1.Background information about Plankton in the North Sea
Plankton are organisms that inhabit the water column. The name “plankton” is derived from the Greek word plankter, which translated into English is equivalent to wanderer or drifter. They are characterized by their incapability of actively moving against the current and by that controlling their position. Plankton is classified by size and can be subdivided into megaplankton, macroplankton, microplankton, nanoplankton, picoplankton and femtoplankton. The biggest species of plankton are jellyfish that have a size of several centimeters and the smallest kinds obtain sizes of less than a nanometer. Although plankton are of such small size they are an essential food source for various marine animals like whales and mussels. The abundance of plankton depends on various factors like nutrient concentrations, the physical and chemical state of the water and the abundance of other kinds of plankton. Generally various kinds of plankton can be found everywhere on the globe. The name for freshwater plankton is limnoplankton and saltwater plankton is called haliplankton. Apart from a classification by size plankton is divided into phytoplankton and zooplankton.
Phytoplankton are free-floating, photosynthetic and phototrophic organisms which generate chemical energy from sunlight. Hence they are autotrophic and usually found in the photic zone of the water column. They are microscopic in size and are characterized by their incapability of swimming against the current. They belong to the taxonomic domains Bacteria and Eukarya – and maybe also Archea. The larger ones, which can be captured in plankton nets, only consist of two kinds of phytoplankton, diatoms and dinoflagellates. A phenomenon of the phytoplankton is the large bloom during spring. Then the populations grow explosively. The growth depends on availability of carbon dioxide, sunlight, and nutrients like for example nitrate and phosphate. Other factors that influence the growth are temperature and salinity.
Diatoms belong to the class Bacillariophyceae. Their body consists of a unique glass “pill box” and does not exhibit any visible means of locomotion. The living part of the diatom is within the “pill box”. The box is mainly made of silicon dioxide (SiO2). Diatoms may occur individually with one individual occupying a single box, or they can form various kinds of chains. Diatoms are particularly abundant during spring.
Rhizosolenia are elongated, unicellular, photosynthetic diatoms. The long straight cells have a spine-like process on each end. The cells have a strong silica shell and contain numerous small chloroplasts. Their sizes range from approximately 13 to 230 µm.
Figure 1: Rhizosolenia
Chaetoceros borealis has cylindrical cells with a broadly elliptical cross-section building straight and close-set chains.
Figure 2: Chaetoceros borealis
Chaetoceros tortissimus are chains, which are straight or slightly bent and strongly twisted about the chain axis. The setae are thin and arise inside the valve margin, perpendicular to the chain axis but then going in all directions. The valve surface is slightly convex and adjacent valves touch in the centre of the valves. An aperture is hence only visible towards the valve margin. Chaetoceros tortissimus obtains diameters (apical axis) of approximately 11 to 20 µm
Figure 3: Chaetoceros tortissimus
Skeletonema are long cylindrical diatoms, which contain chloroplasts. The cells form chains by attaching external tubes or processes. Skeletonema have a size of approximately 2 to 21 µm.
Figure 4: Skeletonema
Coscinodisceae are photosynthetic diatoms. The cells are flat and cylindrical shaped and occur solitary. They sizes ranging from 50 to 500 µm
Figure 5: Coscinodisceae
In contrast to phytoplankton the zooplankton is extremely diverse. It is not photosynthetic but feeds on the phytoplankton, which is usually located further up in the water column. The most important groups of the zooplankton are cladoceras, copepods, rotifers, flagellates and ciliates. Reproduction depends on nutrient supply for example in form of phytoplankton and on temperature. Therefore production increases in spring and reaches it’s maximum in summer until it decreases again in the fall.
Copepods are one to several millimeters in length. The long antennae make them easily recognizable. Those are used to slow the rate of sinking. For swimming the jointed thoracic limbs are used and show a characteristic jerky movement. They graze on phytoplankton by using the anterior appendages or a filtering mechanism.
Acatia clausii belongs to the copepods.
Figure 6: Acatia clausii
Ctenophores are free swimming jellyfish which belong to the plankton. They are transparent or translucent. They typically also exist in shallow parts of the world’s oceans.
Phialella quadrata are found around Helgoland, diameter 1,2mm. They are most numerous between April and September.
Figure 7: Phialella quadrata
2.1 The multiple opening and closing net (Multinet)
The Multinet is a scientific device, which is used to collect plankton and particulate matter from the water column. The sampling can be done at distinct depths horizontally or vertically. In order to have horizontal sampling the Multinet is dragged behind the research vessel at a specific depth. For horizontal sampling it is deployed while the ship is on station. It is then brought down to the starting point of the sampling and then lifted up in specified intervals. The Multinet consists of a stainless steel frame to which five single canvas nets with a certain mesh size are attached with the help of zippers. These nets can be opened and closed separately by an arrangement of levers, which are powered by a motor unit. This is controlled via a deck command unit that stays on board while the Multinet is in the water. The nets can be opened and closed via springs that are tensioned while the device is still on board. In the water the springs can be released one after the other. It can be released by pressing the respective button on the control unit (Figure 9), which is connected to the Multinet via a cable. This closes the previous net and opens the following one so that the sampling can be done continuously.
At the end of the nets there is a second frame. It contains five cups in which the material collected by the nets during the lifting process is stored. The cups have a very fine mesh at their sides so that excessive water can get out of them when the device is on board again.
As the Multinet has a considerable length, it cannot be lowered down to the sea floor completely because the samples should not contain sediments. That’s why you have to keep a certain security distance to the sea floor.
Figure 8: The multiple opening and closing net before deployment
Figure 9: The deck unit for controlling the net during deployment
2.2 Multinet Data
• Net Frame of stainless steel with pressure capsuled motor unit
• Integrated depth meter 0-3000 m
• Canvas part with zip fasteners
• Net opening 50 x 50 cm = 0.25 m²
• Power supply: batteries
• 5 Net Bags with zip fastener, length 250 cm, end of net 11 cm diameter
• 5 Net Buckets 11 cm diameter, side window covered with sieve gauze
• Stainless steel support for Net Buckets
• Net Bucket Holder (for vertical collection) made of stainless steel
• 2 Bridles
• V-Fin Depth Depressor
• Deck Command Unit with push button control for net changing
• Indication of actual net number, depth, battery status
• Display: LCD supertwist with LED backlight
• Main power supply: 85 - 260 V AC
Graph 1: CTD temperature profile for Station 9
In Graph 1 we can see the temperature profile on station. It was measured with the CTD. Unfortunately the measurements seem to be in the wrong temperature range, maybe due to wrong calibration. There is some variation in the temperature in the upper 20m whereas below 20m the temperature is very stable. This might have an impact on the plankton sampling. In general, the water is too shallow to make further assumptions on the thermocline. However, it is nice to have some data on the water properties before deploying the Multinet.
Before the deployment, the Multinet was prepared in the following way.
First, the five nets (length 2.5m, mesh size 55 µm) were attached to the metal frame with zippers and also to the metal cage where the sampling cups are in. Before attaching the nets to the frame, the cups were cleaned with freshwater.
Figure 10: The net has to be attached to the metal frame
Then, they were put into their frame and fixed with screws (Figure 11), which were also fixed by tying them to the frame so that they don’t loosen under water. On the metal frame there are springs, which are tensioned and secured so that they do not loosen until the net is in the water.
Figure 11: The cups have to be attached to the frame and the nets
Figure 12: The net is being deployed
After letting the Multinet down into the water (Figure 12) to the starting depth, by pressing the button “net change” on the deck control unit, one spring is loosened and then one net closes and the next adjacent net opens.
Figure 13: The opening and closing of the net is controlled through a deck unit
The following table gives the data where and when the Multinet was deployed.
The following table gives the data at which depth ranges the individual nets opened and where the water was sampled.
After the Multinet was brought back on deck, the cups were taken off the metal cage and the water is put in plastic containers. Overall we can say that there was sediment mostly in the cups from deeper down in the water. After a first look, some water from each sample was transferred into a petri dish and examined more closely under the microscope.
Figure 14: The five collected water samples
Water depth: 25 – 20 m
The water is turbid and contains many small particles, which are probably organic aggregates and clay particles. Particles settle on the bottom of the sample and form a layer of approximately 1mm thickness. The water gets clearer towards the surface.
With the help of a microscope different kinds of plankton become visible.
In sample 1 we mostly find cylindrical-shaped diatoms. The majority of these cylindrical diatoms – Cuscinodisceae - are flat and orange-colored. The remaining cylindrical diatoms are twice as big as the small ones and rather yellow in color. Additionally, there are long and roundish skeletonema and medium-sized greenish particles, which do not express any kind of symmetry. Also, there is a small number of red particles and a blue-colored thread.
Figure 15: View through the microscope
Water depth: 20 - 15 m
The water is turbid but not as dark as in sample 1. There are tiny particles floating in the water and a very thin pervious layer of particles sediments on the bottom of the sample. The layer is thinner and less dense than the layer in the first sample.
In sample 2, there are almost no cylindrical-shaped diatoms. This is particularly noticeable because the previous sample was dominated by this kind of plankton.
Also, there are short and more compact rod-shaped Guinardia. Additionally, there are skeletonema, whitish and reddish flakes and black aggregates.
Water depth: 15 – 10 m
The water is less turbid than in the first two samples. Particles are floating in the water body. Tiny jellyfish move through the water. Those are probably Phialella quadrata. Particles sink slowly and form a thin layer at the bottom of the sample. Some white flakes are visible without microscope.
In sample 3, there are rarely any particles apart from a few big cylindrical-shaped segmented diatoms. Some are divided into half and others show two thin ring-shaped stripes on their surface. There is a snail-shaped organism. Apart from this, there are a few red and white flakes and black aggregates. Also, there are some segmented transparent needle-like rod-shaped Nitzchia and very few small cylindrical diatoms.
In the sample, there is also a copepod, which seems to be feeding on other plankton.
Water-depth: 10-5 m
The water in sample 4 is relatively clear. Some white particles are visible without a microscope. Black and also yellowish to brownish particles settle at the bottom of the sample.
The sample generally contains little matter. There are some smaller and bigger cylindrical-shaped diatoms, some reddish and whitish flakes of various sizes and transparent skeletonema.
Figure 16: view through the microscope
Water depth: 5 – 0 m
The water is clear with a considerable amount of brown particles floating in the water. There are some very tiny light green algae. Additionally there are particles without a distinct shape, which all have a green nucleus in the center. These particles move actively through the water. Also, there are some transparent rods with yellow dots and a red thread. Apart from this, there is a screw-like animal that moves actively through the water.
The cloudiness of the water is positively correlated to the increasing water depth. Sample 1, which is taken from the greatest depth (25 to 20 m) is the cloudiest water and contains the densest layer of settled particles. Sample 5, taken from 0 to 5 m water depth is clear and contains only a few floating brownish particles. All samples contain cylindrical-shaped diatoms. However, the abundance of these coscinodisceae is different for each sample. Some, for example sample 1, contains a significant amount of small and flat orange-colored coscinodisceae and only a few bigger yellow ones, whereas sample 3, for example, doesn’t include any small and flat ones and sample 2 completely lacks both kinds of cylindrical-shaped diatoms. Sample 1,2 and 4 are the only samples, which comprise the long rod-like skeletonema. Also particularly noticeable is that sample 3 from a depth of 15 to 10 meters is the only one that besides phytoplankton also contains zooplankton. Not only does it include a copepod but it also hosts a considerably high amount of tiny translucent jellyfish – Phialella quadrata-, which are even visible with the naked eye.
Finally, it can be said that the degree of cloudiness of the water of the different samples meets the expectations. The water sample from the greatest depth right on the sea floor contains the most sediment, whereas the water sampled from the top five meters is almost entirely clear and free of settled particles. Furthermore, it can be concluded that the collected plankton samples fit the seasonal plankton distribution. During spring there is a phytoplankton bloom, which is particularly represented by the relatively large amounts of diatoms found in all five samples from the different water depths. The only question that cannot be answered is why there is zooplankton only in the third sample and not in any of the remaining four samples that have been collected at lower and higher water depth respectively.
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