Santiago Alvarez Fernandez

Long term ecosystem changes in the Dutch coastal zone

Marine ecosystems are ruled by non-linear dynamics, both in biological responses to environmental factors (e.g. temperature, salinity, nutrients, etc) and in interactions between trophic levels (competition, predator-prey interactions, etc). Through this non-linearity biological communities can change to the point of shifting to a “new” ecosystem regime with different ecosystem properties, e.g. variations in primary production, reduction of fish stocks, loss of biodiversity, etc.

The North Sea has been reported to have had such ecosystem regime shifts during the last decades (ca. 1978, 1989 and 1998), with drivers such as temperature causing changes in plankton communities and propagating through the food web, ultimately changing how the ecosystem functions. I will present here the community changes and regime shifts detected in the North Sea during the last 40 years, discuss the possible environmental drivers, and show how these overall changes could be affecting the Dutch coast.

Ecogenomics of haloalkaliphilic sulfur bacteria

Gerard Muyzer

Soda lakes are extreme environments with pH values between 9 and 11, and salinities up to saturation. However, despite these extreme conditions, soda lakes are highly productive and harbor diverse microbial communities. The sulfur cycle, driven by sulfur-oxidizing and sulfidogenic bacteria, is one of the most active element cycles in these habitats. Members of the genus Thioalkalivibrio have versatile metabolic capabilities, including sulfide oxidation, denitrification and thiocyanate utilization. We have isolated more than 100 strains, for which the genomes are currently sequenced by the Joint Genome Institute of the U.S. Department of Energy. The availability of these sequence data will allow us to get insight into the diversity of these bacteria, their niche differentiation, and the molecular mechanisms by which they adapt to the extreme halo-alkaline conditions. For this we will use a systems biology approach, combining different 'omics' techniques with physiological experiments under well-defined conditions, and mathematical modeling. The results of these experiments are of paramount importance, both for a basic understanding of life under extreme conditions, as well as for the use of these bacteria in the sustainable removal of noxious sulfur compounds from waste streams. In this lecture, I will discuss the diversity and biogeography of Thioalkalivibrio and present the first results of comparative genomics and proteomics.

Micobes, mud, and methane: Causes and consequences of repeated Early Jurassic anoxia

Bas van de Schootbrugge

Combined geochemical and micropaleontological evidence suggests that there are strong similarities between black shales deposited in the aftermath of the end-Triassic extinction event and those deposited during the better known Toarcian oceanic anoxic event. New core material from Germany and Luxemburg reveals that photic zone euxinia was common during the Hettangian and that its onset was associated with a series of both negative and positive carbon isotope excursions. The Hettangian and Toarcian anoxic events appear to be part of a series of such events in the European Epicontinental Seaway that also includes poorly studied black shale intervals during the Sinemurian and Pliensbachian. The massive dissociation of methane from seafloor clathrates has repeatedly been proposed to explain large negative excursions in carbon isotope records. Alternatively, long-term ocean de-oxygenation during the Early Jurassic has been attributed to greenhouse warming triggered by flood basalt volcanism. However, questions remain regarding the cause(s) of large negative excursions in organic carbon across the Triassic-Jurassic boundary and during the Toarcian OAE, especially where such excursions are associated with important changes in sea level and changes in organic matter composition. Here, I will discuss new evidence for Early Jurassic hydrocarbon seepage and examine temporal and mechanistic links between carbon isotope excursions, methane release, and the emplacement of large igneous provinces.

The top-down role of migrant shorebirds in seagrass-based intertidal ecosystems

Jan van Gils

By reducing activity and numbers of prey, predators often relax competition among their prey, thereby promoting species coexistence and biodiversity. Shorebirds are predators of intertidal flat invertebrates and by their annual migrations they potentially impact biodiversity in multiple ecosystems along their flyway. Here I will discuss the top-down role of the red knot (Calidris canutus canutus), a small shorebird breeding in Siberia and passing through our own Wadden Sea twice a year, but spending most of the year in its wintering quarters on the intertidal flats of Banc d’Arguin, Mauritania.

The mudflats of Banc d’Arguin are covered by seagrass beds, and as most seagrass beds in the world, these are inhabited by lucinid bivalves, in this case Loripes lucinalis. Lucinids live in symbiosis with chemoautotrophic bacteria that use sulfide in their metabolism. As sulfide is toxic to most organisms, Loripes and its endosymbionts thus detoxify the Banc d’Arguin seagrass beds to the benefit of seagrass and its diverse inhabitants.

Having a high flesh-to-shell ratio, Loripes may seem to be the ideal prey for red knots. However, due to its sulfur-based metabolism, red knots feeding on Loripes develop diarrhea and therefore need to find alternative prey. At Banc d’Arguin, they have found a good alternative in Dosinia isocardia, a small algae-feeding bivalve. In spite of differences in diet and metabolism, Dosinia appears to compete with Loripes for resources. I present evidence that by feeding selectively on Dosinia, knots promote the numbers and growth rate of Loripes, thereby potentially stimulating seagrass production and invertebrate diversity.

Judith van Bleijswijk

Bacteria, bivalves, beautiful coral, and booming viruses

some results from the molecular biology laboratory

How do different ballast water treatments influence the microbial community composition and which bacteria survive the treatments: The ones that are relatively abundant  at the start of the treatment or the ones with special characteristics? First and second generation sequencing techniques were used to get the answers.

Sponges harbor micro organisms that produce nitrate. The oxidation of ammonia is the rate limiting step here and both bacteria and archaea are able to do this. We determined the diversity and relative abundance of bacterial and archaeal  nitrifyers in tropical and cold water sponges  and tried to evaluate whether the distribution of the nitrifyers  is influenced by the temperature of the water.

In addition, results of diagnostic tests developed in house are shown for bivalve larvae and viruses, and work in progress on differential gene expression of coral polyps are also presented.

Elisha Moore

Identification of New Membrane Lipid Structures in Planctomycetes from Russian Wetlands

Lipids from microbial membranes can serve as useful sources of information on the biogeochemistry and ecology of natural ecosystems.  Ornithine lipids (OLs) are bacterial membrane lipids that contain the amino acid ornithine in the lipid head group rather than the more common phosphate functional group.  Approximately 25% of bacterial species whose genomes have been sequenced are predicted to have the capacity to form OLs, and to date they have not been observed in eukaryotes or archaea.  While it has been observed that in certain organisms these lipids are produced under phosphorus limitation, or modified under stress conditions, little is known about the specific function of OLs.  We have surveyed a range of metabolically unique and environmentally specialized bacteria, isolated from soil and sediment, for OLs and other amino acid containing lipids by high performance liquid chromatography/mass spectrometry (HPLC/MS).  These microbes were found to contain a variety of known, modified, and previously unknown ornithine lipids.

Four planctomycete species isolated from European North Russia peat bogs shared an apparently abundant but unknown lipid that displayed ornithine like fragmentation in ion trap MS, but yielded different diagnostic mass fragments.  High resolution accurate mass (HR/AM) Orbitrap mass spectrometry and nuclear magnetic resonance (NMR) analysis confirmed that the lipid contains an ornithine head group which is tri-methylated on the d-nitrogen position.  The tri-methylation of the d-nitrogen generates a choline-like moiety, a functional group found in common membrane lipids such as phosphatidylcholines and betaines.  However, this ornithine-choline hybrid structure has not been observed before.  The presence of a quaternary amine gives the lipid a highly charged head group compared to more common ornithine lipids.  This charged head group may be an adaptation to provide membrane stability in the cold temperatures and high acidity of ombrotrophic northern peat bogs.

Co authors: W. Irene C. RIJPSTRA, Ellen C. HOPMANS, Laura VILLANUEVA, Svetlana N. DEDYSH, Hans WIENK, Frans SCHOUTSEN, Jaap S. Sinninghe DAMSTE

Appy Sluijs

Middle Eocene Carbon Cycling, Development of Southern Ocean Sea Ice communities during Antarctic Glaciation and Carbon Isotope fractionation in dinoflagellates as a CO₂ proxy

In this colloquium I will briefly present three lines of research currently carried out by our research team in collaboration with (inter)national partners.

First, I will lay out why a climate - carbon cycle perturbation in the Middle Eocene (the MECO, ~40 million years ago) is very hard to explain considering the present knowledge of the global carbon cycle. This paper resulted from a collaboration with Richard Zeebe (Hawaii) who spent a sabbatical stay in the Netherlands.

Second, I will present micropaleontological work by Sander Houben (received his PhD last fall), who documented dinoflagellate cyst communities in the Soutern Ocean across the glaciation of Antarctica, 34 million years ago. From the moment the ice cap was rapidly established, he found species to dominate assemblages that closely resemble the species that are associated with modern sea ice ecosystems in the Southern Ocean. We interpret this as the first consistent presence of sea ice in the Antarctic realm, as well as rapid ecosystem turnover and likely evolutionary steps.

Finally, I will present dinoflagellate culturing results of Mirja Hoins, who carries out dinoflagellate culturing experiments at the AWI. She generally finds an increase in stable carbon isotope fractionation with increasing CO₂ but with different physiological forcing factors for the various studied species. We aim to quantify these relations as the basis for a new proxy for CO₂ concentrations in the past.

David Chivall

Towards an organic paleosalinity proxy

Paleosalinity is one of the most important oceanographic parameters which currently cannot be quantified with reasonable accuracy from sedimentary records. The fractionation of hydrogen isotopes between growth water and alkenones – a class of biomarker lipids produced by haptophyte algae including Emiliania huxleyi and Gephyrocapsa oceanica - is salinity dependent. As such, the hydrogen isotopic composition of alkenones recovered from sediment cores can be used to reconstruct variations in paleo- sea surface salinity; however, to accurately determine absolute paleosalinity requires a better constraining of the relationship between this hydrogen fractionation, salinity and other competing parameters.

Here, I discuss the results from our culturing work at the NIOZ to constrain the relationship between the hydrogen isotopic composition of alkenones, salinity, growth rate and growth phase for the major oceanic and coastal haptophytes and how these results affect the potential of the paleosalinity proxy.

Dorien Kool

BGC

Nitrite-dependent methane oxidation - In search for lipid biomarkers for a novel link between C and N cycle

In this colloquium, I’ll present our currently ongoing  work on the curious process of nitrite-dependent methane oxidation.

Methane is a strong greenhouse gas, and one of its major sinks in the environment is microbial oxidation. It has already long been known that methane can be oxidized aerobically with oxygen, and in anoxic environments anaerobic oxidation of methane can occur coupled to sulfate reduction. Only recently it was discovered that in the absence of oxygen methane can also be oxidized with nitrite, by the newly identified bacterium ‘Candidatus Methylomirabilis oxyfera’. This seemingly anaerobic processes appeared to be quite special; M. oxyfera oxidizes methane internally produces its own oxygen for the oxidation of methane.

Thus far, only little is known about the occurrence and significance of this ‘new’ methane sink in the environment. To enhance our ability to detect and study this organism in the environment, we are investigating the potential of new biomarkers for M. oxyfera based on its lipid composition and stable isotope techniques. Our results indicate that M. oxyfera indeed contains characteristic lipids that may be used to trace this unique process in the environment. Carbon isotope labeling studies indicate that based on the isotopic composition, a contribution of M. oxyfera to the methanotrophic community may thus far have been overlooked.

Petra Schoon

Impact of CO2 and pH on the distribution and stable carbon isotopic composition of microbial biomarker lipids

Dick van Oevelen

Cold-water coral reef communities: Biogeochemical hotspots in the deep ocean

Cold-water corals form structurally complex reefs along continental margins that are inhabited by a diverse reef community. Ecosystem models suggest that these diverse reef communities are sustained by high organic carbon mineralization. The organic matter supply as measured with sediment traps however, is about two orders of magnitude lower than the demand by the reef community, suggesting a strong demand – supply imbalance. The two questions that I will be addressing in this talk are 1) Are cold-water coral reef communities indeed hotspots of organic carbon mineralization? and 2) How do cold-water coral reefs meet their high organic carbon demands? To answer the first question, in situ measurements of oxygen fluxes (a proxy for mineralization) on a cold-water coral reef on the Norwegian shelf were performed. These measurements were done with a lander-mounted Eddy correlation system, a system that measures the oxygen flux over a large footprint (>10 m2) from simultaneous high- frequency measurements of oxygen concentration and vertical current velocity. To answer the second question, an integrated modelling study was conducted. A hydrodynamic model simulated the flow field around the Logachev Mound Province on the Irish Sea (100x50 km), which was used to simulate the organic matter transport and its reactivity, with special emphasis for the enhanced activity of the reef community.

David Chivall

Towards an organic paleosalinity proxy

Paleosalinity is one of the most important oceanographic parameters which currently cannot be quantified with reasonable accuracy from sedimentary records. The fractionation of hydrogen isotopes between growth water and alkenones – a class of biomarker lipids produced by haptophyte algae including Emiliania huxleyi and Gephyrocapsa oceanica - is salinity dependent. As such, the hydrogen isotopic composition of alkenones recovered from sediment cores can be used to reconstruct variations in paleo- sea surface salinity; however, to accurately determine absolute paleosalinity requires a better constraining of the relationship between this hydrogen fractionation, salinity and other competing parameters.

Here, I discuss the results from our culturing work at the NIOZ to constrain the relationship between the hydrogen isotopic composition of alkenones, salinity, growth rate and growth phase for the major oceanic and coastal haptophytes and how these results affect the potential of the paleosalinity proxy.

Caroline Cleroux

Title and abstract will be published soon

## 11.00 in Noordzeezaal ##

Marjolaine Krug

Natal pulses in the Agulhas Current: origin, evoluation and impact

The Agulhas Current is the strongest western boundary current of the southern hemisphere and a key component of the global climate (Beal et al. [2011]). The Agulhas Current is connected to the variability of the Indian Ocean through its sources regions. Variability from these source regions located in the Mozambique Channel and south of Madagascar, is transmitted into the northern Agulhas Current in the form of large deep sea eddies, which can lead to the formation of Natal Pulses (De Ruijter 1999, Tsugawa 2010). These Natal Pulses are major drivers of variability in the Agulhas Current. They impact on the coastal and shelf circulation and are thought to play an important role in the downstream variability of the Agulhas Current and the subsequent leakage of warm and salty Agulhas Current water into the Atlantic ocean. In this study, we combine satellite observations of Sea Surface Height (SSH) and Sea Surface Temperature (SST) to further our understanding of Natal Pulses and their impact on the coastal and shelf waters. Simple algorithms applied to the SST and SSH datasets allow us to better identify Natal Pulses and follow their evolution from the northern to the southern Agulhas Current regions. We found that Natal Pulses are much more complex than previously anticipated and that more research is required to better understand how energy is lost from the Natal Pulses to the surrounding flow and what the influence of offshore eddies on Natal Pulses are.

Huib de Swart

Dynamics of subtidal currents and net salt transport in estuaries

Observations of subtidal longitudinal flow in coastal plain estuaries typically reveal a structure of seaward currents near the surface and landward currents near the bottom. Traditionally, this vertical structure has been attributed to the presence of a horizontal density gradient resulting from the input of fresh water. However, as was demonstrated in recent observational and model studies, this classical explanation frequently fails and other sources for subtidal flow should be considered as well. The latter include asymmetry in turbulent mixing, induced by tidal straining of the density field, and rectification of tidal currens, in particular due to lateral advection terms.
In this presentation, the role of various forcing agents in generating subtidal flow and net transport of salt is investigated by presenting and discussing output of a numerical model that simulates the hydrodynamics of estuaries under different tidal forcing and freshwater input conditions. It will be shown that both intensity and vertical structure of the residual flow, as well as the net salt transport, strongly depend on the stratification characteristics of estuaries (well-mixed, partially mixed, highly stratified) and on the lateral depth profile of an estuarine cross-section. The potential implications of these results for turbidity and ecology of estuaries will also be discussed.

Robert Poulin

Although not often recognised by marine ecologists, parasites are important components of marine systems, both in terms of biodiversity and (perhaps surprisingly) biomass. They also have significant
impacts on the health, abundance and diversity of free-living organisms. It is therefore reasonable to consider what potential effects global climate change will have on parasitism in marine ecosystems. I will present some of our work on the influence of climatic variables on the multiplication, transmission and impact on hosts of intertidal trematode parasites. Using seasonal field monitoring, single-factor and multifactorial experiments, and mathematical modelling, our work identifies temperature as the main driving factor controlling trematode transmission and infection, although salinity, ocean acidification and ultraviolet radiation all matter as well. In fact, our data indicate that predicted global warming should cause the parasite-induced collapse and local extinction of crustacean populations. Such predictions are, of course, based on present-day biological traits of hosts and parasites, and
ignore the possibility that species will gradually adapt to changing climatic conditions. I conclude with an empirical look at the adaptive potential of trematode parasites faced with warming temperatures, which suggests that predicting the long-term impact of climate change on marine parasitic diseases
is no simple matter.

Laura de Steur

Arctic-Subarctic exchange in the East Greenland Current

Gerben de Boer

An introduction into "Open Earth"

Humankind has been gathering knowledge via empirical methods for 1000's of years, via mathematical methods for 100's of years and via numerical simulation codes for 10's of years. The current Digital Data Deluge will require science to adopt a new, 4th paradigm (Hey et al, 2009). Science has not yet adopted most common practices from IT yet, as discussed in a recent Nature article (Merali, 2010). However, the overwhelming amount of data and model output leaves science no other choice than to start teaming up with IT. However, even in mainstream IT, off-the-shelf tools to deal with the Digital Data Deluge barely exist. A new job has been invented to fill this gap: eScience engineer. OpenEarth is an eScience workflow, organized as a community initiative for marine and coastal science and engineering. We catalyze companies, institutes and universities to team-up to jointly further and disseminate the IT technology needed for the Data Deluge challenges ahead.

First, OpenEarth adopted the Wikipedia crowd-sourcing philosophy to migrate from the common heaps of  'scripts' familiar to scientists, to well-tested and shared libraries. We have now order 1000 users enjoying over 7000 contributions from over 150 active community members, over 250 LinkedIn members and a well-visited wiki.

Secondly, OpenEarth investigates and hosts the techniques to migrate from storage of data at local user PCs to central storage in data centers (private) clouds. A key issue here is to ensure continued smooth real-time usage of the data via the web by fostering 'DataTube' techniques (netCDF-CF-OPeNDAP, OGC WxS, PostgreSQL/PostGIS). The OpenEarth community documents the required modifications in workflow of the scientists on a wiki, larded with code snippets for the analysis languages of a range of disciplines: Matlab (e.g. engineers), Python (e.g. IT) and R (e.g. ecologists, statisticians).

Third, OpenEarth enables scientists to easily plot their geo-data in Google Earth to allow them to share their work with society as a whole (outreach) via this intuitive off-the-shelf touchpad and desktop app. But it also allows traditionally disconnected science disciplines to inspect data from one another, e.g. ecology, governance and engineering in the Building with Nature program.

If you want to experience what “Open Earth” can mean for you, you are cordially invited to join the “hands-on” sprint session (directly following the colloquium) from 14:00 to 16:00 in the Noordzeezaal (BYO laptop)!

Saskia Franken

Open Access: “What’s going on?”

Open Access: what is it, how does it work and why is it beneficial for you and science in general? What distinguishes the different types of Open Access publishing and what are the costs involved? What is the role of publishers of peer-reviewed journals and of the funding agencies?

Saskia Franken, Open Access consultant at Utrecht University, will discuss these issues regarding Open Access publishing during the NIOZ colloquium. Participation of colleagues present at Yerseke will be facilitated by a conference call from the Oceaanzaal (Tx)to the Grote Vergaderzaal (Y).

Cecile Blanchet

Environmental changes in the Nile watershed during the Holocene: insights on climatic mechanisms and human dynamics

The termination of the African Humid Period in northeastern Africa was characterized by the southward migration of the rain belt and the disappearance of the Green Sahara. These changes have exerted major control on Neolithic human population dynamics and were primarily forced by low-latitude summer insolation. On a regional scale, rapid environmental and climatic changes have been explained by precipitation-vegetation feedback and threshold mechanisms. However, the precise timing of precipitation and vegetation changes is still equivocal, which has hampered our understanding of the underlying climatic mechanisms and their impact on human populations. Here we show that the rapid retreat of vegetation in the Sahara did not affect insolation-forced precipitation. Using organic and inorganic proxy data from a sediment core recovered on the Nile deep-sea fan, we reconstructed the changes in river runoff, erosion and vegetation dynamics during the Holocene. Rapid shifts of vegetation and erosion occurred between 8.6 and ~6 ka BP, while rainfall intensity gradually declined following low-latitude insolation changes. This decoupling documents the absence of a strong positive feedback of vegetation to precipitation. The retreat of the vegetation occurred at the same time as major phases of human evolution, such as the Neolithic Revolution in Northeastern Africa, suggesting complex feedback processes between human dynamics and environmental changes.

PRODUS: Jan Drent (MEE) & Aad Smaal (IMARES)

Andre de Roos

Population and Community Ecology of Ontogenetic Development

13 September Richard Zeebe "Ocean Acidification: Past, Present, and Future"

17 September Lennart de Nooijer "Foraminifer-based proxies: development, biomineralization, and application"

4 October Maarten Klunder "Distributions and sources of dissolved iron in the polar oceans"

11 October Tom Jilbert "Anoxic sediments as players in marine biogeochemical cycles and recorders of short-term environmental change"

23 October Caroline Katsman "Towards regional projections of twenty-first century sea-level change"

8 November Lara Pozzato "Prokaryotic, protozoan, and metazoan processing of organic matter in the sediment: a tracer approach

15 November Michio Aoyama "One year tracking of ¹³⁴Cs and ¹³⁷Cs in the North Pacific Ocean: impact of radiocaesium released from Fukushima Dai-ichi NPP accident

29 November Darci Rush "Ladderanes as tracers for present and past anaerobic ammonium oxidation

4 December Brendan Keely A new dawn for sedimentary tetrapyrroles?

6 December Raquel Lopes dos Santos Late Quaternary paleoenvironmental conditions of NW Africa and SE Australia

13 December Daphne van der Wal Structure and functioning of intertidal benthic biota unravelled with remote sensing

21 December Marcel van der Meer Developing new methods to estimate paleosalinity; understanding the past as key to future climate change

9 January Julien Michel "Carbonate-producing organisms and sedimentation of the Golfe d’Arguin (Mauritania)"

12 January Henk Brinkhuis "From Greenhouse to Icehouse and Back Again; lessons from the geological record"

19 January Marco van Hulten "Aluminium in an ocean general circulation model compared with the West Atlantic Geotraces cruises"

26 January Nicole Bale "Tracing nitrogen cycling microorganisms in the North Sea using their intact membrane lipids"

30 January Anne Osborne "Neodymium isotopes as a tracer for past ocean circulation – the Pliocene closure of the Central American Seaway"

2 February Marcel Wernand "From electric bulb to Ipad ‐ The era of visualized trends in ocean colour and chlorophyll"

16 February Mardik Leopold "Record numbers of porpoises in the Netherlands, dead and alive. Is there a problem?"

23 February David Thieltges "Ecology of marine diseases" 

1 March Kees Booij "Passive sampling of organic contaminants: why bother?"

8 March Maarten Loonen "Are geese a key species in arctic ecosystems?" 

15 March Fleur van Duyl "Functional roles of sponges on coral reefs"

22 March Per Palsboll "Population genetic inference methods in marine ecology" 

5 April Rik Tjallingii "New developments for calibration and application of XRF core scanning of sediment cores"

12 April Filip Meysman "Electrogenic oxygen consumption in the seafloor: in search for natural microbial batteries"

18 April Gert-Jan Reichart "Reconstructing hydrological change in lagoonal and open ocean settings"

19 April Hubert Vonhof "Combination of radiogenic and stable isotope techniques for improved paleoenvironmental reconstruction" 

3 May Helena L. Filipsson "Extraordinary high primary productivity off NW Africa during Younger Dryas and its consequences for benthic life"

10 May Henk Bolhuis "Molecular analysis of coastal microbial mat communities" 

24 May Caroline Slomp "Phosphorus recycling and burial in low oxygen settings in the Baltic Sea"

31 May Nienke Bloksma "Escape from the Ivory tower"

4 June Dedmer van de Waal "A sour future for toxic and calcareous dinoflagellates"

6 June Laura Villanueva "Molecular Geomicrobiology: Next challenge in organic biogeochemistry?"

14 June Patrick De Deckker "The role of the Indo Pacific Warm Pool and the Southern Westerlies on Glacial Interhemispheric Asymmetry"

5 July Leon Claessens "The Dodo in 3-D, reconstructing the life habits of the icon of extinction"

13 July Jelle Reumer "The Peruvian mega-toothed killer whale and the evolution of whales"

19 July Eva Niedermeyer "Glacial to Holocene dynamics of the Indonesian monsoon ‐ New insights from plant‐wax dD off Northwest Sumatra"

26 July Arnoldo Valle-Levinson "Adjustment to the paradigms of net circulation and diel vertical migrations in fjords"