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Phytoplankton Dynamics
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| In collaboration with: |
K. H. Wiltshire,
M. Boersma (Biologische Anstalt Helgoland, Alfred-Wegener-Institut für Polar- und Meeresforschung) W. Greve (Deutsches Zentrum für Marine Biodiversitätsforschung, Senckenberg Forschungsinstitut und Naturmuseum) P. C. Reid (Sir Alister Hardy Foundation for Ocean Science (SAHFOS), Plymouth, UK) U. Feudel (ICBM) |
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Facts about phytoplankton:
Phytoplankton are tiny floating plants (algae) that live in the ocean
and in lakes. In the process of photosynthesis, phytoplankton produce
half of the world's oxygen. Moreover, by primary production, death and
sinking they effectively transport carbon from the ocean's surface
layer to marine sediments, a process by which phytoplankton exert a
global-scale influence on climate (carbon dioxide and the greenhouse
effect). Phytoplankton constitute the bottom level of aquatic
foodwebs. There are many species of phytoplankton that can be
distinguished by their morphology.
Under certain conditions algal species can grow abundantly in marine
and limnic environments. The phases of accelerated growth, decelerated
stagnation and rapid decline of cell counts together establish an
algal bloom. Algal blooms are an essential component of biological
productivity in aquatic communities and abundance as well as timing in
the annual cycle are important factors for a proper functioning of the
marine foodweb. Phytoplankton depend upon sunlight, water, and
nutrients to grow and survive. Competition for common resources
couples different species as do grazing by zooplankton or predators
belonging to higher trophic levels. Dominance and succession of
species is controlled by habitat specific conditions and susceptible
to fluctuating or systematically changing environmental
conditions.
Data Resources:
The Helgoland Roads (HR)
Data, records of roughly 300 algal species (cell counts/litre),
nutrient concentrations (ammonium, nitrate, nitrite, phosphate,
silicate), and physical quantities (water temperature, salinity,
Secchi depth, wind, etc.), constitute an exceptional data pool not
only for its density (workdaily records) but also for its long-term
character (1962 until present). The data is collected and hosted by
the Biological Institute
on Helgoland (BAH) and shared with collaborating researchers via
the Publishing Network for Geoscientific & Environmental Data PANGAEA.
As from 1975, Wulf Greve started a zoolankton time series at Helgoland Roads. This involves bi-weekly sampling, at the same site as the phytoplankton and nutrient samples of the daily time series. Analyses of the zooplankton data are also carried out to species level. In addition to these single-site databases with a comparatively high temporal resolution there exists the high spatial-resolution Continuous Plankton Recorder (CPR) database, a synoptic plankton-monitoring programme operated by the Sir Alister Hardy Foundation for Ocean Science (SAHFOS), resident in Plymouth, UK.
A common analyses of both phytoplankton and zooplankton time series
and a combination of high temporal (HR) and spatial (CPR) resolution
data is one of the central issues of our subproject "Match/mismatch of
zooplankton-phytoplankton interactions, based on existing long-term
information in the North Sea" as part of the
DFG-Priority Programme AQUASHIFT.
Research Questions:
Changes Induced by Climate Variability
An increase of sea surface temperature of 1.13° C over the last 40 years (since 1962) and less frequent extremes in winter have recently been shown (see below figure). 
As bifurcation theory teaches, systematic shifts of environmental
conditions like global warming may cause catastrophic ecological
responses. One of such scenarios is linked with the match/mismatch
hypothesis (Hjort-Cushing) that relates survival of fish to the match
between the time of larval occurrence and that of the production
of their food. In fact, more or less pronounced shifts of algal blooms
are revealed by the Helogoland Roads time series of selected algae,
e.g. for Ceratium lineatum
shown in the figure below.
These observations raise the question whether dramatic changes of the
marine ecological system of the North Sea might be expected.
Dominance and Succession Patterns
There is a high inter-annual variability of species specific peak values that can vary over two to three orders of magnitude. The dominance and succession of species within and across years reveals a complex pattern that still awaits explanations in the context of evironmental conditions, nutrients and competition. 
Trigger Mechanisms of Algal Blooms
From recorded cell counts of monitored species it is possible to extract the onset of an occuring principal bloom. Pretreatment of raw data series involves interpolation and appropriate filtering - the technical problem is to accomplish smoothing without significantly shifting the bloom start and the setup of an adequate criterion for the bloom onset (a criterion that is generally accepted within the marine biology community is unknown). After pinpointing the bloom start the immediate prehistory is investigated to detect trigger mechanisms: the method used is similar to reverse correlation analysis and "spike-trigered averaging" as in neurophysiological research of spike discharge events and, in analogy, might be termed "bloom-triggered averaging".
In the framework of process-oriented modelling there are several
variants that generate algal blooms, e.g. the Truscott-Brindley model
that formulates the phytoplankton-zooplankton interaction in the form
of an excitable dynamics (again known from models of neuronal
activity): overcritical deviations from the fixed-point, for instance
by depressing the zooplankton concentration below the stationary value,
elicit a phytoplankton bloom that is followed by a zooplankton bloom
and a return (relaxation) to the fixed-point (see figure below). 
In the context of North Sea research and the Helgoland Roads Data it
is interesting to extend existing analyses to a dynamics that includes
the annual cycle and/or the effect of fluctuating parameters
(multiplicative noise).
References:
Additional Links
Revised 18.10.06 |
