelagic life stages of marine invertebrates is central to understanding their population dynamics and is influenced by hydrodynamic processes at multiple spatial scales. Here we provide a regional scale study that identifies mesoscale oceanographic features linked to the distribution of mussel larvae. We surveyed the distribution of mussel larvae along 12 shelf to slope transects within an alongshore distance of about 700km along the Agulhas Bank off South Africa. Oceanographic and larval abundance data were collected simultaneously to determine how larval distribution is related to physical factors. Larval abundances were most closely correlated with temperature, salinity, beam attenuation (which indicates particle concentrations), and zonal (east–west) water velocity. In most regions, the highest larval concentrations were observed within an average distance of 3.7km from the coastline. Three mesoscale oceanographic mechanisms were identified that could explain larval distribution on separate parts of the coast. These were: a solitary meander whose leading end advects the Agulhas Current onto the continental shelf as an Agulhas Current Meander and whose lagging end advects the Agulhas Current offshore as a Natal Pulse, and the accumulation of larvae in a region with known internal wave activity. The Agulhas Current Meander swept large numbers of recently “spawned” larvae up to 30km offshore, acting as a corridor for larval loss, or a population sink. Onshore retention seems, however, to be generally characteristic of this region, when the Agulhas Current is not on the continental shelf. This study highlights how mesoscale features are associated with larval density, transport, and coupling between the pelagic and benthic systems of the Agulhas Bank.

A model of life cycle, connectivity and population stability of benthic macro-invertebrates in the North Sea/Baltic Sea transition zone

The life cycle of many benthic macro-invertebrates is characterised by a planktonic life stage where larvae are transported by ocean currents and mixing and after some time they reach a stage where they settle onto the ocean bottom and, provided a suitable habitat is available, initiate their benthic life stage as juvenile organisms and complete their life cycle when they mature and spawn as adults. Such age-specific behaviour is in general difficult to include in large scale ocean models unless dispersal is considered on an individual basis, i.e. individual based model coupled with a Lagrangian description of the flow field. However, here we address this issue in an Eulerian framework and develop and apply a new life cycle and population dynamical model of benthic fauna and implement the model in a high-resolution three-dimensional circulation model of the North Sea/Baltic Sea transition zone. The model explicitly describes specific life stages of the population and considers the different processes affecting the organisms during their life cycle, e.g. spawning, dispersion and settling. The model considers different life stages of an idealised marine organism, representing a typical benthic macro-invertebrate species in the area. Populations of juvenile and adult benthic organisms are maintained by spawning, occurring regularly every spring, and subsequent settling of larvae. The pelagic larval stages are simulated by a larval concentration distribution function, i.e. discrete age-classes of the total larval concentration, and age-specific physiological processes, as the onset of their settling behaviour, is explicitly accounted for in the model. Model simulations show, in general, a large connectivity between habitats in the northern and southern part of the area but also that self-recruitment is sufficient to sustain the two populations independently. A sensitivity study were carried out with the spawning rate as a control parameter and two non-trivial quasi-stationary steady states of benthic biomass distributions were identified, characterised by a high and low distribution of organisms in the area, respectively. A stability diagram identifies a bifurcation point when the spawning rate is reduced by 65% and where lower spawning rates implies two different stable equilibria. The existence of multiple quasi-stationary steady states can be explained by the general circulation in the area: when spawning into the surface layer takes place from recruitment areas close to the North Sea where the out-flowing Baltic Sea surface water hinder the southward transport of pelagic larvae. The existence of multiple equilibria support the hypothesis of regime shifts in coupled physical-biological systems where modest changes in critical processes causes rapid and extensive structural changes in the ecosystem. Such changes could occur if a system is close to a bifurcation point such that small changes in critical internal biotic dynamics or in environmental conditions force the system into a new equilibria, for example due to hypoxia or changes in temperature or salinity. Finally, it is shown that model simulations of periods with hypoxic bottom water masses reduces the total benthic biomass distribution in the area significantly.

Seasonality and fine-scale meroplankton distribution off the central Cantabrian Coast

We conducted a high-spatial resolution, monthly sampling survey from July 07 to September 08 at a coastal site in Cudillero, Central Cantabrian Coast, to describe the seasonality and cross shore trends in abundance of meroplanktonic larvae. Larval seasonality was driven by the most common meroplanktonic taxa during the stratification period, the barnacle Verruca stroemia and the veligers of the bivalve Hiatella sp. and littorinids. Across the shelf, distributions differed between species and stages. At least for V. stroemia cyprids, the abundance trends correlate negatively with Ekman transport, suggesting advection to the flow below the thermocline. These relationships were only significant when integrated over periods of several weeks, suggesting that transport effects accumulate during the larval duration. For other abundant taxa, the lack of fit to wind induced currents and the high larval concentrations in very coastal waters may reflect retention mediated by active swimming behaviour throughout the water column.

Three-dimensional distribution of fish larvae in a cyclonic eddy in the Gulf of California during the summer

The three-dimensional distribution of fish larvae in a mesoscale cyclonic eddy was studied in the central Gulf of California during strongly stratified summer conditions (July 2011). The eddy was initially detected using chlorophyll a satellite images. Two transects across the eddy were intensively sampled during a survey consisting of conductivity, temperature, depth (CTD) casts and stratified zooplankton hauls. The latter sampled every 17m layer down to the thermocline (45m depth) and every 50m layer from the thermocline down to 200m depth. Geostrophic velocity calculations revealed that the cyclonic eddy extended 500–600m in depth and 140–160km in diameter. Homogeneous surface temperatures suggested that active vertical pumping was not occurring, most likely because the eddy was mature. The eddy was wide enough to affect both coasts of the Gulf, potentially transporting plankton from one coast to the other. Based on larval abundance, two main larval habitats were clearly defined: the “eddy edge habitat” and the “eddy centre habitat”. The first was located close to the eddy's edge, both offshore and near both coasts. It had the highest larval abundance and number of taxa, consisting of predominantly coastal pelagic and demersal species (e.g., Opisthonema libertate, Anchoa spp., Syacium sp. 1). The greatest larval concentration occurred in the thermocline strata (from 51 to 17m depth) and decreased with depth. These results suggest that the eddy edge captured larvae close to the coasts, predominantly at the thermocline, and transported them around the eddy, enabling the larvae to recruit at either coast. The “eddy centre habitat”, defined in the central region of the eddy, was highly correlated with the highest salinities in the first 50m depth and the lowest salinities from 200 to 50m depth. The dominant species varied with depth; Vinciguerria lucetia and Auxis sp. 1 dominated in the shallowest stratum (with the lowest larval abundance), V. lucetia and Benthosema panamense dominated the thermocline (with the highest larval abundance), and V. lucetia, Triphoturus mexicanus and Diogenichthys laternatus dominated the deepest strata. This stratified larval distribution suggests that the thermocline was an advantageous habitat for some mesopelagic fish larvae, while for others it worked as a barrier to vertical movement. Larval stratification was most likely promoted by the strong physical stratification and the absence of upwelling at the time of sampling. The results suggested a complex, three-dimensional relationship between the dynamics of cyclonic eddies and larval nursery areas, which may change according to the evolution of mesoscale eddies and the species' habits.

Temporal and spatial patterns in ichthyoplankton assemblages in bay and open coastal environments

The larval fish fauna occurring in temperate bay and shelf waters off Victoria, southern Australia, was found to be diverse, comprising taxa from 52 families. The most abundant groups collected were gobiids, tripterygiids, gobiesocids and clupeids. Fish egg concentrations were highest during spring and summer (September to February). Eggs of the Australian anchovy Engraulis australis occurred mainly during spring (September to November). Total larval fish concentrations were highest during summer (December to February), and were significantly higher at 1 km than 2 and 5 km from shore in offshore samples. Larval concentrations of a number of families, mainly reef-associated taxa that attach their eggs to hard substrata, were also higher nearer to shore. These larvae are more developed upon hatching than those of pelagic spawners and more capable of avoiding passive drift. Multivariate analyses found that larval taxonomic composition did not vary significantly with distance from shore, but that seasonal and monthly groupings were evident, with different taxa dominating at different times of the year. Larvae of the families Gobiidae and Tripterygiidae occurred in all months, but were less abundant during winter. Spatial differences in the larval fish assemblage between offshore samples and samples taken in the bay were only apparent during summer. This was primarily due to a higher abundance of seagrass-associated species, such as syngnathids and hemiramphids, utilizing specific habitats in the bay.

Factors contributing to variability in larval ingress of Atlantic menhaden, Brevoortia tyrannus

Annual recruitment levels of age-0 juvenile Atlantic menhaden to Chesapeake Bay, which historically supported >65% of coastwide recruitment, have been consistently low since the 1980s. Diminished larval supply to the Bay is one hypothesized explanation. In a three-year ichthyoplankton survey at the Chesapeake Bay mouth, abundance of ingressing larvae varied nine-fold among years. Larvae were most abundant in 2007–2008 and less abundant in 2005–2006 and 2006–2007. High month-to-month variability in larval concentrations was attributable primarily to seasonality of occurrences. There was no defined spatial pattern in distribution of larvae across the 18-km-wide Bay mouth, but larvae at the south side were longer and older on average than larvae at the middle and north side. Environmental variables measured at the times of larval collections were not correlated consistently with temporal and spatial variability in abundance of larvae at ingress, highlighting complexity and suggesting that abundance may be controlled by processes occurring offshore during the pre-ingress phase. Moreover, the substantial differences in inter-annual abundances of larvae at the Bay mouth were not concordant with subsequent abundances of age-0 juveniles in the three survey years, indicating that important processes affecting recruitment of Atlantic menhaden operate after ingress, during the larval to juvenile transition stage.

Subcellular metal partitioning in larvae of the insect Chaoborus collected along an environmental metal exposure gradient (Cd, Cu, Ni and Zn)

Larvae of the phantom midge Chaoborus are common and widespread in lakes contaminated by metals derived from mining and smelting activities. To explore how this insect is able to cope with potentially toxic metals, we determined total metal concentrations and subcellular metal partitioning in final-instar Chaoborus punctipennis larvae collected from 12 lakes situated along gradients in aqueous Cd, Cu, Ni and Zn concentrations. Concentrations of the non-essential metals Cd and Ni were more responsive to aqueous metal gradients than were larval concentrations of the essential metals Cu and Zn; these latter metals were better regulated and exhibited only 2–3-fold increases between the least and the most contaminated lakes. Metal partitioning was determined by homogenization of larvae followed by differential centrifugation, NaOH digestion and heat denaturation steps so as to separate the metals into operationally defined metal-sensitive fractions (heat-denaturable proteins (HDP), mitochondria, and lysosomes/microsomes) and metal-detoxified fractions (heat stable proteins (HSP) and NaOH-resistant or granule-like fractions). Of these five fractions, the HSP fraction was the dominant metal-binding compartment for Cd, Ni and Cu. The proportions and concentrations of these three metals in this fraction increased along the metal bioaccumulation gradient, which suggests that metallothionein-like proteins play an important role in metal tolerance of Chaoborus living in metal-contaminated environments. Likewise, a substantial proportion of larval Zn was in the HSP fraction, but its contribution did not increase progressively along the metal gradient. Despite the increases in Cd, Ni and Cu in the HSP fraction along the metal bioaccumulation gradient, some accumulation of non-essential metals (Cd and Ni) was observed in putative metal-sensitive fractions (e.g., HDP, mitochondria), suggesting that metal detoxification was incomplete. In the case of Cd, there appears to be a threshold body concentration of about 50nmolCdg−1 dry weight, above which Cd detoxification becomes more effective and below which Chaoborus does not “turn on” its detoxification machinery to the fullest extent. We speculate that acclimation or adaptation of Chaoborus to these highly metal-contaminated environments may have resulted in a capacity to tolerate some metal spillover without comprising essential biological functions such as growth and reproduction.

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

The Role of Season and Salinity in Influencing Barnacle Distributions in Two Adjacent Coastal Mangrove Lagoons

Barnacles are often abundant on roots and branches of mangrove trees in tidal channels and coastal lagoons of the Pacific coast of Panama. Yet, in some coastal lagoons, barnacles are absent. We investigated pre- and post-settlement factors that affect barnacle distributions in two adjacent coastal lagoons in Bahia Honda, Panama, one with moderate to large barnacle populations, and the other with nearly non-existent populations. Although mean barnacle recruitment was higher on mangrove root segments during the dry season (December-April) than in the wet season (May-November), it was not significantly different between the two coastal lagoons. The coastal lagoon with fewer barnacles is considered an estuary, with high freshwater flow and low salinities (0.1) during the wet season that were lethal to barnacle nauplii and cyprids. Furthermore, coastal water was not observed to enter the lagoon, even during flood tides. In contrast, more barnacles were found in the lagoon with higher salinities (8.5). During the dry season, freshwater flow was greatly reduced in both lagoons, resulting in a similar salinity range (22-33). We conclude that the lack of barnacles in the estuarine coastal lagoon is largely due to high flushing rates and low salinities that reduce larval concentrations during the wet season. Moreover, low adult abundance in the lagoon's interior may further reduce larval supply and settlement.

<strong>The drift of Trichoptera larvae in a river with a disturbed hydrological regime</strong>

The aim of the study, carried out in a section of the Drzewiczka River downstream of a dam reservoir and a white-water canoeing track, was to estimate seasonal changes in the number of drifting invertebrates, especially Trichoptera and their body size and weight as compared with individuals living in benthos. It was also important to determine environmental factors which have the most significant influence on transported invertebrate fauna. The abundance of Trichoptera in benthos and drift was estimated for stonegravel habitats at 2 study sites; one just downstream of a canoeing track, which underwent flow fluctuations, marked as disturbed (D), and the other, control site (N), 10 km farther downstream, where no changes caused by 2 hydro-constructions were noticed. The percentage of dominant Trichoptera in drift [U coefficient, %] ranged from 0.0001 (Hydropsyche contubernalis McLachlan) to 0.06 (Halesus sp.). The U value for Trichoptera was 0.0005 and 0.012 at the 2 study sites, respectively, which indicates that when there was no correlation with the flow/current velocity, drifting preferences of Trichoptera were random or dependent on seasons, seasonal reservoir management, and larval concentration in the benthos, and at the same time independent of canoeist training. The highest number of drifting individuals was observed between March/April and August/September. This might be connected with a release of a larger amount of water from the impoundment after spring thaw and autumn rain, especially because sudden increases in water level and current velocity were noticed at that time. The increase in abundance of early developmental stages (juvenile) of caddis larvae in drift took place mainly in autumn and winter, whereas larvae at more advanced developmental stages were more frequently noticed in late spring. The differences in Trichopteran sizes at their 3th, 4th and 5th larval instars representing benthos and drift were observed by comparing the width of the head capsule [mm] and body mass [mg] of the examined individuals. Organisms transported in running water were definitely smaller than benthic ones, with the exception of the most developed larvae of H. pellucidula (Curtis).

Early spawning of Antarctic krill in the Scotia Sea is fuelled by “superfluous” feeding on non-ice associated phytoplankton blooms

The spawning success of Antarctic krill ( Euphausia superba) is generally assumed to depend on substantial winter sea ice extent, as ice biota can serve as a food source during winter/spring and the seasonal ice melt conditions the upper water column for extensive phytoplankton blooms. However, direct observations during spring are rare. Here we studied krill body condition and maturity stage in relation to feeding (i.e. stomach fullness, diet, absorption of individual fatty acids and defecation rate) across the Scotia Sea in November 2006. The phytoplankton concentrations were low at the marginal ice zone (MIZ) in the southern Scotia Sea (Stn. 1, 2, and 3), high in open waters of the Southern Antarctic Circumpolar Current Front (SACCF) in the central Scotia Sea (Stn. 5), and moderate further north (Stn. 6 and 7). Krill had low lipid reserves (∼6.5% of dry mass, DM), low mass:length ratios (∼1.7 mg DM mm −1), and small digestive glands (∼7% of total DM) near the ice edge. The stomachs contained lithogenic particles, diatom debris, and bacterial fatty acids, but low proportions of diatom-indicating fatty acids, which suggest that these krill were feeding on detritus rather than on fresh ice algae. In the SACCF, krill had higher lipid reserves (∼10% of DM), high mass:length ratios (∼2.2 mg DM mm −1), and large digestive glands (∼16% of total DM). Stomach content and tissue composition indicate feeding on diatoms. In the north, moderate food concentrations co-occurred with low lipid reserves in krill, and moderate mass:length ratios and digestive gland sizes. Only in the phytoplankton bloom in the SACCF had the mating season already started and some females were about to spawn. Based on the way krill processed their food at the different stations, we indicate two mechanisms that can lead to fast regeneration of body reserves and oocyte maturation in E. superba. One is “superfluous” feeding at high food concentrations, which maximises the overall nutrient gain. The other is a preferential absorption of crucial fatty acids: 20:5( n−3) and 22:6( n−3) when regaining body condition after the winter, and 14:0 and 16:1( n−7) during accumulation of oocyte yolk. Multi-year time series of phytoplankton distribution show that an early spring bloom, as found in 2006, is not unusual for the central Scotia Sea and coincides with high krill larval concentrations in this area. In conclusion, the Scotia Sea is a main spawning ground of Antarctic krill and this is linked to phytoplankton blooms in open waters of the SACCF rather than to ice edge blooms.

High-frequency observations of early-stage larval abundance: do storms trigger synchronous larval release in Semibalanus balanoides?

The acorn barnacle, Semibalanus balanoides, is thought to release larvae in response to phytoplankton blooms, but there is evidence that another, unidentified cue for release may exist. We conducted high-frequency sampling in Little Harbor, Massachusetts, USA, to determine whether early-stage larval abundance was related to several environmental variables, and to characterize vertical distributions of the larvae. Larval concentrations peaked at 2.52 and 1.02 individuals l−1 during two storms. Larvae were more abundant near the surface than near the bottom. We suggest the hypothesis that turbid conditions and upward-swimming behavior may protect newly-released larvae from predation and cannibalism. Future studies should test this hypothesis with barnacles and other invertebrates.