Pétur M. Jónasson

Physical characteristics of Thingvallavatn, Iceland

The basin of subarctic Thingvallavatn (64°1ON, 21°10W) was originally formed by tectonic subsidence and glacial erosion. Since the glacial recession some 10,000 yr ago, the basin has been strongly modified by volcanic activity. The catchment area is characterized by lava flows, of which four entered the lake, and one dammed it. One eruption occurred within the lake. Fault movements have resulted in horizontal and vertical displacements. The lake, area 83 km 2 , is at 100.5 m a.s.l. It has a mean depth of 34 m, and a maximum of 114 m

Long-Term Population Dynamics and Production of Pisidium (Bivalvia) in the Profundal of Lake Esrom, Denmark

During the 21 years (1953-1973, 1976-1977) of routine quantitative sampling reported in this paper, three species of Pisidium (P. casertanum, P. subtruncatum and P. henslowanum) have been present in the profundal of the eutrophic Lake Esrom, Denmark. From 1955 to 1962 P. casertanum was numerically dominant (peak density of 3273 ind m2 was attained in 1957), at other times P. subtruncatum dominated (peak 3025 ind m2 in 1955). Since 1957-1958 the densities of both species have been below 1000 ind ml. The peak density of P. henslowanum (370 ind m2) was attained in 1957. All the species laid their eggs in spring (April-May), but the young were not released until the following winter, the exact time depending on the temperature and oxygen conditions. Only some of the eggs laid developed into young, and the size of the brood was dependent on parental size. The young reached maturity at the age of 2 yr, and a maximum life-span of 4-5 yr is suggested. The regularly occurring oxygen deficiency in the hypolimnion in summer caused heavy mortality in the dominant species and halted growth of individuals. Both the standing crop and annual production were lower in the population of P. subtruncatum, which is a smaller species. The annual P/B-ratios were ca. 1 in both the dominant populations.

The Growth of Plumatella repens and P. fungosa (Bryozoa Ectoprocta) in Relation to External Factors in Danish Eutrophic Lakes

Introduction 121 Present investigations on Plumatella repens (L.) and Cristatella mucedo Cuvier in Lake Esrom 123 Present observations on vigorous growth of Plumatella fungosa (Pall.) in shallow eutrophic lakes ........ 131 Discussion 133 Summary 135 Acknowledgements 136 References 136

The occurrence and photosynthetic activity of epiphytes on the tips of Nitella opaca Ag. (charophyceae)

Abstract Loosely attached epiphytes were found to play an important role in the metabolism of a Nitella opaca Ag. community in Lake Thingvallavatn, Iceland. The epiphytic coat on the tips of the Nitella thalli on average accounted for one third of both total ( Nitella tips+epiphytes) carbon biomass and 14 C fixation. The epiphytic contribution was highest in May and decreased with the development and increase in height of the Nitella bed. The epiphytes were susceptible to removal by water currents, and, therefore, particular care had to be taken during field sampling and measurement in order to obtain reliable values for the community structure and metabolism.

Thingvallavatn Research History

The research history of the Thingvallavatn area dates almost 300 years back. Geological, limnological and ichthyological research of the lake started 100 years ago. A general limnological investigation including geology, meteorology, vegetation of the lake and the catchment area, physics, chemistry, plankton, benthos, fish, birds and mammals was decided on the occasion of the 1100 th anniversary of the Icelandic settlement 1974. These themes form the chapters of this monograph together with an ecosystem analysis