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Production and Nutrient Supply of Phytoplankton in Subarctic, Dimictic Thingvallavatn, Iceland

The subarctic Lake Thingvallavatn (64°10′, 21°10′W) is 114 m deep, dimictic and located in the North Atlantic rift zone. Primary production was measured during a 7-yr period from 1974 to 1982. The lake is fed mostly by groundwater. The lake is 83 km2 in area with an average depth of 34 m and a volum...

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Bibliographic Details
Published in:Oikos 1992, Vol.64 (1/2), p.162-187
Main Authors: Jónasson, Pétur M., Adalsteinsson, Hákon, St. Jónsson, Gunnar
Format: Article
Language:English
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Summary:The subarctic Lake Thingvallavatn (64°10′, 21°10′W) is 114 m deep, dimictic and located in the North Atlantic rift zone. Primary production was measured during a 7-yr period from 1974 to 1982. The lake is fed mostly by groundwater. The lake is 83 km2 in area with an average depth of 34 m and a volume of 2855× 106 m3. Retention time is 330 d and average ice-cover 95 d yr-1. Mean global radiation is 74.5 kcal (311.7 kJ) cm-2 yr-1. Secchi disc transparency ranges from 6 to 15 m, and the photic layer extends from 20 to 40 m during summer (1% of subsurface light penetration). Consequently, primary production was measured down to 25 m, in a few cases to 30 m depth. The temperature ranges from 0 to 13° C. The nutrients N, P and Si enter the lake at fluxes of 1.4, 0.8 and 240 g m-2 yr-1, or in the ratios of 1.7:1:300. The NO3- N as well as the NH4-N concentrations generally vary from ca. 6-8 μ g l-1 to 0, but may reach 18 and 33 μ g l-1, respectively, after autumn overturn. Phytoplankton production is nitrogen-limited. PO4- P amounts to ca. 10, but may occasionally reach 14 μ g l-1. Large diatom species, as Melosira islandica and M. italica, dominate except in summer. Smaller species, as Stephanodiscus astraea, Asterionella formosa and Nitzschia holsatica, take over in spring and sometimes throughout the summer. The midsummer phytoplankton is mostly Chrysophyceae and Pyrrophyta. Large diatoms return in autumn. Phytoplankton production is bimodal with a spring (max. 1.53 g C m-2 d-1) and an early autumn peak (max. 0.66 g C m-2 d-1) of diatoms; and a summer minimum (0.1 g C m-2 d-1) of mainly Chrysophyceae. Biomass follows the same trend and, measured as Chl a, it ranges from 0.5 to 4.0 mg m-3. In years with unstable thermocline, production stabilizes at 0.3 to 0.4 g C m-2 d-1. Mean annual production is 95 g C m-2. Light-saturated photosynthesis occurs at 1-10 m depth and maximum daily production is 200 mg C m-3. Activity coefficients based on production, Chl a, biomass and P/B ratios were calculated. Data show that the production of North Atlantic Rift lakes considerably exceeds that of natural lakes on the Northern American and European continental plates at similar latitudes.
ISSN:0030-1299
1600-0706
DOI:10.2307/3545051