Tvärminne Zoological Station

Surroundings

Tvärminne Zoological Station is situated in corner of the Hanko peninsula, the southernmost tip of continental Finland. The area is charactrised by exceptionally high diversity in its physical appearance, number of different biotopes and species richness (it harbours about 15% of the endangered species of Finland) thus fulfilling one of the most demanding criteria for a biological field station.

Hanko peninsula itself is a mixture of Precambrian bedrock and a geologically recent end-moraine complex, Salpausselkä, that runs as a continuous ridge from the Russian Karelia through the whole of southern Finland and even further into the northern Baltic Sea. Thus, most of the soils on and around Hanko peninsula are either moraine or sand. However, rich organic soils and luxuriant vegetation can also be found in connection with strips of calciferous minerals in the bedrock.

A vast archipelago of thousands of rocky islands surrounds the southern end of the Hanko peninsula. In the outermost or sea zone, islands are sparse and small, often no more than solid rocks elevated a few meters above sea level. These islands are exposed to winds and waves and their only vegetation consists of grasses and herbs, which grow in the most sheltered crevices. The sea also dominates the outer archipelago zone, but islands are more numerous and often clumped. Most of the largest islands are partly covered by pine forests (Pinus sylvestris). The islands of the inner archipelago zone are large and the forest often stretches down to the sea. Being mostly well sheltered from wave action they often have sedge and reed belts around them.

Part of the archipelago closest to the Station (ca. 140 ha land and 460 ha water) is owned by the University of Helsinki. It has the status of a nature conservation area to preserve its natural state. There are also several small nature reserves and one larger (ca. 4 000 ha) national park, the Ekenäs Archipelago National Park which was established in 1989, in the vicinity. The park aims to preserve a representative part of the archipelago and marine ecosystems of the Gulf of Finland. It also promotes research and environmental education and offers opportunities for boating and other outdoor activities. These conservation areas will be included in the huge Natura 2000 area of 52 000 ha, planned to cover the coastal waters within a range of 20–40 km from the Station.

Since the Station was established in 1902, hundreds of studies have been carried out and a considerable amount of background data has been gathered on the surrounding environments and populations. Most of this data is publicly available at the Station.

Climate

A look at a map is enough to give a rough idea of the local climate: relatively mild and maritime. The appended diagram shows it more precisely for the period 1961– 90. However, the winters of the 1990s have been clearly milder than the long term averages. With a few exceptions, the sea has been ice-covered around the Station for 1–4 weeks only, instead of 3–4 months as is used to be, and outside the archipelago the ice-cover has been even more ephemeral or lacking completely. The same trend holds of course for the snow-cover,

The recent shortening of winters has made the "transitional" seasons even longer than before. Springs have started earlier (regardless of the criteria), but since the warming up of the sea takes time, the temperature rises rather slowly. Thus, spring and also early summer are typically relatively cool. Another characteristic feature is low rainfall during the first half of the year. Mid-summer, on the other hand, may be rather warm and combined with increased evaporation it often results in severe droughts, especially in the archipelago. However, rainfall increases towards autumn and by September there is always enough moisture for a new growing season even on the driest habitats. Occasional snow-showers and some sea-ice formation in the archipelago may occur already in November, but only rarely does the "real" winter begin before the end of the year. During the current decade the winters have mostly been delimited to only a few weeks in or around February.

.Average daily mean temperatures, average daily precipitation (summed over months) and average daily snow depth from the period 1961–1990 (Hanko, Tvärminne; courtesy of Finnish Meteorological Institute)

Accessible aquatic ecosystems

Baltic Sea

In many respects the Baltic Sea occupies a very special position among the seas on the earth. One might even say, that the only thing which justifies us to call it a sea is that it is certainly not a lake - at least not any more. However, when the ancient basin of the present Baltic was released under the cover of the continental ice shield some 11 000 years ago, it first became a lake, then a sea, a lake and a sea again; all these drastic changes occurred in less than 3000 years! This alteration was, in principal, brought about by variations in proportional strengths of the two "competing" phenomena both of which were direct consequences of the melting of the glaciers: land upheaval and rise of the sea level. The last lake phase (Yoldia) ended, when the rising sea pushed salt water upstream to the Danish channels and beyond them. After passing the sill the sea water had free access and gradually the whole basin became brackish. This Littorina Sea was, in fact, even saltier than the Baltic Sea is at present, and also much larger. However, land upheaval has been the indisputable winner since the late Yoldia and it is still conquering land from the sea in many places around the Baltic, especially on the Finnish coasts of the Bothnian Bay.

Apart from being young, shallow and rather small in area the Baltic Sea differs physically from a "real" sea in two important respects: (1) its salinity is mostly less than 10‰ and (2) it has a minimal tide of a few centimetres only. Consequently the Baltic has no intertidal zone, although irregular changes in the water level of up to 1–1.5 m are caused yearly by variations in meteorological conditions, especially wind and air pressure. The biological implications of these physical properties are enormous and resulted in a rather species poor flora and fauna in comparison with fully marine areas. Only a small fraction of marine organisms can endure brackish water and the same applies to fresh water organisms.

A lot of up-to-date background data about the Baltic Sea and its environmental condition can be found in BALLERINA (Baltic Sea region on-line environmental information resources for internet access) and from the home pages of HELCOM, the Finnish Marine Research Institute and the Finnish Environment Institute.

Coastal waters

In the northern Baltic proper, off the coast of the Hanko peninsula, the greatest water depths are 50–80 m and the salinity of the surface water is around 5.5–7 ‰. In the archipelago the water depths are around 20–40 m and the salinity is slightly lower than at the open sea. Thermal stratification usually develops in early spring, but is often disturbed by upwellings, which are common events in this area.

In the vicinity of the station the salinity decreases gradually over a distance of approx 40 km from the open sea through the archipelago and the fjord-like Pojo Bay to nearly fresh water in the innermost part of the bay. This forms an almost unique situation of a relatively undisturbed estuary within the brackish water system of the Baltic Sea. The area has been designated to be of the highest category on the Project Aqua list of waters of international importance for research.

During the winter the sea and other aquatic habitats of the archipelago are usually covered by ice for a period of two to four months. However, as many as nine out of the last ten winters have been exceptionally mild and the area is ususally only frozen over for a few weeks.

Pojo Bay

The Station lies at the mouth of the 40 km long fjord-like inlet, the Pojo Bay (Pojovik), the estuary of the River Svartå (or Karjaanjoki, drainage area 2300 km²). The semi-enclosed Pojo Bay is connected to the outer archipelago by a series of basins with variable depths. The inner part of the bay is separated by a 6 m deep sill at the town of Tammisaari (Ekenäs); from there its depth gradually increases to a maximum of 42 m in Sällvik, ca. 5 km north of Tammisaari. Freshwater outflow from the River Svartå forms an oligohaline (generally less than 4 ‰) surface layer upon the more saline deep water of the bay. This vertical stratification leads to regular summer time stagnation and gradual oxygen decrease in the deep water of the Pojo Bay. The deep water renewal occurs only under special meteorological conditions, mainly during autumn and winter, when hydraulic control at the basin entrance (Tammisaari sill) becomes weaker and allows saline inflows from the archipelago.

Pojo Bay and the outer archipelago have always been amongst the most important research areas of the Station. Therefore, a lot of background data on the biology and hydrography of the bay is available. Nowadays, long-term data is of particular importance when scientists try to explain and understand changes in the coastal brackish-water system of the Baltic Sea.

"Lagoons"

Shallow bays, more or less delimited as so-called gloes and fladas, are common in the archipelago and along the rising coast of the Baltic Sea. These habitats are ecologically important, for example as spawning- and feeding grounds for fish as well as for birds. Both flads and gloes play an important role in the process where the sea is transformed to land through land upheaval. The flads are characterised by the vegetation on their soft gyttja bottoms, which is often luxuriant and exhibits a tendency towards the formation of communities dominated by one or only a few species.

Sandy beaches

Within 3–20 km from the Station there are more than 10 km of sandy beaches and extensive sandy bottom areas reaching far out into the sea. Subfossil dynes also exist in the vicinity.

Rocky shores

Apart from the esker area of the Hanko peninsula, where sandy soils are predominant, the shores are mainly rocky and stony. Precambrian rocky shores (quartz-diorite and granodiorite) dominate the surrounding archipelago. Outermost islets and skerries are exposed to wave actions, whereas the inner shores are more sheltered.

Rock Pools

Hundreds of rock pools varying in size, exposure, salinity and trophic conditions are available in the surrounding archipelago. Rock pool ecology has been a long-standing research topic at the Station.

Aquatic biota

Due to its intermediate salinity, which only a few species can endure, the Baltic Sea has a rather impoverished fauna and flora. The animals and plants inhabiting the brackish water originate both from marine and fresh water environments. Thus marine and fresh water species – for example cod (Gadus morrhua) and pike (Esox lucius), as well as the brown algae Fucus vesiculosus and the pond weed Potamogeton perfoliatus – can be found in the same locations. Fucus-stands are important biotopes as spawning- and hunting-grounds for fishes like perch (Perca fluviatilis) and pike.