Hinweis

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Nahrungsnetz nahe der Wesermündung

- sehr schematisch gezeigt, wie das Ökosystem in der südlichen Nordsee vernetzt ist. Natürlich: es geht nicht nur um Nahrung, sondern um Licht, Signale untereinander, Wärme, Strömungen und Verwirbelung ... und um den Lärm der Schiffe und die un-natürlichen Chemie-Stoffe ...



Herkunft: Schutzgemeinschaft Deutsche Nordseeküste, Autorenrechte by Stefan Wellershaus



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Weser-Ästuar - Weser Estuary

This picture is by Hans Engler (Bremerhaven) and shows the middle stretch of the Weser Estuary at Bremerhaven, 1984. Seen in "Umschau in Wissenschaft und Technik", 1984, 15, cover. View goes upriver at the area of the turbidity maximum. The city of Bremerhaven in the foreground left side. In the bend the Weser is 1 km wide.

A schematic longitudinal picture of the canalized Weser estuary

figure 1: river-km from Bremen, Nordsee = North Sea, pic.: Eurytemora affinis female, Anzahl = number, freshwater resp. seawater species, Trübe, T = turbidity maximum, Salzgehalt = salinity.
figure 2: map, dotted = wadden flats.
figure 3: saltwedge, mud black, upwhirling causing turbidity maximum T, fat dots = ideal layer of "no current", F = freshwater, M = sea water.
figure 4: effect of maintenance dredging, M1 = marine sediments being accumulated in the mud zone, M2 = being re-transported to the sea, F1 and F2 for riverine sediments, Einleitungen: inflow.

The Weser Estuary is a tidal coastal plain estuary. It runs through a soft underground and would form its bed according to the hydrographic situation. But a pure nature-made situation would only occur as long as it is un-disturbed by technical measures. If not influenced by dredging and bank stabilization, it would perhaps flow in meanders and in various arms.

But by man-made deepening, maintenance dredging, and narrowing the banks, a permanent canal is formed to enable deep going sea ships to enter the harbour of Bremen. This canal is much deeper than it naturally would be. The bottom would be sand, but since the bed is made deeper than needed by the water to flow through, fine sediments (clay and organic particles) would occur at a place where a salt wedge would have ist upstream end (SpS in figure 3): the bed is over-deepened.

The so-called salt wedge occurs where salt water meets fresh water and under-flows it. Thus freshwater moves on top of marine water, and along a layer of no-flow it mixes and creates brackish water (see arrows F and M in figure 3). This is a very schematic picture which is disturbed by turbulence and other factors.

Very irregular and heavy tides influence the situation. According to the tides the salt wedge moves upstream and downstream, respectively. The mud which is amassed at the bottom of the canal, is eroded by the tidal currents that in the Weser measure up to 2 m per second. This eroded mud renders the water turbid, and a turbidity cloud (T in figures) occurs and at slack water more or less disappears. Here the water has roughly the colour as in the water colour paintings at the end of this blog. The eroded mud is transported by the currents in the respective directions. So the mud is transported upstream at flood, and downstream at ebb tide.

At the tip of the salt wedge the mud accumulates (figure 3). It is not clear how those fine and light weight particles could settle in spite of the heavy currents and the supposed strong turbulence near the bottom. A logical - but not observed - explanation could be that at this place the bottom is - by dredging - so deep that all turbulence vanishes over long periodes of the tide.

By technical maintenence dredging the mud is removed and deposited farther downstream from where it is partly re-transported into the salt wedge area by the currents (figure 4).

In this difficult habitat live a few plankton species of which the calanoid copepod Eurytemora affinis is of greatest abundance. Though is occurs along the whole estuary, even in low brackish tributaries, it has an impressive maximum in an area of very low salinity up to 100 000 animals per cubic meter (figure 1).



November 1997
Aryaman Stefan Wellershaus
now: Ma.Aryaman@gmx.de

artificial estuaries

Remarks on the estuarine
hydrographic and sedimentary system
– artificial as well as natural estuaries

all my blogs are listed here:
http://mein-abenteuer-mein-leben75.blogspot.com/


All that I mentioned in the Eurytemora paper below refers to canalized estuaries. They are entirely artificial and do not necessarily, or not at all, show the natural situation of an estuary. The following remarks have the rank of a hypothesis that has still to be tested and veryfied.

In nature there are various types of estuaries but many of them can be classified into two main categories according to water types, water flow, sediments and sediment transport and shore form : 1) those that flow in a soft bed and 2) others that flow over or around solid hinderances like rocks or hard bed or mountains or in the fresh water area, trees. What does „soft bed“ mean? All sediments are sandy or muddy and can be lifted by the currents and transported to other places within the estuary or into the sea – whenever there is more water to flow through than the bed form allows at this very moment.

So estuaries that flow in a soft bed may change the form of the bed constantly. A characteristic of them (like for similar rivers) ist that the beds flow in meanders that are permanently changed in their meandric forms. It is obvious that this form exhibits the least strain against the flow, inner and outer friction are smallest and inner and outer tubulence are at minimum. So the stretched canal suffers more resistence than the meandering river. Look only on a car screen when water droplets flow down after a rain or during car washing. This has early been observed when in the North West of India (now Pakisthan) the British engineers constructed long long water canals in the desert sands that again and again changed their bed into meanders. They were meant to irrigate cotton fields.

We do not know much about undisturbed coastal plain estuaries flowing in a soft bed (i.e. without rocks). They seem to be the basic form of an estuary, and studying them may supply us with the deepest insights into the estuarine nature. Most studies have been done in canalized estuaries and may not be really descriptive for „undisturbed (natural) coastal plain estuaries“ in a tidal area.

Such tidal undisturbed coastal plain estuaries (tucpes) flow in ever changing meanders, may hardly have the often observed system consisting of a permanent salt wedge, a mud reach and a turbidity maximum. The flowing water searches for a bed for which it needs the least energy, and this is of the meander type. To obtain this, sediments are permanently reloaded and transported to places with the least turbulence, because the conditions vary permanently: depth, current direction and speed, turbulence, salinity, temperature and viscosity etc. These estuaries are almost not at all polluted by man's technical products, organic waste, or erosive products, they harbour a natural population of species, habitats, including shore habitats, as well as particulate and dissolved species in natural constitution.

In tucpes there are many small creeks and tributaries, back waters, muddy places and reed forests or forests like mangroves etc. In case of freshets or marine gales, a wide range of temporarily flooded low lying land is available as flooding space, consequently in inland, heavy flooding is rarer than in canalized estuaries.

Land- and riverborne substances that are transported by the river, may underlay a soft change during their long stay in the estuarine environment, including swamp and mud areas. So finally they have adapted to marine conditions, much slower than in canalized estuaries or where river water falls directly into marine water such as in fjords.

If we really want to understand Eurytemora and its life, we must go to such an estuary. Not very far from your place there was such an estuary and perhaps still is: The Ogeechee between Savannah and Atlanta. I think, it is worth being studied, and perhaps a longer research plan should be started. May be it already has been done or exists.

In short, I do not know, I do not even have a hypothesis worth mentioning of what are the root causes for the observed peculiar distribution of Eurytemora affinis in our estuaries. But I hope that my letter will help to do better research on estuaries than we could do a generation ago.

by Aryaman Stefan Wellershaus
28th November 1997

The copepod Eurytemora in estuaries

a letter on Eurytemora,
a brackish water plankton copepod

Dear friend,

somebody found your paper "Distribution of Eurytemora americana in the Duwamish River estuary" in internet and sent an outprint to me.

I find your paper very interesting, although I do not do any marine or estuarine studies since many years, I am retired, and my main subject is to look after psychic and spiritual diseases that lead man to exploit and destroy nature. But my former pet was Eurytemora affinis, and I also studied a few other estuarine copepod species.

Enclosed are two papers of my friend Ali and myself, one of which you mentioned but confused it with another paper of ours in the reference list (see our lists). The other you may not yet have seen, it is a kind of final conclusion at the termination of all our Eurytemora work. I am not able to send you any other of my/our papers since they are out of stock.

Your paper is - apart from the RESULTS - mainly based on suggestions of the rank of hypotheses, which is not yet enough for giving a description of, and final conclusion on natural phenomena. This shows that a lot more of this kind of work will have to be done in order to understand fully what is going on in estuaries. And it is worth doing so since still estuarine nature is not well understood. Studying the Eurytemora distribution and transport or retention, respectively, may be promising to understand more about the transport mechanisms in estuaries in general. This will have bearings on pollution studies, especially with regard to changes of the pollutants´ quality during passage through the estuary. Copepods are easily detectable - in comparison to suspended particles (seston) or chemicals.

From our paper in Bull. Plankton Soc. Japan, you can see that many parameters must be measured together with copepod counting etc (1), to obtain evidence (see Karl POPPER's work on scientific methods) for certain relationships as they might have been hypothesized beforehand.

(1) "etc" means that qualitative indicators for
different sub-populations or "herds" are usefull
such as the extent of infestment with ciliata.

By statistical means correlations between copepod density and parameters can be expressed and compared.

If any hypothesis test includes very low salinity (vls) as a parameter or a certain range of salinity within this stretch, a very sensitive method must be used - titration is not enough in this case, electrical conductivity (ec) may be better - although at low salinity, ec is no direct and linear indicator for salinity. Here, ec must not be converted into salinity because this will greatly distort accuracy. But it gives a relative impression and it is a rather exact number for further calculations, may be the exactest available. It does not represent the exact relationship between the copepod and salinity but between the copepod and its position within the estuarine hydrographical system. In this vls region or stretch, if any parameter is correlated against salinity as calculated from ec, a few more parameters may unintentionally be included (that influence ec besides of salinity) and blur the view.

In many published papers a clear conclusion of the relationship between copepod abundance and salinity could not been drawn by the authors - only hypotheses (2) or speculations (3) were given. May be the reasons for this are that salinity was given instead of ec, or that the research was not done far enough upstream so that the upstream slope of abundance was not seen, or the ec or salinity data were not widely enough spread on the abscissa (for example as log log data) so that details were darkened.

(2) often hypotheses were not made according
to the rules of science cognition
theory (see Karl POPPER and others)
and were put francly into the rank of
a theory for which no sufficient
evidence is given. Often such "hypotheses"
are not more than mere speculations.
Scientifically seen and in respect to
practical use of published results,
this is useless and at times even dangerous.

(3) speculations may, however, have good grounds,
they may come from collection of observations
that are not scientific but happen in daily
life accidentally and are stored in the brain
in a diffuse way, not systematically.
Almost unconsciously the brain collects
data from such observations and forms
ideas, experiences of life, pictures of existence etc.
All this is then used to formulate speculations,
they have got only little food from exact observations.
Much biological science works resemble this and

they are, thus, not really exact science.
And speculations are the ground to formulate hypotheses.
These have then to be tested to find the
truth - yes or no must be the answer of such tests.

With regard to migration it is difficult to believe that these tiny animals together with their offspring can migrate actively in an estuary like the Weser - with all its current speed (up to 2 m/sec) and turbulence, the same may be said with regard to vertical migration (as it was seen in crab megalopas that are, however, much bigger and live in low current regimes). May be BAROSS & CRUMP (see AVENT`s paper) have never seen them living or have not kept in mind the enormous mechanical energy prevalent in some tidal estuaries. And even if the copepods would ride on the tidal wave, they would not be retained in their habitat because the net water transport is downstream. But these remarks of mine are as speculative as the remarks of many authors are.

There is no other possibility for Eurytemora affinis in the Weser than to feed on suspended particles, there is nothing else (or on dissolved matter which is not believable - look at the mouth parts). And these particles are not part of the turbidity maximum because this is farther downstream. These particles are riverborne and contain much claiy substances and much organic matter - partly derived from the catchment area of the river (i.e. from erosion), perhaps more from sewage, but hardly from the sea (this seems to be different in mechanically undisturbed estuaries like the Amazone where marine diatoms have been found hundreds of kms upstream). It does not seem that the copepods actively search for a region where such food is densily supplied and rally round there, because where they are amassed there is no special amassment of suspended particles. I can, however, not say anything about the horizontal distribution of quality parameters of these particles in correlation with copepod distribution (except content of organic carbon, but I forgot the results).

Where in the Weser the copepod abundance maximum occurs there is no water stratification - due to high turbulence which is due to canalisation (except a stratification of current speed and turbulence, being lower near the bottom and the shore). And there are practically no prey animals either in this area nor in other places upstream of the salt wedge area due to impoverishment of natural life (due to pollution, current speed etc).

So Eurytemora affinis seems to be almost the only planktonic animal species in the Weser Estuary that can stand man's impact, somehow it seems to play a similar role as rats on land, feeding on man's refuse. But apart from this remark it is a specialized estuarine species which does not occur elsewhere in fresh or marine habitats in considerable numbers.

We have collected quantitative plankton samples by a 1 or 2 l sampler, Ruttner or van Doorn or the like, and poured through a net. This gives a mean value over a much greater mass of water than sampling 1o l in one collection. In addition in situ measurements of ec, oxygen, temperature, light transparency in red over 1 to 1o cm light path, pH, currents and pressure (or wire angle) for depth of the equipment were done from on board and read there. The meters were fixed in a heavy iron frame which was lowered from a crane from the anchored ship. In case of drifting with the current, the assembly need not be so heavy, it can be much simpler.

By Aryaman Stefan Wellershaus
28th November 1997

A few water colour paintings from 1978 in the Weser: a female and a male. The background colour represents rougly the colour of the estuarine water.


The female, pregnant with nine eggs. On the back an empty structure of ciliata (epi-zoons). The smaller red picture shows the eye enlarged. A long bag with spermatozoons is fixed to the abdomen.


The male. The extraordinary structure at the end of the row of swimming-legs is the pair of fifth legs used to fix the bag of spermatozoons to the abdomen of the female - copulation.

For comparison a Calanus from the southern Northsea (near Helgoland).
This species has ca 3 mm length, more than double the size of Eurytemora.

Three water colours by Stefan Wellershaus.



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