Primärproduktion und Seetypen

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Summary

In 2 high-productive lakes in southern Sweden (Uppland: Erken and Görväln) and 7 low-productive lakes in northern Sweden (Lappland: Ransaren, Kullsjön, Nedre [= Lower] Laksjön, Övre [= Upper] Laksjön, Katterjaure, Vassijaure, and Torne träsk), the primary production of organic matter by phytoplankton has been measured in situ with the C¹⁴-technique introduced by Steemann Nielsen. From curves for the vertical distribution of the daily production (figs. 1 and 2), the production rate has been calculated for each cubic meter of the trophogenic Stratum (mg Cass/m³), for the cubic meter in which the light conditions are optimal (mg Cass/m³, max.), and for the whole trophogenic layer (mg Cass/m²). The ratio between the two latter values (cfr. table 1) is mainly dependent upon the optical conditions in the water and roughly related to the transparency (figs. 3–7). The absolute values of the daily production during the ice-free season (fig. 8) are considerably higher in Erken and Görväln (between about 100–2200 mg C/m² and 25–750 mg C/m³, max.) than in the lakes in Lappland (< 100 mg C/m² and < 25 mg C/m³, max.).

While the relationship between daily production rates and corresponding values for the standing crop (fresh weight) of phytoplankton varies within a wide range, the data indicate that in general a proportionate relation does exist between them (fig. 9). The daily production per unit fresh weight — a quotient indicating the productive activity of the total phytoplankton — has an approximately similar range of variation in all the lakes investigated (table 2). Preliminary values for the turn-over time vary between 0.5 and 10 days for the optimal light layer, and between 1 and 50 days for the trophogenic zone as a whole. A slight tendency towards higher activities may be significant for the high-productive lakes in Uppland. Their epilimnic concentrations of phosphate-phosphorus and nitrate-nitrogen, however, may be as low or even lower than the contents in the Lappland lakes (table 3).

For Lake Erken the Efficiency of the Primary Production in Relation to the incident radiation has been calculated for a period of 23 Months (fig. 10). The Decade (10-day) values from different seasons vary between about 0,01% and 0,6% (extreme single-day values are more apart) and for the whole year 1954, when a total of 104 000 mg organic carbon was produced per m², the efficiency of the lake was found to be 0,12%. The amount of organic carbon produced by the spring maximum (during about 2 months, beginning almost a month before the breaking up of the ice) proved to be nearly constant (about 37 000 mg C/m²) in the years 1954, 1955, and 1956 (table 4). If this constancy is significant, the value can be used as a measure of how much the lake can produce when it is at its best.

Some basic concepts of lake typology are shortly discussed. It is claimed that the serious confusion characterizing present use of the “r… trophic” terms is due mainly to the fact that the trophic system still lacks a uniform and quantitatively measurable basis. It is impossible to establish such a basis, if both the conditions of organic production and this production itself (or the standing crop) are selected as criteria for trophic types. We have to decide, upon which aspect of lake metabolism these types are to be founded.

Since only the primary production seems able to offer a uniform, quantitative basis — theoretically correct as well as practically useful — for the trophic system, it is suggested that all “… trophic” lake types except oligotrophic and eutrophic be abandoned and the rate of primary production be used as the measure of the degree of oligotrophy or eutrophy. Other characteristics of lakes (such as their chemical and physical conditions, morphometry, secondary production, as well as qualitative characters) may conveniently be compared and defined for various lakes, if plotted against the trophic axis as abscissa.