Internal loading of nutrients and certain metals in the shallow eutrophic Lake Myvatn, Iceland

Abstract Sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice cover of 190 days per year. In situ, transparent and dark flux chambers were used for direct measurements of benthic fluxes of dissolved oxygen, nutrients, silica and certain metals, taking into account primary production and mineral precipitation. The range of benthic flux observed for dissolved oxygen ͑DO͒, dissolved inorganic carbon ͑DIC͒, ammonium, ortho-P, silica, calcium, and magnesium was -45.89 to 187.03, -99.32 to 50.96, -1.30 to 1.27, -0.51 to 0.39, -62.3 to 9.3, -33.82 to 16.83, and -23.93 to 7.52 mmol m , respectively ͑negative value indicating flux towards the lake bottom͒. Low benthic NH 4 ϩ and ortho-P fluxes were likely related to benthic algal production, and aerobic bottom water. Ortho-P fluxes could also be controlled by the dissolution/precipitation of ferrihydrite, calcite, and perhaps hydroxyapatite. The negative silica fluxes were caused by diatom frustule synthesis. Benthic calcium and magnesium fluxes could be related to algal production and dissolution/ precipitation of calcium and/or Ca,Mg-carbonates. Fluxes of DO, DIC, pH and alkalinity were related to benthic biological processes. It is likely that some of the carbon precipitates as calcite at the high pH in the summer and dissolves at neutral pH in the winter. Mean of the ratio of gross benthic DIC consumption and gross benthic DO production was 0.94 Ϯ 0.18, consistent with algal production using NH 4 ϩ as N source. During the summer weeks the water column pH remains above 10. This high pH is caused by direct and indirect utilisation of CO 2 , HCO 3 Ϫ , CO 3 -2 , H 4 SiO 4°a nd H 3 SiO 4 by primary producers. This study shows that in shallow lakes at high latitudes, where summer days are long and the primary production is mostly by diatoms, the pH is forced to very high values. The high pH could lead to a positive feedback for the Si flux, but negative feedback for the NH 4 ϩ flux

The effect of gross primary production, net primary production and net ecosystem exchange on the carbon fixation by chemical weathering of basalt in northeastern Iceland,

Abstract The overall objective of this study is to define and interpret the annual dissolved inorganic carbon (DIC) flux in selected river catchments in North Eastern Iceland. The flux stems primarily from chemical weathering of basalt. The DIC flux out of the catchments is compared with the spatial distribution of the various vegetation communities and their gross primary production (GPP), net primary production (NPP) and net ecosystem exchange (NEE). There is no correlation between the DIC flux and the GPP, but one between DIC and NPP. The DIC flux is highly dependent on the NEE, which in turn is governed by the area extent of wetlands in these catchments. A variation by a factor 5 of the NEE results in a variation by a factor 2.8 in the river dissolved inorganic flux.

Benthic oxygen flux in the highly productive subarctic Lake Myvatn, Iceland: In situ benthic flux chamber study.

Abstract In situ paired light and dark-stirred benthic flux chambers were used to estimate dissolved oxygen flux across the sediment-water interface in Lake Mývatn, Iceland. Three sampling stations were selected, each station reflecting a specific sedimentary environment, benthic communities, and water depth. During this study the phytoplankton density was low. Spatial and seasonal variations of bottom DO concentration and DO flux have been observed during this study. The oxygen consumption rate at all study sites had a mean of −89 (± 44) mmol m −2 d −1 while the oxygen production rate due to benthic algae had a mean of 131 (± 103) mmol m −2 d −1 . There was a strong correlation (r = 0.91) between oxygen consumption rate and temperature. This was presumably because of the temperature influence on rates of microbial and macrobenthic processes. The mean benthic primary production rate at all study sites was 1216 (± 957) mg C m −2 d −1 between June 2000 and February 2001. Annual gross benthic primary production was estimated from the gross mean daily benthic DO production (P) and Redfield's C:O 2 ratio of 106:138 to be 420 g C m −2 y −1 at station HO, 250 g C m −2 y −1 at B2 and 340 g C m −2 y −1 at station 95. Thus, the mean gross benthic primary production was estimated as 1151 mg C m −2 d −1 at station HO, 685 mg C m −2 d −1 at station B2, and 932 mg C m −2 d −1 at station 95