Dag Lorents Aksnes

University of Bergen | UiB · Department of Biological Science

At high latitudes, the suitable window for timing reproductive events is particularly narrow, promoting tight synchrony between trophic levels. Climate change may disrupt this synchrony due to diverging responses to temperature between, for example, the early life stages of higher trophic levels and their food resources. Evidence for this is equivo...

Mesopelagic fishes and invertebrates contribute to the biological carbon pump (BCP) through direct and indirect effects on the gravitational, diffusive, and migrant (active) fluxes. Here, we analyzed the effect of these organisms on total carbon export and sequestration using an idealized depth-resolved food-web model. We constrained a baseline sce...

Changes in land use, afforestation, reduced acidification, and climate are causing increased terrestrial primary production and vegetation density in the boreal zone. This greening on land contributes to increased exports of natural organic matter that have made inland waters become browner. The brown freshwater ultimately drains to the coast, but...

The glacier lanternfish Benthosema glaciale is a key myctophid with a wide distribution in the northern Atlantic. It is a species that has a strong vertical migration capacity and have the potential to move between the surface waters and mesopelagic depths in a diel cycle (DVM), mainly depending on ambient light conditions. We investigated the feed...

We studied mesopelagic fishes in the Red Sea (22°N), hypothesizing that the rapid shifts between day and night at low latitudes would translate into rapid vertical migration speeds and brief near-surface ‘antipredation windows’. Using a bottom-moored echosounder, we found that diel vertical migration speeds of acoustical scattering layers were up t...

Aim Mesopelagic fishes have a near-global distribution in the upper 1,000 m from tropical to sub-Arctic oceans across temperature regimes. Yet, their abundance decreases poleward and viable populations seem excluded from high latitudes. Why? Location North Atlantic between 50–85°N, with implications for high-latitude oceans globally. Time period...

Variability of mesopelagic scattering layers is often attributed to environmental conditions or multi-species layer composition. Yet, little is known about variation in behaviour among the individuals forming scattering layers. Based on a 10 months high-resolution dataset from stationary echosounders in a Norwegian fjord, we here assess short-term...

Current food production and consumption trends are inconsistent with the Convention on Biological Diversity's 2050 vision of living in harmony with nature. Here, we examine how, and under what conditions, the post-2020 biodiversity framework can support transformative change in food systems. Our analysis of actions proposed in four science-policy f...

The epi- and mesopelagic ecosystems of four sub-polar ocean basins, the Labrador, Irminger, Iceland and Norwegian seas, were surveyed during two legs from Bergen, Norway, to Nuuk, Greenland, and back to Bergen. The survey was conducted from 1 May to 14 June, and major results were published in five papers (Drinkwater et al., Naustvoll et al., Stran...

We studied vertical distribution and diel vertical migration (DVM) behaviour of mesopelagic acoustic scattering layers in relation to environmental conditions in the Norwegian Sea, the Iceland Sea, the Irminger Sea, and the Labrador Sea. Distinct mesopelagic scattering layers were found in all basins, but the daytime depth of the layers varied betw...

Light decays exponentially with depth in aquatic systems, and vertical movement is a widespread strategy among zooplankton for avoiding visual predation. Topographical features may, however, constrain descents and trap zooplankton in light exposed habitats favorable for fish predation. We present an extensive dataset on the vertical distribution of...

Previous studies have shown decline in dissolved oxygen of the ocean basins. A hypothesis for this development is that ocean warming through increased stratification has caused reduced ventilation of the interior ocean. Here we provide evidence that reduced ventilation, which has been associated with a 1 °C warming of the North Atlantic Water (NAW)...

To assess organisms forming mesopelagic scattering layers in the Red Sea, we took advantage of their reactions to light. We used a remotely operated vehicle (ROV) equipped with LED lamps for herding the acoustic targets down to the bottom (700 m), while concurrently monitoring the event by shipborne and deployed echosounders as well as video footag...

With climate warming, a widespread expectation is that events in spring, such as flowering, bird migrations, and insect bursts, will occur earlier because of increasing temperature. At high latitudes, increased ocean temperature is suggested to advance the spring phytoplankton bloom due to earlier stabilisation of the water column. However, climate...

Throughout the oceans, small fish and other micronekton migrate between daytime depths of several hundred meters and near-surface waters at night. These diel vertical migrations of mesopelagic organisms structure pelagic ecosystems through trophic interactions, and are a key element in the biological carbon pump. However, depth distributions and mi...

Light decays exponentially with depth in aquatic systems, and vertical movement is a widespread strategy among zooplankton for avoiding visual predation. Topographical features may, however, constrain descents and trap zooplankton in light exposed habitats favorable for fish predation. We present an extensive dataset on the vertical distribution of...

During the last 20 years, a series of studies has suggested trends of increasing jellyfish (Cnidaria and Ctenophora) biomass in several major ecosystems worldwide. Some of these systems have been heavily fished, causing a decline among their historically dominant small pelagic fish stocks, or have experienced environmental shifts favouring jellyfis...

By definition, the mesopelagic twilight zone extends from 200 to 1000 m depth. Rather than confining the twilight zone to a certain depth interval, we here propose a definition that covers absolute light intensities ranging from 10−9 to 10−1 μmol quanta m−2 s−1. The lowest intensity of this twilight habitat corresponds to the visual threshold of la...

For visual predators, sufficient light is critical for prey detection and capture. Because light decays exponentially with depth in aquatic systems, vertical movement has become a widespread strategy among zooplankton for avoiding visual predation. However, topographical features such as seamounts have been shown to block their descent, trapping th...

Mass mortality events can occur naturally and may have important ecological impacts on local populations. The abundance and stage structure of the coronate scyphozoan Periphylla periphylla in a Northern Norwegian fjord were studied between October 2010 and August 2011. Developmental stage composition varied for the duration of the study, with early...

This chapter begins with a brief description of zonation in the pelagic and benthic realms, followed by a description of the topographies of coastal and fjord biotopes, the continental shelf and slope, and the deep ocean. These biotopes shape the habitats for bottom associated marine organisms. This is followed by a description of the physical char...

This chapter discusses sampling methods for marine organisms. Active gears capture species by actively chasing the species or actively hunting the target species. Passive gears are set, either on the bottom or in the water column, and organisms are caught as they come into contact with the gears. Remotely operated vehicles (ROVs) have become a very...

In marine science numerical models, and especially ecosystem models, have developed into an important tool for policy advice and environmental management applications (Rose et al., 2010; Holt et al., 2014; Robson, 2014; Lynam et al., 2016). The predictive capabilities of these models, in particular under changing environmental conditions, naturally...

Mesopelagic acoustic scattering layers at a 700 m deep location in the Red Sea ascended 70 to 80 m during a passing rain storm that reduced light levels at the surface by more than 2 orders of magnitude. The changes in vertical distribution were observed down to the deepest part of the water column and were interpreted as a response to sudden dark...

The deep scattering layer (DSL) is a ubiquitous acoustic signature found across all oceans and arguably the dominant feature structuring the pelagic open ocean ecosystem. It is formed by mesopelagic fishes and pelagic invertebrates. The DSL animals are an important food source for marine megafauna and contribute to the biological carbon pump throug...

Phytoplankton, the autotrophic component of the plankton community, is a key factor in oceanic ecosystems and in biogeochemical cycling. Over much of the ocean, phytoplankton growth is limited by nitrogen uptake (as nitrate), which is commonly described in ecosystem modelling by the Michaelis-Menten equation: V = Vmax S/(K + S). Previous phytoplank...

We tested the hypothesis that the coronate jellyfish Periphylla periphylla distributes vertically according to a preferential range of absolute light intensities. The study was carried out in Lurefjorden, Norway, a fjord characterized by mass occurrences of this jellyfish. We collected data on the vertical distribution of P. periphylla medusa durin...

It has been well documented that population growth, development of biological subsystems and the utilization of resources in ecology and economy frequently follow a logistic or sigmoid time-development. In the context of oil and gas extraction such development is known as Hubbert's peak oil theory. We observe that the logistic equation describes th...

The oceanic sound scattering layer (SL) is an ubiquitous acoustic signature of mesopelagic fishes and invertebrates that are important food sources for the oceanic macrofauna and players in the biological carbon pump. We investigated the relationship between SL migration amplitude and light in the Norwegian Sea. Incoming surface irradiance was meas...

Nutrient uptake affinity affects the competitive ability of microbial organisms at low nutrient concentrations. From the theory of diffusion limitation it follows that uptake affinity scales linearly with the cell radius. This is in conflict with some observations suggesting that uptake affinity scales to a quantity that is closer to the square of...

We make a comparison of the mesopelagic sound scattering layers (SLs) in two contrasting optical environments; the clear Red Sea and in murkier coastal waters of Norway (Masfjorden). The depth distributions of the SL in Masfjorden are shallower and narrower than those of the Red Sea. This difference in depth distribution is consistent with the hypo...

Plankton are typically divided into phytoplankton and zooplankton in marine ecosystem models. Yet, most protists in the photic zone engage in some degree of phagotrophy, and it has been suggested that trophic strategy is really a continuum between pure phototrophs (phytoplankton) and pure phagotrophs (unicellular zooplankton). Such a continuum of t...

The critical depth concept was first recognized by Gran and Braarud (1935). During summer, in the Bay of Fundy, they observed an unexpected no bloom situation. Their interpretation was that high amounts of detritus of terrestrial origin caused too murky water and insufficient light for the tidally mixed phytoplankton. Almost 20 years later, this wa...

Acoustic scattering layers (SL) ascribed to pearlside Maurolicus muelleri were studied in Masfjorden, Norway, using upward-looking echo sounders cabled to shore for continuous long-term measurements. The acoustic studies were accompanied by continuous measurements of surface light and supplemented with intermittent field campaigns. From autumn to s...

Jellyfish blooms occur in marine environments around the world and have been linked to over-fishing, eutrophication and climatic change. In some coastal areas of Norway, the circumglobal Periphylla periphylla has increased to exceptionally high abundances and has replaced fish as the main planktivorous predator despite the ineffectiveness of its no...

Studies on the relationship between the optimal phenotype and its environment have had limited focus on genotype-to-phenotype pathways and their evolutionary consequences. Here, we study how multi-layered trait architecture and its associated constraints prescribe diversity. Using an idealized model of the emotion system in fish, we find that trait...

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 20...

The vertical distribution and migration of plankton organisms may have a large impact on their horizontal dispersal and distribution, and consequently on trophic interactions. In this study we used video-net profiling to describe the fine scale vertical distribution of Mnemiopsis leidyi in the Kattegat and Baltic Proper. Potential diel vertical mig...

Abstract A central simplifying assumption in evolutionary behavioral ecology has been that optimal behavior is unaffected by genetic or proximate constraints. Observations and experiments show otherwise, so that attention to decision architecture and mechanisms is needed. In psychology, the proximate constraints on decision making and the processes...

Secchi depth is a valuable proxy for detecting long term changes in the water clarity of oceanic and coastal ecosystems. We analyse approximately 40 000 observations, which are available from ICES, from the Baltic Sea and the North Sea in the 20th century. Our results suggest pronounced effects of bottom depth and distance to coast on Secchi depth,...

In marine ecosystem models, the underwater light intensity is commonly characterized by the shading of phytoplankton in addition to a background light attenuation coefficient. Colour dissolved organic matter (CDOM) is an important component of the background light attenuation, and we investigate how variation in CDOM attenuation affects euphotic zo...

We studied the seasonal dynamics of the invasive ctenophore Mnemiopsis leidyi over its distribution range from Skagerrak into the Baltic Proper during 1 yr and related this to ambient physical and biological variables. The appearance of M. leidyi was sporadic in the Baltic Proper, with 60-fold lower abundance than in the Skagerrak and Kattegat (mea...

In microbial competition theory, the Michaelis-Menten (MM) half-saturation coefficient is often considered to be a trait of an organism defining competitive strength in a trade-off conflict with maximum uptake rate. Theoretical studies have shown that a quadratic model characterizes the uptake rate, and that this model can be approximated by a MM m...

We studied the seasonal dynamics of the invasive ctenophore Mnemiopsis leidyi over its distribution range from Skagerrak into the Baltic Proper during 1 yr and related this to ambient physical and biological variables. The appearance of M. leidyi was sporadic in the Baltic Proper, with 60-fold lower abundance than in the Skagerrak and Kattegat (mea...

There is a concern that blooms of cnidarians and ctenophores, often referred to as jellyfish, are increasing in frequency and intensity worldwide and that there is a shift from fish-to jellyfish-dominated systems. We present an idealized analysis of the competitive relationship between zooplanktivorous jellyfish that is based on a generic model, te...

Mesopelagic fishes occur in all the world's oceans, but their abundance and consequently their ecological significance remains uncertain. The current global estimate based on net sampling prior to 1980 suggests a global abundance of one gigatonne (10(9) t) wet weight. Here we report novel evidence of efficient avoidance of such sampling by the most...

We provide evidence that internal waves cause frequent vertical migrations (FVM) in fishes. Acoustic data from the Benguela Current revealed that pelagic scattering layers of fish below similar to 140 m moved in opposite phases to internal waves, ascending similar to 20 m towards the wave trough and descending from the wave crest. At the trough, th...

Nearly 50 yr ago, the Michaelis-Menten (MM) model, originally derived for enzyme kinetics, was adapted to characterize microbial nutrient uptake and has become a framework for defining microbial traits in competition theory, evolutionary dynamics, and ocean ecosystem models. We provide theoretical evidence that microbial traits and environmental pr...

Several Calanus species occupy depths spanning from <100 m to several thousand meters during winter. Here, we report that variations in bottom depth and water clarity accounted for most of the variation in the mean depth of early winter Calanus spp. distributions of Norwegian fjords. We suggest a modification of a previously proposed hypothesis (Mi...

Previous 1 d studies (Giske et al. 1990, Sarsia 75:65-81; Balino & Aksnes 1993, Mar Ecol Prog Ser 102:35-50; Rasmussen & Giske 1994, Mar Biol 120:649-664) of the mesopelagic fish Maurolicus muelleri have suggested that their vertical distribution changes as though they were following a constant light intensity, sometimes termed the 'isolume'. Here...

Jellyfish blooms are of increasing concern in many parts of the world, and in Norwegian fjords an apparent increase in mass occurrences of the deep water jellyfish Periphylla periphylla has attracted attention. Here we investigate the hypothesis that changes in the water column light attenuation might cause local retention and thereby facilitate ma...

Many midwater animals emit ventral light to hide their silhouette in the water column. This phenomenon known as counterillumination typically requires fine control over light emission since it needs a luminescence that closely matches the properties of downwelling light (intensity, angular distribution and wavelength). Here we provide evidence that,...

We evaluate the hypothesis that the vertical migration of Periphylla periphylla is governed by its sensitivity to light intensity. By applying an individual-based model where random walk is combined with assumed individual responses to light, we compare the predicted vertical distributions with acoustical observations. Important features of the obs...

The light regime of the water column has a strong structuring effect on aquatic food webs and it has been previously hypothesized that coastal water darkening has increased the success of tactile predators relative to visual predators such as fish. Due to a general lack of time-series of optical parameters, we applied a proxy for light attenuation...

We document a long-term reduction in Secchi depth of 0.06-0.13 m yr -1 in the southern California Current System (CCS) over the period 1969-2007, reflecting a long-term shoaling of the euphotic zone. Calibrated water clarity observations from 1949 to 1954 reinforce the results indicating a progressive shoaling. For the inshore area, 150 km off the...

We investigated the vertical distribution of Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa at four locations around the archipelago of Svalbard in autumn. The older and larger copepodites of Calanus spp. were generally located deeper in the water column. Differences in vertical distribution between stations partly reflected...

Some western Norwegian fjords host extraordinarily abundant and persistent populations of the mesopelagic, coronate scyphomedusa, Periphylla periphylla. In these environments, from late autumn to spring, the medusae undertake regular diel vertical migrations into surface waters. From unique observations obtained with a remotely operated vehicle (RO...

In models of growth and life history, and in molecular and cell biology, there is a need for more accurate frames of reference to characterize developmental progression. In Caenorhabditis elegans, complete fate maps of cell lineage provide such a standard of reference. To be more widely applicable, reference frames should be easier to measure while...

The transport of nitrate into the euphotic zone is an important regulator of primary production. This transport is facilitated by physical processes that involve the depth and the steepness of the nitracline, but transport is complicated by the dynamical nature of the euphotic zone. Here we derive an analytical model that predicts two optical effec...

The mesopelagic coronate medusa Periphylla periphylla has been very abundant (20-320 individuals m−2), with as many as 2.5 individuals m−3, for more than a decade in certain Norwegian fjords. These abundances are 2-3 orders of magnitude higher than reported from open ocean environments. Comparisons of the size, density and behavior of this jellyfis...

I tested the hypothesis that the abundance of visually constrained fish stocks relate inversely to light attenuation for the Black Sea, where long time series of fish stocks and Secchi depths are available. Variation in Secchi depth accounted for 76-85% of the variation in combined fish biomass, which is higher than any other correlations previousl...

The abundance of midwater fishes in fjord basins, as well as the abundance and size of mesozooplankton, have previously been related to optical properties of the basin water. Herein, we report on concurrent temporal changes in light absorbance and fish abundance for Masfjorden and modest changes in both variables for Lurefjorden and Sognefjorden, w...

Planktonic embryos are unprotected and experience high mortalities. Risk exposure and its consequences for survival depend very much on developmental time, which, in turn, is strongly affected by temperature. Developmental times in planktonic embryos have recently been correlated with cell cycle duration and egg size. Here, we elaborate on this by...

We present a novel modelling approach applied to the appendicularian Oikopleura dioica. Growth and development are represented as separate processes, and a developmental clock is assumed to regulate development. Temperature influences the organism globally through aging and not separately through individual physiological processes as commonly appli...

Gelatinous zooplanktivores (medusae, siphonophores, and ctenophores) and visual zooplanktivores (fish) interact through competition, predation, and commensalism. In the search for key factors governing the outcome of competition, we examined the instantaneous predation efficiency and its light dependency. The visual predator Gobiusculus flavescens...

Aquatic food webs are affected from the bottom up by light through its effect on photosynthesis and productivity. But light also has a top-down effect, because it is crucial for the visual foraging efficiency in many fish. Here we present data suggesting that marine pelagic food webs are primarily structured top-down by light through its effect on...

Gelatinous zooplanktivores (medusae, siphonophores, and ctenophores) and visual zooplanktivores (fish) interact through competition, predation, and commensalism. In the search for key factors governing the outcome of competition, we examined the instantaneous predation efficiency and its light dependency. The visual predator Gobiusculus flavescens...

Pelagic urochordate appendicularians are a vital component of marine zooplankton communities, second in abundance only to copepods. Found in all major ocean systems, they are capable of rapid blooms, attaining densities exceeding 53 000 ind. m(-3). We maintained a widely distributed species, Oikopleura dioica, under controlled laboratory conditions...