Interviews

Teeming ecosystem of miniature creatures

Fresh wind in your face and wet sand between your toes. A hike through the mudflats is pure relaxation. And if you lower your gaze from the horizon to the ground, you discover a world of its own: a teeming ecosystem of tiny creatures. Andreas Schmidt-Rhaesa, head of the Invertebrates I department at LIB, studies life on and between the grains of sand. He is scientific advisor to the exhibition "Experience Mudflats - Understand Mudflats".

When we talk about mudflats, we think of silt and sand. What do you think of?

I think that the mudflats are one of the most fascinating habitats, but only at second glance. The alternation between being covered by seawater at high tide and free-falling at low tide creates a dynamic that exists in almost no other habitat. Despite these challenges, vast numbers of animals live on the tidal flats, most of them somewhat hidden in the mudflats. I am especially fascinated by the very small animals in the so-called sand gap system, collectively known as meiofauna.

What kind of animals do we discover when we look closely?

If you look more closely, you will primarily discover mussels and marine bristle worms burrowing into the bottom. If you have the opportunity to look with a stereomicroscope, you discover an incredible diversity of crustaceans, worms and other animals such as tardigrades, mites and much more.

Who eats whom in the sand gap system - and is this food web still intact?

There are diverse feeding habits in the sand gap system. There are bacteria eaters, algae eaters, or predators that eat other animals. Still others feed on non-living organic matter. All in all, a complete food web that is, of course, part of larger food webs because meiofauna also provide food for larger organisms and thus contribute to the productivity of tidal flats. Meiofauna is also subject to natural fluctuations as well as man-made changes, but we don't always recognize them because we still don't know enough about the meiofauna as a whole.

What can we use the results/data of meiofauna research for?

In addition to more scientifically interesting questions about biodiversity, dispersals, and speciation, many meiofauna organisms are important for understanding evolutionary changes in animals. It is particularly useful for indicating changes, whether due to climate change or pollution, because the composition then changes relatively quickly. But for that we should get to know the meiofauna even better.

What would you like visitors to the exhibition or mudflat hikers to take away with them?

I would be happy if we can show how special and diverse the mudflats are. Maybe the next time you go on a mudflat hike, you'll remember that there are hundreds, thousands of animals under your feet.

Contact

Prof. Dr. Andreas Schmidt-Rhaesa
Department Head Invertebrates
LIB
Martin-Luther-King-Platz 3, Raum R. 234
20146 Hamburg
Tel. +49 40 42 838-3921
E-Mail: Andreas.Schmidt-Rhaesa"AT"uni-hamburg.de

Why we should not trample on the ground

Base, filter, storage, habitat. The soil beneath our feet is more than a foundation. It is crucial for life, the climate and the diversity of species. Prof. Dr. Lars Kutzbach from the Institute of Soil Science at the University of Hamburg is concerned with the central importance of soils. He is the scientific advisor of the exhibition "Experience mudflats - understand mudflat soil".

What and where is actually mudflat - and how is this soil formed?

The word "mudflat" refers to both a habitat type and a soil type. The mudflat soil type is a young soil that forms in the transition area between land and water under the constant alternation of flooding and draining by high and low tide. In addition to the very specific flooding dynamics, the interaction between sedimentation of sinking matter and colonization by organisms is very important for the soil formation of the tidal flat. Depending on the current velocity, mineral particles of different sizes (from clay to sand) and organic material formed in the sea sediment onto the tidal flat bottom at high tide. These sediments, usually quite soft, become fixed by colonization with organisms. Thus, a thin but cohesive skin - called a biofilm - of unicellular algae, bacteria, and mucilage typically forms on the surface of the tidal flats to counteract the flushing away of the sediment. The colonization by animals, e.g. worms or crabs, also promotes the establishment of the mudflats.

How do the very different colored shades in the mudflat bottom develop?

Oxygen can get into burrows of animals living in the tidal flats. This reacts with reduced iron compounds and forms rust-colored iron oxides, which cement the sediment particles and thus give structure to the tidal flat floor. With the exception of burrows, the permanent water saturation of the pore system causes oxygen to penetrate only a few millimeters or centimeters. The absence of oxygen leads to reducing chemical conditions and the formation of finely divided iron sulfide, which colors the lower portions of the tidal flats gray or black. The strong color contrasts between the olive-brown tidal flat surface, the rust-red iron oxides along the burrows, and the gray and black reduced bottom areas are striking and characteristic of tidal flats. Due to the constant alternation of flooding and draining and the lack of oxygen, tidal flat soils are extreme habitats that only specialized species can use as habitat.

What are the functions of soils in general and of tidal flat soils in particular?

Soils fulfill many functions, which can be roughly divided into regulatory functions, habitat functions, utilization functions, and archival functions. Soils can filter pollutants and nutrients from water and thus purify it. They can absorb and store water, thus mitigating the effects of heavy rainfall events. They also provide significant carbon storage, which is important in regulating global climate (see below). They provide water, nutrients, and anchorage for plants, and thus are also the basis for all food production. In addition to plants, soils also provide a diverse habitat for animals. Furthermore, they are storage sites for various raw materials (e.g. clay, sand, peat) and are used as building ground. Their function as archives of natural and cultural history is also important.

The tidal flats in particular fulfill regulatory functions. Organic material formed in sea or river water is decomposed in them and mineralized to carbon dioxide and hydrogen carbonate. The tidal flats are thus a source of hydrogen carbonate for the waters of the Elbe and the North Sea. Nitrogenous nutrient compounds are also converted in the tidal flats. In the Elbe estuary, the tidal flats store pollutants that in the past polluted the water of the Elbe to a much greater extent than today. The relatively stable storage in the tidal flats protects neighboring ecosystems. With their often high population of bottom-dwelling organisms, tidal flats are an important food source for birds and young fish. Tidal flats are not suitable for agricultural production, but have a high value for use by fisheries and tourism. Most tidal flats in Germany are protected by law as valuable biotopes.

And beware, never go into the tidal flats without a local guide! In the tidal flats you can sink surprisingly deep and not be able to get out again on your own. This is especially true for the often very soft river mudflats in Hamburg's urban area!

To what extent do processes in soils (including the tidal flats) interact with the climate system?

Soils play an important role in the climate system because they interact closely with the atmosphere and the hydrosphere and strongly influence the properties of these spheres of the Earth system, which are also very important for the climate. In particular, soils store large amounts of carbon in the form of dead organic matter, e.g., as humus or peat. When this carbon is in the soils, it is not in the form of carbon dioxide (CO2) or methane (CH4) in the atmosphere. These two carbon-containing gases affect the climate as greenhouse gases. Dry soils can actually absorb methane, a potent greenhouse gas, and convert it to carbon dioxide, which is less potent. Wet soils, however, can also be a source of methane. Similarly, nitrous oxide (N2O) is an even more potent nitrogen-containing greenhouse gas. Soils in nutrient-poor wetlands tend to absorb nitrous oxide, while soils in nutrient-rich wetlands often emit nitrous oxide into the atmosphere. Soils continue to be important because they are the site of enhanced weathering of rocks, which has important effects on the Earth's carbon dioxide budget, especially over geologically long periods of time.

Tidal flat soils interact with climate because they are typically sources of carbon dioxide. Depending on ocean influence and nutrient supply, tidal flats can be sinks or sources of methane and nitrous oxide.

Why was the North Sea Wadden Sea designated a World Heritage Site?

UNESCO's International World Heritage Convention aims to promote the protection of the common cultural and natural heritage of humankind. The unique natural landscape of the Wadden Sea on the southern North Sea coast has been designated a World Heritage Site because of its outstanding global value in terms of its exceptional geological evolution, its many important ecological processes, and its significance for the conservation of biodiversity and endangered species. In addition, the status of World Heritage Site recognizes that the Wadden Sea is still highly intact in its entirety and is also under a high protection status under national legislation.

Keyword Elbe deepening. To what extent is the river mudflat habitat particularly threatened?

The deepening of the Elbe will have an impact on current speeds and the tidal range, including in the freshwater zone of the Elbe estuary. Higher current velocities could erode river mudflats in some areas. Even more intensive shipping traffic with even larger ships can likewise lead to greater erosion via increased wave action. On the other hand, a higher tidal range with deeper mean low tides and higher high tides will likely result in an overall greater areal extent of river tidal flats at the expense of permanently flooded shallow water areas and river marshes. Most problematic is the disturbance of polluted sediments by dredging the navigation channel, as this can mobilize pollutants and introduce them into the tidal flats. The negative effect on tidal flats is even more drastic if they are overlaid by dredged sediments during bank flushing or underwater disposal measures. In general, higher flow velocities and thus higher sediment loads pose the risk that problematic oxygen deficits in the Elbe water could occur more frequently. This would also affect the river tidal flats during flooding.

Contact

Prof. Dr. Lars Kutzbach
University of  Hamburg
Institute for Soil Science
Tel.: +49 40 42838-2021
E-Mail: Lars.Kutzbach@uni-hamburg.de( Lars.Kutzbach"AT"uni-hamburg.de)