Náttúrufræðingurinn - 2018, Blaðsíða 63

Tímarit Hins íslenska náttúrufræðifélags
143
til að styrkur súrefnis sé minni við botn
á tímabilum þegar þéttleiki mýlirfa er
lítill á botni Mývatns. Aukið innstreymi
fosfórs frá botni eftir að köfnunarefni
þrýtur í vatninu getur nýst blágrænum
bakteríum, sem þá ná yfirhöndinni.
Þetta ferli getur því hugsanlega skýrt að
nokkru leyti það öfuga samband sem er
á milli mýlirfa (T. gracilentus) og blóma
blágrænna baktería (A. flos-aquae)
í Mývatni.

SUMMARY
The coupling between organic
activity and chemistry in
Lake Mývatn, Iceland
Lake Mývatn is one of the most fer-
tile lakes in the northern hemisphere,
despite being ice-covered for about 190
days a year. The lake is fed with spring
water, rich in dissolved constituents
of which many are essential nutrients.
Biological activity has a large impact
on the chemical composition of the
lake water since primary producers
consume dissolved nutrients until they
become limiting. Decay of organic mat-
ter then releases the nutrients back into
the water column where it can be taken
up by the next generation of primary
producers. The primary production of
green algae and diatoms in Lake Mývatn
is limited by nitrogen but cyanobac-
teria, which is often in high density in
the lake, is independent of the concen-
tration of bioavailable nitrogen in the
lake water, since cyanobacteria can fix
nitrogen from the atmosphere. Phos-
phorus is, in the end, the limiting nutri-
ent for primary production of cyano-
bacteria. Inflow of spring water is one
source of dissolved constituents to the
lake, of which some are essential nutri-
ents. Another source is the diffusive
flux from the bottom sediment which is
rich in most constituents. Many of these
constituents are however immobile in
oxidized conditions and precipitate at
the water-sediment interface. Phospho-
rus is one example, but the precipitation
of iron(oxy)hydroxide scavenges dis-
solved phosphorus from the lake water.
Thus, at oxidized conditions phospho-
rus is adsorbed on the surface of the
iron precipitates. Changes in the redox
state at the sediment-water interface
can thus have an impact on the inner
nutrient cycle of the lake. Several pro-
cesses can affect the oxygen level in the
benthic part of the lake. Chironomid
larvae, which live in tubes in the bottom
sediments, pump oxygenated water into
the top part of the sediment, aerates it
and by doing that, puts “a lid” on the
benthic flux of many dissolved constit-
uents into the lake water. On the other
hand, the decay of organic matter con-
sumes oxygen from the environment
and leaves it reduced. The mobility of
many metals (e.g. Fe, Mn and indirectly
PO4) increases at reduced conditions
and thus the inner nutrient circle is
faster at 1) reduced conditions, e.g. at
high organic matter decay, and 2) in
the absence of Chironomid larvae. This
observation can be one link to explain
the inverse correlation between the
density of Chironomid larvae (T. grac-
ilentus) and the cyanobacteria A. flos-
aquae which has been observed to occur
in lake Mývatn.
11. mynd. Klasar í Mývatni. Þessir hraundrangar stóðu eftir þegar bráðin hrauntjörn tæmdist í lok eldanna í Þrengsla- og Lúdentarborgum. Þau
eldsumbrot sköpuðu Mývatn fyrir ríflega 2000 árum. – Lava Pillars in Lake Mývatn. These pillars stood standing after a lava lake drained out in
the last phase of a volcanic eruption in Þrengslaborgir and Lúdentarborgir volcanones around 2000 years ago. Ljósm./Photo: Árni Einarsson.